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1077-2618/14/$31.00©2014IEEE

LOBAL USERS OF ELECTRICAL

power equipment are increasingly given

the option of using equipment that

implements either the National Electrical

Manufacturer’s Association (NEMA)/the American

National Standards Institute (ANSI) or the International

Electrotechnical Commission (IEC) standards. In many

parts of the world, either set of standards may apply.

IEC and IEEE standards have come substantially closer

over the last decade. However, differences remain. In

this article, the authors will explain the differences in

the applicable standards and what those differences

might mean to a user. The intent is to provide the user

with an analytical basis upon which to understand the

© istockphoto.com/robert churchill

G

REVIEWING EQUIPMENT STANDARDS

by MArcElo VAldEs , Xuhu I rEn, shr IdhAVAl sAprE , MArty tr IVEttE , & stEVEn ME InErs

Digital Object Identifier 10.1109/MIAS.2013.2282565

Date of publication: 31 October 2013

Differences between the IEC and IEEE electrical equipment standards

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tradeoffs that may be made in choosing one set of stan-dards over the other.

Electrical Equipment StandardsFrom a myriad national standards for medium-voltage (MV) electrical equipment, two sets of standards emerge as predominant, those issued by the ANSI and IEC. ANSI standards predominate in North America and are generally composed of documents created by the IEEE or NEMA and adopted by ANSI as broadly accepted con-sensus standards. IEC standards predominate in Europe and much of the rest of the world, either as the accepted standard or as the predominant model for local national standards commonly used within a particular country. IEEE standards are written by IEEE working groups (WGs) sponsored by various IEEE committees, which are in turn sponsored by IEEE Societies, and in the case of C37 standards, sponsored by the IEEE Power & Energy Society’s Switchgear Committee. These committees and WGs are manned by industry experts from manufacturer, user, and consultant ranks that volunteer to create and maintain the IEEE standards. The final vote on IEEE standards is performed by members of the IEEE Stan-dards Association via an online vote handling process to assure balanced participation and inclusion of various perspectives and opinions.

IEC international standards are also consensus based and are written by manufacturing, government, user, and design representatives from many countries around the world. The IEC standard voting process is focused on bal-anced participation from the various participating coun-tries. Each member country has one vote to accept a proposed standard.

The main IEEE/ANSI standards family that defines MV switchgear, circuit breakers (CBs), and related tech-nology are the C37 standards [5]–[14]. Additional related standards are used by NEMA members to document test protocols and criteria for verification of adherence to the standards [17], [18]. The predominant IEC standards are the 62271 standards [19], [21]–[35] written by the 17A and 17C committees.

Over the last decade, these standards have come sig-nificantly closer; however, they remain different in sev-eral ways. Similarly, the equipment that is built to meet these standards and the needs and desires of the various markets that use the equipment also differ. In some mar-kets and applications, users have a choice of buying and installing equipment to either of these standards as well as local national standards. To state that one standard is better than or not as rigorous as the other would not be correct. They are different, and their differences merit recognition. In some cases, manufacturers offer equip-ment that predominantly reflects the traditions and requirements of one set of standards but is offered to the market as meeting the requirements of the other. Manu-facturers often design a product for one set of require-ments and then modify it to meet the requirements of the other. This type of hybrid product is becoming increasingly common in North American markets and is, in fact, influencing how the North American standards are evolving.

Common Standards ScopeBoth sets of standards rely on a base document for com-mon requirements [14], [21], a base document for CBs [7], [22], and a base document (or two in the case of ANSI equipment) for the equipment [9], [10], [35]. Both sets of standards also refer to a long list of other documents for test protocols, specific requirements for special applica-tions and definitions of various conditions, test require-ments, and other items that warrant separate treatment.

IEC 62271 SeriesIEC standards describe various subsets of devices cov-ered by various standards and limit the scope within those standards. The 62271 numbering system has organized the various relevant documents within the IEC standards into one numbering system. The scope of this article will be devices covered by the IEC 62271 family of standards:

▪ IEC 62271-1-2011-08, Common Specifications [21]

▪ IEC 62271-100-2008-04, Alternating Current Circuit Breakers [22]

▪ IEC 62271-200-2011-10, AC Metal-Enclosed Switch-gear and Controlgear for Rated Voltages Above 1 kV and up to and Including 52 kV [35].

This family of standards was first published in 2001 under a new organization using the 62271 prefix; however, they incorporated material from other now-superseded IEC standards that are several decades older. Table 1 lists the common ratings defined within IEC 62271-1 [21, Sec. 4, p. 32] and 62271-100 [22, Sec. 4, pp. 43–44], applicable to. CBs and associated switchgear for 50 or 60 Hz and applications above 1,000 V. CBs may be one or three pole for use in single- or three-phase systems. CBs with con-trolled per-pole mechanisms intended for nonsimultaneous operation are not included at this time but are under con-sideration for future inclusion. Devices that depend on manual operation for closing or opening are not included.

ANSI, IEEE, and NEMAANSI standards for switchgear are written by IEEE WGs, sponsored by the Switchgear Committee of the IEEE Power & Energy Society, and published by the IEEE. The ANSI adopts these IEEE standards. IEEE C37.06-2009 [7], IEEE C37.04 [5], [6] and the C37.20.# series [9]–[13] define the requirements for CBs and equipment. Other standards, C37.5# [15]–[18], define test protocols and acceptance criteria used by Underwriters Laboratories and significantly mimic the test sections of the C37.20 standards. Other C37 stan-dards define applications and other related parameters.

IEEE standards for switchgear assemblies are divided into three categories:

1) metal-enclosed (ME) power switchgear, further divided into three categories:a) low-voltage power CB (C37.20.1)b) metal-clad (MC) switchgear (C37.20.2)c) ME interrupter switchgear (C37.20.3)

2) ME bus (C37.23)3) control switchboards (C37.21).The IEC standards do not have an MC versus ME

gear. The IEC relies on two sets of definitions

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for partition class and service continuity; see the “Switchgear Type: MC, ME, and Service Continuity” section. ANSI MC switchgear is characterized by several features identified in C37.20.2 [9, Sec. 3.1.5, pp. 4, 5] that may be summarized as follows (italics are added by the author):

▪ Drawout self-aligning CB element with  a connected and disconnected position with self-coupling primary and manual or self-coupling secondary terminations. Optional within IEC standards.

▪ Major subcomponents and assemblies are completely enclosed by metal barriers with no intentional open-ings. The main interrupting device or mounting shall include a metal barrier, such that when a cubi-cle door is open on a closed CB, no primary live com-ponents are exposed. Optional within IEC standards.

▪ All live parts are enclosed within grounded metal compartments. Optional within IEC standards.

▪ Automatic shutters, which cover primary circuit elements, are used to cover the primary connections

TablE 1. baSIC SWITCHGEaR RaTINGS STIPUlaTED FOR IEC aND IEEE EQUIPMENT. IEEE c37.100.1-2007, sec. 4, p. 9

IEEE c37.20.2-1999, sec. 5, p. 6

IEc 62271-1-2011-08, sec. 4, p. 32

IEc 62271-200-2011-10, sec. 4, p. 15

a) Rated maximum voltage (V ) or (Ur )

a) Rated maximum voltage

a) Rated voltage (Ur ) a) Rated voltage (Ur ) and number of phases

b) Rated insulation level (Ud), (Us), (Up)

b) Rated insulation levels b) Rated insulation level b) Rated insulation level

c) Rated power frequency (fr )

c) Rated power frequency

c) Rated frequency (fr ) c) Rated frequency (fr )

d) Rated continuous current ( Ir )

d) Rated continuous current

d) Rated normal current ( Ir )

d) Rated normal current ( Ir ) (for main circuits)

e) rated short-time withstand current ( Ik)

e) rated short-time withstand (avg. rms current, 2 sec., 2.6 # rms-first cyc. pk.)

e) rated short-time withstand current ( Ik)

e) rated short-time withstand current ( ,I Ik ke ) (main and earthing circuits)

f) rated peak withstand current ( Ip)

f) Rated momentary with-stand (2.6 # Rated rms short-time withstand)

f) rated peak withstand current ( Ip)

f) rated peak withstand current ( ,I Ip pe ) (main and earthing circuits)

g) Rated duration of short circuit (tk)

g) Rated duration of short circuit (tk)

g) Rated duration of short circuit ( ,t tk ke) (for main and earthing circuits)

h) Rated supply voltage of closing and opening devices and of auxiliary ciruits (Ua )

i) Rated supply frequency of closing and opening devices and of auxiliary ciruits

j) Rated pressure of com-pressed gas supply for insulation or operation

IEEE Standard C37.06— Standard for ac High- Voltage Circuit Breakers Rated on a Symmetrical Basis, table 18, p. 38

h) Rated supply voltage of closing and opening devices and of auxiliary circuits (Ua )

i) Rated supply frequency of closing, opening devices and of auxiliary circuits

j) Rated pressure of compressed gas supply for controlled pressure systems

h) Rated values of the components forming part of the metal-enclosed switchgear and controlgear including their operating devices and auxiliary equipment

IEEE Standard C37.20.7— Guide for Testing Metal-Enclosed Switchgear Rated up to 38 kV for Internal Arcing Faults

j) Ratings of the internal arc classifications (IAc), if assigned by manufacturer.

k) rated filling level (fluid-filled compartments)

i) rated filling level (fluid-filled compartments)

—where multiple symbols are listed as “or,” the first is the IEEE symbol, the second is the IEc symbol.

