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Niche Market Testingby Lynn Hamrick, ESCO Energy Services

A Primer for Testing Medium-VoltageBreakers in the Industrial Environment

During its 20-40 year service life, a circuit breaker must be constantly

prepared to operate. Typically, long periods of inactivity often elapse

during which the breakers mechanical and electrical components never

move. e circuit breaker is the active link in a fault clearance situation. When a

fault occurs on the electrical system, the associated fault current must be inter-

rupted quickly and reliably. is action is referred to as fault clearing. If a breaker

fails to clear a faulted circuit, the resulting damage can be very serious in terms

of both personnel injury and equipment damage. Even though circuit breakers

are comparatively reliable, circuit breaker failures can and do occur. erefore,

circuit breakers must be tested and maintained to ensure proper operation dur-

ing these faults.

Many circuit breakers provide longer service lives than expected. If you can

ascertain that a breaker is in good condition, you can continue to use it ratherthan replace it at its estimated end-of-life. For the remainder of this discussion,we will concentrate on medium-voltage (1 kV up to 35 kV) circuit breakersutilized within an industrial environment.

Different Medium-Voltage Breaker Designs

ere are three basic designs of medium-voltage breakers:

Air Circuit Breaker Air circuit breakers are the prevalent type of medium voltagebreaker in an industrial environment due to the typical age of the equipment andtheir lower cost; however, very few of this type of breaker are still being manu-factured. Air circuit breakers are commonly used in medium-voltage electrical

distribution systems and include operating mechanisms that are mainly exposedto the environment. Since the air circuit breakers are rated to carry current con-tinuously, the exposure to convection cooling air assists in keeping the operatingcomponents within reasonable temperature limits. A typical air circuit breakerincludes a main contact assembly, which is either opened (tripped), interruptinga path for power to travel from the source to the load, or closed, providing apath for power to travel from the source to the load. In many air circuit breakers,the force necessary to open or close the main contact assembly is provided by anarrangement of compression springs. In many air circuit breakers, the mechanismfor controlling the compression springs comprised of a configuration of mechanicallinkages between a latching shaft and an actuation device. e actuation device maybe manually operated, or electrically operated with trip and close coils. In a com-

mon construction of a medium-voltageair circuit breaker, the movable contactis mounted on a contact arm that ispivoted to open or close the contacts bya spring powered operating mechanismtriggered through the trip or closingcoils by a control switch or closing relay(i.e., protective relay). Smaller open-ing springs are put in tension duringclosing and assist in opening whenthe trip coil is given tripping energyfrom the control switch or protectiverelays. ese protective relays may alsobe used to provide auxiliary functionsalong with over-current protection.The current interruption capability

of air circuit breakers is dependent inpart upon their ability to extinguishthe arc that is generated when thebreaker main contacts open or close. Amedium-voltage air circuit breaker inan industrial environment is typicallya draw-out type in which a breaker ismovable so as to be separated (rackedout) from the power source in orderto facilitate maintenance or connected(racked in) to the power source usingeither a manual or motor-assisted rack-ing capability.

Vacuum Circuit Breaker Vacuumcircuit breakers are the most populartype of medium-voltage breaker in anindustrial environment due to theirsmaller size and improved reliabilityover air circuit breakers. Vacuum cir-cuit breakers are similar to air circuitbreakers with the difference beingthat the main contacts are housed in avacuum chamber (bottle). One of thecontacts is fixed relative to the bottle

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and to an external electrical conductor which is intercon-nected with the circuit to be controlled by the circuit breaker.e other separable main contact is movable. e moveablecontact assembly is usually comprised of a stem of circularcross-section having the contact at one end enclosed withina vacuum bottle and a driving mechanism at the other endwhich is external to the vacuum bottle. An operating rodassembly comprised of a push rod, which is fastened tothe end of the stem opposite the moveable contact, and a

driving mechanism provide the motive force to move themoveable contact into or out of engagement with the fixedcontact. Compression springs are provided in connectionwith the operating rod assembly in order to be able toseparate the moveable contact from the fixed contact andto assure the necessary force so that the contacts will notaccidentally open under inappropriate conditions. As withthe air circuit breaker, the actuation device may be manuallyor electrically operated by trip and close coils triggered bya protective relay. ese protective relays may also be usedto provide auxiliary functions along with over-current pro-tection. Vacuum circuit breakers offer many advantages ascompared to other types, including: relatively long life due

