An overcurrent relay is an electrical protection device designed to cut off the power supply to a circuit, appliance, or machine in the event of an overload or failure. These devices typically consist of a manual circuit breaker or contactor and a current sensing relay which interlocks with it. Should the appliance or circuit be damaged or overloaded, it will begin to draw current in excess of its normal operating parameters. This causes the current sensing relay to trip the circuit breaker or deactivate the contactor and cut the power supply. As many machines draw very high current when starting up, most overcurrent relays have a built in “lag” feature to accommodate a high current draw for a preset period before activating.
An overloaded or damaged electrical appliance or machine draws current which gradually begins to rise until it exceeds the design parameters of the device. If left unchecked, this causes additional damage to and the eventual destruction of the machine. It may also cause an explosion or fire and poses a serious electrocution risk to operators. One of the most effective ways of preventing this is an overcurrent relay in the primary supply circuit. This device is designed to interface with a supply circuit breaker or contactor, thereby cutting that power supply if an excessive current drain is sensed.
These relays may be standalone units or integral parts of the primary supply switching devices they work with. Some feature set current ratings while others have a user-adjustable range of current settings. Standalone relays may have removable current sensing coils on each phase or line which can be swapped out for units of differing current ratings. In the case of overcurrent relay protected circuit breakers, a trip requires the circuit breaker to be switched back on when power is restored. Contactor relays generally cut power to the contactor's activation coil in the event of an overcurrent situation and must be reset before the machine or appliance can be restarted.
Many overcurrent relay variants include lag features allowing for an excessive current drain for a predetermined time before they trip the power. This function is designed to accommodate machines with high startup current ratings. The lag settings permit the machine to start and its current drain to return to normal without activating the relay. When an overcurrent relay does trip, it is always advisable to investigate the cause before resetting the system and restarting to prevent possible damage or operator injury.