Understanding the How and Why of Electrical Product Safety Testing

7/29/2019 Understanding the How and Why of Electrical Product Safety Testing

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Understanding the How and Why of Electrical Product Safety

Testing

Homi Ahmadi

Knowing why to perform required electrical safety tests is as important as knowing how to perform them.

With so many electrical product safety standards currently in use and many civil and legalactions pending in various courts around the world, electrical safety testing is more critical

than ever to ensure that all products are safe before they reach the end-user. Fortunately,

the majority of manufacturers are fully aware of the hazards associated with electrical

equipment and the ramifications of noncompliance with relevant safety standards or test

house agreements.

Electrical safety tests can be roughly divided into two areas: those tests carried out during

the approvals process by test houses (known as type tests) and those carried out at the end of each production lin

by the manufacturer (known as routine production tests).

For type tests, a product is subjected to tests and evaluations in accordance with a specific product safety standa

For production tests, a manufacturer can select a few tests, ensuring that each product is subjected to those tests i

accordance with its own procedures. Most manufacturers, in particular information technology (IT) equipment

manufacturers, choose four primary product safety tests to be routine at the end of the production line. These inclu

dielectric, insulation resistance, ground continuity, and leakage current.

These tests are designed to ensure that the user does not get electrocuted or otherwise hurt by operating a piece o

equipment that has hazardous voltages or high fault current as a result of electrical fault. This article looks at the

fundamentals behind each test and analyzes the reasoning behind each test, as well as discussing appropriate limits

and equipment.

Dielectric Strength Test

The dielectric strength test, also known as dielectric withstand test or hipot test, is probably the best known and m

often performed production-line safety test. It is part of virtually every standard, which indicates its importance. Th

hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring

overvoltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure t

the insulation is not marginal. Another reason for conducting the hipot test is that it also detects possible defects su

as inadequate creepage and clearance distances introduced during the manufacturing process.

During type testing, the hipot test is applied after tests such as fault condition, humidity, and vibration to determine

whether any degradation has taken place. The production-line hipot test, however, is a test of the manufacturing

process to determine whether the construction of a production unit is about the same as the construction of the un

that was subjected to type testing. Some of the process failures that can be detected by a production-line hipot tes

include, for example, a transformer wound in such a way that creepage and clearance have been reduced. Such a

failure could result from a new operator in the winding department. Other examples include identifying a pinhole

defect in insulation or finding an enlarged solder footprint.

Most safety standards use the 2U + 1000 V formula as the basis for testing basic insulation, where U is the operat



rms voltage. Although this formula is a guideline, each standardin particular IEC 60950refers the user to a

specific table in the standard showing the exact test voltage based on the working voltage measurements.1 The re

for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-

day transient overvoltages. Experiments and research have shown that these overvoltages can be as high as 1000

Test method. Normally the high voltage is applied between the two parts across the insulation being tested, such

the primary circuit and metal enclosure of the equipment under test (EUT). If the insulation between the two is

adequate, then the application of a large voltage difference between the two conductors separated by the insulato

would result in the flow of a very small current. Although this small current is acceptable, no breakdown of either tair insulation or the solid insulation should take place. Therefore, the current of interest is the current that is the resu

of a partial discharge or breakdown, rather than the current due to capacitive coupling.

Another example would be to test the insulation between the primary and secondary circuits of a power supply. H

all the outputs are shorted together. The ground probe from the hipot tester is placed in contact with this cable bun

and the high-voltage probe is placed in contact with L and N connectors, which are shorted together (see Figure

The EUT does not run during the hipot test. It must also be noted that when applying the high voltage during the ty

test, the ideal situation would require that not more than half of the prescribed voltage be applied, and then raised

gradually over a period of 10 seconds to the full value and maintained for 1 minute. Most test equipment, however

either turns on directly to the full voltage or has an electronically controlled ramp.

Figure 1. Typical hipot test setup.

Test Duration. If the test is part of an agency certification process, then the test duration must be in accordance wthe safety standard being used. For instance, the test time for most standards, including products covered under IE

60950, is 1 minute. However, when testing products in the production line, it is normally not practical to hipot test

each product for 1 minute, and manufacturers normally conduct the test to a much shorter time, such as a few

seconds, but with higher voltages. A typical rule of thumb is 110120% of 2U + 1000 V for 12 seconds. The

duration and procedure should be in agreement with any test houses concerned. It should be noted that although t

reduced time and increased voltage are approximate, experiments and the manufacturers' data sheets indicate that

each insulating material has its own specific voltage-time characteristics.

