DT-3 is used for temperature protection of transformers, or dc generators, and motors, usually rated 1 000 horsepower or more.
The DT-3 relay is an adjustable contact-making ac or dc milliammeter, calibrated in degrees Centigrade. The relay operates to initiate an alarm or tripping function (most commonly through an auxiliary type SG or MG-6 relay) when the stator temperature o( the protected machine reaches the temperature setting of the relay. The relay is applied as the center leg of a Wheatstone dc energized bridge circuit consisting of balancing resistors within the relay and a temperature search coil embedded in the stator of the machine.