The sheath loss of a cable is a power loss occurs in the lead sheath as a result of the induced e.m.f. produced by the flux linkage with the lead sheath. When single core paper insulated lead covered and served power cables are used on a.c. system the presence of a lead sheath around each insulated conductor has an important effect on the electrical characteristics and current rating of the cable. If the lead sheaths are bonded together or earthed a voltage will be induced in the open circuited sheaths by transformer action between the conductors and the sheaths, the value of which depends on the flux interlinked with the lead sheaths and increases with the increase in the interaxial spacing of the cables. The induced sheath voltages are short circuited through the bonding resulting a flow of current along the sheath and thus dissipating energy as heat through the cable serving and surrounding earth or air. The eddy currents are also generated in the lead sheaths due to the lack of perfect symmetry in the lead sheath circuit relative to the magnetic field. Therefore the heating losses which occur in the lead sheaths are of two types e.g. (a) sheath circuit loss for bonded sheaths only and (b) sheath eddy loss. The sheath eddy loss is generally very small compared with the sheath circuit loss and can be neglected in many cases.
