Overload Relay – Definition, Types, and Principle

Overload relay is very common when we are dealing with electric motors. Why is that?

Electric motors are often used in equipment that include rotating parts. Because motors are often fairly costly, it’s critical to avoid them failing due to carrying more electric current than their rated amperage.

Ground faults (short-circuits in the motor windings or peripheral cables) can cause electrical overload, but jamming or inappropriate operation are more prevalent causes.

Every motor must be protected against all possible flaws in order to ensure long-term, safe operation and save time lost due to breakdown. Almost all industries rely on electric motors to regulate their processes and output. As a result, the engine must be made fail-safe.

Overload relays are one type of device that prevents a motor from being damaged by overloads and overcurrents. It’s found in motor control centers and motor starters, and it’s used with contactors.

What is Overload Protection?

Overload relay is one types of relays we will encounter one day.

Overload relay is mostly used as an overload protection. But what is overload protection?

An overload occurs when the motor consumes too much current. This may cause the motor to overheat, causing damage to the windings. As a result, it’s critical to protect the motor, motor branch circuit, and motor branch circuit components from overload.

Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat caused by an overload. The motor starter includes overload relays (assembly of contactor plus overload relay).

They protect the motor by keeping an eye on the current going through the circuit.

If the current exceeds a preset limit for an extended length of time, the overload relay trips, activating an auxiliary contact that terminates the motor control circuit and de-energizes the contactor.

As a result, the motor’s power is turned off. The motor and motor circuit components do not overheat and become damaged when there is no power.

Overload relays can be manually reset, and some overload relays will reset themselves after a period of time. After that, the motor can be turned back on.

Overload Relay Definition

Overload relays are devices that protect electric motors from overloads and phase failure.

When the motor is overloaded, it detects this and terminates the power flow, preventing the motor from overheating and winding damage.

It can also protect the motor from phase loss/failures and phase imbalance, in addition to overloads. They’re usually referred to as OLR.

What is an overload, exactly?

An overload occurs when a motor draws a current that is greater than its rated value for an extended length of time.

It’s the most common problem, and it might cause the motor winding to overheat. As a result, a quick return to regular operation is critical.

Read also : basic direct current circuit

Overload Relay Parts

Aside from the bimetallic strip and contacts indicated in the working principle section, there are a few other components in an overload relay worth mentioning.

Observe the thermal overload relay below:

overload relay 1


Input terminals L1, L2, and L3 are available. It can be mounted directly to the contactor. Terminals T1, T2, and T3 can be used to supply power to the motor.

Ampere range toggle

Over the overload relay, there is a rotary knob. The rated current of the motor can be set with this knob. The current can be adjusted between the supplied upper and lower limits. In the case of an electronic overload relay, there is also a knob for selecting the tripping class.

Reset button

The overload relay has a reset button that can be used to reset it after a trip and fault clearance.

Automatic-Manual reset mode

We can choose between manual and automatic reset of these relays after a journey using the manual/auto reset selection button. A remote reset of OLR is available if the device is set to auto.

Auxiliary contact

They come with two auxiliary contacts: one NO (97-98) and one NC (97-98). (95-96). The NO contact is used to signify a trip, whereas the NC contact is used to disconnect the contactor. NC contacts should be able to switch the contactor coil directly.

Testing button

The control wiring can be tested by pressing the test button.

Overload Relay Symbol

One thing you should know and understand is that every electrical component, element, and device have their symbol.

This symbol will make it easier to use when drawing, designing, and analyzing.

Observe the symbol of thermal overload relay below:

overload relay 2

Terminals 1, 2, 3, 4, 5 and 6 are power terminals
Terminals 95 and 96 are trip contacts
Terminals 97 and 98 are signal contacts.

Overload Relay Connection Diagram

The overload relay connection diagram is given below, and the overload relay sign is represented by two opposite question marks.

Otherwise, the overload relay function is addressed using the ‘S’ symbol. Although there are many different types of overload relays on the market, the “bimetallic thermal overload relay” is the most commonly utilized.

overload relay 3

This relay’s design incorporates two different types of metal strips, which can be interconnected and grow at various speeds when heated.

