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!

 

[littlespark]

If you’re not sure that everything is unplugged, IR test at 250v, 500 could fry the appliance.

know what you’re looking for.... a dead short, or open circuit where there shouldn’t be is a tell tale.


An intermittent fault is the hardest to find.... unless you can pinpoint a sequence of events that always causes the fault.... otherwise you could be testing for a fault that isn’t there at the exact moment you perform the test.

 

[loz2754]

Just to add that a simple test to determine whether there are any connected loads would be a low reading continuity test from line to neutral of the circuit in question.

 

[timhoward]

It comes down to building a picture a bit at a time.
Is it MCB or RCD tripping? (so are you looking for an overload issue or leakage issue)
Is it intermittent or repeatable?
Is fixed wiring ok -
-normal resistance measurement L to N (looking for open circuit to prove everything isolated unless there are USB sockets, dimmers, neons)
-IR at 250v of L+N together to E (this won't damage anything connected)
-IR at 500v of L+N together to E (a very different reading to 250v might indicate a surge protected extension lead connected)
-if open circuit L to N, IR test other permutations at 250v first.
What happens with most likely items disconnected (anything with heating element, anything outside etc.)
PAT testing suspicious items
Ramp testing RCDs
Meausuring current with clamp meter

 

[telectrix]

IR test L/E, have a beer
ditto L/N, have another beer
scratch head, have another beer
cinsinyooity l/n, have another beer.
next test, have another beer... who cares about it any more.

 

[timhoward]

I'm working on a very large EICR at the moment with two other sparks. I always like working with others, I always learn stuff. Both the others are older than me and very good sparks but I will risk stating the obvious by sharing something neither of them had ever twigged in all their years. When we were comparing MFT's, boys toys and all that, they both said they barely used the ramp testing function which prompted the discussion. So:

RCD Ramp testing - it doesn't just tell you the trip current of an RCD. Well, it does, but it tells you the trip current in the present circumstances. This can be so useful.
So if you ramp test with nothing connected, then add appliances one at a time, and re-test, any reduction is the leakage of the last item connected. A jump down suggests the last item connected has some notable leakage and may be contributor to any occasional random tripping.
Or do the opposite, start with everything connected and turned on, the test value is basically then the fault current 'headroom". Turn things off and look for a significant improvement.

It sounds obvious. I had thought this was widely known. But in case it's not I will accept ridicule for sharing it!

 

[pc1966]

RCD Ramp testing - it doesn't just tell you the trip current of an RCD. Well, it does, but it tells you the trip current in the present circumstances. This can be so useful.

That is a good way to check if you don't have a clamp ammeter that goes down to mA to see leakage directly. Most of the cheaper ones, and the very high current ones, only resolve 1A or maybe 0.1A at best, you need to look out for the ones for leakage testing that can see 1mA (0.001A) or so.

Another good point mentioned above was PAT testing suspicious appliances. Many folks don't have a PAT tester if it they don't offer the service (difficult to compete with the 80p/test monkeys) and while you can IR test with a bit of effort, the testers make is so much easier, safer, and more reliable.

Just don't get the cheapest pass/fail ones for office staff, get one that actually tells you the leakage as you can see what a few are adding up to, and if you get a "fail" at say 3.5mA for a odd electronic item it might just be fine and as-designed for the era.

Doing that along with EICR might be good for the better class of landlord who actually wants a professional job done to ensure safety as well by a trusted spark.

 

[timhoward]

Another good point mentioned above was PAT testing suspicious appliances. Many folks don't have a PAT tester if it they don't offer the service (difficult to compete with the 80p/test monkeys) and while you can IR test with a bit of effort, the testers make is so much easier, safer, and more reliable.

A half way house is a Kewtech PAT Adaptor. Or make one (forgive the language!)

Just don't get the cheapest pass/fail ones for office staff, get one that actually tells you the leakage as you can see what a few are adding up to, and if you get a "fail" at say 3.5mA for a odd electronic item it might just be fine and as-designed for the era.

I'd be glad of a recommendation for a good one that isn't super expensive. In my head I want a battery powered and more compact version of an old Megger PAT32...but they do all seem quite pricey.

