low-voltage circuit breaker testing strategies-part2.pdf

7/27/2019 Low-Voltage Circuit Breaker Testing Strategies-Part2.pdf

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Fall 2002 1

High currents are those that exceed 200 percent of the breaker’s

rating and are segregated into short-time and instantaneous faults.

Ground faults are characterized by an excessive amount of cur-

rent flowing in the ground system. These types of faults are of the most

concern in industrial and commercial applications and, therefore, these

protection modes are most often found on three-phase breakers.Part one of this series (Summer 2002) described the general technol-

ogy of circuit breakers and how long-time overcurrent conditions aredetected. This article will specifically focus on the sections of the tripunit dedicated to high currents and ground faults. Electromechanicaltrip units rely on the magnetic field generated by the fault current toactivate what is essentially a relay armature. Closure of this magneticcircuit activates the tripping mechanism, and the breaker opens. For theinstantaneous function, this action is not time delayed and normally hap-pens within 200 milliseconds after the fault is initiated. Electrohydraulicmechanisms commonly respond in less than one second. Tripping timesare shorter when the strength of the magnetic field generated by the

current pulls the slug through the liquid at a faster rate. The higher themagnetic field, the shorter the trip time. Finally, electronic trip units imple-ment all of these functions by measuring the actual current and compar-ing it to a reference table of values to determine the appropriate response.

Short-time Overload PerformanceThis test verifies that a breaker will trip in less than one second when

subjected to an overcurrent condition in the range of 200-1000 percent of its rating. This is accomplished with a single short pulse of current. It isgenerally best to measure the duration of the current pulse and confirm

Part 2 — High-Current Overloads and Ground Fault Detection

This three-part article addresses testing requirements, methods,

and challenges for low-voltage circuit breaker testing strategies.

that the breaker tripped within therequired time frame. Limiting thepulse to one second by means of electronic controls is essential to ac-curate testing because if the faultis applied for too long the breakerwill trip via the long-time mode.In contrast, relying on a quicktrigger finger to limit the pulse du-

ration can produce inconsistentresults. Figure 1 diagrams thepossible test results and theirinterpretation.

by Adam R. Fleder

Electronic Device Corporation

Low-Voltage Circuit Breaker

Testing Strategies



2 NETA WORLD

Figure 2

Instantaneous OverloadPerformance

This test verifies that a breaker

will normally trip in less than 200milliseconds when subjected to anover-current condition in the rangeof 300-2400 percent of its rating. Per-formance is verified at both hold andtrip levels. The hold level test veri-fies that a breaker will not trip pre-maturely whereas the trip level testindicates the breaker will clear ap-propriately. There are three primaryways to accomplish this test, andeach has its benefits and problems.The three types are ramp, dual

pulse, and multiple pulse.

Ramp TestThe ramp test is the original type of instantaneous current test. Th

current is turned on somewhere near 70 percent of the target trip poinand continuously ramped until the breaker trips. Figure 2 illustrates ramp test waveform. There are two benefits to this method:

1. It determines an actual trip value.

2. The test rate is fairly high.

There are three disadvantages to this method:

1. The current source must be sized larger because the duty cycle is thgreatest of the three methods.

2. The accuracy of the test is questionable because the person performing the test can influence the results by adjusting the rate of rampAlso, some breaker’s trip mechanisms activate prematurely whenexposed to this waveform.

3. The metering resolution is affected by a fast ramp. As an exampletake a test ramping from 7000-13000 amperes with a one second ramptime. The current must rise 6000 amperes in one second and a minimum of one half cycle is required to make a measurement. Assuming a 60 Hz test, this yields 120 half-cycle periods in which the current may be measured. Dividing 6000 amperes by 120 results in amaximum current resolution of 50 amperes for this example. Theris also the question of which cycle caused the breaker to trip. Was ithe last one measured or was it actually the one before?

Dual Pulse TestThe dual pulse test evaluates the breaker at the extreme limits. Th

first current pulse is applied at the hold level, and the breaker shouldremain closed. The second pulse is applied at the trip level, and the breaker should open. This is a fast and convenient way of determininthat a breaker complies with a specification but this does not producan actual trip current level. All that can be confirmed is that the breaketrips somewhere in between these two levels.