Current Setting. Most modern hipot testers allow the user to set the current limit. However, if the actual leakage



current of the product is known, then the hipot test current can be predicted. Choosing the trip level really depend

on the product being tested. The best way to identify the trip level is to test some product samples and establish an

average hipot current. Once this has been achieved, then the leakage current trip level should be set to a slightly

higher value than the average figure. Another method of establishing the current trip level would be to use the

following mathematical formula:

The reason for the factor of 2 is that the line leakage current provides current through a single Y capacitor while thhipot test provides current through capacitors on each line. By solving the equation for I (hipot), one can predict t

hipot test current. Therefore, the hipot tester current trip level should be set high enough to avoid nuisance failure

related to leakage current and, at the same time, low enough not to overlook a true breakdown in insulation.

Test Voltage. The majority of safety standards allow the use of either ac or dc voltage for a hipot test. When usi

ac test voltage, the insulation in question is being stressed most when the voltage is at its peak, i.e., either at the

positive or negative peak of the sine wave. Therefore, if one decides to use dc test voltage, one must ensure that t

dc test voltage is 2 (or 1.414) times the ac test voltage, so the value of the dc voltage is equal to the ac voltage

peaks. For example, for a 1500-V-ac voltage, the equivalent dc voltage to produce the same amount of stress on

insulation would be 1500 x 1.414 or 2121 V dc.

One of the advantages of using a dc test voltage is that the leakage current trip can be set to a much lower value th

that of an ac test voltage. This would allow a manufacturer to filter those products that have marginal insulation, wh

would have been passed by an ac tester. It must be noted that when using a dc hipot tester, the capacitors in the

circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good

practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is hand

Another advantage of a dc hipot tester is that it applies the voltage gradually. By monitoring the current flow as

voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage

the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slig

higher than that of an ac tester.

One of the advantages of an ac hipot test is that it can check both voltage polarities, whereas a dc test charges the

insulation in only one polarity. This may become a concern for products that actually use ac voltage for their norm

operation. The test setup and procedures are identical for both ac and dc hipot tests.

A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of Y capacitors, then,

depending on the current trip setting of the hipot tester, the ac tester could indicate a failure. Most safety standard

allow the user to disconnect the Y capacitors prior to testing or, alternatively, to use a dc hipot tester. The dc hipo

tester would not indicate the failure of a unit even with high Y capacitors because the Y capacitors see the voltage

don't pass any current.

Insulation Resistance

The insulation resistance test is also known as a Megger test. Its objective is to measure the total resistance betwe

any two points separated by insulation. The test, therefore, determines how effective the insulation is in resisting th

flow of electrical current. The voltage is typically around 5001000 V dc; hence, the current is very low. Because

current is so low, this test is useful for checking the quality of the insulation not only when a product is first



manufactured, but also over time as the product is used.

Test Procedure. The EUT is connected to the measuring instrument, and the voltage is ramped up from zero to th

final value, which in most cases is 500 V dc. Once the voltage reaches the selected value, it is kept at that value fo

brief period (typically up to 5 seconds) before the resistance test is measured. The measured value should be very

high (typically in the megohm region). The insulation resistance test is mandatory in some product safety standards

including IEC 60065 and UL 6500.2,3

Ground Bond Test

Also known as the ground bond continuity test, the ground bond test must be conducted on all Class I products. T

purpose of the test is to ensure that all accessible conductive parts of the product that could become live in the ev

of a single insulation fault are connected securely to the final earth point of the supply input. In other words, a grou

bond test verifies integrity of the ground path by applying a high-current, low-voltage source to the ground-path

circuit.

Compliance is checked by measuring the resistance of the connection between the protective earthing terminal or

earthing contact and the parts to be earthed to ensure that resistance does not exceed certain values when subject

to a high current as specified in various product safety standards. It is important to bear in mind that from theconstructional and design points of view, the protective earthing conductors should not contain switches or fuses.

Test Requirements. Most safety standards require the following parameters for conducting the ground bond tes

The EUT must be subjected to a high ac or dc current with a low test voltage for a certain period.

The voltage drop between the protective earthing terminal

or earthing contact and the part to be earthed must be

measured.