—where "or" is not stated, the symbols are common.

—Arc-resistant(Ar) preferred ratings are covered by this c37.20.7 standard.

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when a CB is removed. Optional within IEC standards.

▪ Primary bus conductors and con-nections are covered with insulat-ing materials. Optional within IEC standards.

▪ Mechanical interlocks are provided for proper operating sequence under normal operating conditions.

▪ Instrumentation and wiring are iso-lated by grounded metal barriers from all primary circuit elements. Optional within IEC standards.

Within IEC equipment, many of these characteristics are currently avail-able, but some are not mandatory by standard in the base product, and hence the user should specify the features if they are desired. It is common in IEC regions to specify MC to gain these features. However, the traditional IEC MC con-struction does not fully satisfy the requirements of C37.20.2 MC construction. It is recommended to be specific on each feature desired when expecting that type of equipment.

The main IEEE standards germane to MV and high-voltage switchgear are as follows:

▪ IEEE C37.100.1-2007, Standard of Common Require-ments for High-Voltage Power Switchgear Rated Above 1,000 V [14]

▪ IEEE C37.04-1999 (R2006), Standard Rating Struc-ture for AC High-Voltage Circuit Breakers [5]

▪ IEEE C37.04b-2008, Standard for Rating Structure for AC High-Voltage Circuit Breakers Rated on a Symmetri-cal Current Basis Amendment 2: To Change the Descrip-tion of Transient Recovery Voltage for Harmonization with IEC 62271-100 [6]

▪ IEEE C37.06-2009, Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis-Preferred Ratings and Related Required Capabilities for Voltages Above 1,000 V [7]

▪ IEEE C37.20.2-1999, Standard for Metal-Clad Switchgear [9]

▪ IEEE C37.20.3-2001, Standard for Metal-Enclosed Interrupter Switchgear [10].

The ANSI standards establish a symmetrical current rating structure and construction requirements for all indoor and outdoor types of ac high-voltage CBs rated above 1,000 V. Similar to the IEC standards, they are only applicable to three-pole devices for three-phase systems and single-pole devices for single-phase systems. The CBs covered are for use at 50 or 60 Hz. Generator CBs are cov-ered in a dedicated standard, C37.013. Reclosers are cov-ered by C37.60-2003-IEEE standard requirements for Overhead, Pad-Mounted, Dry Vault, and Submersible Auto-matic Circuit Reclosers, and Fault Interrupters for Alternating Current Systems up to 38 kV, jointly published by IEC as IEC 62271-111:2005 in 2005.

Voltage Ratings and Tests in GeneralThe methods for testing the dielectric capability of switchgear are similar within the two standards and

mostly consist of two series of tests. One series is at power frequency, requiring a voltage to be applied for a fixed period of time, and the other series requires a voltage pulse to be applied with a defined rate of rise, duration, and decay. Both standards define different tests for dry versus wet conditions. Wet conditions apply to CBs specifically designed for outdoor applications, which are typically used for overhead lines. Outdoor CBs have traditionally been referred to in North America as distribution CBs. However, currently both the IEC and IEEE stan-dards refer to this class of devices as “S2-type outdoor circuit breakers.” S2-type devices are not the main sub-ject of this article.

The standards also include CBs for applications above 38 kV. This arti-

cle, however, shall focus on devices rated for application at 38 kV and below in MC or ME equipment, which has traditionally been called MV switchgear in North America and within IEC markets as well.

Service ConditionsService conditions within both sets of standards are defined as normal and special as well as usual and unusual within IEEE documents. Normal conditions are divided into indoor and outdoor in both standards. The defined condi-tions are mostly defined within the “common” documents and seem to closely align between the two sets of docu-ments. Both standards put the responsibility to identify special conditions on the specifier or user.

Temperature and Humidity (IEC 62271-1, Sec. 2.1.1, 2.1.2, and 2.2.3; IEEE C37.100.1, Sec. 2.1.1, 2.1.2, and 2.2.3; and IEEE C37.20.2, Sec. 8.1.4.3)Both standards provide for an upper-normal temperature of 40 °C. The IEC standard also defines the 24-h average maximum temperature as 35 °C. For both standards, the normal maximum is the same for indoor and outdoor equipment.

For outdoor equipment, the normal low-temperature preferred values are defined by the IEC standard as -10  °C, -20 °C, -30 °C, and -40 °C. For indoor equipment, the preferred minimums are -5 °C, -15 °C, and -25 °C. This indicates that user specifications need to identify which temperature rating is desired. The IEEE standard only defines one normal range as +40 °C to -30  °C. Both standards define special conditions as -50 °C to 40 °C for very cold climates and 5 °C to 55 °C for very hot climates. The IEEE standard also identifies the rating for cold climates as -40 °C to 40 °C and that for hot climates as -15 °C to 50 °C. Since multiple rat-ings are possible under each standard, it is up to the speci-fications to refer to the proper rating for the application when the equipment is specified.

The IEC standard defines normal solar radiation on outdoor gear at 1,000 W/m2. The IEEE standard

MAnuFActurErs oFtEn dEsIgn A product For

onE sEt oF REquIREmEnts

And thEn ModIFy It to

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thE othEr.

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defines normal as 1,040 W/m2. IEEE provides guidance on how to account for higher levels of solar radiation in IEEE C37.24. The IEC standard refers to IEC 60721-2-4 [39].

Normal indoor relative humidity conditions are defined in both standards as not to exceed an average daily value of 95% and an average monthly value of 90%. The IEC standard also defines values for water vapor pressure: 2.2 kPa maximum daily average and 1.8 kPa maximum monthly average. Both standards indicate that particularly dry or humid conditions can affect equipment dielectric performance, and allow-ances should be made for such environments. Both standards suggest a space conditioning or heating apparatus to minimize the effect of condensation or extreme humidity.

Altitude (IEC 62271-1, Sec. 2.2.1; IEEE C37.100.1, Sec. 2.2.1; and IEEE C37.20.20, Sec. 8.1.3)Both sets of standards provide for altitude correction factors to be used to derate the dielectric capability of equipment when used at altitudes above 1,000 m that both define as normal. However, the formulas differ slightly, as the for-mula within IEC 62271-1 [21, Figure 1, p. 15] assumes a starting point of 1,000 m and the formula in IEEE C37.100.1 [14, Sec. 2.2.3, p. 6] is applicable from sea level. The IEEE guidance in C37.20.2 extends the range to 6,000 m above sea level; the IEC information in 62271-1 and C37.100.1 only extends up to 4,000 m above sea level. Both reference IEC 60071-2. IEEE C37.20.2 [9, Table 8, p. 40] provides a table with dielectric correction factors and continuous current correction factors from 1,000 to 6,000  m. IEEE C37.20.3 only provides factors up to 3,000 m for ME switchgear [10, Table 5, p. 29].

Pollution (IEC 62271-1, sec. 2.1.1, 2.1.2, and 2.2.2; IEEE C37.100.1, sec. 2.1.1, 2.1.2, and 2.2.1; and Annex C)Under the normal conditions for indoor equipment, the air is expected to be relatively free of pollutants. Within the IEEE standard, an annex providing guidance for classi-fying environments for relative pollution levels is pro-vided. Table C1 within the annex identifies four levels of pollution and the associated minimum required creepage distances. The minimum level is considered normal for indoor gear, and level II is considered normal for outdoor gear. The IEC standard defines normal as none for indoor and level II as described in IEC 60815 [36, Sec. 8.3, p. 17] for outdoor. Special conditions within the IEC standard are levels III and IV, as defined within IEC 60815. Both stan-dards refer to IEC 60071-2:1996 for data on how to deal with pollution to achieve proper dielectric performance.

Vibration, Shock, Tilting—Seismic (IEC 62271-1, Sec. 2.1.1, 2.1.2, and 2.2.4; I EEE C37.100.1, Sec. 2.1.1 and 2.2.4; and IEEE C37.20.2, Sec. 8.1.4.6)Both standards define normal conditions as relatively free of vibrations, shock, and tilting. Seismic activity is expected to be negligible. Special conditions are to be identified by the user. Where seismic activity is expected, both standards refer to other documents for definitions of severity level. The IEC standard refers to IEC 62271-300

or IEC 62271-2. The IEEE standard refers to IEEE 693 for severity level definitions and IEEE C37.81 for nuclear applications. The IEEE standard also mentions that the user must identify what the operational requirements are for seismic events, i.e., is the gear only required to operate properly after an event, class 1 per IEEE C37.20.2 [9, Sec. 8.1.4.6, p. 43], or during and after the event, class 2 [9, Sec. 8.1.4.6, p. 43]?

