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One of the key components of the flash hazard analysis which is

required by NFPA 70E-2000, Part II, paragraph 2-1.3.3 is the clear

ing time of the protective devices, primarily circuit breakers and

protective relays. Fuses, although they are protective devices, do not have

operating mechanisms that would require periodic maintenance; there-

fore, this article will not address them. The primary focus of this article

will be the maintenance issues for circuit breakers and protective relays.

Molded-case and low-voltage, power circuit breakers (600 volts or less)will generally clear a fault condition in three to eight cycles. To be con-

servative a clearing time of eight cycles should be used. The majority of older medium-voltage circuit breakers (2300 volts or greater) will clear afault in around eight cycles with the newer ones clearing in three to fivecycles. Protective relays will generally add approximately three to fourcycles to the clearing time of the medium-voltage circuit breaker. Wherecorrect maintenance and testing are not performed, extended clearingtimes could occur, creating an unintentional time delay that will affectthe results of flash hazard analysis.

All maintenance and testing of the electrical protective devices ad-dressed in this article must be accomplished in accordance with themanufacturer’s instructions. The NETA Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems is an excellent sourceof information for performing the required maintenance and testing of 

these devices. Visit the NETA website for more information at http://www.netaworld.org.

This article will address some of the issues concerning the correctmaintenance and testing of these protective devices. It will also ad-dress how protective device maintenance relates to the electrical arc/flash hazard.

by Dennis K. Neitzel, CPE 

 AVO International TrainingInstitute

Protective Devices Maintenanceas it Applies to the Arc/Flash Hazard

Molded-Case CircuitBreakersGenerally, maintenance on

molded-case circuit breakers islimited to the correct mechanicalmounting, electrical connections,and periodic manual operation.Most lighting, appliance, andpower panel circuit breakers haveriveted frames and are not de-signed to be opened for internal in-spection or maintenance. All other

molded-case circuit breakers thatare UL approved are factory-sealedto prevent access to the calibratedelements. An unbroken seal indi-cates that the mechanism has not been tampered with and that itshould function as specified by UL.A broken seal voids the UL listingand the manufacturers’ warrantyof the device. In this case, the in-tegrity of the device would be

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questionable. The only exception to this would be aseal being broken by a manufacturer’s authorized fa-cility.

Circuit breakers installed in a system are oftenforgotten. Even though the breakers have been sit-ting in place supplying power to a circuit for years,there are several things that can go wrong. The cir-cuit breaker can fail to open due to a burned outtrip coil or because the mechanism is frozen due to

dirt, dried lubricant, or corrosion. The overcurrentdevice can fail due to inactivity or a burned outelectronic component. Many problems can occurwhen maintenance is not performed and the breaker fails to open under fault conditions. Thiscombination of events can result in fires, damageto equipment, or injuries to personnel.

Low-Voltage, Power Circuit BreakersLow-voltage, power circuit breakers are manufac-

tured under a high degree of quality control, of the best materials available, and with a high degree of 

tooling for operational accuracy. Manufacturer’s testsshow these circuit breakers to have durability beyondthe minimum requirements of standards. All of thesefactors give these circuit breakers a very high reliabil-ity rating. However, because of the varying applica-tion conditions and the dependence placed upon themfor protection of electrical systems and equipment aswell as the assurance of service continuity, inspectionsand maintenance checks must be made on a regular basis. Several studies, including those by IEEE, haveshown that low-voltage power circuit breakers whichwere not maintained within a five-year period, havea 50 percent failure rate.

Maintenance of these breakers will generally con-sist of keeping them clean and appropriately lubri-cated. The frequency of maintenance will depend tosome extent on the cleanliness of the surrounding area.If much dust, lint, moisture, or other foreign matterwere present then, obviously, more frequent mainte-nance would be required.

Medium-Voltage, Power Circuit BreakersMost of the inspection and maintenance require-

ments for low-voltage, power circuit breakers alsoapply to medium-voltage, power circuit breakers.

Manufacturers recommend that these breakers be re-moved from service and inspected at least once peryear. They also state that the number and severity of interruptions may indicate the need for more frequentmaintenance checks. Always follow themanufacturer’s instructions because every breaker isdifferent.

Protective RelaysRelays must continuously monitor complex powe

circuit conditions, such as current and voltage magnitudes, phase angle relationships, direction of poweflow, and frequency. When an intolerable circuit con

dition, such as a short circuit (or fault) is detected, threlay responds and closes its contacts and the abnormal portion of the circuit is deenergized via the circuit breaker. The ultimate goal of protective relayinis to disconnect a faulty system element as quickly apossible. Sensitivity and selectivity are essential tensure that the right circuit breakers are tripped athe right speed to clear the fault, minimize damage tequipment, and to reduce the hazards to personnel.

Flash Hazard AnalysisAs noted at the beginning of this article, NFPA 70E

2000 requires a flash hazard analysis be performed before anyone approaches exposed electrical conductors or circuit parts that have not been placed in anelectrically safe work condition. In addition, Paragraph 2-1.3.3.2 requires a flash protection boundaryto be established. All calculations for determining thincident energy of an arc and for establishing a flashprotection boundary require the arc clearing time. Thiclearing time is derived from the engineering coordination study which is based on what the protectivdevices are supposed to do.

***************************************************************This program is made available to the general public for the

purpose of calculating heat flux received at a surface some

distance from an electric arc. The use of this program is theresponsibility of the user. The author makes no warranty to the

accuracy of the results and accepts no responsibility any dam-age that may arise from its use.