The resistance must be calculated from the current and the resulting voltage drop using Ohm's law.

The resistance should not exceed certain values as stated in various safety standards. For example, IEC 60950

requires that the test voltage not exceed 12 V. The current can be either ac or dc at 1.5 times the current

consumption of the product or 25 A, whichever is greatest. The test duration must be 1 minute, and the resistance

the connection between the protective earthing terminal or earthing contact and parts required to be earthed must

exceed 0.1 W. This value does not include the resistance of the power cable. Some standards, such as CAN/CSA

C22.2 No. 60950-00 or UL 60950 with Canadian deviation, require the test to be conducted at 30 A and for 2

minutes if the current rating of the circuit under test is 16 A or lower.4,5

Understanding Resistance Values. With the exception of the Canadian standard, most standards require 25 A

1 minute. The value of 25 A for 1 minute represents the worst current and the longest operation time of a mains

overcurrent device. The maximum 25 A is approximately 1.5 times the mains circuit breaker value installed for mo

pluggable type A cord-connected equipment rated up to 16 A. The Canadian National Wiring Code requirement

are very similar to these in the sense that they assume that fuses are expected to operate no more than 1 minute at

twice their rated current. Because most mains circuits are protected with a 1516 A fuse, the fault current would b

30 A for no more than 2 minutes.

Earth Leakage Current

Some standards, including IEC 60950, 3rd ed., have named the leakage current test "touch current." This refers t



the electric current through a human body or through an animal body when it touches one or more accessible part

installation or equipment. There is also another concept known as "protective conductor current," and this refers to

the current that flows in a protective conductor. A protective conductor current, therefore, can never be the sourc

an electric shock because, by definition, the protective conductor is connected to earth.

If touch current is excessive, an operator could receive an electric shock, which could result in a serious injury,

depending on a person's body weight. Typically, currents of more than 1.0 mA can cause an electric shock to an

operator. The shock may or may not be serious, depending on the amount of the current and the body weight.

Like the other tests, the leakage current test is also a very important safety test, and most safety standards require

test to be conducted under various conditions such as normal operating condition, switches open as well as closed

reversed line polarity, and so on. The measured earth leakage current must not exceed specified limits during any

the tested conditions. Table I shows some typical limits.

Type of Equipment

Terminal A of

Measuring

Instrument

Connected To

Maximum

Touch

Current

mA rms1

Maximum

Protective

Conductor

Current

All equipment

Accessible parts

and circuits not

connected to

protective earth

0.25

Handheld 0.75

Movable (other than

handheld but

including

transportable

equipment)

3.5

Stationary, pluggable

type A

Equipment main

protective earthing

terminal (if any)

3.5

All other stationary

equipment

not subject to theconditions of 5.1.7

subject to the

conditions of 5.1.7

3.5

5% of input

current

1 If peak values of touch current are measured, the maximum values

are obtained by multiplying the rms values by 1.414.

Table I. Typical earth leakage currents.



One of the biggest contributors to leakage is the capacitance between ac lines and earth, i.e., the Y capacitors. Th

Y capacitors are normally placed in the circuit to control electromagnetic interference. It should be noted that som

standards, such as IEC 60950, allow higher earth leakage current than 3.5 mA only for Class I stationary equipm

that is either permanently connected equipment or that is pluggable equipment type B, provided certain conditions

met. These conditions are listed in Clause 5.1.7 of IEC 60950.

Equipment. When measuring the earth leakage current of IT equipment, it must be noted that the measuring

instruments should satisfy the requirements of Annex D of IEC 60950, which simulates the worse-case impedance

the human body. The use of an isolating transformer during the test is also highly recommended (see Figure 2).

Figure 2. Typical earth leakage current setup.

Any capacitive leakage in the transformer must then be taken into account. If for any reason the use of an isolatingtransformer is not possible, then the EUT must be mounted on an insulating stand, and appropriate safety precauti

must be taken. Such measures compensate for the possibility that the body of the EUT may carry a hazardous

voltage.

Test Method. Most standardsin particular IEC 60950require that the EUT be energized for this test. The in

voltage applied to the EUT is typically adjusted to 110% of the highest rated mains voltage and the highest rated li

frequency. As mentioned previously, for safety reasons, it is highly recommended that the EUT be powered via an

isolating transformer.

