Current limiting reactors with fire-proof insulation on the conductor

In a previous paper, tests were described which proved conclusively that if conducting material were lodged between the turns of a reactor having bare conductor, the reactor would flashover at the instant a failure occurred on the circuit in which the reactor was placed. In this paper, tests are described which were made during the development of a proper insulation for the conductor of reactors. Short Circuit Tests. Reactors tested consisted of one reactor with enameled conductor and two reactors with asbestos insulated conductor; one having a thin covering of asbestos; the other a thicker covering. The reactor with enameled conductor flashed over during the first short circuit test. That with a thin covering of asbestos stood one short circuit and arced over in the second short circuit. The reactor with the thick covering stood many short circuit tests without any failure or sign of distress. These tests established the fact that thin insulation on the conductor will not prevent such failures, even though it has sufficient dielectric strength to withstand the voltages placed across it for the reason that the magnetic force exerted on iron and steel objects will cause them to break through thin insulation. On the other hand, thick insulation will adequately protect the reactor from failure due to foreign substances. Thermal Tests. Thermal tests on the asbestos insulation established the following facts: First: That this insulation does not smoke excessively at temperatures below 350 deg. cent. Second: That it does not burn even at temperatures of melting copper. Third: That its insulation and mechanical strength is not appreciably affected when heated rapidly as high as 350 deg. cent. Thermal Capacity. The thermal capacity of the conductor is affected by the insulation, as follows: First: Under the effects of extremely high short circuit currents for a very brief interval, the thermal capacity is not affected by the asbestos. Second: With a moderate short-circuit current for a longer length of time, the thermal capacity of the insulated conductor is increased due to the storage of heat in the insulation. Third: During normal operation, at rated current, the temperature rise of conductor is increased due to the drop in temperature in the insulation. Costs. The asbestos insulation increases the cost of the reactor directly by the addition of the cost of the insulation itself and indirectly by making it necessary occasionally to increase the size of the conductor. However, this increase in cost is not a large percentage of the total cost of the reactor.