***************************************************************

Enter the arc current(amps) ? 20000Enter the arc gap(inches) ? 3Enter the supply voltage(volts) ? 480

Arc column area 43.03264 sq. inchesArc column cir. 14.34421 inches

Arc diameter 4.565908 inches

Arc power in watts - 1781250Arc power in calories/sec - 425540.6

Heat flux on surface of arc 1533.146 cal/cm^2-secEnter the distance from the arc to the receiving surface ? 18

Transfer Shape Factor 1.482744E-02Heat Flux at Receiving Surface 22.73263 cal/cm^2-sec

Enter the number of cycles for the arc duration ? 5

Arc Duration 8.333001E-02 seconds

Total Calories per Sq. Cm. at Receiving Surface 1.89431

 Do You Wish To Run Another Case? (Y or N) ?

Calculation with a Five-Cycle Clearing Time

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Summer 2002 3

Maintenance is a critical part of the flash hazardissue. Evidence has proven that inadequate mainte-nance can cause unintentional time delays in the clear-ing of a short-circuit condition. If, for example, a low-

voltage, power circuit breaker had not been operatedor maintained for several years and the lubricationhad become sticky or hardened, the circuit breakercould take several additional cycles, seconds, minutes,or longer to clear a fault condition. The following is aspecific example.

If a flash hazard analysis is performed based onwhat the system is suppose to do, let’s say five-cycleclearing time, and there is an unintentional time de-lay, due to a binding mechanism, and the breakerclears in 30 cycles, the worker could be seriously in- jured or killed because he/she was underprotected.

If the calculation is performed for a 20,000 ampere

fault, 480 volts, three-inch arc gap, the worker is 18inches from the arc, with a five-cycle clearing time fora three-phase arc in a box (enclosure), the resultswould be approximately 6.5 cal/cm2  which wouldrequire an arc/flash Category 2 protection based onNFPA 70E-2000, Part II, Table 3-3.9.3.

Figure 1 uses the heat flux calculator (developed by Alan Privette) and the values above for a five-cycleclearing time. This value of 1.89431 cal/cm2 is basedon a single-phase arc in open-air. As a general rule of thumb, the value of 1.89431 would be multiplied by afactor of two for a single-phase arc in a box (2 x 1.89431= 3.78862 cal/cm2 – Category 1) and by a factor of 3.4for a multiphase (three-phase) arc in a box (3.4 x1.89431 = 6.440654 cal/cm2 – Category 2).

If the clearing time is increased to 30 cycles (Figure2) then the results are approximately 38.7 cal/cm2,which requires an arc/flash Category 4 protection. Thevalue of 11.36586 cal/cm2 is based on a single-phasearc in open-air. Again, as a general rule of thumb, thevalue of 11.36586 would be multiplied by a factor of two for a single-phase arc in a box (2 x 11.36586 =22.73172 cal/cm2 – Category 3) and by a factor of 3.4for a multiphase (three-phase) arc in a box (3.4 x1.89431 = 38.643924 cal/cm2 – Category 4).

Therefore, as can be seen, maintenance is extremelyimportant to an electrical safety program. Mainte-nance must be performed according to the

manufacturer’s instructions in order to minimize therisk of having an unintentional time delay in the op-eration of the circuit protective devices.

SummaryWith the appropriate mixture of common sense,

training, manufacturers’ literature, and spare parts,correct maintenance can be performed and powersystems kept in a safe, reliable condition. Circuit break-ers, if installed within their ratings and correctly main-tained, should operate trouble-free for many years.However, if operated outside of their ratings or with-

out maintenance, catastrophic failure of the powersystem, circuit breaker, or switchgear can occur caus-ing not only the destruction of the equipment but se-rious injury or even death of employees working inthe area.

Dennis K. Neitzel, CPE, Director of AVO Training Institute,Dallas, Texas, earned his bachelor’s degree in electrical engineer-ing management and his master’s in electrical engineering ap-plied sciences from Columbia Pacific University. He earned hisCertified Plant Engineer (CPE) through the AFE and his CertifiedElectrical Inspector-General through the IAEI. He has been a Prin-

ciple Committee Member for the NFPA 70E, Standard for ElectricalSafety Requirements for Employee Workplaces since 1992 and is co-author of the Electrical Safety Handbook, Second Edition, McGraw-Hill Publishers. He is a member of the Institute of Electrical andElectronics Engineers, the American Society of Safety Engineers,the Association for Facilities Engineering, the International Asso-ciation of Electrical Inspectors, and the National Fire ProtectionAssociation.

***************************************************************This program is made available to the general public for the

purpose of calculating heat flux received at a surface somedistance from an electric arc. The use of this program is the

responsibility of the user. The author makes no warranty to the

accuracy of the results and accepts no responsibility any dam-age that may arise from its use.

***************************************************************

Enter the arc current(amps) ? 20000Enter the arc gap(inches) ? 3

Enter the supply voltage(volts) ? 480

Arc column area 43.03264 sq. inchesArc column cir. 14.34421 inches

Arc diameter 4.565908 inchesArc power in watts - 1781250

Arc power in calories/sec - 425540.6Heat flux on surface of arc 1533.146 cal/cm^2-sec

Enter the distance from the arc to the receiving surface ? 18

Transfer Shape Factor 1.482744E-02Heat Flux at Receiving Surface 22.73263 cal/cm^2-sec

Enter the number of cycles for the arc duration ? 30Arc Duration .49998 seconds

Total Calories per Sq. Cm. at Receiving Surface 11.36586

 Do You Wish To Run Another Case? (Y or N) ?

Calculation with a Thirty-Cycle Clearing Time