Low-Voltage Circuit Breaker Testing Strategies-Part3.pdf

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Winter 2002-2003 1

The first two parts of this series (Summer and Fall, 2002) focused on

verifying circuit breaker performance when subject to current over

loads. Interrupting excessive current is what circuit breakers are

designed to do and is, therefore, their main mission. However, this final

installment will look at some peripheral areas, which are no less impor-

tant when it comes to ensuring the continued safe operation of switchgear.

These areas are:

• Dielectric performance, important in residential, commercial, and in-dustrial products; and

• Accessory device function, important in commercial and industrialareas.

StandardsSome clarification is in order regarding standards and their applica-

tion. UL489 is limited to molded-case circuit breakers operating on 600volts ac nominal voltage or less, whereas ANSI C37.50-1989 would ap-ply to metal frame construction as well. IEC 60947-2 covers both molded-case and metal-frame circuit breakers that operate at less than 1000 volts

ac. The distinction becomes important when considering dielectric test-ing levels because the final applied voltage is based on the nominal ratedvalue.

Dielectric TestingDielectric testing is often ignored. Manufacturers perform this test

when the unit is built to ensure safety, but that is not sufficient for a long-term guarantee. An example of such a case occurred when I was calledto a lumberyard to troubleshoot a switchgear installation. Lumberyardsare more sensitive to fire hazards than many other businesses because of the large amounts of ready kindling lying about. The problem appeared

to be a miswired transformer thatwas instantaneously tripping the branch breaker whenever it wasenergized. The wiring error wascorrected, but the problem per-sisted. Finally, the entire circuit wasdisconnected, but the branch breaker still tripped when ener-gized. This was caused by a dielec-tric failure that over time depositedsufficient carbon to produce a line-to-line fault inside the breaker case.Dielectric testing of this deviceprior to installation should havefound the weakened insulationprior to complete failure.

Dielectric testing consists of ap-plying the appropriate voltage tothe required points on a breakerand verifying the insulationstrength. The appropriate voltagevaries among the standards rang-

Low-Voltage Circuit Breaker

Testing Strategies

by Adam Fleder Electric Device Corporation

Part 3 — Dielectric Performance and Accessory Device Function

This three-part article addresses testing requirements, methods,and challenges for low-voltage circuit breaker testing strategies.



2 NETA WORLD

ing from 1500-3500 volts ac depending on the nomi-nal voltage and the duration of application. Unfortu-nately, universally applying 3500 volts ac to cover allconditions can result in damaging circuits that werenot designed to handle voltages beyond 1500 voltsac. The following points have to be checked to ensureproper performance:

• Line-to-load with the breaker contacts in the openposition. This verifies that the breaker can properlyisolate the load from the line supply.

• Line-to-line with the breaker contacts in the closedposition. This verifies the integrity of the internalpole-to-pole insulation.

• Line-to-ground with the breaker contacts in theclosed position and the breaker housed in agrounded enclosure. This verifies the integrity of the external case insulation.

Accessory devices such as bell alarms orundervoltage releases must also have their leadsgrounded when performing dielectric tests. This veri-

fies that there is no current path from the internal con-ductors through the accessory device that could cre-ate a safety hazard. In addition, accessory devices areoften field installed, which can create additional li-abilities for the service firm if dielectric performanceis not verified.

Accessory DevicesAs the name itself implies, accessories are a grab

bag of devices that provide additional helpful func-tions. A comprehensive list of these devices is quitelong. Here are the most common devices that are en-

countered on switchgear:• Shunt-trip release: Trips the breaker when voltage

is applied.

• Undervoltage-trip release: Trips the breaker whenvoltage is too low.

• Overvoltage-trip release: Trips the breaker whenvoltage is too high.

• Electrical operator: Typically, a motor or solenoidthat turns the breaker on and off.

• Auxiliary switch: An additional set of contacts thatmimic the operation of the main breaker contacts.

• Bell alarm: A set of contacts that indicate when the breaker is tripped.

One issue to be aware of is the application of anaccessory for other than its prescribed use. Many ac-cessories, developed to meet specific needs, have beenmore broadly applied over time. If these accessorieshave become part of critical safety circuits they mustnot fail to operate. Accessory practices may also vary between countries. For example, in Europe it is com-mon to use the undervoltage-trip release as a safetydevice in an interlock circuit, whereas US manufac-

turers discourage this application of the device.

Accessory Device Testing MethodsTesting accessory devices is not very complicated.

