Finding the Not So Obvious Fault

We have all had that fault that simply “disappears” when you try to find it right? Well, here is one for your archives.

The Fault

The supply to the control circuit would trip just about every day, but when it is turned back on, it stays on! The fault is no longer there and this makes it impossible to find. Having looked at the installation, I had the following:

The control circuit is supplied from a RCD.

The total load on the circuit is only 1 Amp

It only trips during the day.

The First Steps

Having power back on, I decided to turn off all non essential circuits to start the process of elimination. This did not work as the power tripped again the next day. Testing prior to restoring power again revealed no fault present and, as before, the power stayed on overnight. This had not highlighted the fault, but it has shown me that every time it tripped, the ambient temperature was reasonably high. At this stage that did not present a solution in itself though, but let’s head to the next morning.

Found It

Determined to find the culprit for this intermittent fault, I arrived on site early the next morning and started testing. As there was no constant fault, I decided to focus my attention on the neutral bar first. At first there was nothing obvious. No loose connections at all which lead to bringing out the old trusty magnifying glass. And there it was, one of the mounting screws had almost no gap between it and the neutral bar. Seeing this prompted me to touch the neutrals with almost no pressure at all and it tripped the RCD instantly.

Now the puzzle started falling into place. Hot weather and/or slight vibration was all that was needed to make the screw touch the neutral bar and trip the RCD

This is not something you expect, but handy to remember the next time you have an intermittent fault.

Till next time – stay safe out there and look after your test equipment – your life depends on it!

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