Why single core underground cable armored is earthed in one side only in 3 phase system.?
We connect the cable armor to earth at only one end of the cable because we want to prevent circulating currents in the armor that would occur if the armor were earthed at both ends. These circulating currents increase the heating of the cable and reduce its effective load current capacity. There are certain details that we look at carefully when choosing to earth the armor at only one end. In particular, the unearthed end of the armor will have an induced voltage on it relative to the local earth. If this induced voltage is greater than 25 V ac (rms), special precautions are taken to do one of the following: Insulate the armor, Earth the armor at both ends of the cable, Connect an impedance in the armor-to-earth connection to limit the voltage and circulating current. If the armor is earthed at both ends of the cable, we must de-rate the continuous current capability of the cable to account for the additional heating that occurs from the circulating currents.
