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What is an Overcurrent Relay?

Paul Scott
Paul Scott

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.

Circuit breakers.
Circuit breakers.

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.

Discussion Comments


@allenJo - I used to play with electronics way back when, just as a hobby. I often built solid state relay circuits. They were certainly not high voltage devices; I rarely needed more than a 9 volt battery to power up the circuit.

However, the principle of relay technology is still the same, whether you’re talking about 9 volts or thousands of volts. A relay basically shuts off a circuit. In this case it’s to prevent things from blowing up, but in other cases it simply acts as a basic on and off switch.


@everetra - That sounds cool. I can imagine that an overload relay is one of the most important components in the substation. I’ve heard of stories where a substation took a large spike of voltage and set the equipment on fire, and cut out power to thousands of customers.

I think that relay technicians who handle that equipment have quite a bit of responsibility on their hands.

I imagine that it would be a potentially dangerous job too with the amount of electricity that courses through the power grid, and those technicians would have to take precautions to make sure that they are not injured in anyway.


I work at a software company that services the electrical utilities industry. Our software is used to test power relays to make sure that they work properly.

While I am not an engineer myself, I have gained a bit of an education while working there. One of my projects was to expand our software so that it would work with a particular test set.

We have a lot of these units in the workplace that we can play with. The software module I built tested an overcurrent relay to make sure that it captured the trip times and reported them accurately.

Of course all of my testing was done in simulate mode. I didn’t actually have a “live wire” to work with; that’s reserved for the field, but the simulate mode was just as effective.

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