Short Circuit Stymies First Shift

I was working the second shift at P&F Tool and Die, when I came across a production line that was halted due to a short circuit. The first shift could not pinpoint the problem and had used most of the spare fuses trying to figure it out.

The machine that was down welded an inner and outer shell, as well as bushings, into an assembly. There were 11 stations that had a walking beam transfer. The wire with the short was labeled No. 1, and it had a 115V AC supply. It is only a slight exaggeration to say there were almost 100 wires at the No. 1 terminal bank.

I started by replacing the fuse with a circuit breaker of similar rating. Then I disconnected all of the No. 1 wires from the terminal bank. I got a 115V AC incandescent light bulb and connected it to the 115V AC after the circuit breaker. I then connected each No. 1 wire individually to the light bulb. Most of the time, the light bulb did not light, or did so only dimly.

So these wires did not have a short circuit, and I reconnected them to the terminal bank. A few wires caused the bulb to light brightly, which indicated a possible short. The next test was to remove the light bulb, turn off the circuit breaker, connect an individual wire, turn on the breaker, and see if it tripped. That left me with one wire that was shorted -- somewhere.

I reconnected the power connection to the No. 1 wires that were not shorted. When the circuit breaker did not trip, I re-installed the fuse into the circuit. Now I had power to all of the sections of the machine, except the section with the short circuit. Next I had to check each section to find the section without power. Then I had to do a visual inspection to try to spot a problem.

I eventually found a proximity sensor that that been damaged by weld expulsions. After replacing that sensor, I had the machine up and running.

This entry was submitted by Glenn Aitchison and edited by Rob Spiegel.

Glenn Aitchison has worked with automotive robotics and other industrial automation and machinery. He has received his Certificates of Qualification as an industrial electrician and as an industrial mechanic (Millwright).

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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