The user should identify the operational requirement regardless of the equipment standard specified.

Other Service Condition RequirementsWind is handled similarly within both standards, with 34  m/s (76 mi/h) [14, Sec. 2.1.2 f, p. 14] within IEC standards and 40 m/s (90 mi/h) [21, 2.1.2 f, p. 5] within IEEE standards considered normal. Higher wind speeds would be considered special, and no preferred ratings above 40 m/s are proffered by either standard.

Permissible ice coating for outdoor equipment is mentioned in both standards. Normal is defined as up to 20 mm for the IEC standard. The IEEE standard defines three classes of ice coating: up to 1 mm, up to 10 mm, and up to 20 mm, as class 1, 10, and 20, respectively.

Exposure to damaging fumes and dust of various kinds requires proper coatings as well as heaters for the former and proper enclosure accommodations for the latter. Totally enclosed nonventilated enclosures may be required. The current ratings may be decreased by as much as 30%, depending on design. Forced ventilation with appropriate filters may be required in other cases. Within IEC stan-dards, dust ingress control is provided by an IP5X degree of protection, where the 5 indicates dust protection as well as a 1-mm protection against access to hazardous parts [14, Table 7, p. 55].

Service Conditions SummaryBoth standards define normal and special conditions very similarly, with minor differences. The main conclusion is that neither standard seems to offer significantly different capability, and both put the responsibility on the user to identify unusual conditions. Both standards also advise the user to provide conditions that are as close to normal as possible, especially with respect to temperature, humidity, and pollution. IEEE C37.20.2 [9, Sec. 8.1.4, p. 41] advises that “the emphasis should be on eliminating such conditions, if at all possible. However, if these undesirable conditions cannot be eliminated, more frequent mainte-nance may be required.”

Maximum Application Voltage for EquipmentThe IEC standards list two voltage ranges: series I, which includes the traditional IEC voltage ratings, and series II, which adds voltages found in other markets such as North America. The IEEE standards have one list of preferred voltages described in C37.20.2 and C37.20.3. Both lists are shown in Table 2. When applying equipment, the rated voltage must be equal to or higher than the expected maximum system voltage, and attention must also be paid to insulation levels described later in this article.

IEEE C37.100.1-2007 [14] includes a broader list of voltages ratings harmonized with the IEC 62271-1

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ratings. However, these voltages are described as “the upper limit of the highest voltage of systems for which the switchgear is intended.” This same IEEE standard suggests a list of voltage ratings to be used in future IEEE switchgear standards that is fully har-monized with IEC 62271-1 with respect to rated maximum voltage and rated insulation levels.

There is one noticeable difference in the test requirements between the two sets of standards. IEEE C37.09-1999(R2007) includes single-phase tests at 58% V [8, Table 1, p. 17]. The IEC standards do not include similar tests.

basic Switchgear RatingsRatings for equipment are generally covered by two sets of documents: 1) the common requirements, IEC 62271-1 [21] and IEEE C37.100.1 [14], and 2) the switchgear-spe-cific requirements, IEC 62271-200 and IEEE C37.20.2 [9]. IEEE covers arc-resistant (AR) switchgear testing in a separate document, IEEE C37.20.7 [13]. The IEEE guide for testing does define preferred AR ratings for current and time: equal to short circuit withstand for current [13, Sec. 4.2, p. 5] and 0.5 s for time [13, Sec. 4.3, p. 6]. The IEEE guide states that 0.1 s is the minimum recom-mended time, and more than 1 s is considered unnecessary. The various ratings specified within these standards are summarized in Table 1. This article will only discuss rat-ings applicable for equipment rated at 52 kV and lower.

Rated Insulation LevelsInsulation levels refer to various measurements of dielectric capability under a variety of test conditions. The test condi-tions include specific ranges of temperature, humidity, alti-tude, and air pressure that are similar across both sets of standards and fall within the normal equipment service con-ditions. It is important to realize that if service conditions are not normal, the rated insulation levels may be affected. Lack of maintenance, the build-up of dirt, cumulative effect of pollutants, cumulative effect of high temperatures, or temperature cycling can affect insulation performance.

Rated insulation levels are defined three ways: light-ning impulse (U p ), often referred to as BIL in IEEE mar-

kets; switching impulse (Us ), which applies to circuit breakers above 245 kV (i.e., not the MV CB used in MC equipment discussed in this article); and power frequency (Ud ). The volt-age ratings can then be tested against phase to ground (earth), phase to phase, and across the switching device. Tests, in some cases, are also applied across the isolating distances where devices are drawn into a test or disconnect position. The preferred rated insulation voltages and associ-ated maximum rated voltages are shown in Table 3.

One difference between the relevant IEC and IEEE standards is that the IEC standards identify two levels of impulse withstand for each rated voltage. The higher withstand ratings generally exceed or match the closest equal or lower IEEE-rated voltage, and the lower IEC withstand levels generally are lower than those for the next lower or equal IEEE-rated voltage. When specifying or accepting IEC equipment in lieu of IEEE equivalent equipment, the available withstand voltage should be evaluated. Another difference is that the IEC standards allow 3% tolerance around test parameters. The IEEE test procedures do not allow any parameters to be below rated values during qualifying tests.

For use at common voltages in North America, equiva-lent slightly higher rated voltage IEC gear may be applied. However, there are small differences in the dielectric test levels between the various ratings when using the higher of the two choices available at these ratings in IEC equipment (Table 3). In North America, equipment selected for 2.3-kV applications is usually suitable for 4.16-kV applications and is rated per the 4.76-kV IEEE requirements. These exceed the IEC 3.6-kV requirements but align well with the IEC 7.2-kV-class requirements. At 6.9-kV, North American users implement 8.25-kV- or 15-kV-class equipment, both of which exceed the dielectric ratings of equivalent 7.2-kV equipment. For applications at 12–13.8 kV, IEEE offers 15-kV-class equipment, and IEC offers 17.5-kV-class equip-ment. The dielectric ratings for both of these are very simi-lar. For applications at 21 kV, ANSI offers a 27-kV rating, and IEC offers a 24-kV rating. The dielectric requirements are similar. At 27, kV ANSI offers a 27-kV class of equip-

ment, but in IEC ratings, one needs to jump to 36-kV class equipment. At this rating, the 36 kV-equipment designed with lower insulation levels would be comparable to the ANSI 27-kV class of equipment. For 34.5-kV app l i c a t ions , the ANSI 38-kV class and the IEC 36-kV class at higher insulation levels are comparable.

Rated Power Frequency ( fr )IEC 62271-1 identifies possi-ble frequencies as 16 2/3, 25,

TablE 2. SWITCHGEaR VOlTaGE ClaSSES.

IEC series I (kV)

3.6 7.2 12 17.5 24 36 52

IEC series II (kV)

8.25 15 15.5 25.8 27 38 48.3

IEEE (kV)

4.76 8.25 15 27 38

From IEEE 62271-1, IEEE c37.20.2 & c37.20.3, IEc 62271-200 refers to –1 for these ratings.

IEEE stanDaRDs ArE wrIttEn by IEEE workIng

GRoups sponsorEd by VArIous IEEEcoMMIttEEs.

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50, and 60 Hz. IEEE C37.100.1 identifies 50 and 60 Hz as preferred. However, IEEE C37.20.2 and C37.20.3 base all ratings and test criteria on 60 Hz only.

Application at 50 Hz based on 60-Hz ratings, or the opposite, may imply changes in the device capabilities. For example, higher frequencies may negatively impact temperature rise, and lower frequencies my impact peak current related ratings. Whenever possible, the ratings used to evaluate application should be based on the actual application frequency or adjusted based on manu-facturer recommendations. When converting from cycles to milliseconds, the appropriate period should be used. The relay timing may be in milliseconds or cycles; the manufacturer should be consulted if there is doubt on how to apply.

Rated Continuous Current (Normal)The rated current and temperature rise are integrally related. The rated current is the current that conductors within the equipment are able to carry continuously without any parts of the equipment exceeding allowable temperature rise or allowable maximum temperature while the equipment is operated within normal service conditions.

