Believe it or not, air circuit breaker problems are the cause of almost 20% of electrical power system failures, according to a survey conducted by the Hartford Insurance Company. In addition, they cause property damage and/or personal injury, not to mention lost revenues from unscheduled downtime.
Why do low voltage power circuit
breakers fail?
Low voltage power circuit breakers fail because dust, hardened grease, corrosion, and/or frozen parts prevent the proper operation of trip linkage. Some of the most common failures result from leaking dashpots, maladjusted linkage, broken cases exposing live parts, loose internal connections, defective or broken parts, metal fatigue, age, overuse, and/or misapplication. Symptoms include spurious tripping or no tripping at all.
The best way to counter these potential problems is to inspect and test power circuit breakers on a scheduled basis per the following International Electrical Testing Association (NETA) guidelines.
Visual and mechanical inspection
You first should verify that the breaker is deenergized and that all maintenance devices are available for servicing and operating the breaker. Then, the following procedures should be carried out.
* Look for any broken parts and replace as needed. Clean and lubricate all mechanical parts.
* Inspect breaker anchorage, alignment, and grounding; inspect arc chutes; and inspect moving and stationary contacts for condition, wear, and alignment.
* Verify that contact wipe as well as other dimensions critical to the proper operation of the breaker are correct per the manufacturer's recommendations.
* Perform all mechanical operator and contact alignment tests on the breaker and its operating mechanism.
* Verify correct tightness of bolted bus connections by using a calibrated torque-wrench. For proper torque levels, you should refer to the manufacturer's instructions or use the levels noted in Tables 1 through 3 (Tables 2 and 3 are on page 106), depending on the type of fastener used.
* Check cell fit and element alignment.
* Check the racking mechanism for proper operation.
* Lubricate all moving current carrying parts.
The recommended way of testing power circuit breakers is to use a portable high current (1000A to 50,000A rms) test set. The test set should be connected across the circuit breaker one pole at a time, with the following tests conducted.
* Contact voltage drop at rated current. This measurement will help detect loose connections, improper contact pressure, and/or damaged main contacts.
* Short-circuit protection capability. This is done by pulsing higher currents until instantaneous tripping occurs.
* Overload tripping capability. This is done by applying 300% of rated current to the breaker and measuring the overload tripping time.
Insulation resistance (IR) testing should also be done per the following.
* IR testing at 1000VDC across opened and closed contacts, both phase-to-phase and phase-to-ground. Compare microhm or millivolt drop values on any one pole to adjacent poles as well as to poles on similar breakers. If a deviation of more than 50% is noticed, further investigation is needed.
* IR testing at 1000VDC on all control wiring. (Do not do this testing on any wiring connected to solid-state components.) Insulation resistance should not be less than 100 megohms.
* Determine minimum pickup current; long-time delay; short-time pickup and delay; ground-fault pickup and delay; and instantaneous pickup by employing primary current injection.
* Verify calibration of all functions of the breaker trip unit by employing secondary current injection.
* Make necessary adjustments for final breaker trip settings per existing or new coordination study.
* Additional testing includes the following.
* Verify operation of shunt-trip devices by activating any auxiliary protective devices (ground-fault or under-voltage relays, for example). Check operation of electrically-operated breakers in their cubicles.
* Verify operation of auxiliary features (trip and pickup indicators, zone interlocking, electrical close and trip operation, trip-free, and anti-pump function).
* Check charging mechanism.