Abridgment of protection of supervisory control lines: Against over-voltage

Supervisory control lines used in the remote or automatic control of electric plants are subject to overvoltages dangerous to insulation and to operators. These overvoltages may be caused by lightning, crosses with the power lines, or induction from them. Open wire control lines are influenced by all of these, cables with grounded sheaths are immune to disturbances caused by lightning or other electrostatic induction, and are more or less safe from crosses. However, the ordinary cable is still subject to high voltages by electromagnetic induction when a fault occurs on the power line. These voltages may be dangerous not only to apparatus, but also to the cable insulation. Protection by means of lightning arresters is therefore necessary. Calculations show that even if the apparatus locations are protected, high voltages may occur along the line if the transmission line fault is between stations. This may cause a cable failure. It may, therefore, be advisable to protect the cable at certain intervals. Supervisory control protectors may be called upon to discharge heavy currents of appreciable duration. This requires extremely sturdy arresters. The requirements are met by a spark-gap of special design in argon at a reduced pressure. Specially made cables will shield the line against extraneous disturbances. In large installations, the use of such cables may be worthy of consideration.