IEC 62271-1 uses the IEC 60059 R10 series of values. The values in this series are listed in Table 4. The first col-umn is the R10 series, with each number approximately 25–28% larger than the previous. The ratings are created by multiplying the R10 series by ten, raised to a whole

TablE 3. EQUIPMENT VOlTaGE ClaSSES aND aSSOCIaTED DIElECTRIC RaTINGS, INDOOR, S1 Cbs.

na application Voltages

rated Voltage V or Ur (kV rms)

power Frequency withstand Ud (kV)

Impulse withstand Up (kV peak)

Common Isolating Gap Common Isolating Gap

ansI IEC (sI) ansI IEC ansIa, b IEC ansI IEC ansIa, b IEC

column 1 2 3 4 5 6 7 8 9 10 112.3 kV 3.6 10 12 20 23

40 464.16 kV 4.76 19 20.9 60 666.9 kV 7.2 20 23 40

604670

8.25 36 39.6 95 104.5

12.47 kV,13.8 kV

15 36 39.6 95 104.5

17.5 38 4575 8595 110

21 kV 24 50 6095 110125 145

21 kV, 27 kV 27 60 66 125 137.5

34.5 kV36 70 80 145 165

170 19538 80 88 150 165

atest not required if equipment has grounded metal shutters.b110% of common voltage withstand.

TablE 4. R10 NUMbER SERIES USED TO DERIVE VaRIOUS IEC CURRENT RaTINGS.

r10 number series, IEc 60060101 102 103 104

1 10 100 1,000 10,0001.25 12.5 125 1,250 12,5001.6 16 160 1,600 16,0002 20 200 2,000 20,0002.5 25 250 2,500 25,0003.15 31.5 315 3,150 31,5004 40 400 4,000 40,0005 50 500 5,000 50,0006.3 63 630 6,300 63,0008 80 800 8,000 80,000preferred ratings are in bold. other rated current values such as short-circuit interrupting current are also from the r10 series, specifically the 104 columns.

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integer power. In the case of continu-ous current ratings, the exponent may be two or three.

The preferred ratings for IEC and IEEE equipment and CBs are shown in Table 5. IEC standards allow any number in the R10 series to be used. Larger device ratings such as 4,000 A may be fan assisted (cooled).

The allowed temperature rise is based on a normal service condition of 40 °C ambient in both sets of stan-dards.  IEEE C37.100.1 and IEC 62271-1 are substantially harmonized and may be considered relatively equal when similar materials and equipment technology are compared.  However, C37.20.2 limits temperature rise in bolted connections to 65 °C above ambient [9, Table 3, p. 9] versus the 75 °C that the harmonized “Common Requirement” standards allow. IEEE C37.20.3 has the same 65 °C require-ment. When a specific standard describes different requirements than the “common requirements” the iden-tified requirements take precedence.

Rated Short-Time Withstand Current (Ik), Rated Peak-Withstand Current (Ip), Rated Interrupting Current (Isc), and Rated Close and Latch Current (Rated Short-Circuit Making Current in IEC Terms)The rated short-time withstand cur-rent is the maximum rms current that the equipment can carry for a specified period of time under pre-scribed “normal” conditions of use and behavior. Within IEC standards, the short time withstand currents may be selected from the R10 series of numbers.

For IEEE standards, preferred rat-ings may be seen in C37.06-2009 [7, Table 1, p. 4]. Rated short-circuit interrupting amperes and rated short-time current are the same in this table. The interrupting time for the CBs may be three or five 60-Hz cycles, as specified by the manufacturer. The maximum permissible trip-ping time delay is 2 s. Table 6 shows the preferred cur-rent values for the associated IEEE preferred voltage and continuous current ratings. Application at 50 versus 60 Hz changes the peak current that the CB experiences during fault conditions. That difference is small and is shown in Table 6. I p and rated close and latch current are the same. The peak-withstand current for IEC and IEEE standards is essentially the same. Short circuit cur-rents are made up of a symmetrical rms component and a dc component (dc offset) that is more substantial in MV

applications than is typical for low-voltage applications. The degree of dc offset is the same for IEC and IEEE standards, with the only difference being driven by the application frequency and longer dc time constants used for special application with high X/R ratios. The stan-dard time constant (L/R) is 45 ms in both standards with the IEC standards identifying longer special constants of 60, 75, and 120 ms.

Rated Duration of Short Circuit ( tk)The time that the switchgear or CB can carry a current equal to its rated short-time withstand current is called

TablE 5. PREFERRED CONTINUOUS CURRENT RaTINGS FOR IEC aND IEEE Cb. Equipment/ Device

rating in rms A, normal service Conditions standard

IEC

switchgear 630 1,250 1,600 2,000 3,150 4,000 62271-1-2007

cbs 630 1,250 1,600 2,000 3,150 4,000 62271-1-2007

IEEE

mE switch-gear

600 1,200 2,000 c37.20.3-2001

mC switch-gear

1,200 2,000 3,000 4,000 c37.20.2-1999

cbs 1,200 2,000 3,000 4,000 c37.06-2009

TablE 6. PREFERRED CURRENT RaTINGS (S1 Cb).

Ur kV rms

Rated Continuous Current ( Ir ) a rms

rated short- Circuit and short-time cur-rent ( Ik) kA rms

Rated Closing and Latching a ( Ip), kA

60 hz 2.6X

50 hz 2.5X

4.76 1,200, 2,000 31.5 82 79

1,200, 2,000 40 104 1001,200, 2,000, 3,000, 4,000 50 130 1251,200, 2,000, 3,000, 4,000 63 164 158

8.25 1,200, 2,000, 3,000 40 104 10015 1,200, 2,000 20 52 50

1,200, 2,000 25 65 631,200, 2,000 31.5 82 791,200, 2,000, 3,000 40 104 1001,200, 2,000, 3,000 50 130 1251,200, 2,000, 3,000, 4,000 63 164 158

27 1,200 16 42 401,200, 2,000, 3,000 25 65 63

38 1,200 16 42 401,200, 2,000, 3,000, 4,000 25 65 631,200, 2,000, 3,000, 4,000 31.5 82 791,200, 2,000, 3,000, 4,000 40 104 100

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the rated duration of short circuit in IEC and IEEE stan-dards. The standard value for IEC standards is 1 s; for IEEE it is 2 s. However, the IEC standards allow other val-ues to be used. The preferred values are 0.5, 2, and 3 s.

IEC 62271-100 adds that self-tripping CBs need not specify a value for tk , provided that when connected in a circuit the CB shall be able to carry the prospective cur-rent for the maximum time the integral trip could take to operate. This is also addressed in IEC 62271-1 Section 3.4.118, where a definition of self-tripping is provided as “any CB that is tripped by current in the main circuit without the need for auxiliary power” [22, p. 28]. This is similar within North American standards to the perfor-mance provided by a molded case CB with integral trip-ping or any CB with override protection. The CB provides its own protection and, hence, it does not need to specify a time limit to guide external relay application. Whenever

applying this type of CB, it is incumbent on the system designer to understand the self-imposed time limitation this type of CB may have and what the effect will be on expected coordination (discrimination). CBs with integral trip capabilities are available for North American markets as well, although the sensor locations may vary and may not be integral to the CB mechanism as they are in low-voltage CBs.

Rated Supply Voltage (Ua) and Rated Supply Frequency, for Secondary DevicesThe rated values for frequency in both standards are dc: 50 Hz and 60 Hz. The control power tolerances are gener-ally 85–110% for IEC devices, specified differently for tripping versus other for IEEE. Within IEEE S2, CBs have different ratings, and CBs with self-contained dc control also have different ratings. Table 7 provides a detailed list

TablE 7. CONTROl POWER PREFERENCES.

IEc VdcIEEE and IEc Vdc

Expected supply tolerance IEC and IEEE IEEE IEC

85% 110%Closing and auxilliary tripping

Closing and auxilliary tripping

24 20 26 14–28 20–26 17–2648 41 53 38–56 28–56 41–53 34–53

60 51 66 51–66 42–66110 or 94 121 94–121 77–121

125 100–140 70–140 106–138 88–138220 or 187 242 187–242 154–242

250 200–280 140–280 213–275 175–275IEEE note 1— Equipment having self–contained dc control sources shall operate over the range of 85–115% of nominal

voltage and the table above shall not apply.

IEEE note 2—c37.06, table 18, p. 38 identifies slightly lower dc V for s2 outdoor cb (38–>36 V, 100–90 V, 200–>180 V).

IEc Vac IEEE and IEc VacIEC tolerance Expected

IEEE tolerance Expected

auxilliary devicesand functions shalloperate up to thestated tolerances

120 102–132 104–127380/220 323/187–418/242230 196–253400/230 340/196–440/253415/240 353/204–204/264

240 204–264 208–254480 408–528 (1)208/120 177/102–229/132 180y/104–220y/127240d 204–264 208–254480d 408–528 (1)480/277 408/235–528/305 (1)

600/347 510/295–660/382source: IEc 62271-1-2007 & c37.100.1-200.

(1) per IEEE c37/20.2, sec. 7.2.8, p. 33, >254 Vac and >280 Vdc control or instrument voltage is not allowed.

