AUXILIARY TRIPPING RELAYS FOR BUS PROTECTION

Bus protection relays usually energize a multicontact auxiliary relay which has individual tripping contacts for each breaker connected to the bus.

Lockout Relays
A common practice is to use a lockout type tripping relay to prevent reenergizing the bus until an inspection is made.

It may also be desirable to interrupt the breaker closing circuits by separate lockout relay contacts connected in the closing circuit of each breaker to prevent breaker closing, even though the breaker would be immediately tripped by the unreset lockout relay tripping contacts.

One type of lockout tripping relay is spring actuated, with a latch released by a tripping solenoid. In one version of this relay a manually operated handle is used to reset the relay.

In another version, a motor performs this reset function so that the relay can be reset remotely.

A second type of lockout relay uses an electrical seal-in coil to lock itself in the operated position. The seal in coil is energized by one of the tripping contacts on the relay.

A reset pushbutton momentarily opens the energized seal-in coil to reset the relay. A third type of lockout relay has a mechanical latch that is reset electrically.

Nonlockout Relays
Nonlockout (self-reset) tripping relays are used in some installations to permit automatic reenergization of a bus. A self-reset relay automatically resets when its coil is deenergized (in the case of bus protection, this means when the fault is cleared).

It is often used at unattended locations where there is open bus work, no supervisory or remote control facilities are provided, and operator travel time is long. Hence, automatic reenergization of a bus will reduce outage time if successful.

Ratings and Connections
Bus tripping auxiliary relays are usually selected with underrated trip coils to obtain fast and positive operation. With a 130 V dc tripping supply a 24 V rated coil is frequently used.

With lockout type relays two b contacts of the relay are usually connected in series with the operating coil to interrupt the coil current after the relay operates. This sudden interruption of the coil current may produce a high transient voltage across the coil.

Diodes are sometimes used to minimize the effect of this transient voltage not only on this coil, but also on other portions of the control circuitry or other solid state components.

As with any auxiliary tripping relay, it may be necessary to use resistors in parallel with the relay coil to be sure that the target coils of the bus protection relays receive sufficient current to operate. If these resistors are used, they may eliminate the need for diodes to limit the aforementioned transient voltages.