When it comes to the placement of high-voltage fuses, it's all about location, location, location
Sometimes it's not enough to make sure the equipment you're working on or near has been de-energized. Being aware of your surroundings and the hazards that nearby equipment can present is equally important. But even the most cautious worker can't always be held responsible for his injuries when poor equipment placement is the real culprit.
As an employee of a waste-disposal company prepared to drain oil from the tanks of three de-energized circuit breakers in a 3,000kW outdoor rectifier substation that served an electrified railroad system in New York, he never gave the fused disconnect switch above his head a second thought. That is until the fuse blew open and dumped arc byproducts on him as he was arranging his oil draining equipment. The arc products were blown out of the lower end of the vertically mounted fuse. And although the breakers on which he was working had been disconnected from their source, the switches and bus work in the vicinity of the breakers had not been de-energized.
The substation at the train station is served by two overhead 33kV power lines. The voltage is stepped down to 750VDC inside the substation, which serves the third rails of the railroad. As shown in the Figure above, the 3-phase oil-filled circuit breaker on the primary side of the rectifier transformer had been replaced with three high-voltage pull fuses prior to the accident. The electrical team at the station had left the 750V rectifier DC breaker behind to protect the rectifier from third-rail faults.
As originally installed, the de-energized circuit breaker was located directly under the common 33kV bus and the primary terminal bushings of the rectifier transformer. The 33kV power could have been shut off by the manual switches mounted at the top of the substation structure or at the utility substation feeding the 33kV lines to the substation.
The breakers were removed as part of an upgrade process, thus altering the arrangement of the equipment in the substation. The two 33kV lines terminate at the top of the structure. The substation is normally supplied by one 33kV line; the other line is the alternate. The selection is made by the switches shown in the Figure. The worker was located at the position designated A in the Figure. The transformer supplied the rectifier mounted in the building at the opposite end of the substation. The mountings and contacts for the high-voltage fuses are shown in the Photo at left. The fused disconnects were mounted vertically. They swing down around the lower contacts when they blow or are pulled.
As it turned out, the utility company's records indicated that the circuit breaker at the utility substation that supplied the 33kV line to the railroad substation tripped and reclosed due to a fault. Inrush current to the rectifier transformer when the utility circuit breaker reclosed probably caused the fuse to blow.
For safety reasons and to ensure compliance with the Code, the train station's maintenance crew relocated the high-voltage fused disconnect switches to a horizontal position to prevent future arc products from discharging and showering a person in the substation as happened in this situation.
Although workers in all trades and professions are often warned not to work on or around energized equipment, the railroad shouldn't have permitted the worker to drain the oil from the circuit breakers with the substation energized. The circuit breakers may have been disconnected, but that didn't eliminate all potential for harm. The manual 33kV switches should have been opened, and the DC breaker to the third rails should have been flipped.
The worker was absolved of responsibility and the suit against the railroad was settled before trial, but the case still illustrates the point that no matter how safe you think you are, you always need to be aware of your surroundings.
Kusko is vice president of Exponent Failure Analysis Associates, Natick, Mass.