N/E fault RCD not tripping ??

[Buzz1980]

Hi all

can anyone give me the logic why the RCD wouldn’t trip on a neutral/earth fault. Testing a extension where they have added in 5 additional sockets to the upstairs ring had been powered up and nothing tripping with household appliances plugged in and working. Carrying out IR tests to find had dead short on one leg of the neutral and earth between 2 sockets but there was no issue of tripping ? RCD checks out ok with test button and RCD tests. Have disconnected this leg and then fitted fuse spur off the ring to carry the additional sockets now as to late to add in additional cable, so extension spurred off ring using fuse spur. Would it be the case that it had seized a bit RCD or that its connected to a TNCS system?? Replies much appreciated ?

 

[Grant1987]

Sorry no the consumer unit is 6months old, this was a extension that got done and they require test cert. yeah think best to disconnect but then test kind of pain in the arse tbh is it disconnecting circuits ?

Ah I wish I was local to you now I would help you out. Are you at the property now?

 

[Buzz1980]

But I have IR test with line and neutral connected and down to earth all clear only coming dead short when I do live to earth, which if it was damaged cable then it would show up on line/neutral together to earth ??

 

[Buzz1980]

Must be USB port that brings it down, I’m thinking when IR testing down the earth it’s coming back on the neutral and causing it to short through the USB ?? ?

 

[Grant1987]

But I have IR test with line and neutral connected and down to earth all clear only coming dead short when I do live to earth, which if it was damaged cable then it would show up on line/neutral together to earth ??

Right forget about joining them, do them individually I’m certain your lays between N-E put your 2 legs into a connector block for earth and then the other 2 legs for neutral in a connector block. Test them with your probes. You will probs get a big fat 0. Are you still at the property if you are I will talk you through it

 

[Buzz1980]

Sorry not there anymore, left 4 hours ago ?

 

[Grant1987]

Right forget about joining them, do them individually I’m certain your lays between N-E put your 2 legs into a connector block for earth and then the other 2 legs for neutral in a connector block. Test them with your probes. You will probs get a big fat 0. Are you still at the property if you are I will talk you through it

Fault finding is

Sorry not there anymore, left 4 hours ago ?

Ah gutting hopefully they got power?

 

[Buzz1980]

Yeah it’s not tripping hasn’t tripped in years ?

 

[Grant1987]

Fault finding is

Ah gutting hopefully they got power?

If you’re struggling I’ll message you direct with my phone number and you can ring me direct tomorrow ?

 

[Grant1987]

Yeah it’s not tripping hasn’t tripped in years ?

With a dead short? ?

 

[Grant1987]

With a dead short? ?

Ok I’ll leave you to it lol, doesn’t sound good to me

 

[Buzz1980]

I know that’s why I don’t think it’s a dead short and the reading is from the USB points, live to earth IR test, goes down the earth which is also connected to the neutral at the supply TNCS will go through this neutral back around all socket and through the line conductor hence giving me a short circuit, but when line and neutral connected together all clear ??

 

[Ziggie]

If you know there are usb sockets in the circuit, testing N-E insulation is a bad idea.
you can't say there is a dead short if testing with an insulation resistance tester.

I would only consider something to be a dead short if measured below 1 ohm.

But regarding the USB sockets. If I have not disconnected the neutrals from neutral bar and tested between live and earth and I am getting low reading basically zero near enough, this should be because the USB points are bringing the reading down as I will be going through the earth and being TNCS will be linked to neutral so will be testing the USB point. As when I link out live and neutrals at the consumer unit and go to a socket then test live to earth insulation resistance it comes out clear. >999mega ohms. Now if that was a damaged cable that would be picked up, does that make sense ?

The USB installation blurb differs between brands. I fitted a LAP usb socket and it states that you can do installation test at 500 V but others state that they must be disconnected.

first thing mentioned on my insulation test is that dimmers should be disconnected for the test.

Short circuit between live conductors after the RCD will not ensure it’s tripping ( RCD guide Eaton.EU) may apply and example shown is N to E.

 

[richy3333]

This might be of use to you:

https://uk.megger.com/electrical-te...on/june-2015/testing-usb-wall-mounted-sockets

It should also be noted that the electronic circuitry inside the SMPS could easily be damaged by a 500 V insulation test injected across it. Consequently the USB port may fail to function following a 500 V Line to Neutral insulation test