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of preferred voltages, expected supply tolerances, and expected operated range for auxiliary devices and functions. The different standards reflect common sec-ondary control voltages used within the various markets as well as the different tolerance allowances. During the opera-tion of a device, the control power volt-age must remain within the expected tolerance.

The differences are not significant, and as in any CB control system, it is imperative that control power be within tolerances and highly reliable to ensure that devices operate when called upon to do so, even under serious fault condi-tions [40].

Operating SequenceOperating sequence is similar in both standards, either defined as Open-t-Close-Open-t'-Close Open or Close-Open-t''-Close-Open. The IEC standards define t and t' as follows:

▪ t = 3 min for CB not intended for rapid auto reclosing

▪ t = 300 ms for CB intended for rapid reclosing

▪ t' = 3 min, alternatively 15 or 60 s for CB intended for rapid reclosing

▪ t'' = 15 s for CB not intended for rapid reclosing.IEEE uses the O-t-CO-t'-CO sequence for rapid auto

reclosing and CO-t''-CO for CB not intended for rapid reclosing, with the following t values:

▪ t = 300 ms

▪ t' = 3 min

▪ t'' = 15 s.Most vacuum CBs in industry today meet the rapid

reclosing requirements. However, these requirements are different from those imposed on reclosers.

Classes of Cbs and Special applicationsThe IEC 62271-100 [22, Sec. 3 and 4, pp. 28, 29] stan-dard specifically addresses several types of CBs above 1,000 V. IEEE standards have equivalent ratings for most of these. Table 8 is a summary of the various CB types and the main standard references.

Transient Recovery VoltageTransient recovery voltage (TRV) is the voltage impressed across a switching element’s contacts by the current interruption process interacting with the source and load circuit impedances and available power. This is a complex subject of significant importance for distribu-tion CBs (S2-type CBs) and breakers in particularly stressful applications where out-of-phase conditions, sig-nificant capacitor switching, or overhead cable connec-tions are required. Typical S1-type indoor CBs (Figure 1) used in industrial and commercial applications have less demanding needs.

The IEEE WG has significantly aligned TRV standards with IEC requirements over the last decade, resulting in IEEE C37.04b-2008 [6], Amendment 2: To Change the

Description of Transient Recovery Voltage for Harmonization with IEC 62271-100 [22] and C37.06-2009 [7], which has greatly incorporated the IEC methodol-ogy, particularly for S1 CBs below 100 kV. The interested reader is encouraged to also read IEEE C37.011-2005 IEEE Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers.

Maintenance Grounding—Earthing Switches Versus Ground and Test DevicesMaintenance grounding accommoda-tions provided by IEC switchgear ver-sus IEEE switchgear vary. IEEE switchgear typically relies on acces-

sory devices called ground and test (G&T) devices defined in C37.20.6-2007 [12]. These devices may be inserted into a CB cubicle and used to effectively ground and short circuit either the main bus in the switchgear or the load side connections of a switchgear cubicle, i.e., the load circuit. The G&T devices may be manually or elec-trically operated. The electrically operated devices are rated with a close-and-latch capability and may or may not have an open or interruption capability. They must be applied so their close-and-latch capability matches or exceeds system requirements. If an interruption capabil-ity is required, the user must specify it.

Manually operated devices do not carry a close-and-latch rating and must be operated on a system already known to be de-energized. Devices may be provided with three or six rear terminals. Devices with six terminals will have a way to select which three are grounded. The selection is made when the device is “not” installed in the gear. Shutters protecting the potentially energized front terminals shall be interlocked with the grounding termi-nal mechanism. The device must have position indication that is fully visible when installed in the switchgear [12, Sec. 6.2, p. 5]. A power-operated G&T device shall have a colored position indication clearly stating if the device is closed-grounded, or open-ungrounded. A power-oper-ated G&T may have a user selector switch, which can only be operated outside the equipment, to select which terminals are  grounded. Devices that employ stored energy mechanisms for operation shall have a charged/discharged status indication.

G&T devices must have mechanical interlocks so they cannot be installed in higher-rated CB cubicles that exceed the G&T device’s rating. The devices must also have inter-locks that prevent them from being racked-in or racked-out in the closed position. Many G&T devices will include test ports for access to measure voltage. Test ports are not required by standard, but if provided, they must have bar-riers to prevent inadvertent access. User-defined interlocks to ensure that the G&T devices are operated under proper system topology conditions are normal but should be care-fully specified. Part of the system design process is to identify the interlock sequence that is needed for proper system interlocking.

spECIaL ConDItIons

wIthIn thE IEc stanDaRD aRE

lEVEls III And IV, As dEFInEd

wIthIn IEc 60815.

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TablE 8. Cb TYPE DESCRIPTIONS aND REFERENCES.

IEC Definition

62271-100 2007

IEEE Definition

c37.06 2009

s1 cb intended for cable system protec-tion, typical commercial and industrial

sec. 3.4.119

Equivalent defined similarly in IEEE c37-06.

sec. 5, pp. 4–10

s2 cb intended for line (overhead) pro-tection, typical of utility applications and very large distribution systems. Also similarly referred to in IEEE c37.06-2009.

sec. 3.4.120

Equivalent defined similarly in IEEE c37-06.

sec. 6, pp. 11–18

E1 cb with “basic” electrical endurance sec. 3.4.112

standard cb as defined in c37.06. s2 cb at 15.5kV and higher have different requirements.

sec. 9, pp. 35 and 36

E2 (1) cb with “extended” electrical endur-ance. required for auto reclosing distri-bution cb rated # 52 kV, optional for those not required to perform auto reclosing

sec 3. 4. 113

Limited reclosing application discussed for indoor and out-door cbs intended for external relaying control. reclosers with dedicated integral controls intended for network applica-tions defined in c37.60.

sec. 11, p. 40

c1 cb with “low” probability of restrike during capacitive current breaking as demonstrated by specific type tests

sec. 3.4.114

IEEE c37.06-2009 defines s1 cb as c0 (general purpose).

table 4, pp. 8–10 and table 8, pp. 16–18c2 cb with “very low” probability of

restrike during capacitive current breaking as demonstrated by specific type tests

sec. 3.4.115

IEEE defines an optional s1 cb as c1 or c2 (formerly definite purpose). c1 and c2 cb have greater capacitive inrush capability, per table 4 (<100 kV). similarly for s2 cb, table 8.

M1 cb with “normal mechanical endur-ance” type tested for 2,000 operations

sec. 3.4.116

no similar classification. All c37.06 compliant cb must meet 2,000 operations with no maintenance.

sec. 9, pp. 35–37

M2 cb with “extended mechanical endur-ance” type tested for 10,000 opera-tions requiring only limited mainte-nance during operations

sec. 3.4.117

standard classification. All c37.06 compliant cb must meet 10,000 operations with minimal maintenance.

self- tripping

cb that is tripped by a current in the main circuit without the aid of any form of auxiliary power (commonly done with low-voltage cb)

sec. 3.4.118

Mentioned within c37.06 but not specifically defined by a dedicated standard to be substantially different than standard c37.06 compliant devices except with respect to control power and trip control source (relaying).

c37.100.1, sec. 4.8.2 7

Inductive switching cb for switching of highly inductive loads

62271-110

Capacitor bypass switching

cb for use as bypass switches for line series capacitors

62271-109 and 60143-2

notes—IEc: (1) number of operating sequences varies based on % short circuit current, earthing, and other factors. see table IEc 62271-100, sec. 6.112.2, pp. 163, 164.

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Design endurance tests are made per Section 7 of C37.20.6. For electrically operated G&T devices, tests are 500 insertions and removals from a cubicle and 500 open/close operations including five operations each at maxi-mum and minimum control voltage. Routine mainte-nance is allowed after 250 operations with no parts replacement. For manual devices, the tests include 500 insertions/removals from the cubicle and 250 operations with maintenance allowed at 125 operations.

Within IEC standards, grounding (earthing, in IEC terms) switches are often fixed mounted in the equipment and may be provided on a per-circuit basis within each CB assembly. Though removable G&T devices are available for enclosed switchgear, they are not commonly used. IEC 62271-102-2003 [24], [25] is the standard for earthing devices. This standard covers a wide range of applications including high-voltage distribution line applications.

IEC earthing devices are rated for 2,000 operations and have withstand ratings to match the CB capabilities. How-ever, different close and latch ratings are possible, E0, E1, and E2, each rated at zero, two, or five times carrying capacity, respectively. The lowest rating, E0, is the most commonly offered by manufacturers. The CB-earthing switch combination must have interlocks to ensure that the grounding switch is never closed while the corresponding CB is closed. In the case of a separate permanently installed grounding switch, the interlocks must also be supplied and coordinated with the system source devices to ensure a live bus is never grounded and proper sequencing is observed. If there are multiple sources, electrical interlocks may be required to enforce proper sequencing.

