OVERCURRENT RELAY RATCHETING

Effects of autoreclosing on disk type overcurrent relays (ratcheting)

Electromechanical disk type time overcurrent relays respond to current above their pickup level in a time inversely proportional to the current level. When the current level is above the pickup, the disk begins to turn and continues to turn until the rotating contact meets the stationary contact to cause a breaker trip or until the current drops below the pickup level, as would occur if the fault were cleared by downstream protection.

Disk reset will occur when the current has dropped below the pickup level for a period of time. The disk resets to its original position at a relatively slow rate by the action of a coil spring.

This relatively slow reset action of the disk should be taken into consideration when autoreclosing is applied to any breaker or circuit recloser downstream of this breaker.

The most common application that requires special care is the circuit recloser operating to clear sections of a feeder downstream of a circuit breaker having disk type overcurrent relays applied. The circuit recloser can autoreclose the faulted section multiple times, resulting in several periods of fault current flow through both the circuit recloser and the upstream circuit breaker.

In a coordinated system, the circuit recloser trips the fault and removes the fault current flow before the disk type overcurrent relays reach their trip point. A minimum safety margin is normally provided to assure good coordination. At the point the overcurrent condition is removed, the disk has rotated some percentage of the amount required to provide a trip.

If the disk is not fully reset by the time the circuit recloser operates to re-energize the faulted section, the disk will not have as far to travel as during the original fault. This ratcheting effect could lead to a loss of coordination resulting in the unnecessary tripping of the circuit breaker.

Another example of ratcheting is an application in which two or more breakers having disk type overcurrent relays are in series. When both encounter an overcurrent condition, different time-dial settings or unequal currents due to load could cause the percentage of disk travel of the upstream relay to vary considerably.

Once again, if autoreclosing of the downstream breaker occurs before the upstream device is reset, a loss of coordination could occur.

Solutions for these loss of coordination events include use of larger safety margins to allow for partial reset of electromechanical relays, application of inverse time relays with fast resets on the upstream breakers, or longer dead times for autoreclosing.

Microprocessor or static relay designs may have an instantaneous reset of the timing function as well as other selective reset characteristics, which is normally considered in the coordination process.