When this strip reaches a specific temperature, it can provide enough turns to break the circuit.

After a few seconds, the overload is identified when the flow to the motor exceeds the charge for the heater. The time of the relay exploration is used to classify overload relays into three sorts.

Overload relays for 10 seconds, 20 seconds, and 30 seconds can be found in the Class 10, Class 20, and Class 30 categories.

This relay’s key characteristic is that it prevents the engine from starting right away. The overload relay, for example, probes into the bimetallic relay, and the NC (normally closed) bimetallic connections release the circuit until the strip cools.

The motor will not start if any contactor tries to push the start switch to turn it off.

Principle of Overload Relay

The electro-thermal characteristics of a bimetallic strip are used to operate a thermal overload relay. It is wired into the motor circuit so that the current flows through the poles of the motor.

The current heats the bimetallic strip directly or indirectly, and it bends when the current flow surpasses the predetermined value.

They’re always used in tandem with contactors. When the bimetallic strips heat up, the trip contact opens, cutting off power to the contactor coil, de-energizing it and interrupting current flow to the motor.

The current flow through the OLR is always inversely proportional to the tripping time. As a result, the stronger the current flow, the faster it will trip.

overload relay 4

A = Indirectly heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip

Thermal overload relays are thus described as current-dependent and inversely time-delayed relays.

Overload Relay Types

The following are the several types of overload relays:

  • Bimetallic thermal overload relays
  • Electronic overload relays
  • Eutectic overload relays
  • Solid state overload relays

Each of the above works on a somewhat different principle. Let’s have a look at it in the parts below.

Bimetallic Thermal Overload Relay: How it Works

A bimetallic thermal relay, as previously stated, relies on the heating properties of a bimetallic strip. The complete current to the motor flows through the OLR in the direct heating approach.

As a result, the current heats it up directly.

Indirect heating, on the other hand, keeps the bimetallic strip in close contact with the current-carrying wire inside the OLR. The conductor and, as a result, the bimetallic strip heats up when there is too much current flowing to the motor.

overload relay 5

Because the conductor is insulated, no current can pass through the strip.

Electronic Overload Relay: How it Works

This is also known as solid state overload relays.

There is no bimetallic strip inside electronic overload relays. Instead, it detects the amount of current flowing to the motor using temperature sensors or current transformers. It protects you using microprocessor-based technologies.

PTC is used to sense temperature and to trip the circuit in the event of an overload fault. Current transformers and Hall effect sensors are included in some electronic overload relays, which directly sense the amount of current flowing.

The lack of a bimetallic strip in electronic OLR leads to low heat losses inside the relay, which is a major advantage over thermal OLR.

Electronic relays are also more accurate than thermal relays.

Electronic relays from some manufacturers come with a variety of characteristics, such as earth fault protection, motor stall protection, and so on.

Earth fault protection can also be achieved using CBCT on the secondary three phase windings.

Electronic overload relays are ideal for applications where motors must start and stop frequently.

They are built to sustain the motor starting current (which is normally 6 to 10 times the full load current) for a limited time (typically 15-30 seconds depending on the threshold of current).

Eutectic Overload Relay: How it Works

A heater winding, a mechanical mechanism for activating a tripping mechanism, and a eutectic alloy make up this sort of overload relay. An eutectic alloy is made up of two or more components that solidify or melt at a given temperature.

overload relay 6

The eutectic alloy is housed in a tube in the overload relay, which is frequently used in conjunction with a spring-loaded ratchet wheel to trigger the tripping mechanism during overload operations.

The little heater winding conducts the motor current. The heater winding heats the eutectic alloy tube during the overload.

As a result of the heat, the alloy melts, releasing the ratchet wheel and allowing it to turn. The overload relay’s closed auxiliary contacts are opened as a result of this action.

After tripping, Eutectic overload relays can only be manually reset. This is commonly accomplished by pressing a reset button located on the relay’s cover.

The heating unit put on the relay is selected based on the motor’s full load current.

Overload Relay Trip Class

The trip class specifies how long it takes them to open the contactor during overloads.