In the meantime my DIY adaptor box of plenty serves the purpose. A double socket with a 4mm banana socket attached to each conductor inside, and a short lead with stackable 4mm plugs. (Originally built to allow a Megger CBT2 RCD tester to test other circuits but has found many uses since)

 

[LastManOnline]

It sounds obvious. I had thought this was widely known. But in case it's not I will accept ridicule for sharing it

 

[brianmoooore]

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

 

[SparkyChick]

Have the users of the installation keep a diary of what trips and when, have them include things like if it was raining or not, and whether it just tripped or whether they had just turned something on or off, a rough list of what was on at the time.

Also if you find nothing that could indicate a fault, have them (or do it yourself) have a chat with their neighbours. I've had a recent case of two clients on the same road where they were having seemingly random RCD trips. The trips started at around the same time (Christmas Eve after we had the heavy rains which caused some heavy flooding in their area). I tested both properties, one I found some issues with outside lighting which we disconnected but the trips continued... we got the DNO involved, they came out checked the service heads etc., found nothing so they installed monitoring gear. They've since dug up the road, done something and the tripping has stopped.

 

[Dartlec]

Edited into below message for formatting...

 

[Dartlec]

That is a good way to check if you don't have a clamp ammeter that goes down to mA to see leakage directly. Most of the cheaper ones, and the very high current ones, only resolve 1A or maybe 0.1A at best, you need to look out for the ones for leakage testing that can see 1mA (0.001A) or so.

Another good point mentioned above was PAT testing suspicious appliances. Many folks don't have a PAT tester if it they don't offer the service (difficult to compete with the 80p/test monkeys) and while you can IR test with a bit of effort, the testers make is so much easier, safer, and more reliable.

Just don't get the cheapest pass/fail ones for office staff, get one that actually tells you the leakage as you can see what a few are adding up to, and if you get a "fail" at say 3.5mA for a odd electronic item it might just be fine and as-designed for the era.

Doing that along with EICR might be good for the better class of landlord who actually wants a professional job done to ensure safety as well by a trusted spark.

A half way house is a Kewtech PAT Adaptor. Or make one (forgive the language!)

I'd be glad of a recommendation for a good one that isn't super expensive. In my head I want a battery powered and more compact version of an old Megger PAT32...but they do all seem quite pricey.

In the meantime my DIY adaptor box of plenty serves the purpose. A double socket with a 4mm banana socket attached to each conductor inside, and a short lead with stackable 4mm plugs. (Originally built to allow a Megger CBT2 RCD tester to test other circuits but has found many uses since)

I have a Seaward PrimeTest 100 which may not do all the fancy stuff necessary if I did full PAT testing service, but is ideal for this sort of thing. I bought it new 12 years ago, but going for around £100 or less on eBay now.

It does say Pass/Fail, but also gives you the leakage so you can decide whether it's the length of the lead, or the age of the appliance etc.

Very handy to rule out some of the likely candidates (dishwashers, washing machines, toasters, Fridges etc)

 

[LastManOnline]

It sounds obvious. I had thought this was widely known. But in case it's not I will accept ridicule for sharing it!

Widely known but perhaps not widely used. Your post is a helpful reminder of its use. I mucked up my first response due to someone shouting in my ear and I prematurely pressed the "post" button. By the time I get back online I find "Brian Moore" and "sparkychick" have summarised everything I wanted to say very nicely.
Wanted to add thought that 20 years ago we used to solve many intermittent faults by converting from rcd protected circuits to rcbo populated CU, s and hence greatly expanding the parameters for "natural leakage". "ramp resting" was still a slightly unfamiliar science back then. Not so any more.
On intermittent faults I also find water (or some similar product) is, responsible for vast majority of intermittent faults. Be it rain or plumbing leaks accessing places it should, nt.

 

[SparkyChick]

I'd also like to add... if the issue is nuisance RCD tripping, check all accessible incoming supply connections and get an insulated inspection mirror and make sure all devices are correctly located on the bus bars.

Have a feel of the service head as well, of course only if it is safe to do so. If it's warm to the touch or it's oozing goo, could be an indication of a loose connection internally. Have the DNO come and check it.

 

[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.