Low-Voltage Circuit Breaker Testing Strategies-Part1.pdf
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Summer 2002 1 T his three-part article addresses testing requirements, methods, and challenges for low-voltage circuit breaker testing strategies. Amazingly, the electric power industry is now in its third century of operation. However, as author Irene Peter noted; “Just because every- thing is different doesn’t mean anything has changed.” The basic mis- sion of the circuit breaker has remained unchanged since Henry Cutter developed the first one at the end of the 19 th century. Protecting life and property from fire damage is the ess ential mission of the circuit breaker. Over time, methods and specifications have been developed to verify that a breaker still provides the level of protection for which it was originally designed. One of the primary values that testing provides is the mitigation of risk. This article provides a general overview of the r equirements. The current testing standards are well established and fairly explicit. Y ou are encouraged to become familiar with their guidelines and apply them to provide the highest level of protection for your customers and your or- ganization. Products The types of products that exist now use multiple technologies to ac- complish the functions of overcurrent detection and interruption. A breaker is, basically , divided into two parts, the overurrent element and the contacts. Overcurrent element designs use either an electromechani- cal, electrohydraulic, or an electronic assembly to set the overload cur- rent point and delay time. • Electromechanical designs are typically based on a bimetallic element that bends as it is heated by the current flowing through the breaker. • Electrohydrau lic desig ns utiliz e a slug of metal movi ng throu gh a liq- uid-filled tube. The viscosity of the liquid and the strength of the mag- netic field generated by the current are balanced to s et the trip current and time. • Electronic d esigns use a curre nt transformer to measure the am- perage. The electronics then de- termine whether an overload has occurred. The overcurrent element then triggers a spring-loaded me- chanical latch to open the breaker contacts. Laimon Kalning, a re- tired breaker test engineer from Westinghouse Electric once re- marked, “One way or another, they’re all just mouse traps.” Mouse traps maybe, but today’s electronic trip units add quite a bit of sop his tic atio n. The se tri p units monitor current, voltage, and power, and they communi- cate those values to data collec- tion and control equipment. This Part One — Products, Applicable Standards, Heat Rise, and Long-Time Overload Testing L ow-Voltage Circuit Breaker Testing Strategies by Adam R. Fleder Electric Device Corporation
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Transcript of Low-Voltage Circuit Breaker Testing Strategies-Part1.pdf
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Summer 2002 1
This three-part article addresses testing requirements, methods, and
challenges for low-voltage circuit breaker testing strategies.
Amazingly, the electric power industry is now in its third century of operation. However, as author Irene Peter noted; “Just because every-thing is different doesn’t mean anything has changed.” The basic mis-sion of the circuit breaker has remained unchanged since Henry Cutterdeveloped the first one at the end of the 19th century.
Protecting life and property from fire damage is the essential missionof the circuit breaker. Over time, methods and specifications have been
developed to verify that a breaker still provides the level of protectionfor which it was originally designed.One of the primary values that testing provides is the mitigation of
risk. This article provides a general overview of the requirements. Thecurrent testing standards are well established and fairly explicit. You areencouraged to become familiar with their guidelines and apply them toprovide the highest level of protection for your customers and your or-ganization.
ProductsThe types of products that exist now use multiple technologies to ac-
complish the functions of overcurrent detection and interruption. A breaker is, basically, divided into two parts, the overurrent element andthe contacts. Overcurrent element designs use either an electromechani-cal, electrohydraulic, or an electronic assembly to set the overload cur-rent point and delay time.
• Electromechanical designs are typically based on a bimetallic elementthat bends as it is heated by the current flowing through the breaker.
• Electrohydraulic designs utilize a slug of metal moving through a liq-uid-filled tube. The viscosity of the liquid and the strength of the mag-netic field generated by the current are balanced to set the trip currentand time.
• Electronic designs use a currenttransformer to measure the am-perage. The electronics then de-termine whether an overloadhas occurred.
The overcurrent element thentriggers a spring-loaded me-chanical latch to open the breakercontacts. Laimon Kalning, a re-tired breaker test engineer fromWestinghouse Electric once re-marked, “One way or another,
they’re all just mouse traps.”Mouse traps maybe, but today’selectronic trip units add quite a bit of sophistication. These tripunits monitor current, voltage,and power, and they communi-cate those values to data collec-tion and control equipment. This
Part One — Products, Applicable Standards, Heat Rise,and Long-Time Overload Testing
Low-Voltage Circuit Breaker
Testing Strategies
by Adam R. Fleder
Electric Device Corporation
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2 NETA WORLD
allows a more sophisticated coordination of circuitprotection. In addition, there are many accessoriesthat add various functions and complexity.
