Air Circuit Breakers for Alternators

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ELECTRICAL PROTECTION DEVICES

Air Circuit Breakers for Alternators

A majority of circuit breakers used onboard ships are air circuit breakers rather than oil-immersed or vacuum-break types which are common ashore. Figure 11.6 (a) depicts a typical air circuit breaker’s construction while figure 11.6 (b) is a pictorial diagram. It comprises of fixed contacts, arranged in such a manner that the arching contacts make before and break after the main contacts. The main contacts are usually silver-faced copper, copper with silver inserts or silver-cadmium oxide and the arching contacts are usually silver-tungsten or silver-cadmium oxide. These combine to provide minimum contact resistance and carry current with reduced crosion.

If severe burning or pitting occurs on the main contacts, they need to be filed. The manufacturer's handbook will give instructions to rectify this. It is often caused by misalignment of the contacts.

The arcing contacts are normally subject to burning and can be dressed with a smooth file, but not emery cloth. The abrasive particles from the emery cloth may get dislodged and embed themselves in the smooth silvered surfaces of the contacts. Circuit breakers are capable of breaking under the influence of very large short-circuit currents. This is achieved by providing a fast break with long travel to hinder arc formation. Sometimes silver-coated roller-contacts are used to achieve this. Arc extinction occurs as the arc rises into the splitter (arc chute). Never allow a circuit breaker to operate with the arc chutes removed.

Sometimes, blow-out coils, which are connected in series with the circuit breaker contacts, form an electro-magnetic field which reacts with the arc to give a deflecting force that tends to blow the arc upwards and thus outwards. The increase in effective length of thearc causes it to extinguish more quickly. The blow out coil is protected from the arc by are resistant material which may be in the form of an air chute.

Hot ionized gases around the arc and contacts are displaced by cold air thereby forming an eddy current air flow. This helps to increase the resistance between contacts. Contact design and other innovations have rendered blow out coils obsolete but they may still befound.

The air-break circuit breakers used for marine installations are frame-mounted and arranged for isolation from the bus bars and alternator input cable contacts by being moved horizontally forward to a fixed position. The isolating plugs are not designed for making or breaking contact on load and so the breaker must be open before the assembly is withdrawn.

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