However, it can be time consuming if there are sev-eral accessories in the switchgear. Having the righttools and instruments handy is the best way to getthe job done quickly and accurately. The wide varietyof voltages encountered in the field requires a powersupply that can provide both ac and dc voltages. Typi-cally 6-300 volts dc and 6-700 volts ac at 500 VA willcover most requirements. Occasionally, the electrical

operators on large switchgear frames will requirecloser to 2000 VA for proper operation. In addition tothe power supply, a 24 volt dc continuity circuit orhandheld meter is necessary to verify the state ofswitches.

Shunt-Trip ReleaseThis device is normally a solenoid that must trip

the breaker when energized throughout some rangeof its nominal voltage. It is often used in safety cir-cuits to interlock guards and doors as well as toremotely trip off breakers in an emergency. Shunt-

trip releases must be able to tolerate a large devia-tion in the nominal input voltage. The standardsvary quite a bit but require these devices to oper-ate as low as 50 percent of the rated voltage to ashigh as 110 percent or more. The device must tripthe breaker at both the low and high limits to beacceptable. In addition, the shunt-trip release mustalso open its own circuit once the breaker is tripped.This is verified by using the continuity circuit. Ifthe shunt-trip release does not disconnect itself itmay overheat and cease to function.

Undervoltage-Rrip ReleaseThis device is normally a solenoid that is held in

the retracted state and mechanically trips the breakerwith a spring mechanism when the voltage falls be-low a specific threshold. Typically, the device shouldnot trip the breaker when the voltage is above 60-70percent of rated voltage and should trip the breakerwhen it is less than 30-35 percent. The test is per-formed by applying the rated voltage and slowly de-creasing it until the breaker trips. The voltage at the



Winter 2002-2003 3

trip point is compared to the limits to judge the ac-cessory. Improperly functioning devices may causenuisance trips when motor starting or other short-term overload events draw down the line. Addition-ally, the breaker should not be able to be closed whenthe undervoltage-trip release is not powered.

Overvoltage-Trip Release

This device is normally a solenoid that is held inthe retracted state and mechanically trips the breaker with a spring mechanism when the volt-age exceeds some percent of its nominal value.There is not a strong convention on what is an ov-ervoltage condition, but 110 percent of rated volt-age is not uncommon. The test is performed byapplying the rated voltage and slowly increasing ituntil the breaker trips. The voltage at the trip pointis compared to the accessory’s limits. Improperlyfunctioning devices will fail to allow the breaker toremain closed at the nominal voltage.

Electrical Operator This device allows the breaker to be remotely

opened and closed when needed. It typically does notact quickly enough to be used in safety circuits butcan create safety hazards if it does not function prop-erly. The greatest concern is that the device cannotgenerate sufficient force to actuate the breaker. De-vices are run at 85 percent and 110 percent of theirrated voltage to verify that they can successfully openand close the breaker. In addition, the breaker shouldstart from a tripped condition to verify the device canovercome the reset force, which is often the highest.

Auxiliary SwitchThis device is normally operated by means of a cam

inside the breaker through some mechanical linkage.Switches are commonly found in form A or B and allstates should be verified. Fortunately, this is quite easyto do with a simple continuity circuit. Normally, 24volts dc is sufficient but some recommend 110 voltsdc. The 24 volts dc is a common control voltage anddoes not require special safety precautions. The testfails if the form A switch contact states do not corre-spond with the main contact state, and the form Bswitch contact state is not opposite to the main con-tact state. Please note that normally closed contactstates will be inverted.

Bell AlarmThis device is a special form of auxiliary switch. It

is designed to activate when the breaker is in thetripped condition. Again, a simple continuity circuitis sufficient. The test fails if the switch contact statesdo not change when the breaker is tripped.

Closing

This three part series provided an overview of thechallenges of circuit breaker testing. Today’s require-ments are quite extensive and commonly not thor-oughly performed once a piece of switchgear leavesthe factory. These requirements will continue tochange as new applications and functions are added.Our litigious society impels us to mitigate the risksinherent in working with electrical systems by prop-erly performing circuit breaker testing and continu-ing to stay current with industry developments.

Adam R. Fleder is the CEO of the Electric Device Corporationand a member of the UL489 STP. Electric Device Corporationmanufactures a variety of circuit breaker testing instruments. Hecan be contacted at [email protected].

Low-Voltage Circuit Breaker Testing Strategies-Part3.pdf

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