Circuit breaker servicing: Is it for you?

Electrical Apparatus, Jun 2007 by Nailen, Richard L

When to give attention to a device that only rarely is called upon to perform

TRANSFORMERS AND CIRCUIT BREAKERS: Whatever else any commercial or industrial electric power system may include, those two components will always be there. The transformer is a static device-no moving parts, except for accessories such as a tap changer, and failure of a transformer, though sometimes spectacular, seldom affects the primary and secondary circuits other than by de-energizing the load.

In contrast, failure among a circuit breaker's many moving parts may result in destruction of the downstream circuit and even of the entire premises. Yet despite their importance, breakers rarely call attention to themselves (no noise, no vibration) and consequently are all too often neglected. Such treatment invites deterioration, which then leads to failure.

Most electrical apparatus is doing something, performing some task, during most of its useful life. Lights are on, motors are running, transformers energized (with intermittent idle periods). A circuit breaker, in contrast, sits in idleness for months, years, even decades without operating. When called upon to open quickly and safely to interrupt a fault, the mechanism must function as designed. Failure may be catastrophic.

Maintenance, then, is of the highest importance for these guardians against short-circuits and ground faults. We won't discuss here the small circuit breakers on panelboards, or the molded case units controlling many industrial motor circuits, lighting feeders, or other relatively low-energy applications. Rather, the concern is with "power breakers"-the enclosed units safeguarding power circuits rated 600 volts or below, carrying currents of hundreds or thousands of amperes, as well as the larger medium-voltage devices housed in switchgear cubicles, their contacts operating in either air, vacuum, gas, or oil environments.

Maintenance intervals should be suited to the type of equipment. For air-magnetic breakers, servicing is typically recommended at intervals of no more than a year, or a single fault interruption, whichever occurs first. Vacuum interrupters may safely go ten times that long. Consider the difference between a circuit breaker and a contactor. The contactor is nothing more than an on/off switch. It's designed to make and break rated current a large number of times, typically tens of thousands. Even in a continuous process industry, it may operate one or more times every day. Eventually, fatigue sets in, and a spring or some other part will break. The contacts themselves will erode. Deterioration will be rather quickly noticed because of the frequent usage.

Infrequent operation means that, unlike a contactor, a circuit breaker seldom "wears out." But ample evidence confirms that the breaker will often malfunction when called upon to interrupt a fault on the circuit.

Vacuum interrupters are widely used in both circuit breakers and motor starting contactors. Many thousands have been installed since the first one in 1963. Smaller and more closely spaced than air-break contacts, contacts sealed in vacuum bottles can last much longer because of the elimination of contact bounce or burning. No arc chutes or blowout mechanisms are needed. The contacts generate less heat while carrying normal current, and require less operating power. Moisture and contamination are also eliminated.

Because the contacts cannot be visually inspected, periodic checks on their condition are more difficult to perform. Wear influences contact movement during operation; therefore, precise measurements on movable contact travel are a useful maintenance tool.

Initial vacuum in the bottle is typically less than a billionth of an atmosphere. Although leakage leading to loss of vacuum is rare, it can happen, and will result in reduced interrupting capability. To check on that, a hipot test across each open contact is recommended (typical a-c test voltage is 25,000). A "pull test" is also possible for some vacuum bottles. This is done by disconnecting the movable contact from its operating mechanism and manually pulling on it. Atmospheric pressure against the exposed side of the contact bellows resists that pull, and the net force is measurable; it will be zero if all vacuum has been lost, because there will then be atmospheric pressure on both sides of the bellows. Unfortunately, partial loss of vacuum decreases the pulling force only slightly while seriously diminishing the interrupting capability.

Despite the advantages of the vacuum contacts themselves, their operating mechanism requires the same kind of maintenance as with any other breaker design. Linkages, springs, pivots, solenoids-all operate in the same way.

One IEEE survey of circuit breaker performance in industrial and commercial facilities showed that half of the units were receiving either "fair" or "poor" maintenance, while 15% got no maintenance at all. Another IEEE survey (reported in 1996) of maintenance testing on 1,174 low-voltage circuit breakers revealed that nearly 25% failed to operate properly.