Any advice on how to diagnose or find leakage onto the neutral line.
I have 1 led light that turns on when the switch is off, measuring with the multi-meter i have about 2.5 - 3V leakage between neutral and earth. Enough for this little led strip to shine very dim at night. Not enough current it seems to trip the E/L
You don’t really measure leakage in volts. A small voltage between neutral and earth can sometimes be explained easily: Voltage drop over the neutral connector you are measuring on.
For example, if you have a TN-S earthing arrangement (TN bond is at the transformer, several meters from your house), and you have 2% drop over your combined conductors, then will show as around 2V between live and neutral. Of course (ideally), with low to no load, you want to see no potential difference.
The best device to hunt for earth leakage issues is this one. Or similar. That was the cheapest one I could find. I convinced myself I need it. I’ve used it exactly twice (for its intended purpose), but it is a very nice clamp meter too.
Guess ill go looking for a clamp meter to borrow. I cant convince myself I need that 1 yet.
Looking for these faults does test one’s patience!
But you can try fault finding by isolating circuits. However this will only work if the fault trips your earth leakage promptly.
The idea is to isolate all circuits: Switch off all CBs in your DB and check if there are no trips. Then switch on the CBs slowly one by one until it trips. Then switch off the last CB and reset the RCD. If it is then stable there’s a good chance that the fault is on that circuit…
Had a similar problem here when we moved in. The electrician used his tester to check at which point the EL trips, then turned off all circuits except the one he was on and tested the value. I expected him to walk the house like that, but he only flipped a circuit disconnected a neutral one at a time while retesting from the same spot. Found the problem in less than 5 minutes and another minute or two to confirm.
Actually saw afterwards that the COC electrician noted that the EL trips on 16mA and just called it a day. 
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That’s a very specific kind of meter that is optimised to measure earth leakage current. It is way too expensive for the ordinary guy. I’m not ordinary! 
That’s the cheaper way. Trial and error. Takes a bit more time, but you can buy half a day’s time at premium rates for the price of that clamp meter.
Where the clamp meter is very useful, is to help find the offending circuit faster. The last time I used it, it was to split my circuits from one block with a 14mA standing loss, into two blocks of 7mA each. A standing loss is normal in a modern home.
Pretty typical. The rules says the RCD may trip at 15mA, and must trip at 30mA. So most RCDs end up tripping around the 15-20mA mark.
I also like isolating the fault like your electrician. If you’re methodical it’s a kind of simple way of fault finding.
Personally I would reach for my multimeter and having disconnected the live & neutral measure the resistance to ground. It should be in the 1000’s of Ω… (where else could the current go??)
If you went by the moniker of TTT, he would have said … Here, hold my beer …
Unfortunately it is not that simple. Much of the coupling could be capacitive. Dielectrics break down past a certain voltage - that is why you measure isolation with a megger, which measures leakage while applying a voltage.
Then to make things even more complicated, capacitive coupling is frequency dependent, so if you have an inverter, some high frequency components end up on the lines which can cause even more leakage. This leakage is also not measured the same by different RCD devices.
I suspect the problem the OP is having is capacitive coupling, but not related to leakage - only in the wiring between the switch and the lighting fixtures.
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Always good to find someone that know themselves
I presume you mean insulation? The size of capacitor to draw 15mA at 50Hz is massive…
I want to add here, the idea of poking around in a DB with a hardware-store multimeter that is supposedly CAT-II (according to someone in China) gives me the shivers. You’d have to completely isolate the installation (or risk blowing up a meter), and even then, a multimeter uses a very low-voltage DC signal to measure resistance, and you’re going to measure nothing but dead air… or you will see something and then be unable to replicate it (because the capacitor on the other end has charged with that DC current you sent into the circuit).
A megger (which is a brand-name, but like Jacuzzi and Jik it has become the defacto name for the thing) is the better option to see if there is leakage to ground.
It is hard to really give an answer, as the range of problems are wide. If you have a very definite leak that is enough to trip the RCD, then use the divide and conquer method of pulling all the lives and neutrals and bringing them in one by one.
But sometimes you have nuisance tripping. There isn’t a specific leak on a specific circuit, it seems to be all over. Divide and conquer doesn’t work because it isn’t just one specific circuit. That’s why I eventually did the crazy thing of buying an expensive clamp meter. I needed to SEE what is going on!
What helps here (and is very cheap) is to get an earth leakage tester (they are cheap, around R400), set it to its lowest setting (10mA usually). This is not enough to trip the RCD on its own (RCDs trip from 15mA onwards), but if the circuit you are currently testing/eliminating has a > 5mA leak it starts to trip. Much cheaper than the clamp meter. I think that is what @mariusm was talking about?
But of course, the megger remains one of the best tests, but that is also an expensive tool.
I agree. It might have nothing to do with earth leakage. Might even just be a cheap LED strip…
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?
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Just ensure that all appliances are unplugged. I mean you go around the house and if you see a plug in a wall socket you physically unplug it before you start…
And I recommend not feeding your leaky outbuildings with a feed from your protected supply in the house. (Those who work in those environments just have to take their chances 
)
All insulation is also a dielectric…
Well, that is why I add the bit about the higher frequencies. If you have an inverter or even just some switch mode power supplies, there will be some higher frequency components on the lines. IME, it does not need to be nearly 15mA to lower your trip point my 15mA.
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Thanks for this - very interesting topic 
For those curious for a good demonstration/explanation, which is potentially quite relevant to SA homes, have a look at this utube…
WARNING: contains questionable language, visuals and humour but I found the overall content very enlightening.
Those who prefer a more sanitised version can read his static web write-up
So, for glowing LED when the switch is off, some possible reasons to consider:
- Capacitive coupling (more prominent on lighting circuits where only a live runs to the switch)
- Smart switches of the “does not need a neutral” variety
- Insulation resistance breakdown
- Dimmer switches
Family friendlier reading