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.
There is a reason that old fuse holders used the same screw thread as lightbulbs.
When a fuse blew, you put a high wattage bulb in and then tried plugging in/out until you discovered the appliance or cord that caused the bulb to go from "off" to "full on".
With today's breakers it is not so easy to go through a set of fuses and blow them all, then to stop and consider that maybe the fuse was not the problem.
and listen for the sound. That would identify a short to ground (or shielding if shielded cable, which would then be terminated to ground at the source point.)
I like to use the diode check, as do not have to look at the screen. Using this and the "bundling" technique, and you can soon locate the short. Have done this in many control cabinets.
Can use Ohm mode to see level of resistance as you described. The light bulb check does help to identify a short as opposed to a flow to ground through another component, but I have seen where using the bulb actually created enough working load that it burned the few wire strands that were contacting the ground, and then went out. Now you measure to ground, and the wires that had shown a short do not, and you have still identified the faulty wire.
Of course, back in the old days, many of us did not have multimeters and it was standard practice to use bulbs for testing, including to trace wires, etc. on our automobiles.
I agree. It's nice to see an example of a well run and methodical investigation into a failure. So often, someone goes charging off towards a fix without proper information: an approach I've often heard described as "ready"-"shoot"-"aim". Well done and well described.
putting a higher wattage lightbulb is a trick that I was taught way back in the 1950's, when we had fuses at home and it was a great saver. I used that exact technique just this past Monday for servicing a house-wiring compaint that the circuit breaker would not stay reset.
The faster way to find the short would have been using the "divide and conquer" approach, which would be to split the hundred terminals into two groups and check each for shorts, then split the shorted group in half and check again. Just two checks and disconnecting three places cuts the number of suspects down to 25 from 100.
Of course, in some machines the switches circuit feed wire is daisy-chained through the remote wiring, which can make it a bit more work because of needing to open remote boxes.
The one other technique that I have seen is to use a hundred watt lamp with a flasher and a clampon ammeter to determine which wire is drawing the current. That method allows for checking groups of wires without any unhooking, which sometimes reconnecting wires can be a pain.
Of course, good controls design would always separate output device power feeds from sensor switch feeds. That way the E-Stop could switch off all the motion drive functions.
Some of the power wires went to operator panels, some to sensors, some to solenoid valves. Some proxi sensors have an LED indicating the sensor has power, and some others have an LED to indicate it is detecting the target. Any section of the machine that had LED's on had to be okay. When I noticed sensors without LED's on, I just had to look more closely at that area. It just happened that the sensor had obvious damage from weld expulsions, and was in a location where it was readily visible. If the sensor had been hidden, or there had been another sensor that was also shorted, the story would have been longer.
Sounds like you have some great instructors, Gsmith120. This is a consistent theme in the Sherlock Ohms blogs. Logic used with discipline and thoroughness seems to be the specical sauce for solving engineering problems.
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