Overload relay

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

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 circuituntil 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

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

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

Related Content

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Overload Relays

What is a relay switch?

An overload relay, also referred to as a relay switch, is a device that opens the circuit in the event of an electrical, thermal or power overload. When mounted with a contactor they create a motor starter. Overload relays are normally closed, meaning they only open if they experience an overload. They are used to protect motors from damage in a number of applications across industrial and commercial sectors, among others. They are available in a number of current ratings which can be adjustable on some models.

Common types of overload relays.

Overload relays are typically one of two types: thermal relays or electrical relays. The overload mechanism inside thermal relays consists of a bimetallic strip in conjunction with a heating element. When the heating element experiences an excessive current, the bimetallic strip bends and opens the normally closed contact, thus interrupting the circuit. These have adjustable current ratings to meet a variety of application requirements. On the other hand, electronic relays rely on electrical components which measure the current flowing through. While these are more accurate and are available across more current ratings, thermal relays can be more cost-effective.

Sours: https://www.baypower.com/motor-and-lighting-control/overload-relays
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Every motor must be protected from all possible faults to ensure prolonged and safe operation as well as time loss caused due to breakdown. Almost all the industries rely on the electric motor to control its processes and production. Hence it is necessary to make the motor fail-safe.

Overload relay is one such device that protects a motor from damages caused due to overloads and over-currents. It is used with contactors and can be found in motor control centers and motor starters.

Thermal Overload relay

Definition of overload relay

An overload relay is a device that protects an electric motor against overloads and phase failure.

It senses overloading of motor and interrupts the power flow to the motor, thus protecting it from overheating and winding damages. Apart from overloads, it can also protect the motor from phase loss/ failures and phase imbalance. They are very commonly known as OLR.

What is an overload?

An overload is a condition at which a motor draws a current above its rated value, for a prolonged period.

It is the most encountered fault and can result in temperature rise in the motor winding. Hence the rapid return to normal operation is important.

Principle of operation

A thermal overload relay works in the principle of electro-thermal properties in a bimetallic strip. It is placed in the motor circuit in such a way that the current to the motor flows through its poles. The bimetallic strip gets heated up by the current directly or indirectly and when the current flow exceeds the set value, it bends.

They are always work in combination with contactors. When the bimetallic strips heat up, the trip contact is activated that in turn breaks the power supply to the contactor coil, de-energizing it and breaking the current flow to the motor. This tripping time is always inversely proportional to the current flow through the OLR. Hence higher the current flow faster shall it trips. Therefore, thermal overload relays are referred to as current dependent and inversely time-delayed relay.

Indirectly heated overload relay

Types of overload relays

Overload relays can be classified as follows:

  1. Bimetallic thermal overload relays
  2. Electronic overload relays

The working principle of the above differs a little from each other. Let us discuss it in the following sections.

Working of bimetallic thermal overload relay

As explained above, a bimetallic thermal relay works on heating property of bimetallic strip. In the direct heating method, the full current to the motor flows through the OLR. Therefore, it gets heated up directly by the current.

But in the case of indirect heating, the bimetallic strip is held in close contact with the current-carrying conductor inside the OLR. Excessive current flow to the motor heats up the conductor and hence the bimetallic strip. The conductor shall be insulated hence no current flow through the strip.

Principle of operation of overload relay

Working of electronic overload relay

Electronic overload relays do not have a bimetallic strip inside. Instead, it uses temperature sensors or current transformers to sense the amount of current flowing to the motor. It uses microprocessor-based technology for protection. Temperature is sensed using PTC and the same is used to trip the circuit in case of overload faults. Some electronic overload relays come with current transformers and Hall effect sensors that directly senses the amount of current flow.

The major advantage of electronic OLR over thermal OLR is that lack of bimetallic strip results in low heat losses inside the relay. Also, Electronic relays are more precise that the thermal relays. Some manufacturers build electronic relays with extensive features such as earth fault protection, motor stall protection etc. Electronic overload relays are much suited for applications that require a frequent start and stop of motors.

They are designed in such a way to withstand the starting current (which is typically 6 to 10 times the full load current) of the motor for a limited period (typically 15-30 seconds depending on the threshold of current).

