June 2007 Electrical Apparatus

June 2007 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2007 featured technical article,  by Richard L. Nailen, P.E.    

Circuit breakers are essential components of all industrial and commercial electric power systems. Of the air-magnetic, vacuum, oil, or SF6 types, at voltages from 480 through 34,500, they protect against damaging short-circuit or ground faults, and in some applications (particularly in power generating stations) also serve as motor starting contactors.

Transformers and most rotating machines spend most of their lifetimes in actual operation. A circuit breaker, in contrast, may be inactive for months or years. Its often complex operating mechanism is therefore subject to malfunction caused by deterioration of lubricant and by corrosion. Though often neglected, an important maintenance activity is periodic manual operation to make sure all parts move freely.

However, that does not test the electrical devices that initiate breaker opening and closing. The breaker may be incapable of tripping open during a short-circuit. Industrial plant surveys have repeatedly found many circuit breakers unable to provide fault protection because of neglect. Even when a breaker does interrupt fault current, its operation may be slowed so as to greatly increase the arc energy supplied to the downstream fault.

Many electrical service centers have therefore made circuit breaker servicing and repair an important part of their business, Such work usually includes making sure parts are properly lubricated and any defective hardware is replaced; refinishing or replacing pitted or worn contacts; cleaning or replacing damaged acr chutes; and replacing old electromechanical trip units with microprocessor-based electronic auxiliaries. Some shops also modernize air circuit breakers by replacing the contact assemblies with vacuum interrupters.

Repaired or rebuilt breakers are tested in several ways, including primary current injection (in which a fault is simulated by passing several values of a-c current through each breaker pole at six or more times rated breaker amperes, and comparing the observed trip times with the breaker’s design characteristic). Secondary injection is also used — a test in which current flows only in the breaker trip circuit.

Circuit breaker maintenance guidelines are available in standards of the National Fire Protection Association (No. 70B); the IEEE (Nos. 493, 902, 1015, C37.10); and the International Electrical Testing Association (Maintenance Testing Specifications).

To order a back issue with the full article, “Circuit breaker servicing: Is it for you?” call 312-321-9440 or visit our online webstore.

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