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Temperature Monitoring Relay: What you Need to Know

PT-100 temperature monitoring relay
PT-100 temperature monitoring relay

The high temperatures caused by excessive currents can damage electrical equipment –or even cause safety hazards. To prevent these dangers, temperature monitoring relays are used to monitor the temperature of an electrical device and disconnect power if the temperature exceeds a certain threshold. How do these types of monitoring relays work and where are they used? Let’s take a closer look.

What is a Temperature Relay?

The temperature relay, or temperature monitoring relay, is a temperature-sensing device that is used to protect electrical equipment from overheating.

Overheating usually happens when there’s overload, or it can occur if an appliance draws too much current. Overcurrents cause overheating, and if the temperature gets too high, it can cause damage to the equipment or, in the worst of cases, start a fire.

The temperature monitoring relay is installed in the circuits of devices that could overheat. It constantly measures the temperature and if it exceeds a safe limit, it will automatically disconnect the power to prevent further damage.

There are two main designs of the temperature monitoring relay based on the user interface and controls: digital temperature relay and analog temperature relay.

Digital Temperature Relay

A digital temperature protection relay is a type of temperature monitoring relay that uses a backlit screen to show the current temperature and other information. It also has buttons or a keypad so that you can set the temperature threshold at which the relay will trip.

Some digital temperature relays also have an alarm that sounds when the temperature gets too high. The advantage of a digital temperature relay is that it’s easy to set the temperature threshold and monitor the current temperature.

Analog Temperature Relay

An analog temperature sensing relay is a type of monitoring relay that uses an analog dial to monitor the temperature of a system or appliance and detect a rise. To set the temperature threshold, you need to use a screwdriver to adjust a potentiometer, usually located on the relays front.

Analog temperature relays are often used in industrial settings because they’re more durable, with thousands of hours of service life expectancy. They can also be used in many other different settings based on the protection requirements.

How Does a Temperature Monitoring Relay Work?

A typical temperature monitoring relay system has three main parts: a sensor, a control unit, and a switch. The sensor is often a thermistor or RTD (resistance temperature detector) that measures the temperature of the equipment. In some temperature relays, a thermocouple (bimetallic strip) or a melting alloy sensor can be used. Here is how it works:

  • The control unit compares the measured temperature to a set point or threshold, and if the temperature exceeds the threshold, it activates the switch.
  • The temperature relay switch then disconnects the power to the equipment, preventing it from overheating.
  • In order to avoid tripping due to momentary spikes in temperature, most relays have a time delay feature. This means that the switch will only activate after the temperature has exceeded the threshold for a certain amount of time.

The time delay can be set by the user, and it is usually between 0.1 and 10 seconds. The working and usability of a temperature sensor relay is, to an extent, dependent on whether it’s an electromechanical relay or a solid-state relay.

Electromechanical Temperature Relay

An electromechanical temperature relay uses an electromagnetic coil to close (or open) the switch contacts. There are advantages and disadvantages to each type of relay. For example, electromechanical relays are more rugged and operate with higher currents without overheating.

Solid-State Temperature Relay

A solid-state overtemperature relay, on the other hand, uses semiconductor components to control the switch contacts. Solid-state relays are smaller and more precise. They can be used for fast switching applications, but they are not as rugged as electromechanical relays and can easily overheat when in operation.

Temperature Monitoring Relay function

The temperature monitoring relay works by sensing the temperature of the winding and other parts and disconnecting the power if the temperature exceeds a certain threshold. High temperatures can result from various situations including:

Overloading: in the event a piece of equipment is overloaded, more current will pass through it and generate more heat than normal. The temperature relay will shut it down to prevent damage.

Faulty parts: if there are any faulty parts in the equipment, they can cause a short circuit and generate extra heat, and the temperature relay will trip to avert possible damage or a fire.

Blocked ventilation: With the ventilation blocked, the equipment will overheat. The temperature relay action is meant to react to this problem and cut off power to the appliance or circuit.

Environmental conditions: if the ambient temperature is too high, it can cause the equipment to overheat, and the temperature relay will trip to prevent damage.

Application of Temperature Monitoring Relay

The temperature monitoring relay finds important use in a broad range of settings, including industrial applications. Some common applications of the relays include:

HVAC (heating, ventilation, and air conditioning) systems: the temperature relay can be used to prevent the system from overheating.

Motors: electric motors can overheat if they are overloaded or have faulty parts. By using a motor winding temperature relay, the power can be disconnected before the motor overheats and is damaged.

Transformers: the transformer winding temperature relay is used to prevent damage to transformers by disconnecting the power if the temperature gets too high.

Switchboards: switchboards are used to distribute electricity, and they can overheat if they are overloaded. By using a temperature relay, the power can be disconnected before the switchboard overheats and is damaged.

There are many benefits of using a temperature monitoring relay or temperature protection relay, including:

  • Preventing damage to equipment: if the temperature exceeds the safe limit, the power will be automatically disconnected, preventing further damage.
  • Avoiding costly downtime: by disconnecting the power before the temperature gets too high, you can avoid costly downtime due to repairs.
  • Improving safety: by preventing equipment from overheating, you can avoid potential fires and other hazards.
  • Reducing energy costs: by preventing equipment from overworking, you can avoid wasting energy and lowering your overall operating costs.


A temperature monitoring relay is a type of temperature monitoring and protection device that uses a temperature-sensitive mechanism to disconnect the power. The advantage of using a temperature monitoring relay is that it can prevent damage to the equipment before it becomes too hot. This can avoid costly downtime and repairs, and it can improve safety by preventing fires and other hazards.

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I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.


    Merci beaucoup

    Hello, im noob 🙂

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