What is an Overload Relay : Types & Its Applications

An overload relay is an electrical device used to protect an electric motor from overheating. So it is essential to have sufficient motor protection. An electrical motor can be operated safely with the help of overload relays, fuses otherwise circuit breakers. But this relay protects the motor whereas the circuit breaker otherwise fuse protects the circuit. More purposely, fuses, as well as circuit breakers, are intended to detect the overcurrent within the circuit, whereas the relay is intended to detect overheat if an electric motor gets heated. For instance, an overload relay can explore without the tripping of a CB (circuit breaker). One does not restore the other. This article discusses an overview of overload relay, types, and its working.

What is an Overload Relay?

An overload relay can be defined as, it is an electrical device mainly designed for imitating the heating prototypes of the electric motor, as well as breakups the flow of current when the heat-detecting device in the relay attains a fixed temperature. The designing of an overload relay can be done with a heater coupled with generally closed connections that unlock when once the heater acquires too hot. The connections of this relay can be done in series as well as placed among the motor & contactor itself to avoid the motor from restarting when the overload trips.

Connection Diagram

The wiring diagram of an overload relay is shown below, and the connections of an overload relay symbol may seem like two opposite question marks otherwise like the ‘S’ symbol. The overload relay working/function is discussed below.

Although there are several types of overload relays available in the market, however, the most frequent type of relay is the “bimetallic thermal overload relay”. The designing of this relay can be done by using two dissimilar kinds of metal strips, and these strips can be connected mutually as well as to enlarge at diverse rates while heated. Whenever the strip is heated at a particular temperature, then the strip can twist far enough for breaking this circuit.

Overload Relay Wiring Diagram
Overload Relay Wiring Diagram

Whenever the flow of current toward the motor is more than what the heaters are charged for, the overload explores later than some seconds. The classes of overload relay can be classified into three types based on the duration of relay explore. Class 10, Class 20, and Class 30 overload relays can be explored later than 10 secs, 20 secs, and 30 secs correspondingly. One main security characteristic of this relay is that stops the motor from instantly restarting. For instance, when the overload relay explores within a bimetallic relay, then the NC (normally-closed) bimetallic connections will unlock the circuit until the strip gets cool. If anyone tries to push the start switch to shut the contactor switches, then the motor will not be switched on.

Overload Relay Working

The working principle of an overload relay depends on an electro-thermal property within a bimetallic strip. The arrangement of this in the motor circuit can be done like the flow of current to the motor can be done using its poles. When the flow of current increases the fixed value then the bimetallic strip gets heated up then it bends.

These relays always work with contractors. Once the bimetallic strips get heat, then the contact trip can be activated and breaks the power supply toward the contactor coil, deactivates it & breaks the flow of current toward the motor. The time taken for tripping is always inversely proportional to the flow of current throughout the relay. Therefore, these relays are called current dependent as well as the inversely time-delayed relay.

The connection of this relay can be done with the motor in series so that the flow of current will be there towards the motor. When the motor activates then the flowing motor throughout the OLR will be there. Once the surplus current flows through the relay then it will trip at a certain level so, the circuit will be opened among the power source as well as the motor. After a prearranged period, this relay can reset automatically or manually. Once the overload has been recognized and corrected then the motor will be activated again.

Parts of Overload Relay

Apart from the contacts as well as bimetallic strip, there are some more parts available in the overload relay which is discussed below.


In the relay diagram, the input terminals are denoted with L1, L2 & L3 which are directly mounted toward the contactor. The motor supply can be connected to T1, T2 & T3 Terminals.

Ampere Range Setting

A rotating knob can be available on the OLD. By utilizing this, the rated flow of current toward the motor can be set. The flow of current can be set among the provided higher & lower limits. In electronic OLD, an extra knob is also provided for class selection tripping.

Reset Button

This button is available over the OLD which is used to reset the relay once a trip & fault clearance.
Manual or Auto-Reset Selection Button

By using these buttons, one can select among manual as well as the automatic reset of relays after a trip. Once the device is fixed to auto-reset, then a remote reset of the relay is achievable

Auxiliary Contact

This relay includes two auxiliary contacts like one NO & another one is NC. For trip signaling, NO contact is used whereas disconnecting the contractor, NC contact is used. NC contacts are capable of contactor coils direct switching.

Test Button

The test button is used to check the control wiring.

Overload Relay Types

These are classified into two types namely thermal overload relay and magnetic overload relay.

Thermal Overload Relay

A thermal type relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.

To achieve this, these relays include an NC (normally closed) relay. Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor, a temperature of the relay, otherwise detected overload current, based on the type of relay.

Thermal Overload Relay
Thermal Overload Relay

This relays are related to circuit breakers in construction as well as an application; however, most of the circuit breakers disturb the circuit if overload happens even for a moment. These are equally designed for calculating the heating profile of the motor; thus, overload should happen for a complete period before the circuit is broken up. Thermal overload relays are classified into two types namely solder pot as well as a bimetal strip.

