Cable Sheath Jacket Requirements to Withstand Abnormal Voltage Stresses

Some popular types of high-voltage single-conductor duct-lay cable have insulating jackets over the sheaths. Examples are aluminum-sheathed cable and reinforced lead-sheathed cable. The jacket may be subjected to high abnormal a-c and impulse voltage stresses. The abnormal a-c voltage occurs on the cable sheaths during a ground fault in the line. The impulse voltages occur on the sheaths during switching. Several 230-kv and 60-kv oil-filled cable lines have been designed and installed and a 132-kv cable designed with sheath insulation to withstand these abnormal voltages. The voltage withstand levels adopted for the insulation of the sheaths and accessories are given. The means used for insulating the various parts of the sheath circuit are described and field testing to prove the quality of the completed installation is discussed. Naturally, in this virtually unexplored field, many questions remained unanswered. Some of these may be of vital importance. For example, one may ask whether it is really necessary to insulate the sheath circuit against the abnormal voltage which occurs during a fault to ground. After all, ground faults have occurred in high-voltage cable circuits having special bonding and bare lead sheaths; no secondary damage is known to have occurred because of the abnormal sheath voltage. Again, it may be asked whether it is sound economy to spend a lot of money to insulate the sheaths against fault voltages. The fault voltage will probably never occur, since most oil-filled lines have never had a failure.