Phase Loss and Phase Unbalance

The most important feature offered by a solid-state overload relay (SSOLR) is phase loss protection. When a phase loss causes a significant current increase in the remaining phases of the motor circuit, there is a major increase in rotor current that can cause motor damage.

The time it takes for a melting alloy device to trip is determined only by the level of current in the remaining phases. The majority of the motors installed (world-wide) run at about 70% of their full-load capability. In these situations, the phase loss condition may result in a level of current in the remaining phases just slightly above the actual FLA of the motor and, therefore, only slightly above the rating of the thermal unit. Therefore, it could take a substantial amount of time for the melting alloy device in this application to respond to phase loss.

The bimetallic device offers a limited form of phase loss protection by means of a differential tripping mechanism, where the device trips slightly faster when an overload is detected on only two of the phases. This device contrasts with an SSOLR with phase loss protection, which trips in less than 3 seconds and alerts the operator of a potential distribution system problem in advance of motor failure. Consequently, the problem does not propagate to affect other equipment on the system.

The SSOLR also provides phase unbalance protection, where the device trips if the current on any phase is 25% greater than the average of all three phases. Phase unbalances are typically caused by an unbalanced upstream single-phase load that can disturb phase voltages. This condition can also lead to excessive rotor currents and motor damage.