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 ?

 

[telectrix]

it has been known for a N-E fault to inhibit the RCD from tripping, even on the test button. don't know the theory behind it, but i have seen it a couple of times.

 

[westward10]

It may not be creating an imbalance. Just because the neutral and cpc have become connected does not necessarily mean enough will be diverted away from the neutral for the imbalance.

 

[Buzz1980]

Aww ok thanks for the reply ?. Had also a couple of other issues never seen before.

Trying to carry out Zs test on upstairs lights and found that it would carry out test as error code of >65 fre. Looked at the manuals and states supply frequency to high for test. So I disconnected the dimmer switch and managed to carry out test. Seems the dimmer is causing the frequency issue but don’t know why this would be as should be rated for 50hz ?

another issue think I know the issue on this one though IR test on socket circuit live to earth dead short. Join live and neutral together test down to earth >999mega ohms. Thinking with me going live to earth IR test travelling though USB ports on the sockets ? I noticed behind a cabinet was causing reading to be dead short. This is the only reason I can think of as unable to switch off USB ports ?

 

[James]

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.

 

[Buzz1980]

Hi yeah I tested live to earth but coming up 0.00mega ohm but link neutral and live together then test then all clear >999 mega ohm so it’s like I’m testing through the USB when doing live to earth on the IR tests ?

 

[Buzz1980]

I’m thinking I would have to disconnect sockets and connector block to carry out live to earth then neutral to earth ?

 

[Grant1987]

I’m thinking I would have to disconnect sockets and connector block to carry out live to earth then neutral to earth ?

You should be doing a Bridge test or disconnecting sensitive equipment or putting it down as limited on test certificate.

 

[westward10]

Hi yeah I tested live to earth but coming up 0.00mega ohm but link neutral and live together then test then all clear >999 mega ohm so it’s like I’m testing through the USB when doing live to earth on the IR tests ?

If your neutral is disconnected from the neutral bar you should not get zero as your linked neutral/line to earth was clear.

 

[Grant1987]

If your neutral is disconnected from the neutral bar you should not get zero as your linked neutral/line to earth was clear.

Good point ?

 

[Wilko]

Sorry I may not have followed things here fully, but if you are reading zero N - E but everything works etc then I’m thinking perhaps you’re recording the excellent N - E connection at the cutout?

 

[Buzz1980]

I don’t think so as I managed to trace the fault and link between two sockets N to E short. But that may be a case in future works.?

 

[Buzz1980]

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 ?

 

[Grant1987]

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 ?

What voltage are you doing insulation resistance test?

 

[Buzz1980]

250volts to see if ok then 500volts after that test ?

 

[Grant1987]

250volts to see if ok then 500volts after that test ?

Ok well where did you get the >999Mohms?

 

[Wilko]

I don’t think so as I managed to trace the fault and link between two sockets N to E short. But that may be a case in future works.?

Oops, sorry I missed that

 

[Buzz1980]

On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense

 

[Grant1987]

Ok well where did you get the >999Mohms?

Googling it now are you? Lol

 

[Buzz1980]

Sorry googling what ???