On deviations from standard practise in lightning arresters

This paper is an endeavor to answer questions of practise and criticism of arresters brought out by an investigation conducted by the Protective Devices Committee. For the most part practise in lightning arresters is standardized. In fundamental principles there have been no changes for many years. Improvements in details, especially of construction, are still being made. A new arrester, the oxide film arrester, gets rid of the oil and electrolyte and avoids the necessity of daily charging, but fundamentally it is designed along the same principles as the aluminum arrester. The important principle is the electric valve action — there are but a few milliamperes of discharge rate at normal line voltage, but at abnormal line voltages the discharge current rise to hundreds of amperes. In answer to criticisms made by a few prominent engineers, it is maintained as fundamental that a large discharge rate for an arrester is an absolute essential. The burden of proof falls on those engineers who use arresters of low discharge rate. These arresters cannot discharge the dangerous lightning surges on overhead lines. Since there are lightning arresters of low discharge rate in apparently satisfactory use, an explanation for this anomaly is found in the use of insulators of low arc-over voltage. Either the lightning potential is relieved locally at the insulator or the resultant traveling wave is of too low voltage when it reaches the transformer greatly to endanger the insulation. Poor line insulation is not a solution of the problem of continuity of service. Why not save the cost of the useless lightning arrester? The current in such a traveling wave is about two amperes for every thousand volts of lightning potential, 600 amperes for 300 kv. One to twenty-five ampere discharge rate of arresters has little effect in reducing the lightning voltage. How many arresters should be used, to protect a six-feeder system? It depends on the conditions of insulation in circuit breakers and the importance of continuity of service. According to the conditions discussed in the body of the paper, from one arrester connected to the busbars to seven arresters with auxiliaries are needed. The use of no arresters is discussed from three standpoints. 1. If it is contended that lightning is not of sufficient voltage to cause damage. 2. If it is considered a better investment to put lightning arrester money into spare transformers. 3. If it is considered good practise to so highly insulate a transformer as to give it immunity from lightning. The conclusion reached is that each of these three arguments is dangerously faulty. A new method of inspection of aluminum arresters is proposed. The experiments given in the paper show that the power factor of the cells examined is very sensitive to their condition. There are promises of effecting economies in overhauling aluminum arresters and of lengthening their life. Experiences are given with a 33-kv. arrester in service thirteen years without overhauling. The plates are still in good condition. The usual damaging deposits of decomposed oil on the aluminum film were prevented by using an initial rush of charging current great enough to throw them off. The electrolyte is partially exhausted in strength and needs changing. The discharge rate is still high.