Tips for Intermittent Fault Finding

[IAmSparkytus!]

Good evening!

Majority of the work I've done over the past 10 year or so has been new build full rewires and rewires of existing premises and because of this there are very rarely any faults to be found when carrying out the testing of the installations.

In a very short space of time, I've had a few jobs where the customers have had circuits intermittently tripping. Dodgy appliances have been found and a dodgy MCB.

Whats your best tips for finding intermittent tripping faults on circuits where you can't guarantee that all appliances have been unplugged and all fuses have been turned off etc?

Sometimes struggle a bit on these. How are you supposed to take an IR reading for the circuits when you can't be sure that everythings disconnected/switched off?

Cheers for any tips!

 

[LastManOnline]

Best tool for finding intermittent or obscure electrical faults is experience from many years of doing so.

Yes. On honest reflection of the faultfinding "learning curve" we all must make, I can look back at a number of "near misses" I had due to inexperience. Loz 2744, s point about using 250 for IR testing instead of 500 volts. It cost me a few detector heads but it could have been much, much more painful.
I learned many years ago that effective electrical faultfinding must always be preceded with by a principle all surgeons apply before medical faultfinding "FIRST do no harm..."

 

[loz2754]

Loz 2744, s point about using 250 for IR testing instead of 500 volt

As much as I'd love to take the credit for saying this, I believe it was @littlespark who said it, post #2.
Of course, it's something I always do anyway...?

 

[IAmSparkytus!]

Had another job today for the same thing... the law of attraction must be real - ask and you shall receive!

Fault finding in an old pub on a ring main tripping whenever it felt like it. Unplugged everything believed to be connected, tested at the board, test results showing that there were still more items to be disconnected but like hell could I find them!

Stuck to the sockets i could find, disconnected a few things and found some bar equipment to have 5megohms IR and 0.00megohms IR. Left these items unplugged and haven't heard anything back since (touch wood!).

Hopefully one of the 2 has caused the tripping.

Also found a faulty socket with nothing connected in showing a neutral fault on my socket tester. Could this have caused it if it wasn't in use? I'm doubtful.

What do you all do when it looks like there are other things plugged in/turned on but you can't find them?

 

[pc1966]

What do you all do when it looks like there are other things plugged in/turned on but you can't find them?

Things the don't look plugged in, like outside lights or internal stuff in the form of lights, hand dryers, etc.

Some places are bad for having years of stupid modifications, and from other sparks' comments pubs seem to be #1 for that.

Or switch the ring MCB off and get the bar folks to check everything to see what has stopped working?

 

[SparkyChick]

What do you all do when it looks like there are other things plugged in/turned on but you can't find them?

Open every cupboard, look behind every piece of furniture, look in every void above, between, beneath and behind kitchen units and appliances, walk the exterior of the property looking for anything coming out the wall, particularly if it's in the vicinity of an accessory inside, ask them (three times - once when you get there, once somewhere in the middle and once more for good measure a bit later) whether they have had or have done anything in terms of the electrics, decoration, hanging things on the wall etc. etc. and if so what.

 

[Dartlec]

Things the don't look plugged in, like outside lights or internal stuff in the form of lights, hand dryers, etc.

Some places are bad for having years of stupid modifications, and from other sparks' comments pubs seem to be #1 for that.

Or switch the ring MCB off and get the bar folks to check everything to see what has stopped working?

Not just pubs - there have been at least two properties in the last year with a random breaker with cable attached to it that I turned off and nothing has been spotted not working when I've gone back weeks or months later. One of them was even labelled "unknown" at the CU years ago so if the installer didn't know then I don't have much hope of finding it!

 

[UNG]

IMO faultfinding is an art and a skill that is developed over many years and a good memory of past faults found on previous jobs helps a lot. After that it is down to having a good knowledge of the test equipment you have and how best to apply it to get the results you need to locate the fault
Over the years I have built up an assortment of test equipment that is mainly used for faultfinding, I find good cable tracers that will work on both open and closed circuits are a very useful piece of kit to have (can be useful to find plugged in appliances, cable faults or identify circuit cables) after that a logging multimeter with a current clamp is useful for those faults that happen when your at home and in bed other bits I've added over the years are a non contact thermometer, power logger, thermal imager and a scopemeter to list a few bits

In recent years on of the most common intermittent faults I've had is the auto defrost fridge / freezer with the failed heating element, they can trip between 12 to 72 hours after the RCD is reset so it is difficult to find without accessing the element for a direct test

 

[Marvo]

With a lot of intermittant faults you're more likely to find them by visual inspection and a bit of experience than just by using a tester. There's many faults a tester won't help whatsoever in locating.