Parts of a thermal overload relay

Apart from the bimetallic strip and contacts discussed in the working principle section, there are few more parts in an overload relay that needs to be mentioned.

Parts of an overload relay


Terminals L1, L2, L3 are input terminals. It can be directly mounted to the contactor. Supply to the motor can be connected to Terminals T1, T2, T3.

Ampere range setting

A rotary knob is present over the overload relay. Using this knob, the rated current of the motor can be set. The current can be set between the upper and lower limits provided. In the case of electronic overload relay, an additional knob for tripping class selection is also provided.

Reset Button

A reset button is present over the overload relay to reset the overload relay after a trip and clearance of fault.

Manual/Auto reset selection

With the manual/auto reset selection button, we can choose between manual and automatic reset of these relays after a trip. If the device is set to auto, a remote reset of OLR is possible.

Auxiliary contact

They are provided with two auxiliary contacts – one NO (97-98) and another NC (95-96). NO contact is for trip signaling and NC contact is for disconnecting the contactor. NC contacts should be capable of direct switching of contactor coil.

Test button

Using the test button, it is possible to test the control wiring.

Symbol of an overload relay

Symbol of an overload relay

Here 1, 2, 3, 4, 5 and 6 are power terminal 95 & 96 are trip contacts and 97 & 98 are signalling contacts.

What is trip Class of overload relay?

The time taken by them to open the contactor during overloads is specified by the trip class. It is commonly classified into Class 10, Class 20, Class 30 and Class 5. The OLR trips in 10 seconds, 20 seconds, 30 seconds and 5 seconds respectively at 600% of full load current to the motor.

Class 10 and Class 20 are very commonly used ones. Class 30 overload relays are used for protection of motors driving high inertia loads and Class 5 relays are used for the motors requiring very fast tripping.

trip class currents
Trip class curve

How to use an overload relay in a circuit?

They are always used in combination with the contactors in the circuit. It is connected in line with the motor such that the current to motor fully flow through it. Below are the various types of connections for single-phase and three-phase motors.

DOL starter with overload relay

What causes the OLR trip?

As discussed above, there are three major conditions for overload trips:

  1. Overloading of the motor.
  2. Input phase loss
  3. Phase imbalance.

Apart from these, there may be some additional protection feature available. This varies from one manufacturer to the other.

How does an overload relay protect from phase failures?

During normal operation, the current flowing through each pole of an overload relay to the motor remains the same at a time. If anyone of the phase is interrupted, the current through the other two phases rises to 1.73 times the normal value. Hence the overload relay gets heated up and it trips. Phase failure is also known as single phasing of motor or phase loss.

Phase loss in overload relay

Can OLR protect from short circuits?

Overload relays cannot protect against short circuits. They should always be used with short circuit protection devices. Otherwise, any short circuits in the motor can potentially damage them. They can protect against overloads, phase loss, and phase imbalance, but not short circuits.


Overload relay is a device that can protect a motor from overloads, phase failure and phase imbalances. Based on the principle of operation they are classified into thermal and electronic overload relay. Thermal OLR is based on the principle of deformation of a bimetallic strip on heating and the electronic overload relay is a microprocessor-based device.

OLRs are used in combination with the contactors. It opens the contactor whenever it senses a fault. The time taken by them in opening the contactor during overloads is specified by its trip class. Overload relays cannot protect against short circuits.

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Overload Relays: Types & Tripping & What is Overload Protection?

Introduction to Motors

Electric motors are an integral component of industrial equipment, toys, vehicles, and electronic devices. They are designed to convert electrical energy into mechanical energy. These devices may be powered by AC or DC sources. Blowers, fans, compressors, cranes, extruders, and crushers are a few important devices equipped with electric motors.

What is an Induction Motor?

An induction motor, also referred to as a synchronous motor, is one of the main types of AC electric motors used in commercial and industrial environments. These motors feature armortisseur windings, and work on the principle of electromagnetic induction. The electro-magnetic field in the rotor is produced by the rotating field of the stator. In short, the power is transferred to the rotor winding by stator through induction. There are two main types of induction motors — single-phase induction motors and three-phase induction motors.