to controlled contact erosion; maintenance-free operationprovided by enclosure of the contacts within the hermeti-cally sealed housing; excellent operational reliability due tothe use of hard contact materials; little or no atmosphericcontact contamination, which contamination can detri-mentally form oxides and corrosion layers on the contacts;little or no noise during operation due to containment ofarcing within the hermetically sealed housing; relatively fewenvironmental effects; and very low current chop, resultingin a minimal induced transient voltage spike during circuitinterruption so that surge suppressors are typically notrequired. A medium-voltage vacuum circuit breaker in an

industrial environment is also typically a draw-out type.Oil Circuit Breaker Oil circuit breakers, which aretypically located outdoors, were the past standard but arebecoming more rare in the industrial environment as theyare being replaced with air or vacuum circuit breakers. Oilcircuit breakers have all contacts and mechanism housedwithin an oil-filled tank, which provides for both an in-sulation and arc-quenching capability. A typical oil circuitbreaker includes the same operational characteristics as anair circuit breaker. A medium-voltage oil circuit breaker inan industrial environment is typically a stand-alone typeand requires that oil be drained and the breaker de-tankedto perform maintenance.

Suggested Electrical Testsere are three basic electrical tests that should be per-

formed on medium-voltage circuit breakers as part of apreventive maintenance program.

Contact resistance is test should be performed fromline-to-load terminals for each phase with the breakerclosed. e values should be within 50% of each other andcomparable to similar circuit breakers.

Insulation resistance is test should be performedphase-to-phase and phase-to-ground (i.e., case, if appli-cable) with the breaker closed, as well as across the opencontacts for each phase. For medium-voltage breakers, thmanufacturer should provide the testing voltages and ac-ceptance values for this test. When not available or whereno recommendations are provided, the following abbreviatedtable from ANSI/NETA MTS-2007 may be used

Insulation Resistance Test Values MV Breakers

Nominal Rating ofBreaker in Volts

Minimum TestVoltage, DC

RecommendedMinimum

Insulation Resistancein Megohms

2,500 1,000 500

5,000 2,500 1,000

8,000 2,500 2,000

15,000 2,500 5,000

25,000 5,000 20,000

35,000 and above 15,000 100,000

Dielectric withstand voltage is basically an ac or dchigh potential test. is test should be performed fromthe line side of each phase with the breaker open andthe load side of that phase and line and load sides of theother phases tied together and tied to ground. For vacuumcircuit breakers, this test will also provide an indication ofthe vacuum bottle integrity. For medium voltage breakersthe manufacturer should provide the testing voltages forthis vacuum bottle integrity. It should be noted that fo

a medium voltage vacuum breaker, only use of an ac highpotential is recommended. When not available or where norecommendations are provided, the following abbreviatedtable from ANSI/NETA MTS-2007 may be used

Dielectric Withstand Test Voltages MV Breakers

Nominal System(Line) Voltage

Maximum FieldApplied AC Test

(kV)

Maximum FieldApplied DC Test

(kV)

1.2 6.0 8.52.4 9.0 12.7

4.8 11.4 16.1

8.3 15.6 22.1

14.4 20.4 28.8

18.0 24.0 33.9

25.0 30.0 42.4

34.5 42.0 59.4

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In addition to these basic tests, the protective relaysshould be tested for functionality with all implemented pro-tective functions being tested. In an industrial environment,over-current protection is typically the main, and possiblyonly, function provided by the protective relay; however, ifthere are other protective functions being provided, theyshould also be tested. It is also important that the breakeractually be tripped and closed by the manual controls, andtripped by the protective relay to ensure that the relay and

breaker are operating correctly. is will functionally test thetrip and close coils as well as the protective circuit.Power-factor or dissipation-factor testing should be

considered as an additional test of each breakers insulationsystem. For oil circuit breakers, a tank loss index should bedetermined and, where individual mechanisms are providedby phase for operation, time-travel testing should be con-sidered for each phase. Oil sampling is also recommendedannually with oil sample quality analysis being provided fordielectric breakdown, acid neutralization number, interfacialtension, moisture, color, and power factor. Where cubicleheaters are provided, functional testing of the heaters shouldbe verified.

In conclusion, even though circuit breakers are reliablewhen compared to other electrical devices, circuit breakerfailures occur. When a breaker fails to operate, the result-ing damage can be very serious in terms of both personnelinjury and equipment damage. erefore, circuit breakersmust be tested and maintained to ensure proper operationduring these faults.

As Operations Manager of ESCO Energy ServicesCompany, Lynn brings over 25 years of workingknowledge in design, permitting, construction, andstartup of mechanical, electrical, and instrumenta-

tion and controls projects as well as experience inthe operation and maintenance of facilities.

Lynn is a Professional Engineer, Certified En-ergy Manager and has a BS in Nuclear Engineeringfrom the University of Tennessee.