RCD tripping on new CU

[Evostick]

I have just fitted a new 17th edition CU to a customer's house.
Everything checked out ok except insulation resistance test on the single ring main in a two story dwelling. I double checked everything was unplugged, got a reading of >99 ohms on a continuity test but could only get a reading of 50 ohms on the insulation resistance test, and when connected to the CU, the RCD trips instantly.

I am going back tomorrow to start checking all the sockets.

Any advice would be gartefully received.

This my 2nd CU in two days, yesterday's had no CPC on any of the circuits and was a rental property.

 

[Amp David]

Well for a start you've got an open circuit if you got >99 ohms on the continuity test. Best of finding this baby first I reckon.

Was the insulation resistance ohms or Meg Ohms?

Always best to do a pre job check before starting too.

 

[Evostick]

I was looking for an open ciruit betweem live and nutral.

 

[Hawk81]

I was looking for an open ciruit betweem live and nutral.

But you said continuity test. That would have suggested you were doing a ring continuity test. ie live to live, neutral to neutral ect. An insulation test of 50M ohms would be an ok reading.

 

[Amp David]

Ah ok, thout you meant testing continuity of the ring. I'd split the ring in half the ins res test each half, to find you dodgy run.

Does it trip with the ring not connected?

 

[Evostick]

Having verified nothing was still plugged in and all switches were in the off position, I performed an insulation resistance test and got a result of less than 50 ohms, and then when connected the RCD trips immediately.

 

[Evostick]

No, it doesn't trip when the ring is not connected.
Like the idea of splitting it in two, thanks for that.

 

[Johno]

Yep, sounds like you need to split the circuit up and trace the fault that way. Can be a real pain in the ****, but you might get lucky and find it straight away (it does happen sometimes).

 

[Evostick]

Thanks for tips guys.

 

[Spudnik]

What were your L-E & N-E IR results?

 

[pushrod]

Having verified nothing was still plugged in and all switches were in the off position, I performed an insulation resistance test and got a result of less than 50 ohms, and then when connected the RCD trips immediately.

How do you get a reading of less than 50Ω when you are doing an IR test and the meter is reading in MΩ

 

[Funkymonkeytoo]

Split the ring in half do an insulation resistance on both halfs find the faulty side then split the faulty side in half again retest insulation if ok add one or two sockets at a time to the faulty side testing untill you reach the faulty leg, if tested and circuit is open once split in half split in half again.

Easier to find than it sounds

 

[Amp David]

How do you get a reading of less than 50Ω when you are doing an IR test and the meter is reading in MΩ

Thats what sent me of course too. Thought at one point I was getting me continuity and ins res mixed up.

 

[Monkey1984]

I had a CU Change the other day and that had 8 Circuits / 6 With Faults. That's the sort of Board you think why did I get the call.

If you were getting low IR Readings you shouldn't have proceeded to change the board without locating the fault first. For the exact reason you have stated.

 

[nickblake]

The chances are that there is a trapped Neutral , carry out all your continuity tests , you know youve an IR problem , disconnect the tails from the CDU , put your meter into ohms and test accross the legs of the ring as you would with an IR test , if you get a reading , in ohms you then you have a short in a socket or jb even may be a damaged cable ,IE N to Earth fault , if you have a lead for your meter that has seperate leads to a plug top , plug it in ,N to the live input of the meter and Earth as normal you can now do a continuity test on the fault at each socket ,go round each socket till you get the lowest reading that will be the area the fault is in , chances are its a trapped neutral , so look at the sockets with the lowest readings could be more than 1 i found 6 on one ring main last week ,im asuming its a N to E fault , it could be a faulty appliance thats plugged in , but doubt it will be a L to E fault or it would have blown a fuse / MCB , if all is well and no shorts then check the sockets by as has been mention splitting the ring and removing them , sockets can fail especially when humans and animals pee over them