Introduction to Three-Phase Induction Motors

It is one of the most widely used types of electrical motors; and, is an integral part of almost 80% of the industrial applications. Its popularity is due to the rugged construction, excellent operating characteristics, speed regulation, and absence of commutator. Like any regular induction motor, this motor also comprises a stator and rotor.

  • Stator: This is the stationary element of the induction motor. The stator is a small cylindrical frame which carries the cylindrical core of the rotor. It features different slotted stampings to carry three-phase windings. The windings of the stator have 120 degrees separation.
  • Rotor: This is the rotating part of the motor. The rotor features laminated cylindrical slots with copper or aluminum conductors that have joined ends. It is the shaft of the motor.

The rotor of the three-phase induction motor is classified as phase wound rotor or slip ring rotor and squirrel cage rotor. Among the two, the squirrel cage rotor is one of the most common ones.

Squirrel Cage Induction Motors

Induction motors equipped with a squirrel cage rotor are known as squirrel cage induction motors. They get their name because the rotor resembles the rotating cylindrical “cage” that you might find in a pet squirrel or hamster cage. These motors are available in sizes ranging from fractional horsepower (HP) less than one kilowatt to 10,000’s HP (tens of megawatts). Factors such as simplicity, rugged construction, and constant speed in different load sizes have contributed to their popularity. Like other induction motors, the squirrel cage motor consists of:

  • Rotor: It is a cylindrical-shaped component mounted on a shaft. It contains longitudinally organized conductive bars. The bars are made of copper or aluminum, and are set into grooves, which are connected at ends to form a cage-like structure. The rotor has a laminated core, which helps avoid power loss due to hysteresis and Eddy currents. Conductors of the rotor are skewed, which helps prevent cogging during the start of the equipment. Also, this skewing assures improved transformation ratio between the rotor and stator.
  • Stator: It consists of a three-phase winding along the core. The stator is placed in a metal housing. The windings in the stator are organized such that they are 120-degree apart in space, and mounted on a laminated iron core. This iron core provides reluctance path for flux generated by AC currents.

What is Overload Protection?

When the motor draws excess current, it is referred to as an overload. This may cause overheating of the motor and damage the windings of the motor. Because of this, it is important to protect the motor, motor branch circuit, and motor branch circuit components from overload conditions. Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat from the overload condition. Overload relays are part of the motor starter (assembly of contactor plus overload relay). They protect the motor by monitoring the current flowing in the circuit. If the current rises above a certain limit over a certain period of time, then the overload relay will trip, operating an auxiliary contact which interrupts the motor control circuit, de-energizing the contactor. This leads to the removal of the power to the motor. Without power, the motor and motor circuit components do not overheat and become damaged. The overload relay can be reset manually, and some overload relays will reset automatically after a certain period of time. After which, the motor can be restarted.

How an Overload Relay Works

The overload relay is wired in series with the motor, so the current that flows to the motor when the motor is operating also flows through the overload relay. It will trip at a certain level when there is excess current flowing through it. This causes the circuit between the motor and the power source to open. The overload relay can be manually or automatically reset after a predetermined time duration. The motor can be restarted after the cause of the overload has been identified and rectified.

Types of Overload Relays

Bimetallic Overload Relay

Many overload relays include bimetallic elements or bimetallic strips, also referred to as heater elements. The bi-metallic strips are made of two types of metals – one with a low coefficient of expansion, and another with a high coefficient of expansion.These bimetallic strips are heated by a winding around the bimetal strip, which carries the current. Both of the metal strips will expand due to the heat. However, the metal with a high coefficient of expansion will expand more in comparison to the metal with a low coefficient of expansion. This dissimilar expansion of the bimetallic strips causes the bimetal to bend towards the metal with a low coefficient of expansion.As the strip bends, it actuates an auxiliary contact mechanism and causes the overload relay normally closed contact to open. As a result, the contactor coil circuit is interrupted.The amount of heat generated can be calculated by the Joule’s Law of Heating. It is expressed as H ∝ I2Rt.

  • I is the overcurrent flowing through the winding around the bimetal strip of the overload relay.
  • R is the electrical resistance of the winding around the bimetal strip.
  • t is the time period for which the current I flows through the winding around the bimetal strip.

