When a single E/F is detected in the DC field circuit of a machine, the machine has to be taken out of service at the first opportunity. This is because, if allowed to run with an E/F on the rotor, a subsequent second E/F can cause severe damage to the machine. However, a relay like CAEM-33 which can detect such a second E/F and trip out the machine can make it possible to run the machine even with a single E/F, without any such risks, thus helping to preserve the generation capacity. The start of the second rotor earth fault detection scheme is a very sensitivity transductor element. The AC winding of the transductor is connected in series with a rectified AC voltage relay. The Dc winding of the transductor, on the other hand, is connected in series with the rotor E/F circuit. Under normal conditions- i.e. with no DC flowing, the AC wining of the transductor presents a high impedance, and the AC voltage applied is mostly dropped across this winding. Hence the relay remains deenergised. When a second rotor E/F occurs, a DC current flows through the transductor dc winding which causes the impedance of the AC winding to reduce considerably by driving the transductor core into saturation. Hence, the applied voltage is fully available across the relay and it operates.
