Department of Labor
Mine Safety and Health Administration

Accident Info-Gram
May 2003

Circuit Breakers Shorting Phase-to-Phase on Line Side

Since January 2003, four circuit breakers have shorted phase-to-phase on their line side. These instances have occurred at three separate underground coal mines. The last instance occurred on April 22, 2003. No injuries to personnel have occurred during the short circuits.

The power supply leads involved in two of the instances were attached to the top (line side) of Westinghouse, 1000VAC, 225 Ampere, AB De-Ion circuit breakers. The terminations were made using crimp-on terminal ends bolted to the circuit breakers' line side terminals. The un-insulated barrels of the terminal ends extended approximately one inch above the horizontal plane of the circuit breakers' molded case.

At the other two mines, the circuit breakers were Cutler-Hammer, E2K, 400AMP, 1000VAC. The E2K breakers are the same physical size as the 225 AMP Westinghouse De-ion circuit breakers. The leads had been attached to the line side of these breakers, but excessive conductor insulation had been removed at their terminations. This left approximately one-half inch of exposed conductor above the horizontal plane of the circuit breakers' molded cases, and the vents for the arc chutes on these circuit breakers are located directly below these line-side terminations. (As circuit breakers open under a load, hot ionized gases are released upward through these arc-chute vents.)

Each of the four transformers supplying power to these breakers was damaged when the phase-to-phase fault condition occurred. This fault condition occurred when either a 1000VAC feeder cable or a shuttle-car cable was ribbed (smashed), causing a fault in their cables that in turn caused their respective circuit breakers to open. As the circuit breakers opened, a phase-to-phase fault occurred across the top of the circuit breaker frames where the line side terminations are located. It appears that the ionized gases and the voltage spikes that were produced when the circuit breakers were opening, the approximate one-inch air gap between the circuit breaker terminations on the small-frame 1000-VAC circuit breakers, and the un-insulated portion of the conductors above the case of the circuit breaker frames caused these phase-to-phase faults.

• Ensure proper termination devices are used at circuit breakers.
• Make sure no uninsulated conductors or terminations are above the horizontal plane of the circuit breaker.
• Prevent and/or remove combustible, conductive, or other materials such as rock dust and float coal dust from interior of transformers.

This information was provided by concerned miners in an effort to eliminate accidents. Accident or "close call" information within District 8 may be shared by contacting the MSHA office at (812) 882-7617.