Magnetic Overload Relay

Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current toward the motor. This relay can be built with a variable magnetic core within a coil that holds the motor current. The flux arrangement within the coil drags the core up. As the core increases far enough, then it trips a set of connections on the summit of the relay.

Magnetic Overload Relay
Magnetic Overload Relay

The major difference between thermal type as well as magnetic type relays is that magnetic type overload relay is not responsive toward ambient temperature. Generally, these are used in the areas where extreme changes exhibit within the ambient temperature. Magnetic overload relays are classified into two types namely electronic as well as dashpot.

Bimetallic Thermal Overload Relay

The working of a bimetallic thermal overload relay mainly depends on the bimetallic strip’s heating property. In the straight heating technique, the complete flow of current toward the motor can be supplied using the overload relay which is also called OLR. As a result, directly it gets heated due to the flow of current.

However, in the case of not direct heating, the strip can be arranged within close contact through the conductor within the relay. The extreme flow of current toward the electric motor gets heated by the conductor & the bimetallic strip. Here, the conductor shall be insulated therefore no flow of current will be there throughout the strip.

Electronic Overload Relay

Usually, electronic overload relays are referred to as solid-state overload relays. The inside of these types of relays does not contain a bimetallic strip. As an alternative, it includes current transformers otherwise temperature sensors to notice the sum of flow of current toward the motor. For protection, this kind of relay uses a technology based on a microprocessor. Here PTC plays a key role to detect the temperature as well as tripping the circuit once overload errors occur. Some types of overload relays come with Hall Effect sensors as well as current transformers to detect the flow of current directly.

The main benefit of an electronic overload relay over a thermal overload relay is that lacks the bimetallic strip which results in fewer heat losses in the relay. Also, these types of relays are more accurate as compared to thermal relays.

Some of the manufacturers of electronic OLD design by including additional features like protections of earth fault & motor stall. Electronic overload relays are used where a start & stop of motors are required frequently. The designing of these relays can be done in such a way that to withstand the initial current of the motor for a restricted period.

Eutectic Overload Relay

The eutectic overload relay includes a winding heater, a eutectic alloy, and a mechanical device to activate the tripping mechanism. Here, a eutectic alloy is a blend of two otherwise more materials, which melts otherwise, hardens at a precise temperature. In the OLR, the eutectic alloy is enclosed within a tube to use frequently through a ratchet wheel loaded with a spring to make active the tripping device throughout the overload process.

The current in the motor supplies through the small heater winding throughout the overload, the eutectic alloy tube can be heated through the heater winding and the alloy dissolves because of the heat so that the ratchet wheel turns. This act begins to open the closed auxiliary contacts within the OLR. This kind of relays can simply reset manually once tripping is done. So, usually, this reset can be done using a reset button, which is arranged on the relay cover. The heater unit which is connected over the relay can be selected based on the motor’s full load current.

Fridge Overload Relay

A protection device like an overload relay is used within the compressor circuit of the refrigerator. The power supply is given to the windings of the compressor motor using the overloaded machine. This kind of relay is mainly used to include the start winding within the circuit till the compressor is at running speed.

How does an OLR Guard from Phase Failures?

In normal operation of OLR, the flow of current throughout every pole to the electric motor remains similar at a time. If any phase is interrupted, then the flow of current throughout the remaining two phases increases to the usual value. Therefore the relay gets heated up & trips. Phase failures are also called phase loss otherwise single phasing of the motor.

These relays cannot defend from short circuits but, they must be used through protection devices of a short circuit to protect them or any short circuits within the electric motor can injure them easily. These relays can defend from phase loss, phase imbalance, overloads, but not from short circuits.

What Causes the OLR Trip?

From the above discussion, there are three main states for excess trips:

  • Overloading of the Motor.
  • Input Phase Loss
  • Imbalance of Phase

And also there are some extra protection features available but changes from one designer to the other.

Overload Relay Tripping

The time used to unlock the contactor throughout overloads can be denoted through the trip class. Generally, it is divided into different classes like Class5, 10, 20 & 30. This relay trips in 5 secs, 10 secs, 20 secs & 30 secs correspondingly at full load current toward the electric motor.

The commonly used overload relays are class 10 & 20 whereas class 30 OLR is mainly used for protecting the motors while operating high inactivity loads. The Class 5 type relays are mainly used for the motors which require extremely fast tripping.


The applications of an overload relay include the following.

  • It is extensively used to protect the motor.
  • It can be utilized for detecting both overload conditions as well as fault conditions & then declare trip commands for a protective device.
  • This relay has developed into microprocessor systems as well as solid-state electronics.
  • These relays deactivate the device whenever it pulls extreme current.

Thus, this is all about an overview of the overload relay. From the above information finally, we can conclude that these are electromechanical overload protection relay devices used for the circuits. These devices provide consistent protection for motors while the failure of phase otherwise overload occurs. Here is a question for you, what is the function of the overload relay?

Image Sources: Temco Industrial

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