Figure 1



Fall 2002 3

Multiple Pulse TestThe multiple pulse test was devised to overcome the deficiencies of the

preceding two tests. Individual current pulses are applied at increasingincrements until the breaker under test trips. The resulting trip current isthen compared to the hold and trip level requirements to determine com-pliance. The power supply does not have to be sized as large and the testmore closely simulates actual instantaneous faults when compared to theramp test. The actual trip current is also determined with a high degree of certainty, but the drawback is found in the test rate. The multiple pulsetest is the slowest of the three methods.

Instrumentation Concerns

Current Measurement

The current waveforms used in overload tests present some interest-ing measurement challenges. The actual rating of a circuit breaker is basedon the root-mean-square (rms) value. The rms level can be calculated intwo ways. The first method uses the sinusoidal ratio of peak to effectivecurrent (.707). The peak value of current is captured and scaled accord-ingly. This relationship holds if the waveform is not distorted which inturn changes the ratio of peak to rms value.

Secondly, the true rms method most accurately represents the actualcurrent that will affect the device under test. The rms method involvessumming the area under the sine wave to accurately calculate the effec-tive current value. This method will work equally well on symmetric anddistorted sine waves.

Offset Compensation

Offset refers to the distortion inthe first cycle of the current wave-form. The main output transform-ers that make up the typical pri-mary injection test sets are thecause of this problem. The mainoutput transformer is turned off

prior to testing. The transitionfrom the off to the on state createscurrent transients that enlarge thefirst cycle. This is typically cor-rected by delaying the start of thefirst half cycle of current throughan electronic control. This delayis referred to as the phase-onangle, and its effects are shown inFigure 3.

The phase-on requirementswill vary with load currents aswell as load impedance. Variabletransformer regulated test setswill exhibit both the zero-crossingand the peak distortions, whileelectronically regulated test setscan dynamically compensate forthe distortion. When verifying set-tings, always measure the widthof the first half-cycle to be sure of proper phase-on adjustment.

Ground Fault PerformanceGround fault is actually a wide

field and encompasses everythingfrom the ubiquitous GFCI devicethat can be found in kitchens and baths to dedicated devices thatlook for excessive current ingrounding conductors. Thisarticle will be restricted toground fault sensing in multi-pole breakers.

Balancing loads in an electricalsystem reduces costs and is oftenrequired by the utility company.A perfectly balanced three-phasewye circuit will have no currentflowing in the neutral conductor.As the circuit becomes unbal-anced the current in the neutralconductor will rise. Ground faultsensing monitors the sum of thephase currents and opens the breaker when it exceeds a presetlevel.

Figure 3 — These waveforms depict three different phase-on angles. The top signal(phase-on angle too small) is that of a load that requires more phase-on angle. In thissituation, the peak of the first half-cycle is seen to be larger than the average peak value.The peak of the second half-cycle may also be affected. Note also that the zero-crossingsare shifted in time causing the period of the first half-cycle to be more than 8.333 milli-seconds. The second signal is the opposite condition. Here the phase-on angle is toolarge and the first half-cycle is too small. The period of the first half-cycle is also shorterthan 8.333 milliseconds.



4 NETA WORLD

Testing this mode varies with the type of sensing.Four-wire systems utilize a separate neutral currenttransformer (CT) connected to the trip unit. Currentmust be passed through the CT in the correct direc-tion and the breaker’s response noted. The breakershould not trip. In the opposite direction, the breakershould trip at half the rated ground fault pickup.Double-ended substations require additional tests forthe mains and tie breaker. The short-time test men-

tioned above is a good choice for verifying the opera-tion. A fixed duration of current is applied, and thetime to trip is compared to the specifications. Apply-ing the fault current to one pole at a time, which pro-duces the necessary unbalance condition, tests theseproducts. The ground fault mode must be disabledon these types of devices when making other tests because the breaker may trip for ground fault whenone is testing for long-time overload.

ClosingHigh-current and ground fault testing can be som

of the most difficult testing for a technician to consistently perform. It is recommended that good controland instrumentation be used to achieve and monitothe testing process.

Part three of this article will examine dielectric testing and the safety implications of various accessoriewith which breakers can be outfitted.

Adam R. Fleder is the CEO of the Electric Device Corporatioand a member of the UL489 STP. Electric Device Corporatiomanufactures a variety of circuit breaker testing instrument

Joseph Bouch and Paul Miller also contributed material for tharticle.

low-voltage circuit breaker testing strategies-part2.pdf

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