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Principle of operation

Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism after a certain travel which depends on the current-setting of the relay.

Working principle of thermal motor protection relay
Working principle of thermal motor protection relay (photo credit:

The release mechanism actuates an auxiliary switch that breaks the coil circuit of the motor contactor (Figure 1). A switching position indicator signals the condition “tripped”.

Principle of operation of a three pole thermally delayed bimetal motor protection relay with temperature compensation
Figure 1 – Principle of operation of a three pole thermally delayed bimetal motor protection relay with temperature compensation

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

The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly through the bimetal, in the second through an insulated heating winding around the strip. The  insulation causes some delay of the heat-flow so that the inertia of indirectly heated thermal relays is greater at higher currents than with their directly heated counterparts. Often both principles are combined.

For motor rated currents over approx. 100 A, the motor current is conducted via current transformers. The thermal overload relay is then heated by the secondary current of the current transformer.

This means on one hand, that the dissipated power is reduced and, on the other, that the short-circuit withstand capacity is increased.

The tripping current of bimetal relays can be set on a current scale – by displacement of the trip mechanism relative to the bimetal strips – so that the protection characteristic can be matched to the protected object in the key area of continuous duty.

The simple, economical design can only approximate the transient thermal characteristic of the motor.

For starting with subsequent continuous duty, the thermal motor protection relay provides perfect protection for the motor. With frequent start-ups in intermittent operation the significantly lower heating time constant of the bimetal strips compared to the motor results in early tripping in which the thermal capacity of the motor is not utilized.

The cooling time constant of thermal relays is shorter than that of normal motors. This also contributes to an increasing difference between the actual temperature of the motor and that simulated by the thermal relay in intermittent operation.

For these reasons, the protection of motors in intermittent operation is insufficient.

Temperature compensation

The principle of operation of thermal motor protection relays is based on temperature rise. Therefore the ambient temperature of the device affects the tripping specifications.

As the installation site and hence the ambient temperature of the motor to be protected usually is different from that of the protective device it is an industry standard that the tripping characteristic of a bimetal relay is temperature-compensated, i.e. largely independent of its ambient temperature (see Figure 2 below).

Tripping tolerances for temperature-compensated overload relays
Figure 2 – Tripping tolerances for temperature-compensated overload relays for motor rotection under IEC 60947-4-1

I = Overload as a multiple of the set current
δ = Ambient temperature
Limit values– Limit values under IEC 60947-4-1

This is achieved with a compensation bimetal strip that makes the relative position of the trip mechanism independent of the temperature.

Sensitivity to phase failure

The tripping characteristic of three-pole motor protection relays applies subject to the condition that all three bimetal strips are loaded with the same current at the same time.

If, when one pole conductor is interrupted, only two bimetal strips are heated then these two strips must alone produce the force required to actuate the trip mechanism. This requires a higher current or results in a longer tripping time (characteristic curve c in Figure below).

Typical trip characteristics of a motor protection relay
Typical trip characteristics of a motor protection relay

Ie= Rated current set on the scale
t = Tripping time

From a cold state:
a = 3-pole load, symmetrical
b = 2-pole load with differential release
c = 2-pole load without differential release

From the warm state:
d = 3-pole load, symmetrical

If larger motors (≥10 kW) are subjected to these higher currents for a longer time, damage should be expected.

In order to also ensure the thermal overload protection of the motor in the cases of supply asymmetry and loss of a phase, high quality motor protection relays have mechanisms with phase failure sensitivity (differential release).

Resource // Low Voltage Switchgear and Controlgear – Rockwell

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About Author


Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Yiannis Karyotis
    Jul 17, 2018

    Sorry. Is that a blog for engineers of electrical cable fitters? The explanation in the site is so clear. Why these questions?

  2. Dimitris Adamos
    Jul 11, 2016

    Hello i want to ask something that i m a bit confused. If i have a three phase motor and a thermal protection relay and only 1 phase is above the current of the thermal protection relay (the other 2 are below) will it trip ?Or must the all three currents be above to trip the relay? Thanks for the answer.

  3. avwunukpa
    Dec 10, 2014

    yes i am and i want to become one some day dat is why i love researching thanks to the author

    Apr 15, 2014


  5. Steven Harris
    Dec 25, 2013

    I have a question that my electrical partner and I are in disagreement. I do hope someone can settle this. In the event that you did not use the normally closed contact to open the starter, will protection be present. In other words when the mechanism is activated, does it not also open the phase to the motor despite the state of the motor starter?

    • Earl Wenger
      Feb 27, 2014

      NO, there is no protection if you dont use the N.C. contact, as the unit does not have contacts in it, it depends on the starter to do that.

  6. shivkumar1585
    Mar 09, 2013

    Dear Sir,

    Please advice ,

    In ac distribution system if bus bar are paint with black paint can it increase the current capacity of bus bar?? For example if we are using 3x100x10 copper bus bar and if same busbar are painted with mat black paint then in that case we need 2x100x10 instead of 3x100x10???

  7. kvsridhar
    Mar 09, 2013

    Thanks Edvard. Perhaps a little bit about the two-slider relays (differential sliders) which give a superior phase-loss/ phase unbalance protection would be nice?

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