These tests are performed on both Class I and Class II products. For Class II equipment, the test is made toaccessible conductive parts and to a metal foil with dimensions of approximately 10 x 20 cm, which is attached to

enclosure of the product. The metal foil simulates a human hand contact.

This test should also be conducted in all possible combinations such as normal operating condition, switches open

well as closed, reversed line polarity, etc. Equipment designed for multiple power sources, only one of which is

required at a time (e.g., for backup), must be tested with only one source connected. Although most standards do

require the earth leakage current test to be carried out for 100% of the units in a production line, some standards,

such as those for medical products, do require a 100% production-line test.



Safety Precautions

Because any electrical safety test involves some risk of electrical shock, it is crucial that certain precautions be tak

to avoid shock and injury to operators. Listed below is a sampling of precautions that can minimize the danger of

electrical shocks and ensure all-around safety:

Train operators in the basic theory of electrical circuits and explain the object of each test.

Review and update all safety test procedures regularly.

Locate the testing area away from walkways and crowded areas on the shop floor.Guard the testing area with nonconductive barrier.

Mark the testing area with a clear and visible sign such as "Danger" or "High Voltage Present."

Mark the testing area with a clear sign indicating "Qualified Personnel Access Only."

Ensure that all test equipment is properly connected to a reliable earth.

Reconfigure all testers (where possible) with push-button switches so that operators must use both hands to

activate the test equipment or, alternatively, provide the equipment with a safety interlock that automatically

shuts down the high voltage when a safety switch on the EUT is opened.

Connect the complete test setup to a palm-type switch that can shut off the power to the test bench in case

an emergency.

Record Keeping and Identification

CENELEC has released a standard, EN 50116, which essentially defines the routine electrical safety tests and the

procedures to be applied during or after the manufacturing process of IT equipment certified or declared as

complying with EN 60950.6,7

It is extremely important for manufacturers of electrical products to ensure that all test results, including routine

production tests, are clearly and adequately documented and kept on file for possible inspection. Although this ma

not be required by all agencies, keeping accurate test records is not only good engineering practice, but could also

vital in defending a legal action should such a case arise. Most modern test equipment can produce test results in a

electronic format that can be stored or printed when needed.

It is also necessary for manufacturers to ensure that all test equipment is calibrated regularly and that such records

kept on file. In fact, most test houses expect to see a log for daily calibration of the hipot tester on the production l

This log confirms that the hipot tester has indeed been applying high voltage to the EUT. It is also vital to ensure th

all electrical safety tests are carried out on units that have been returned for repair or service.

On the production line, there are generally three product states: not tested, tested-passed, and tested-failed. The

status of any product must be clearly apparent to ensure that untested products are not shipped and that only teste

passed products are shipped. For instance, a red tag attached to the unit can indicate tested-failed. Untested

products are those that have neither a tested-passed nor a tested-failed indicator.

Conclusion

Electrical safety testing at the design and development stages as well as at the production stage is vital to ensure th

all products are safe before reaching the end-user. The four tests described in this article are among the most

fundamental tests that manufacturers of electrical and electronic products should conduct routinely. It is important

use correct test equipment and adequate and accurate test procedures so that sufficient testing is conducted and

operator safety is considered.



Acknowledgement

The author wishes to thank Rich Nute of Hewlett-Packard for his valuable help.

References

1.IEC 60950, 3rd ed., "Safety of Information Technology Equipment," International Electrotechnical Commission

(IEC), Brussels, 1999.

2.IEC 60065, "Audio, Video and Similar Electronic ApparatusSafety Requirements," IEC, Brussels, 1998.

3.UL 6500, "Audio/Video and Musical Instrumental Apparatus for Household, Commercial, and Similar General

Use," Underwriters Laboratories Inc. (UL), Northbrook, IL.

4.CAN/CSA-C22.2 No. 60950-00, "Safety of International Information Technology Equipment," CSA

International, Toronto, ON, Canada.

5.UL 60950, "Safety of Information Technology Equipment," UL, Northbrook, IL, 2000.

6.EN 50116, "Information Technology EquipmentRoutine Electrical Safety Testing in Production," EuropeanCommittee for Electrotechnical Standardization (CENELEC), Brussels, 1996.

7.EN 60950, 2nd ed., "Safety of Information Technology Equipment, including Electrical Business Equipment,"

CENELEC, Brussels, 1992.

Understanding the How and Why of Electrical Product Safety Testing

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