Switchgear Type: MC, ME, and Service ContinuityIEEE standards divide indoor-type MV switchgear into two types, MC defined by IEEE C37.20.2 and ME switchgear defined by IEEE C37.20.3. IEC relies on the service continuity definitions provided in IEC 62271-200-2011 [35, Sec. 8.103.3, pp. 52–54].

Within the IEC definition, there are two major catego-ries, LSC1 and LSC2, with LSC2 divided into three subcat-egories, LSC2, LSC2A, and LSC2B. None of the definitions limit the CB to a stationary or draw-out configuration. IEC definitions are as follows:

▪ LSC1: Not intended for maximum service continuity during the opening of any accessible compartment and may require complete disconnection from sources before opening any compartment doors.

▪ LSC2 (family): Intended to allow maximum continuity of service. Opening of accessible high-voltage com-partments in a functional unit is possible while other functional units of the same section are energized.

▪ LSC2: Minimum requirement is that it is possi-ble to open the connection compartment while keeping the bus bars live. There may or may not be other accessible high-voltage compartments.

▪ LSC2A: All accessible high-voltage compart-ments, other than the main bus bar compart-ment, can be opened with the bus bars energized.

▪ LSC2B: In addition to the requirements of LSC2A, the high-voltage connections (e.g.,

cables) to the functional unit being accessed may be kept energized. This implies that there also is a point of disconnection, as well as proper parti-tioning, between the accessed compartment and the high-voltage connections.

Note that the definitions above do not specify drawout or stationary, nor the location and type of earthing switches. Figures 2–4 are from IEC 62271-200 [35, Sec. 8.103.3, p. 54] and show various possible CB, earthing switch, section barriers (compartmentalization), and isolation or drawout connections combinations under the standards definitions. These are not the only possible variations. Users may specify CBs without earthing switches and may rely on earthing trucks similar to those commonly used in ANSI applica-tions or may use earthing switches for CB and feeder cable earthing and a separate truck for main bus earthing. It is interesting to note that the strictest IEC definition for com-partmentalization and isolation of the various devices and sections may be achieved with or without a drawout ele-ment. When implementing a stationary CB with this degree of compartmentalization it is implied that the CB requires little maintenance when used as expected, is rela-tively easy to remove, or can be maintained in situ. The nor-mal IEEE expectation of having CB elements easily removable for maintenance or replacement is not easily achieved without drawout elements.

Earthing switches may have two or three positions. Switches used in series with CBs often have three positions, the third for isolation only. Earthing and isolation switches fulfill the role of the drawout function, shutter function and grounding function. In industrial applications, drawout equipment of the LSC2B type is most common.

Switchgear Bus

CB Line-SideEarthing Switch

Interlock System toEnsure Main Bus IsNot Grounded

CB Load-SideEarthing Switch

Load-Side Cable Connections

CB Contacts

a simplified IEC Cb symbol as may be used in MV indoor switchgear.

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The IEC standard (62271-200) also defines partitions as PI (nonmetallic), [35, Sec 3.109.2, p. 11] or PM (metal-lic) [35, Sec. 3.109.1, p. 11]. Both types shall provide IP2X protection per IEC 62271-1 [21, Table 7, p. 55]. Conductors passing through the partitions shall be pro-vided with suitable bushings or insulation. Metallic barri-

ers (PM) and shutters (if intended as earthed) shall be earthed so that no electric field is present in the un-ener-gized open compartment.

The market traditionally served by switchgear meeting IEEE standards has had two clearly differentiated prod-ucts available. MC switchgear based around drawout vac-uum CBs and ME switchgear based around fused switches. However, in the last decade, various products demonstrating an IEC influence have appeared in the market, most importantly, the use of stationary CBs within equipment meeting ME standards. IEEE C37.20.4 is for the switches used in the ME switchgear. C37.20.3 is in the process of revision today to address the use of fixed and drawout CBs. Future versions of ME switchgear will incorporate CBs installed and meeting this new revision to the standard. It would be reasonable to see significant influence from the corresponding IEC standards. This trend has the effect of bringing the equipment platforms closer to each other, which has benefits for the user as maintenance and system design practices can become more similar. The advantages for the manufacturers are that similar components, and eventually equipment, can be more easily designed and manufactured for both markets, providing design and manufacturing efficiencies that should result in lower costs for all.

aR RatingsArc resistance in switchgear is a subject with increased levels of attention in IEEE markets over the last ten years. The IEC standards have considered this subject for several decades. IEC 62271-200 [35, Sec. 8.104.1–8.104.6, pp. 55–59] lays out the basic IEC require-ments. In IEC standards, equipment that has been

Isolating andEarthing Switch

Bus Bar, CBand CableConnections in1 Compartment

CableConnectionCompartment

CB

Bus BarCompartment

CBCompartment

Isolating andEarthingSwitch

(a) (b)2

The lSC1 and basic lSC2 section layouts are described in IEC 62271-200, Section 8.103.3. (a) Figure 101-lSC1—Stationary Cb without compartmentalization, grounding of switch and load capability. No compartment may be opened with bus energized. (b) Figure 102-lSC2—Stationary Cb with compartmentalization. This does not allow the Cb compartment to be opened with energized bus, but the cable compartment may be opened.

CableConnectionand CBCompartment

Bus BarCompartment

CableConnectionCompartment

CBCompartment

Isolating andEarthingSwitch

Bus BarCompartment

Isolating andEarthingSwitch

(a) (b)3

The lSC2 with partial compartmentalization and lSC2a sec-tion layouts as described in IEC 62271-200, Section 8.103.3. (a) Figure 103-lSC2—Stationary Cb with partial compart-mentalization. This does allow the Cb/cable compartment to be opened with live bus. Typical of ring main units (RMU) designs. (b) Figure 104-lSC2a—Stationary Cb with compart-mentalization. This does allow the Cb and cable compart-ment to be opened with energized bus.

CableConnectionCompartment

CableConnectionCompartment

Bus BarCompartment

Bus BarCompartment

DrawoutConnection=

CBCompartment

CBCompartment

Isolating andEarthingSwitch (1 of 2)

(a) (b)4

lSC2b sections with stationary or with drawout Cb, IEC 62271-200, Section 8.103.3. (a) Figure 105-lSC2b—Stationary Cb with full compartmentalization. This does allow the Cb and cable compartment to be opened with energized bus. (b) Figure 105-lSC2b—Drawout Cb with full compartmental-ization. This does allow the Cb and cable compartment to be opened with energized bus.

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tested to sustain an internal arcing fault is referred to as “internal arc classified” (IAC) and defined [35, Sec. 3.132, p. 14] as “metal-enclosed switchgear and control gear for which prescribed criteria, for protection of per-sons, are met in the event of internal arc as demon-strated by type tests.”

Under general comments in Section 8.104.1, the IEC standard states:

When selecting a metal-enclosed switchgear and control gear, the possibility of the occurrence of internal arc faults should be properly addressed, with the aim of providing an acceptable protection level for operators and, where applicable, for the general public.

This protection is achieved by reducing the risk to a tolerable level. According to ISO/IEC Guide 51, risk is the combination of the probability of

occurrence of a harm and the severity of the harm. Therefore, the selection of adequate equipment, in relation to internal arc, should be governed by a procedure to achieve a level of tolerable risk. This procedure is based on the assumption that the user has a role to play in the risk reduction.The standard also states that the faults have been shown

to occur more in some locations inside the switchgear than in others. A table of locations and probable causes is pro-vided in the standard and summarized in Table 9.

Furthermore, the IEC standard directly addresses sup-plementary protective measures such as

▪ rapid fault clearing based on fault detection via light, pressure, heat, or differential protection

▪ application of fuses in combination with switching devices to limit the let-through current and fault duration

TablE 9. lOCaTION, CaUSES, aND EXaMPlES OF MEaSURES TO DECREaSE THE PRObabIlITY OF INTERNal aRC FaUlTS. IEC 62271-200, SEC. 8.104.3, TablE 102, P. 56.location where Internal Arc Faults Are Most likely to occur

possible Causes of Internal Arc Faults Examples of possible preventive measures

Connection compartments

Inadequate design selection of appropriate materials and use of adequate dimensions.

Faulty installation Avoiding crossed cables. worksmanship supervision. proper torque.

Insulation failure worksmanship supervision. dielectric testing/maintenance.

disconnectors, switches, and earthing switches

maloperation Interlocks, delayed reopening. Independent manual operation. sufficient device-making capacity. personnel training.

bolted connections and contacts

Corrosion use of corrosion-inhibiting coating and/or greases. use of plating. Encapsulation where possible. supple-mental heating to prevent condensation.

Faulty assembly checking of worksmanship by suitable means. correct torque, adequate locking means.

rack-in/out due to damage or distortion of the plug-in contact and/or shutters

worksmanship supervision.

