Sparks Electrical News

The fictional detective Sherlock Holmes is credited with saying that, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth ..."

Many of you have been there. The factory or the plant has stopped working – come to a complete halt. The normally thundering machines are silent. And nobody knows why the plant has stopped working. Finally, it is decided that something has gone wrong with something electrical in one of the control cabinets. The shift electrician is working on the problem. Breathing over his shoulder is the foreman. In the background there is the production manager, and the general manager. All of them hoping for a solution.

The longer all of this goes on, the greater the tension and the greater the pressure. The situation is not helped by the fact that the drawings of the control panel are poor or do not exist at all; or the panel has been modified out of recognition compared to what it used to be. All that is really known is that it used to work and now doesn't.

I'm going to give you some ideas about how to find faults in a control panel or distribution board when you don't have a drawing, or the drawing is out of date.
The first thing to do if you're the lucky person who has to solve the problem is to get everybody who is not involved in the matter to move away. This won't go down well with the managers, but what the heck. Next, get some good lights so you can see into the distribution board/control panel. If necessary, get a chair or stool so you can sit while you work (in thirty five years I have yet had to stand while working on a distribution board/control panel). Once you have done this, take off your rings, watch, bangle, etc. and roll down your sleeves. Following this, have a little safety discussion with your back-up guy or helper. Explain that (a) if you suddenly collapse, possibly been shocked, make sure that somebody does CPR until a doctor says you're dead (b) if there is a huge bang or bright flash - just stay put.

This sounds a bit fussy but it only takes a minute.

To fix the problem you first have to understand what the problem is. This means you should be measuring things - like voltages and so on. So start at the beginning – measure the main incoming supply voltage and all the voltage supplies to components like programmable logic controllers (PLCs), transducers, and so on. Especially measure from earth to the neutral of all the components – there should be no voltage. If there is, one of the neutrals is disconnected.

Next inspect all the PLCs to see that the power light is on.

And now ... we get to the deductive logic part.

In no particular order, the following: Generally, PLCs do not fail. If the process is not working, 90% of the time it is due to a false input from a field transducer or limit switch ... it is seldom the PLC. So if the process is not working first check on the field transducers.

Next, if an earth leakage protection (ELU) keeps on tripping this often because there is in fact an earth fault. But, electricians often decide that if the ELU trips it must be faulty. Similarly with circuit breakers: if it trips, it's probably overloaded, not faulty.

Check the history of what happened before the outage. Was there a power failure? If so, a battery might have gone flat. Were there trips and power restorations? If so, one or more transducers may have failed. Has there been a lightning storm? If so, follow the process as for trips and power restorations.

Oh, and do check that the micro switch that disconnects the power if the cubicle door is open has been by-passed.

So you will work through the fault. If you follow the steps as outlined above, 80% of the problems will solve themselves. In my next column: some unusual electrical problems ...


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Title: Editor
Name: Gregg Cocking
Phone: +27 11 622-4770
Fax: +27 11 615-6108

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Name: Carin Hannay
Phone: +27 11 622-4770
Fax: +27 11 615-6108