Making contact with high-voltage equipment when your feet are grounded can prove disastrus.
While servicing TV equipment in a designated equipment room near the top of a skyscraper in a major U.S. city, a service technician suffered internal and external injuries when he attempted to operate a makeshift insulating shaft on an auxiliary device connected to a TV transmitter. The Photo at right shows the cabinet housing the TV transmitter. At first glance, everything appears to be in order, but looks can be deceiving.
The auxiliary device, called a pulser, was connected to the cathode of another piece of equipment, called a klystron. A pulser reduces the overall energy consumption of a TV transmitter. The pulser waits until the video signal needs an extra power boost to reach maximum value, or peak sync, and then boosts the klystron current to 100%. Because a video signal only needs this maximum current for a very brief period — about 8% of the total duty cycle — the pulser significantly reduces transmitter power consumption.
In this case, however, there was a problem. Several years before, someone had replaced the klystron and modified the transmitter. Because of these modifications, the pulser couldn't be installed under the carriage assembly of the klystron. Instead, it was mounted on the back wall of the cabinet in a plastic box that rested on insulators.
The metal enclosure and internal parts of the pulser operated at 30kV, DC-to-ground — a voltage level high enough to kill a man in just a few seconds. To put this into perspective, electric chairs typically operate at only 2.4kVAC, and a current flow as low as 60mA is enough to kill a grown man.
Because of the danger this situation presents, an insulated — in this case, plastic — shaft with a short brass coupler and knob at the end is used to operate a switch and potentiometer when setting the pulser level for the transmitter. When the pulser is installed under the carriage assembly in the equipment cabinet, as intended, the shaft used to control the device is typically 24 in. long. Because most TV transmitter equipment operates at high-voltage levels, the length of the shaft is critical — it protects technicians from inadvertent flashovers. However, on this modified installation, the shaft was only an inch or two long — not long enough to provide the proper withstand distance needed to protect technicians from shock hazards.
The subsequent investigation revealed that when the service technician reached into the cabinet to operate the switch, an arc jumped across the shortened shaft (from the pulser to his finger). At the time, his feet were in contact with the grounded base of the cabinet, completing an electrical circuit through his body. The arc mark, clearly visible on the bearing and nut where the insulated shaft entered the pulser, indicated the presence of a current of several amperes.
The arc severely burned the bottoms of the service technician's feet, and his spleen was so badly damaged from the accident that it had to be removed. These serious injuries prevented him from returning to work on a full-time basis.
The litigation on this case lasted more than 7 yr, during which time a host of experts inspected the installation in question, inspected similar transmitter installations at other facilities, and reviewed numerous installation and maintenance manuals to better understand who was to blame for the accident and how it could have been prevented in the first place. In the end, the concerned parties agreed that a lead technician, who was responsible for overseeing the work of several technicians, should have better managed the installation of the pulser and ensured the modified installation met all safety requirements.
The lesson to be learned here is that even an experienced worker can get into trouble if he doesn't follow proper safety precautions and procedures when working on energized high-voltage equipment. In this case had the technician made sure his feet weren't grounded, this accident would not have occurred.
Kusko is vice president of Exponent Failure Analysis Associates, Natick, Mass.