工程师，别让自己成了导体

Engineer as conductor

by Dwight Bues

It is quite often enjoyable to relate stories wherein someone else “screwed up.” If we were all honest with ourselves, we would find plenty of occasions where we were the culprits. This is one such example.

My first job out of college was working for the power company in Roanoke, VA. I was assigned to the Communications group, responsible for the company’s carrier relay, cable carrier, telephone, microwave link, and VHF radio systems. One day we were sent on a trouble call to Lynchburg, VA, to repair their cable carrier system. This system consisted of 24 narrowband frequency-division multiplexed (FDM) channels modulated on a 50 MHz carrier, to provide good noise immunity. This signal was then transmitted on a pilot wire system throughout the city of Lynchburg.

The pilot wire was actually a bundle of signal cables that were mounted on the electrical power poles about 10 feet (3 m) below the 12 kV distribution lines. Since this wire ran a long distance, very close to the phase conductors of the power lines, it is no wonder that a significant voltage could be induced on the pilot wire. Normal circuits that relayed the state of certain distribution circuits were fashioned in such a way that they were immune to this induced voltage (current loop systems, for example).

The cable carrier system required additional protection to prevent blowing the RF transmitter (or maybe even the receiver, as it was quite sensitive). To that end, we had installed two AC drain coils, at each cable carrier terminal, to couple the induced voltage to ground (thus protecting the equipment). The normal failure mode of the AC drain coil (and its condition on the day we received the service call) was to short the signal wire to ground as a fail-safe to protect the equipment. These drain coils were mounted on a 19-inch rack panel with a terminal block in between them. Only one would be hooked up at a time so that when one failed, it would be a simple job to swap in the new one, then to replace the old one.

On the day in question, we discovered that the failure was located at the Reusens Power Station, a few hundred yards away from the old Reusens Hydroelectric Plant. It was a rather noisy environment, as there was a 6-ft-diameter, 10-ft-long, synchronous condenser running in the room. It resembled a large motor, but was used to compensate the Power Factor for the rather old-fashioned generators at the hydro plant. I had to actually shout at my buddy standing next to me to be heard over the din. I went behind the rack to where the drain coils were located, got out my insulated screwdriver, put my watch and ring in my pocket (a highly-advised safety precaution), and proceeded to disconnect the failed AC drain coil.

When I went to attach the new coil, I must have gotten across the circuit, for the next thing I remember, I was on my back on the floor. My buddy didn’t hear, so much as feel me hit the floor (it was made of steel deck-plate, so the vibration would have been easy to sense). When he looked around the back of the rack, I was sitting up and dusting myself off. You see, I should have connected the new drain coil first, THEN disconnected the old one. What I had done, when I disconnected the shorted drain coil first, was to allow the induced voltage to build up to about 1000V and then, when I tried to connect the new drain coil, I actually shunted the voltage to ground all at once, apparently through myself!!!

After that experience, I have never forgotten to eliminate the charge first, then to replace the failed equipment.

Dwight Bues is a Georgia Tech Computer Engineer with almost three decades’ experience in Computer Hardware, Software, and Systems and Interface Design.