Observe the table below from Schneider:

overload relay 7

Class 10, Class 20, Class 30, and Class 5 are the most popular classifications. At 600 percent of full load current to the motor, the OLR trips in 10 seconds, 20 seconds, 30 seconds, and 5 seconds, respectively.

overload relay 8

Class 10 and Class 20 are the most widely utilized. Motors generating high inertia loads are protected by Class 30 overload relays, while motors requiring very fast tripping are protected by Class 5 relays.

Overload Relay Connection Diagram

They are always utilized in conjunction with the circuit’s contactors. It is wired in series with the motor, allowing the electricity to flow freely through it.

The many methods of connectors for single-phase and three-phase motors are listed below.

overload relay 9

Overload relays K1 and K1M are used. A single phase motor connection is shown in the first and second pictures, while a three phase motor connection is shown in the third.

Overload Relay Advantages

Overload protection relays come in a variety of shapes and sizes. Fuse, thermal relays, electromechanical relays, and electronic relays are examples.

Low-current devices, such as domestic appliances, are frequently protected by fuses.

High-current machinery, such as industrial motors, are protected by thermal, electromechanical, and electronic relays.

The following are the key advantages of relays:

When a high-current scenario arises owing to a ground fault, short circuit, phase failure, or mechanical jamming, overload relays turn off electricity to the motor. They’re a low-cost technique to reduce downtime while repairing or replacing motors that have failed due to excessive current.

Contractors carry the main circuit’s high operational currents. They contain built-in systems to prevent arcing caused by heavy motor current interruption. When contactors and thermal relays are appropriately matched, the result is a good motor starting circuits

Manual motor starters are used to turn on and off motors. These electromechanical devices are simple to set up and reset once they have tripped.

For several types of overload protection relays, application-specific mounting kits are provided.

To control their activation threshold, overload protection relays have adjustable current setting ranges. They can detect and defend against phase faults in addition to preventing electrical overload.

Because these relays are frequently used in hot situations, they have ambient temperature tolerances of up to 60 degrees Celsius.

Relays also include sealable automatic or manual resets to protect them from the hazardous conditions in which they work.

While not carrying an electrical current, relays have stop and test operations to ensure their operational integrity.

Overload Relay vs Circuit Breaker

What is the difference between an overload and a current that is too high?

A type of overcurrent is an overload. Overload is an overcurrent that is sustained within the boundaries of the equipment’s permitted current rating yet causes the equipment to overheat.

Overcurrent is usually instantaneous. A type of overcurrent is a short circuit.

What is an overload in a circuit, for example?

When you draw more electricity than a circuit can safely handle, you have an overload. Wiring, a breaker (or a fuse in older wiring systems), and gadgets make up a circuit (such as light fixtures, appliances, and anything plugged into an outlet).

Is overload protection provided by a circuit breaker in this case?

A circuit breaker is an electrical switch that operates automatically to safeguard an electrical circuit from harm caused by an overload or short circuit.

Its primary purpose is to stop current flow when a defect is identified.

What is an overload protection device, and how does it work?

When the current reaches a level that causes an excessive or dangerous temperature rise in conductors, an overcurrent protection device opens to protect the circuit.

Short-circuit or ground-fault current values, as well as overload circumstances, are detected by most overcurrent protection devices.


1. Why is the OLR tripped?

As previously stated, there are three key factors that lead to overload trips:

  • The motor is overloaded.
  • Loss of input phase
  • Unbalanced phase.

Aside from this, various more security features may be available. This varies depending on the manufacturer.

2. How does overload relay protect from phase failures?

The current flowing through each pole of an overload relay to the motor remains constant during normal operation.

If one of the phases is disrupted, the current in the other two phases increases to 1.73 times its typical value.

As a result, the overload relay heats up and trips. Single phasing of the motor or phase loss are other terms for phase failure.

3. Is an overload relay able to protect from short circuits?

Short circuits are not protected by overload relays. Short circuit protection devices should always be utilized with them.

Otherwise, any short circuits in the motor could cause them to fail. Overloads, phase loss, and phase imbalance are all protected, but short circuits are not.

2 thoughts on “Overload Relay – Definition, Types, and Principle”

  1. Thanks, these notes are very helpful for the people how learn electrical installation course or electrical engineering.


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