<p style="text-align:justify"><span style="font-size:14px">Thermal relays measure the heat developed in windings of electrical equipment. It means that they measure the temperature reached in electrical equipment due to passage of current. These relays basically protect the equipment against the overloading.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">There are important differences between an overload and overcurrent due to fault. Using these differences, the protective relaying system discriminates between faults and overload. The differences are:</span></p>

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	<li style="text-align:justify">
	<p><span style="font-size:14px">If current passing through electrical equipment is more than (usually any electrical equipment should withstand 110 % of CMR continuously) its CMR, equipment is said to be overloaded. If a transformer with a primary current rating of 100 amp starts drawing 125 amp, it is called overload.</span></p>
	</li>
</ul>

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<p style="margin-left:48px; text-align:justify"><span style="font-size:14px">When fault occurs, a current is diverted to an undesired path because of failure of insulation.</span></p>

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<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">The magnitude of fault current varies from 2 to 8 times (in case of heavy short-circuits it can be more than 8 times) the CMR of equipment.</span></p>
	</li>
</ul>

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<p style="margin-left:48px; text-align:justify"><span style="font-size:14px">Overload, usually, is 115 to 200 % (or in certain cases little more) of CMR.</span></p>

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<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">An electrical equipment can withstand overload for many minutes or sometimes (if overload is just 115 % to 130 % or so) for few hours depending upon the magnitude of the overload. A typical distribution transformer can withstand 115 % overload for 2.5 hours without being damaged. Hence longer time of operation is envisaged for thermal relays as against time of operation of few seconds (or even fraction of a second) which is quite common for overcurrent relays.</span></p>
	</li>
</ul>

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<p style="text-align:justify"><span style="font-size:14px">The thermal relay uses a bimetal strip, thermocouple or RTD (Resistance Temperature Detector). But these devices take a very long time to reset. This makes hot restart of equipment (transformer or motor) impossible and it is required to wait till the relay resets. The modern numerical relays use certain algorithms which use mathematical formula to calculate the temperature reached in the equipment to be protected. The resetting time, in this case, is negligibly small, practically zero.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">The setting range of thermal relays is usually 70-130 % of relay rating in steps of 5 %. Steps of 1 % are possible in numerical relays.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">The details about overload protection will be given in equipment protection later.</span></p>