DC Battery Trip
The battery is probably the most reliable source of control power when it is properly maintained and serviced. It uses single contact protective relays to energize the breaker trip coil. It is unaffected by voltages and current during fault conditions. Generally, a 125 or 250 V battery is recommended for MV switchgear when both closing and tripping is required. When such is not available, a 48 V battery may be used for tripping only. However, it must be sized to meet the required load of the switchgear. A battery trip circuit is shown in Figure 7.30a. Long service can be obtained from batteries when they are serviced regularly, fully charged, and the electrolyte level maintained at the proper level.
Capacitor Trip
The capacitor trip device is commonly used where a DC battery source is not available or uneconomical, such as in outdoor switchgear or where only few circuit breakers are installed. The capacitor device simply consists of a capacitor and half-wave rectifier charged from an AC control power trans- former. When using a capacitor trip device, a separate capacitor trip unit is required for each breaker.
The capacitor trip device comes in two types: nonautocharge and autocharge. The nonautocharge retains adequate charge for a short time (about 30 s) after the AC supply is lost. The autocharge consists of a regulated charge. It con- tains a voltage amplifier, a battery, and a battery charger. In case of loss of AC supply, the voltage amplifier steps up the battery voltage to maintain an adequate charge of the capacitor for several days. The simple capacitor trip device circuit is shown in Figure 7.30b.