The above equation defines that heat produced by the winding will be directly proportional to the time period of the flow of overcurrent through the winding. In other words, the lower the current, the longer it will take the overload relay to trip and the higher the current, the faster the overload relay will trip, in fact it will trip much faster because the operation of the relay is a function of the current squared.

Bimetallic overload relays are often specified when automatic reset of the circuit is required, and occurs because the bimetal has cooled and returned to its original state (form). Once this happens the motor can be restarted. If the cause of overload is not rectified, the relay will trip again, and reset at predetermined intervals. It is important to be careful during the selection of an overload relay, because repeated tripping and reset can reduce the mechanical life of the relay and may cause damage to the motor.

In many applications, the motor is installed at a location with a constant ambient temperature, and the overload relay and motor starter may be installed in a different location, which experiences different ambient temperatures. In such applications, the trip point of the overload relay can vary depending on multiple factors. The current flow through the motor and the temperature of the surrounding air are two factors, which may cause premature tripping. In such cases, ambient compensated bimetallic overload relays are used. The relays of this type feature two types of bi-metal strips – a compensated bi-metal strip and a primary non-compensated bi-metal strip. At ambient temperatures, both these strips will bend equally, thereby preventing the overload relay from nuisance tripping. However, the primary bi-metal strip is the only strip that gets affected by the current flow through the heater element and the motor. In the condition of an overload, the trip unit will be engaged by the primary bi-metal strip.

Eutectic Overload Relay

This type of overload relay is comprised of a heater winding, a mechanical mechanism for activation of a tripping mechanism, and a eutectic alloy. A eutectic alloy is a combination of two or more materials, which solidifies or melts at a specific known temperature.

In the the overload relay, the eutectic alloy is contained in a tube, which is often used along with a spring loaded ratchet wheel to activate the tripping mechanism during the overload operations. The motor current passes through the small heater winding. During the overload, the eutectic alloy tube is heated by the heater winding. The alloy melts due to the heat, thereby releasing the ratchet wheel, and allowing it to turn. This action initiates the opening of the closed auxiliary contacts in the overload relay.

Eutectic overload relays can only be manually reset after tripping. This reset is usually done through a reset button, which is positioned on the cover of the relay. The heater unit installed on the relay is chosen on the basis of the full load current of the motor.

Solid State Overload Relay

These relays are commonly referred to as electronic overload relays. Unlike the bimetallic and eutectic overload relays, these electronic overload relays measure current electronically. Although available in various designs, they share common features and benefits. The heaterless design is one of the main advantages of these relays. This design helps reduce the costs and efforts of installation. In addition, the heaterless design is insensitive to the change in ambient temperatures, which helps minimize nuisance tripping. These relays also provide protection from phase loss – more effectively than bimetallic or eutectic alloy overload relays. These relays can easily detect a loss of phase, and operate an auxiliary contact to open the motor control circuit. Solid state overload relays enable easy adjustment of trip times and set points.

Overload Relay Tripping

The tripping time of an overload relay will decrease when the current increases. This function is plotted on the inverse time curve below, and is termed as the trip class. The trip class also indicates the time taken by the relay to open in an overload condition.

Trip Classes 5, 10, 20, and 30 are common. These classes suggest that the overload relay will trip in 5, 10, 20, and 30 seconds. This tripping usually occurs when the motor is running 720% of its full load. Trip Class 5 is suited for motors that demand fast tripping, whereas Class 10 is usually preferred for motors of low thermal capacity like submersible pumps. Class 10 and 20 are employed for general purpose applications, whereas Class 30 is employed for loads with high inertia. Class 30 relays help avoid nuisance tripping.

We hope that this short paper has given you a good, basic understanding of overload relays. Look for other informative papers from c3controls at c3controls.com/blog.

The content provided in this white paper is intended solely for general information purposes and is provided with the understanding that the authors and publishers are not herein engaged in rendering engineering or other professional advice or services. The practice of engineering is driven by site-specific circumstances unique to each project. Consequently, any use of this information should be done only in consultation with a qualified and licensed professional who can take into account all relevant factors and desired outcomes. The information in this white paper was posted with reasonable care and attention. However, it is possible that some information in these white papers is incomplete, incorrect, or inapplicable to particular circumstances or conditions. We do not accept liability for direct or indirect losses resulting from using, relying or acting upon information in this white paper.

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