You are currently viewing What is Overload Relay : Full Explained

What is Overload Relay : Full Explained

Friends, if you are doing a job in electrical, then you must have seen an overload relay connected with all starters of the motor. So in this post, we will talk about this OLR.

Full-Form of OLR: Overload Relay

In some places, this is called thermal overload relay.

What is Overload Relay?

Overload relay is an electrical device used to give safety to the motor. Its Working is very simple. It sense current going in the motor. Suppose motor draw more than set current. In that case, OLR trip the circuit. So our motor will remain safe from any fault.

Overload relay working

There are bai-metallic elements inside the overload relay. The current going to the motor goes through these bi-metallic elements. Now, if the motor draws more current, then this bi-metallic element gets heated and bends. As soon as this happens, the circuit breaks, and the motor stops.

Overload relay Connection

Mostly OLR is used with the connector. While using an overload relay there is two types of connection.

  1. power connection
  2. control connection

Power wiring: Inside the power wiring, we connect the three-phase coming out of the contactor to the L1 L2 L3 terminal of the relay. After this, we give three-phase supply to the motor from the T1 T2 T3 terminal of Relay.

Control Wiring: In control wiring, you have to connect the relay in series with the motor stop button (NC push button). We all know that to turn the contactor on/off, we give supply with NO NC push button. So now you do not have to directly connect the NC output wire with NO push.

The phase that comes from NC button first goes in OLR relay’s NC contact and after output from relay NC contact will contact from NO button.

Types of OLR Relay

Overload relay is mainly two types:

  1. thermal overload relay
  2. magnetic overload relay


Thermal Overload relay- The working of a thermal overload relay is quite simple. In this type of relay, the current going inside the motor first goes through the contact of the relay. The contacts of this relay get heated due to the flow of that current and the thermal overload relay works on how much hot it is getting.

Magnetic Overload relay- magnetic overload relay does not work on heat. It works on the magnetic field. As we all know, if current flows through a conductor, a magnetic field is created around it. So this magnetic overload relay works on this.

It senses the strength of the magnetic field created by the current and detects whether the motor is running overloaded or not. And if there is an overload condition, it trips immediately.

Overload relay protection

Overload relay mainly gives protection from 3 types of fault:

Overloading Fault- When we connect our equipment to the overload relay. Now if that device draws more current than the set ampere on the relay, then at that time the relay trips the circuit considering it to be an overload fault.

Input Phase loss- This protection is very important for an electrical motor. As we all know, if we give 2 phase supply inside a three-phase motor, that motor will burn in less time. So overload relay also provides protection against this fault. If a phase is missed, then this relay trips at the input phase loss fault.

Phase Imbalance- If voltages in all three phases are coming differently, more in one and less in the other, then OLR relay trips and protects the system. Along with this, if our motor also takes more current in one phase and less current in the other, then this relay trips due to phase imbalance fault.

Can OLR protect from short circuits?

Many people think that can overload relay give protection against short circuit? So guys, the answer is NO. Overload relay provides only three protection to our equipment overload, phase loss and phase imbalance.

If you want short circuit protection in addition, you should also install MCB along with it so that the MCB protects the system against short circuit fault.

Parts of an overload relay

Terminal- There are total six terminals inside the relay. In this, the phase supply wire is connected on L1 L2 L3. Along with this, there are T1 T2 T3 terminals. On these terminals the wire of the motor is connected.


Ampere Range Setting- You will definitely find a rotary knob in all types of overload relays. With its help, we can set the relay, that how much it will allow maximum current to pass through.

Reset Button- When the overload relay trips due to some reason, then we have to reset the relay to use it back. So to reset the OLR relay, there is a reset button on it.

Manual / Auto reset selection- This is a button. You can set it to either auto or manual. If it is set to manual, you will need to reset the relay yourself if a fault occurs. But at the time of auto, this relay automatically resets after it cools down.

Auxiliary contact- This auxiliary contact has NO and NC contacts. The main use of its NC contact is to start and stop the contactor from the relay.

Test button- With the help of this button, we can check the relay whether it is working properly or not.

We hope that you understand. If still any doubt or question remains then feel free to ask in the comment section or you can ask this question on Instagram Electrical Dost”

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Thank You 🙂

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