Faults that aren't there?

[Bob Geldoff1234]

Over the past couple of years i have been called to houses where the rcd has periodically tripped.I have carried out all insulation resistance tests and have been greater than 20m ohms across all conductors.I have then tested the rcd tripping times and ramp tests.
Everything is showing clear and working.I then might have been called back to the same job a couple of times with sometimes months in between tripping.I have asked the usual questions about if they had plugged anything in recently that they don't normally use and that usually draws a blank.
Eventually,on one such job,the customer said that when he reached down to switch the tv off at the socket the rcd had tripped.
I removed the socket and remade off the connections(no sleeving,grommets etc) and put a new socket front on and checked the plug tops.(I still couldn't really see any definite problem that could have caused it)
This has now finally cured the tripping rcd and they have had no problems since.
My question is how do other electricians try and trace such a fault when technically when you test there are no faults present?
My insulation resistance tester inserts a small current at 500v when testing and quite often this is enough to jump across the faulty item and show up on the meter.
Are there any meters on the market that might inject more current so that in these hard to find faults it might show up better?
In other words how do you trace a fault that isn't there?
Most of the faults i get called to show as a poor reading on the meter and you can set about fault finding,knowing that when you find it,the fault will be cleared.
Sorry for the long post but just interested what other do.
Also any stories of hard to find faults and what you did to find them would be most appreciated.

 

[Ian1981]

You could be getting earth leakage from various appliances that combined reach the tripping threshold of the rcd.

 

[LankyWill]

I think i know what you mean the amount of times i have been fault finding, only for the fault to disappear and fix itself is mind boggling. Electricity is a strange thing and sometimes i think my megger plays tricks on me.

 

[Deleted member 9648]

Anyone who has an RCD has to accept occasional nuisance tripping, if it's not regular and everything tests ok there's not a lot you can do. I had a call today to one that had been out '5 or 6 times since Christmas'....global IR with everything still connected around 10 megs, no worries there. There were 11 circuits with a single up front 30ma main switch and lots of outside stuff. Random tripping almost guaranteed. Quoting for a board change with RCBO's. The thing as well is the RCD is at ceiling level while the DB is about 4ft off the floor......lady of the house is borderline dwarf, and it always trips when hubbys not there!

 

[7029 dave]

Hope you mean meg ohms.?

 

[Ian1981]

I've had faults where a low reading has been obtained during an IR test on a ring circuit only for you to take of various sockets then the fault clears as if it's been maybe an impact/crushing of the wires behind a socket causing the issue

 

[charlie76]

I had a customer with a randomly tripping RCD. Spent nearly 2 days testing and hunting. They thought it was related to the shower but nothing found there. In the end I removed various sockets, switches and the cooker isolator. On removal of the cooker switch the Live fell out, it was not screwed down. Re-inserted this, screwed it nice and tight, they never had an issue again.

 

[gutterball]

once had one where if you stood on a particular spot in the hall the rcd would trip. it was at the side where you Wouldnt normally walk.

 

[SparkyChick]

once had one where if you stood on a particular spot in the hall the rcd would trip. it was at the side where you Wouldnt normally walk.

Don't leave us hanging Did you get to the bottom of it?

 

[Risteard]

Hope you mean meg ohms.?

Or perhaps even Megohms.

 

[Marvo]

I think rather than faults that 'aren't there' you've got to look at it that there's sometimes a fault that isn't picked up by the standard way of testing or sometimes the fault is intermittant and you'll only pick it up if you happen to be in the right place at the right time.

You get moisture related faults that only cause tripping at certain times of the day or during certain weather conditions. You get mechanical faults like screws through cables or crushed cables or rodent damaged wiring that, due to expasion and contraction, will only trip when it's warm or cold. You get appliances that intermittantly cause tripping but they're not plugged in and hidden away when you happen to be there. I've also had an RCD that occasionally tripped when the street lights came on outside, it was sensitive to phase angle changes with inductive upstream loads and a like for like replacement was exactly the same. As already mentioned poor terminations can cause a range of intermittant faults including RCD tripping. These are just a few problems that are unlikely to show up during normal testing proceedures, the best chance of finding most of them is to expose and visually inspect.

I don't think modern day testers aren't doing people any favours in many cases. You could get a lot more info about a circuit from the behaviour of the needle on an analogue tester if you knew what you were looking at, the tendancy toward go/no-go and software interpreted tester outputs can make it more difficult to find these faults.

.

 

[gutterball]

Don't leave us hanging Did you get to the bottom of it?

It was damaged cables in a notch under the floor. they only crushed together enough to trip if you stood on that particular spot.

 

[7029 dave]

Or perhaps even Megohms.

Yep that too.

 

[D00DY_T00DY]

once had one where if you stood on a particular spot in the hall the rcd would trip. it was at the side where you Wouldnt normally walk.

Did you advice to mark the spot so they could jump over it instead of trip the rcd

 

[Deleted member 72666]

@BG1234 Fault finding would be easier with an earth leakage clamp meter for intermittent faults on rcds/rcbos.

 

[NDG Elecs]

Finding intermittent faults can be tricky indeed. Sometimes it's a real fluke how you do find it. A had a mega fault once, I was flummoxed and nearly rang my mates Martin and Dale, and sometimes Robin gets a bell too! It was caused by a brown uPVC door frame that expanded after having a few hours of sun on it, just enough to crush down on a cable that shouldn't have been where it was. Took months on and off to find that one. Only found it because the customer could barely open the door on that particular visit, and clutching at straws I dug around.
8
I always try to explain that looking for intermittents can be a real needle in a haystack, and it certainly does no harm to test ccts regardless. Which is a good angle to approach it from I think. I would wager that a lot of the ccts I test have never seen a MFT before.