Fault finding - in particular tripping RCD's?

[HappyHippyDad]

My confidence levels and ability levels have increased hugely in the past year. In others words I have gone from knowing 0.1% of all there is to know about electrics to around 1.5%!!

I would just like to gain more confidence and knowledge regarding ‘tripping RCD’s’!

I have only changed one CU and it went smoothly but throughout the change and afterwards I did not feel confident, solely because I was fearful of one of the RCD’s tripping when all connected and not being able to solve the problem. So I will not change another one until I have a better understanding of ‘How to find the fault of a tripping RCD’. I did a prework survey which included a satisfactory IR test between live conductors and CPC but maybe next time it wont be satisfactory and I will need to fault find before proceeding.

I guess I would really like to know these 3 things:

1. How to use my clamp meter (has earth fault capability) to detect earth fault. Is it as simple as clamping on relevant earth and reading? What results should I see? Any reading greater than 30mA? Would this residual reading only show if the circuit was live? If so how can you get a reading if its always tripping?

2. How do I narrow down the search? Finding the circuit at fault should be straightforward but how do I locate where on the circuit the fault is (i.e on a ring/radial or lights), how do I narrow it down?

3. Any other tips on fault finding?

I know this is a pretty lame question, but I'd rather look a bit daft and know the answer!

Cheers guys..

 

[alanl]

1) I've got an earth leakage clamp meter, but have only used it once in 2 years or so to measure earth leakage current. In usual use, there is no need to know the earth leakage, especially in domestic situations where appliance leakage is unlikely to cause problems.
But yes, that is all you do, clamp around the relevant CPC/earth cable, and run the circuit/appliance. You will not get a reading over 30mA as the RCD will trip before then.
2) Narrowing down. Not always easy. Unplug/disconnect all appliances, then plug them in one by one, and you'll see which one is causing the fault if it is an appliance.
If it is the installation causing the RCD tripping, then your Insulation Resistance testing should show which circuit is at fault.
If cumulative earth leakage from appliances, then this is where your earth leakage meter comes in. You do a ramp test on your RCD tester, to find out when it trips. You can then turn on appliances to see if there is one causing a large leakage, or just a cumulative effect, which causes a trip when everything is on.
Extra, high integrity earthing may help there. Unlikely to be this though, unless in an office/commercial environment.

Insulation resistance is your biggest pointer to Installation faults. If you havent got >50M Ohm resistance between conductors/earth, then there is likely to be a fault (yes, I know 2M Ohm is allowable, but who ever sees such a low figure on a single circuit?)

 

[Rampantchilli]

As above but also if you are doing a pre work survey, then I'd go round switch off everything and unplug whatever you can.
do a global IR test at 230-250v with each CB switched on but main switch off, remember you are looking for above 2M ohm so you decide what is low here. If you haven't got 200+ OR WHATEVER VALUE YOU DECIDE IS ACCEPTABLE then isolate each breaker and do IR on each circuit one at a time. Hopefully you pinpoint the circuit/s with issues and can then do further investigations.

if you've done all your pre work tests and gone ahead and done the job, but now you have a problem then first check your board layout, and your connections are in the proper place and your RCD links are correct( it's happened to most of us) then you should assume borrowed neutrals on lights, obviously you've done your ring tests so they will be right and not possibly crossed,so the borrowed neutral will normally be around newer wall lights, landing lights or hall lights but could be somewhere else too.

check connections to links, and busbars and ensure your CBs are mounted on your bar correctly, rcds don't like bad connections on CBs.

thats my experience of faults following a board change,

 

[Thefailsafe2012]

The others have nailed it, sometimes if there are existing lights/sockets whatever outside then those are always good to check for water ingress etc as that can play havoc with RCD's

 

[tony mc]

IMO when you use your earth leakage meter clamp the tails for a more accurate reading and get the cables in the centre of the loop as reading can vary. I have a extension lead where the cores are broken (so I can clamp them)so I can see what leakage most appliances have remembering that 0.75 mA for hand held and a max of 3.5 mA for fixed as per the IET guidelines. IR testing is by far the best way in tracing faults and once you ID the cct or cct's at fault disconnect both the line & neutral from the CU and start you testing. Remember just because your meter tells you its flat with 0.00 Meg you need to switch over to ohms and obtain the reading either low or in K ohms. If it is a dead short then it can be much easier to locate using maths with the resistance of the cables etc but when its high in the K ohms you have to start breaking he cct down to get nearer the fault. We had one last week with a neutral to earth fault of 8.5k ohms on one leg of a ring final and my mate traced it to a joint box with a loose neural which had been arching causing carbon and tracking across to the cpc especially when IR tested on 500 v. Remember with a 30 mA RCD it will hold above 7666 ohms so with our fault the RCD was stable but the cct could not go back in for obvious reasons! As mentioned make sure your connections are good and tight & outside equipment not on double poled switches with water ingress! Also remember if you do a global IR test and it comes back low all your ccts could be >300 but it only takes one cct to be low which will bring your total IR readings down 1/RT etc> Remember if testing at 500 v with line & neutral connected that surge protecting will dump to earth and give you a false reading (Extension leads etc) Hope this little bit helps.