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New supply to outbuilding

I have a 30ma RCD protected 2 way CU providing a new supply to an outbuilding in 6mm 3 core SWA. At the outbuilding there is a 6 way CU with another RCD. The SWA has been terminated using the correct glands with earth continuity maintained. The 3rd core has been used as an earth too. If I remove the earth at the CU end in the outbuilding both RCD's hold in, as soon as the earth is connected both RCD's trip. Can anyone help? No circuits are connected at this point so there is nothing causing this other than the supply cable from the house to the outbuilding. Does anyone have a clue, is it the RCD's causing discrimination, should I replace the house side of the supply with an isolator? Suggestions would be appreciated.

To all, thanks for the replies

Previously I error I suggested changing the house side to an isolator, when it's been my intention to remove the RCD at the outbuilding and replace with an isolator then install an RCBO for the sockets as there is an outside socket requiring protection.
How would removing the CPC from the house side of the distribution cable give a fault path, once an earth rod is placed at the outbuilding end, as surely earth continuity would no longer exist?

I will carry out tests and add the results in the next day or two but the suggestion of a neutral to earth fault seems most likely. Sadly the owner had already buried a section of SWA leaving me no choice but to join and take it into the house with another join into 6mm T&E to the 2 way CU.

5 Answers from MyBuilder Electricians

Best Answer

Firstly the use of two RCD's in series is very poor electrical design! From what you describe, itspounds like there is a neutral to earth fault on the SWA distribution cable. What are the insulation resistance values between L-N, L-E and N-E. If you have a PME (TNC-S) su[pply at the house end then you can not export the earth! What you will need to do for a PME supply is leave the CPC (earth) off at the house end and drive an earth rod into the ground at the shed end. Carry out an earth rod test to check the value of RA and then connect the earth at the shed consumer unit to the earth rod. As all this comes under part P of the building regulations and needs top be notified to local building control (£5000 fine if not done), it maybe easier to hire a competent electrician to do the work.


Answered 19th May 2015

You have 2 rcd causing a inbalance if you replace house side rcd with a isolater the armoured cable will be unprotected,

I would Replace rcd in shed to a isolater instead


Answered 19th May 2015

The rcd protects you in a earth to live fault, I would disconnect the cable and test it between the wires to ensure there is no continuity between the neutral or live to the earth wires, you don't really need a rcd in the shed as you have one in the consumer unit and any fault to earth should trip it if the sizing and length of cable are correct, however should you bypass the house rcd you would have no protection of the cable and this would not meet the regulations, it could be a faulty rcd but check your wiring is correct 1st.


Answered 19th May 2015

remove the RCD from the outbuilding and replace with a new main switch, if there's an RCD in the cu in the house then that is already protecting any circuit it is feeding there for no need for another RCD on the circuit, so yes it is causing discrimination.


Answered 19th May 2015

Although it sounds like a N-E fault on the SWA this would still be present if removing the earth at the DB in the out house so tripping would still occur. You have not given enough info. Also you should not have more than one RCD on the supply line. That is not actually causing the problem as you are describing it but needs rectifying.


Answered 22nd May 2015

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