No power to circuits - RCD not tripping

[kickaha]

Domestic CU garage unit. Power is incoming at 237v and registers Live on input and output of RCD. It supplies 3 MCB's on a busbar feed all of which show Live on output as well. Earth is TT and Neutral is on a common bar. Its been fine for 10 years but all of a sudden the lights and ring have just not worked (despite Live output on all of the MCB's). The I changed the RCD as the test button on the old one failed to trip, but the new RCD fails to trip as well.
I used to be an electrician some years ago and only have basic test eqipment now. I suspect a Neutral problem but am a bit flummoxed!

the only odd reading I get is RCD off. L - E at incoming is 237v N - E 37v
RCD on L - E at incoming is 237v N - E 0v
RCD on L - E at outgoing is 237v N - E 0v

I've forgotton a lot of theory so any hints and suggestions as to how I could proceed would be very much appreciated as I can't work on my old Land
Rover!

thanks, Clive

 

[TPES]

What voltage do you get at the top of each MCB on the cable going out? L-E, L-N? I would also check the earth you have.. You need someone with the correct tester to check you still have a good earth from your rod

 

[telectrix]

almost certainly a missing neutral.

 

[TPES]

Check the neutral from where it's supplied from.. What V do you get L-N

 

[Welchyboy]

a neutral has become disconnected somewhere, first I would take a closer look at the bar at your local cu or at the supply end of the sub main

 

[kickaha]

What voltage do you get at the top of each MCB on the cable going out? L-E, L-N? I would also check the earth you have.. You need someone with the correct tester to check you still have a good earth from your rod

I get 237v on each. Ive disconnected all the N and L circuits now then wired direct to the RCD - still nothing. connected E to N to trip the RCD and it tripped the supply RCD.

 

[Welchyboy]

I doubt it an earth issue

 

[Deleted member 9648]

Do a continuity test on the neutral from source to the CU and to the final circuits...as above it is almost certainly a poor (high resistance) or broken neutral. If that test proves a low resistance (<1ohmish) do the same with the lives. You can use a standard multimeter if you have one,just make sure you have safely isolated the supply.

 

[kickaha]

Check the neutral from where it's supplied from.. What V do you get L-N

a nice healthy 237v. But decided that it must be a problem between the supply CU and the garage unit. Found a junction box, opened it up and it was a molten mess inside. Electrics are about 15 years old to the stables and workshop the armoured cable looks even older, probably fitted second hand which I would always refuse to do. Will change the box and see if it fixes the problems. Will let you know. - weeks of sunshine and the one job I have to do outside and its p***ing down........

 

[kickaha]

Do a continuity test on the neutral from source to the CU and to the final circuits...as above it is almost certainly a poor (high resistance) or broken neutral. If that test proves a low resistance (<1ohmish) do the same with the lives. You can use a standard multimeter if you have one,just make sure you have safely isolated the supply.

Yes - thanks for this advice - tested and high resistance! see reply to Welchyboy - burnt out junction box.

 

[JB_]

Like Welchyboy says, neutral missing. Check at the origin of the supply first and if you have 230V L-N there on the outgoing cabling you have a joint box of some description between the origin and your CCU with a broken connection on the neutral or possibly damage elsewhere. Have you had any other work done recently? If so look in the local area of where the work was done and check for damage cabling etc.

 

[JB_]

Must type faster must type faster must type faster!!

 

[telectrix]

think it may be the RCD. insert key in right hand side and turn clockwise until fully wound up.

 

[Guitarist]

The test button simply creates an imbalance within the RCD itself. If the supply is ok and the new RCD doesn't trip using the button, then this is faulty also.

 

[telectrix]

if a new RCD not tripping, sounds like there's no power to it. i.e lost N.