StandardsThere are four primary standards that are used
to qualify the design of a circuit breaker: ANSIC37.50-1989, UL489, IEC947-2, and IEC898. Thesestandards act as a guide to define what is criticalfor correct operation. The ANSI and UL standardsapply to the US market, whereas the IEC standardsare used in the EU countries, Asia and Africa. Thedifferences between these standards are notable al-though not mutually exclusive. Generally, UL489and IEC898 are oriented toward residential and lightindustrial applications whereas IEC947-2 and theANSI standards are directed at heavy industrial andhigh power applications. The scopes of these stan-dards do overlap somewhat but manufacturers gen-erally follow these distinctions.
The areas that are critical to the correct operationof a circuit breaker in the field are:
• Heat rise
• Long-time overload performance
• Short-time overload performance
• Instantaneous overload performance
• Ground fault sensing
• Dielectric withstand
• Accessory operation
Manufacturers do perform many more tests toqualify a design. These include destructive testingsuch as short-circuit and mechanical endurancetests. For obvious reasons, destructive tests are notapplicable to field testing. Also, all breakers do notrequire all tests. For example, many light industrial breakers do not have short-time settings.
Heat RiseThe heat-rise test verifies that the breaker does
not generate excessive amounts of heat in an en-closed space when carrying rated current. As a breaker ages, the contacts or terminals may corrodeand create high resistance connections. The elevatedtemperatures this causes can degrade wiring insu-lation and, in extreme cases, be the cause of a fire.Verifying conformance involves passing the ratedcurrent through a breaker connected with appro-priate conductors and monitoring the temperatureof the enclosure until it stabilizes. The stabilized tem-perature is then compared to the appropriate stan-
dard to determine acceptance. Performing a heat-ristest requires quite a bit of equipment and instrumentation and is not practical in the field. A reasonable approximation can be obtained by measuring the contacresistance (R) with the precision digital low-resistancohmmeter (DLRO) utilizing the four-wire resistancmethod. The amount of power (P) the breaker uses arated current (I) can be calculated from the equationP=I2R. Contact the breaker manufacturer for actual con
tact-resistance values and expected operating temperatures if a problem is suspected.
Long-Time Overload PerformanceThis test verifies that a breaker will trip within an
acceptable time when subjected to an overcurrent condition in the range of 100-300 percent of its rating. Therare really two concerns when verifying long-time overload performance. The first is that the breaker will remain closed when rated current is flowing through it. Ia breaker is improperly calibrated, then the breaker maytrip when it should not. This would interrupt electricaservice at inopportune times.
The second is that the breaker will trip within an acceptable time when subjected to an overcurrent condition in the range of 135-300 percent of its rating. UL andIEC standards require tests in the 100-135 percent areto be performed with the proper lengths of the appropriate conductors. The conductors can act as a heat sinand draw some of the thermal energy out of the breakeIn extreme cases this will prevent the breaker from tripping. Electrohydraulic and electronic breakers are nosensitive to this condition but can be sensitive to thactual routing of the conductors. High currents flowinin nearby conductors can interfere with the sensing ele
ments inside the breaker. The best practice is to keepthe conductors away from the body of the circuit breakeduring tests.
Finally, the standards specify overload testing in th200-300 percent range that do not require special conductors. Obviously, the conductors have to be sized tocarry the test current without excessive heating.
The key to all of these tests is regulating the currenaccurately for extended periods of time without an operator monitoring it. A typical test may apply rated current for one hour and then apply 135 percent rated foan additional hour.
Data CollectionTesting and installation of circuit breakers also bring
some liability. Even more liability is assumed if th breaker is modified. Correct documentation of test results is a good way to mitigate the risk of litigation. Apaper form that captures the essential data is usuallysufficient to certify the testing process. Process integrity is highest when this is combined with equipmenthat is also certified to NIST standards. Primary injection sets that record data for later retrieval are benefi
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Summer 2002 3
cial. This allows the technical personnel to get asystem running now and access the test resultswhen things are calmer. The results can also bestored in a database for easier access and preser-vation.
ClosingThe safety implications regarding the applica-
tion and use of circuit breakers have produced clear
standards for their expected performance. Heat riseand long-term performance tests are primarily di-rected toward verifying a breaker’s ability to pre-vent fires.
Part two of this article will review high-currentoverload and ground fault detection.
Part three of this article will examine dielectric test-ing and the safety implications of the various acces-sories with which breakers can be outfitted.
Adam R. Fleder is the Vice Pesident of Technology for the ElectricDevice Corporation and a member of the UL489 STP. Electric DeviceCorporation manufactures a variety of circuit breaker testing instru-ments. Joseph Bouch and Paul Miller also contributed material for
this article.