Instrument transformers Ferro resonance suitable design of the circuit.short circuit on low- voltage side of Vts

Avoid short circuits by using covers, low-voltage fuses, and other means.

cbs Insufficient mainte-nance

regular programmed maintenance as needed. training.

all locations human error limiting access via compartmentalization. Embedding energized parts in insulation. training.

aging partial discharge testing.pollution, moisture, ingress of dust, vermin, etc.

measures to ensure that specified service conditions are maintained. use of gas-filled compartments.

overvoltage surge protection and adequate insulation coordination. dielectric tests on site.

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▪ fast elimination of the arc by diverting it to a metal-lic short circuit crowbar by means of fast-sensing and fast-closing devices

▪ remote operation instead of operation in front of the switchgear and control gear

▪ pressure-relief device

▪ transfer of the withdrawable part to or from the ser-vice position only when the front door is closed, closed-door drawout.

The IEC standard also points out certain considerations the user should take into account.

▪ Not all switchgear and control gear will be IAC classifiable.

▪ Not all switchgear and control gear is of withdraw-able design.

▪ Not all switchgear and control gear is fitted with a door that can be closed when in the service/connected position, the earthing position, or the test position.

Although the above considerations are based on common IEC equipment practices, they can easily be identified with parallels viable regardless of the equipment standards such as the following.

▪ Not all equipment in an electrical room may be AR/IAC, and hence a hazard may exist regardless if one piece of equipment is so rated.

▪ Not all equipment will require the same operating or maintenance procedures, and some may require pro-cedures that invalidate an AR/IAC rating.

▪ Not all circuits and devices that may require mainte-nance or verification will be in compartments free from arc hazard, even in AR/IAC rated equipment.

This should lead the user to consider the recommenda-tions in the IEC standard and C37.20.7 for supplementary protection and other application considerations regardless of which standards the applied equipment meets.

IAC Classification, IEC 62271-200Classification under IEC and IEEE standards is intended for protection of personnel under normal operating con-ditions. It is not intended for protection of personnel under maintenance conditions nor for protecting service continuity [35, Sec. 8.104.6, p. 57]. IEEE C37.20.7 [13, Sec. 1.2.2, p. 2] describes a similar intention. The classi-fications may be described as follows:

▪ general: classification IAC

▪ accessibility: A, B, or C per 62271-200, Section 4.101.2 and Annex AA.2.2

▪ accessibility type A: restricted to authorized person-nel only

▪ accessibility type B: unrestricted accessibility, includ-ing that of the general public

▪ accessibility type C: restricted by installation out of reach and above a general public area. The minimum approach distance to be stated by the manufacturer and the minimum installation height is the declared minimum approach distance plus 2 m.

Arc-fault current is expressed in (kA) and arc test duration in seconds. Single-phase values (IAe ) may be used where construction prevents three-phase arcs as demonstrated during the internal arc test. Single-phase values may be 87 or 100% of the three-phase current

( ) .IA Users must identify the need based on how the sys-tem on which the switchgear will be applied is grounded. Impedance and solidly grounded systems require 100% phase-to-earth arc resistance, and isolated neutral (float-ing) systems need 87%. It should be noted that the stan-dard does not specify a particular current value nor a particular time period, although the current values must be taken from the R10 table previously described, and the suggested values for time are 0.1, 0.5, and 1 s. Where the manufacturer only states a single value of three-phase IAC rating the single-phase rating shall be 87%. In the case where the switchgear design is such that only single phase arcing fault can occur, only a single phase arc rating needs to be provided.

Designation of the gear is based on the accessibility that it is intended for, the fault current it was tested with, the time it was tested for and the sides that passed the test. Sides are indicated by letters: F, front; R, rear; and L, lateral. Equipment certified for unrestricted accessibility, with a successful IAC test for all four sides at 12 kA, for 0.5 s would be designated IAC BFLR 12.5 kA, 0.5 s. The IEC standards do not have a specific designation for front only and front/sides/rear other than the individual listing of the qualified sides per test.

IEEE Guide for Testing Metal-Enclosed Switchgear Rated up to 38 kV for Internal Arcing Faults, IEEE C37.20.7-2007 [13]The defining IEEE document for arc resistance is IEEE C37.20.7-2007. As listed in the title to this section, it is a guide for testing, not a standard. Manufacturers may offer varying ratings. However the IEEE guide provides guidance on how to qualify those claims by test, not how to achieve them or what they must be.

Section 1.2.3, “Application of This Guide,” includes some important points such as that the guide does not address all effects that constitute risk, such as release of toxic materials or excessive sound pressure. The guide also does not address the failure of components, espe-cially liquid-filled components or other materials inter-nal to components that may fail. Section 1.2.4 goes on further to state that it is not possible to test equipment for all possible conditions that can produce an internal arcing fault. An assembly proven by these tests cannot guarantee to withstand all internal arcing faults that may occur in service.

The IEEE standard [35, Sec. 4, p. 5] identifies two types by accessibility:

▪ Type 1—switchgear with AR designs or features at the freely accessible front of the equipment only

▪ Type 2—switchgear with AR designs or features at the freely accessible exterior (front, back, and sides) of the equipment only.

Passing the basic tests for either type of accessibility allows the switchgear to be labeled as Type 1A or Type 2A. Annex AA defines additional tests that allow a user to open a compartment door for compartments identified as low voltage control or instrumentation only. Successful passing of this second set of criteria allows the equipment to be labeled 1B or 2B. A third criterion is described in the annex for suffix C designation. Suffix C indicates that the

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effect of an internal arc fault is confined only to the compartment where the fault originally occurs. Type C suffix implies that the equipment is fully compartmentalized and that each com-partment is independently AR. A suffix D (for type 1 only) is also described for gear that, due to its location or other considerations, does not need to be AR on all sides but has been tested for the sides that need the arc resistance.

The IEEE guide also includes an application section (Annex B) similar in content to what is found in IEC 62271-200. An important point is described in B.2.2, p. 29 [35]. “To properly apply the switchgear, both the physical instal-lation and the electrical protection devices must be coordinated with the ratings of the arc-resistant switchgear.” This is true regardless if the gear is based on IEC or IEEE standards. There is always a desire for the fastest possible protection to minimize damage to the equipment and mini-mize probability of a failure. Plus, it should be taken into account that the equipment is only rated for a specific amount of time, and hence it is imperative that all protec-tion fully clear the fault within that time. For the main sec-tion where the protection may be on the line side of a transformer at a higher voltage, achieving sufficiently fast protection without a sophisticated protection scheme may not be easy. Special attention should be paid to protection of the line side of the incoming compartment, where that pro-tection resides.

A difference in the AR tests is that the IEC uses 300 mm and IEEE C37.20.7 requires 100 mm between the cotton test panels and the equipment enclosure. The cot-ton panels ascertain that excessive heat or hot materials do not exit the enclosure. Also, the AR tests under IEEE are performed on a system that can deliver full phase-to-ground current, whereas the IEC tests are performed on an impedance grounded system with limited ground fault current available.

ConclusionOver the last decade, IEEE and IEC standards for MV and switchgear have been significantly harmonized. The IEEE standards currently being revised by the IEEE WGs will further harmonize the standards. However, it is likely that the standards will not achieve complete harmonization in the near future. The complex physics and even more com-plex testing of characteristics like TRV dictate that com-mon standards facilitate the generation of common science, knowledge, and testing. Harmonizing the stan-dard simply recognizes that we all must deal with the same physics problems, test in the same laboratories, and often use the same components or subassemblies regardless of what standard the product shall meet or what market the final product targets. However, traditions and some differences in the standards still create distinctly different equipment choices. Currently, one of the main areas of dif-ferences is the ways that CBs are installed in equipment.

Fully front-connected assemblies are more common in IEC equipment, although the standards do not directly promulgate that construction. Station-ary CBs are more common in IEC equipment, but recent products intro-duced in the North American market are bringing that practice to the tradi-tionally IEEE market. Changes expected in C37.20.3 will further stan-dardize that practice in North Amer-ica, bringing IEC and IEEE practices closer together.

Traditionally, North American equipment relies heavily on doughnut current transformers (CTs) mounted within the CB cubicles. This requires the CBs to be drawn out to access the CTs and also requires the CB cell con-nections to allow sufficient space to

mount multiple CTs in one cubicle. Common IEC prac-tices have been to rely on bar CTs more often so as to get enough power from the CT’s iron to power electromag-netic protective relays. But modern electronic relays pro-vide lower burden and are facilitating common CT practices in both markets.

