Arc Furnaces 15 kV or 15 MVA and Less
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This control is for arc furnaces that are operated by one 15 kV 600 A three pole switch. It is a stored energy control with a fast charging circuit to enable frequent operation of the furnace switch.
The use of this control prevents problems caused by an inadequate current source to operate the control. A single solenoid operated switch requires 60 to 65 amperes peak for one and one half cycles to operate properly. If this current is not available, it will operate slowly and may have intermittent malfunctions and failures which are difficult to explain.
Schweitzer™ Relays Can Prevent Catastrophic Switch Failures on Small Arc Furnaces
Catastrophic switch failures are common on small arc furnaces as a result of a switch’s attempting to interrupt a current exceeding its interrupting rating of 4000 amperes. These failures happen because in an effort to save money a circuit breaker is not installed in series with the vacuum switch. Over current relays are then connected to the vacuum switch in the absence of the breaker. Since normal switch currents are less than 600 amperes, this arrangement works well most of the time because fault currents are usually less than the vacuum switch’s rating. But occasionally the rating is exceeded, and the result is catastrophic.
The number of failures of this type can be substantially reduced by installing a Schweitzer™ over current relay as part of the above control. The intelligence in the Schweitzer relay can recognize whether a fault current is within the capability of the switch. If the current is too large, the Schweitzer relay prevents the vacuum switch from opening and allows a fuse up stream from the vacuum switch to do the interruption. The prevention of a single switch failure will pay for having a Schweitzer™ relay.
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