The three-phase, interconnected star neutral earthing transformer is similar in its construction to a three-phase power transformer, but on each limb it has a single winding only, which is split up into two equal interconnected portions.

The apparatus is therefore a 1 to 1 autotransformer with its windings so arranged that, while the voltages to each line to earth are maintained under normal operation conditions, a minimum impedance is offered to the flow of a single-phase fault current, such as is produced by an earth fault on one line of a system which has a grounded neutral.

Under normal operating conditions the current flowing through the windings is the magnetizing current of the earthing transformer only, but the windings are designed to carry the maximum possible fault current to which they may be subjected for a specified interval (usually 30 s).

For the purpose of fault current limitation, resistors may be used in conjunction with the earthing transformer, inserted between either the neutral point and earth or the terminals of the apparatus and the h.v. lines. If an l.v. supply is required for the local load at an h.v. substation and an earthing transformer is to be employed, it is possible to incorporate in the latter a star-connected auxiliary winding of, say, 100 to 300 kVA rating.