The IEC practice of combining isolation switches with grounding switches in the CB section can take the place of a drawout mechanism and a ground and test device. Com-bined with a SF6 insulated tank, the combination can yield very compact equipment that can be built for full-front connectivity. These designs may be very attractive from the size and even cost perspective. However, the user will need to determine if the complexity of having multiple devices that form one integral assembly, which cannot be drawn out of the cubicle for fast replacement, is consistent with the reliability requirements of the system. North American users are accustomed to the positive verification of separa-tion provided by a CB in the drawn out position or visible blade switch contacts viewable through an inspection win-dow in the switchgear. For an IEC-style fixed mounted CB, the user will often need to rely on position indication pro-vided by a common shaft or other mechanism that provides feedback on the status of the CB, isolating switch, and grounding switch assembly. The adoption of this type of equipment may require changes in U.S. maintenance prac-tices. But those practices have been in use for many years in markets where IEC equipment is routinely used. A more substantial barrier to the adoption of IEC practices is the National Electrical Code’s (NEC’s) requirement for visible verification of isolation found in articles 225.1 and 230.34(A) [15, pp. 70–71 and pp. 70–87, respectively]. This requirement is fulfilled by MC equipment’s drawout CBs or ME, visible-blade knife-type switches. The com-mon IEC practice of using shaft position or other less posi-tive indication of device position does not normally meet the requirements in the NEC. Additional requirements in NFPA 70E [38, Sec. 120.1(3)].

A user today has more choices, with finer gradations between them. The responsibility for choosing the optimum product for an application, in a particular

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region, still falls on the user, and it is the user that must become informed on the choices available. Regardless of the choice, many detailed decisions are still required to procure the optimum equipment solution regardless of the standard referenced.

acknowledgmentThe authors wish to thank Ted Olsen for his careful review and many valuable suggestions.

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current ratings of high-voltage circuit breakers rated according to ANSI and IEC standards,” IEEE Trans. Ind. Applicat., vol. 29, no. 1, pp. 195– 201, Jan./Feb. 1993.

[2] B. Bridger, Jr., “Comparison of ANSI/IEEE and IEC requirements for metal-clad switchgear,” IEEE Trans. Ind. Applicat., vol. 33, no. 1, pp. 216–225, Jan./Feb. 1997.

[3] D. Dufournet, “Transient recovery voltages for high voltage circuit breakers,” presented at the IEEE Mega Projects Workshop, Calgary, AB, Canada, Oct. 2008.

[4] L. van der Sluis, Transients in Power Systems. New York: Wiley, 2001.[5] IEEE Standard Rating Structure for AC High-Voltage Circuit Breakers,

IEEE Standard C37.04-1999, R2006.[6] IEEE Standard for Rating Structure for AC High-Voltage Circuit Breakers

Rated on a Symmetrical Current Basis Amendment 2: To Change the Descrip-tion of Transient Recovery Voltage for Harmonization with IEC 62271-100, IEEE Standard C37.04b-2008.

[7] IEEE Standard Rating for AC High-Voltage Circuit Breakers Rated on a Symmet-rical Current Basis-Preferred Ratings and Related Required Capabilities for Volt-ages Above 1,000 V, IEEE Standard C37.06-2009.

[8] Standard for Testing Procedures for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis, IEEE Standard C37.09-1999, R2007.

[9] IEEE Standard for Metal-Clad Switchgear, IEEE Standard C37.20.2-1999.[10] IEEE Standard for Metal-Enclosed Interrupter Switchgear, IEEE Standard

C37.20.3-2001.[11] IEEE Standard for Indoor AC Switches (1 kV–38 kV) for Use in Metal-

Enclosed Switchgear, IEEE Standard C37.20.4-2001.[12] IEEE Standard 4.76kV to 38kV Rated Ground and Test Devices, IEEE

Standard C37.20.6-2007.[13] IEEE Guide for testing metal-enclosed switchgear rated up to 38 kV for inter-

nal arcing faults, IEEE Standard C37.20.7-2007.[14] IEEE Standard of Common Requirements for High Voltage Power Switchgear

Rated Above 1000 V, IEEE Standard C37.100.1-2007.[15] National Electrical Code, NFPA Standard 70, 2011.[16] Conformance Test Procedures for Indoor Alternating Current High-Volt-

age Circuit Breakers Applied as Removable Elements in Metal-Enclosed Switchgear Assemblies, NEMA ANSI Standard C37.54-2002, R2010.

[17] Switchgear—Metal-Clad Switchgear Assemblies—Conformance Test Proce-dures, NEMA ANSI Standard C37.55-1989, R2010.

[18] Switchgear—Metal-Enclosed Interrupter Switchgear Assemblies—Confor-mance Testing, IEEE Standard C37.57-2003, R2010.

[19] Indoor Medium Voltage Switches for Use in Metal-Enclosed Switchgear— Con-formance Test Procedures, IEEE ANSI Standard C37.58-2003.

[20] High-Voltage Switchgear and Controlgear—The Use of Electronic and Asso-ciated Technologies in Auxiliary Equipment of Switchgear and Controlgear, IEC/TR Standard 62063, 1999.

[21] High-Voltage Switchgear and Controlgear—Part 1: Common Specifications, IEC Standard 62271-1, 2011.

[22] High-Voltage Switchgear and Controlgear—Part 100: Alternating Current Circuit-Breakers, IEC Standard 62271-100, 2008.

[23] High-Voltage Switchgear and Controlgear—Part 101: Synthetic Test-ing, IEC Standard 62271-101, 2010.

[24] High-Voltage Switchgear and Controlgear—Part 102: Alternating Current Disconnectors and Earthing Switches, IEC Standard 62271-102, 2001.

[25] Amendment 1—High-Voltage Switchgear and Controlgear—Part 102: Alternating Current Disconnectors and Earthing Switches, IEC Standard 62271-102-am1, 2011.

[26] High-Voltage Switchgear and Controlgear—Part 103: Switches for Rated Voltages Above 1 kV up to and Including 52 kV, IEC Standard 62271-103, 2011.

[27] High-Voltage Switchgear and Controlgear—Part 104: Alternating Current Switches for Rated Voltages of 52 kV and Above, IEC Stan-dard 62271-104, 2009.

[28] High-Voltage Switchgear and Controlgear—Part 105: Alternating Current Switch-Fuse Combinations, IEC Standard 62271-105, 2002.

[29] High-Voltage Switchgear and Controlgear—Part 106: Alternating Current Contactors, Contactor-Based Controllers and Motor-Starters, IEC Standard 62271-106, 2011.

[30] High-voltage Switchgear and Controlgear—Part 107: Alternating Current Fused Circuit-Switchers for Rated Voltages Above 1 Kv up to and Including 52 kV, IEC Standard 62271-107, 2005.

[31] High-Voltage Switchgear and Controlgear—Part 108: High-Volt-age Alternating Current Disconnecting Circuit-Breakers for Rated Voltages of 72.5 kV and Above, IEC Standard 62271-108, 2005.

[32] High-Voltage Switchgear and Controlgear—Part 109: Alternating-Cur-rent Series Capacitor By-Pass Switches, IEC Standard 62271-109, 2008.

[33] High-Voltage Switchgear and Controlgear—Part 110: Inductive Load Switching, IEC Standard 62271-110, 2009.

[34] High Voltage Switchgear and Controlgear—Part 111: Overhead, Pad-Mounted, Dry Vault, and Submersible Automatic Circuit Reclosers and Fault Interrupters For Alternating Current Systems up to 38 kV. IEC Standard 62271-111, 2005.

[35] High-Voltage Switchgear and Control Gear—Part 200: AC Metal-Enclosed Switchgear and Control Gear for Rated Voltages Above 1 kV and up to 52 kV, IEC Standard 62271-200, 2011.

[36] Selection and Dimensioning of High-Voltage Insulators Intended for Use in Polluted Conditions—Part 1: Definitions, Information and General Principles, IEC Standard 60815-1, 2008.

[37] Insulation Coordination Part 2: Application Guide, IEC Standard 60071-2, 1996.

[38] Standard for Electrical Safety in the Workplace, NFPA Standard 70E, 2012.

[39] Classification of Environmental Conditions—Part 2-4: Environmental Conditions Appearing in Nature—Solar Radiation and Temperature, IEC Standard 60721-2-4, 2002.

[40] G. H. Fox, “Applying microprocessor-based protective relays in switchgear with AC control power,” IEEE Trans. Ind. Applicat., vol. 41, no. 6, pp. 1436–1443, Nov.–Dec. 2005.

Marcelo Valdes (marcelo.valdes@ge.com), Marty Trivette, and Steven Meiners are with GE Industrial Solutions in Plainville, Connecticut. Xuhui Ren is with GE Industrial Solutions in Shanghai, China. Shridhaval Sapre is with GE Industrial Solutions in Hyderabad, India. Valdes is a Senior Member of the IEEE. Trivette and Meiners are Mem-bers of the IEEE. This article first appeared as “IEC & IEEE Medium Voltage Distribution Equipment Standards: A Review and Analysis” at the 2012 IEEE IAS Petroleum and Chemical Industry Technical Conference.