Up front RCD tripping with sub main connected.

[Toblerone]

Hello all

I have recently installed a sub main in 4mm 3-core SWA, approx length 45 metres, to feed a small garden office.

Final circuits in the cabin are protected by 30mA RCBOs and the supply is fed from a 32A MCB on the non-RCD side of the DB in the house. As the installation is TT I put in another earth rod at the cabin and go satisfactory readings (below 40ohms). There is a 100mA up front RCD at the house (not S-type).

I tested everything before leaving (still need to go back to finish the job but it’s in Sussex and I am London so can’t go back until January). All was fine in the cabin and got good readings on the supply cable for IR between all conductors, no continuity between conductors, good R1+R2. Energised everything, lights on, power on - all ok.

The next day I get a call saying the power has tripped out in the house at the incoming RCD. My first thought was a N-E fault on the sub-main but was confused as all had tested ok before I left. The main switch in the cabin was off, and after a few more days of nuisance tripping multiple times a day, I explained how to disconnect N and E from the DB (bloke is competent, just not a spark) and this seems to have cleared the fault for now.

It would be good to hear if anyone can elucidate further or has had a similar issue before? Does the fact the up front RCD is not time delayed make any difference?

Many thanks.

 

[James]

If there is an existing fault in the house, adding the second earth rod may be enough to increase the fault current and trip the rcd.

 

[pc1966]

As @James said It sounds like there is something up with the house.

However, not having the up-front RCD with any delay means if there is a fault that could otherwise be cleared by a down-stream RCD it will probably go as well (on TN the delay potentially also allows clearing hard faults on a MCB as well, if the main RCD was for fire reasons, etc).

Try clamping the tails with a meter that can resolve down to a few mA to see what the overall leakage is, also ramp-test the house RCDs just to check they are all behaving as they should. It could be a fault that a 30mA should clear but it is stuck and it is getting enough now for the incomer to bother.

 

[Toblerone]

Appreciate the responses. Had not considered the potentially lower impedance introduced by the second rod.

Will investigate further with my clamp meter etc. when next on site.

 

[yorkieone]

Hello all

I have recently installed a sub main in 4mm 3-core SWA, approx length 45 metres, to feed a small garden office.

Final circuits in the cabin are protected by 30mA RCBOs and the supply is fed from a 32A MCB on the non-RCD side of the DB in the house. As the installation is TT I put in another earth rod at the cabin and go satisfactory readings (below 40ohms). There is a 100mA up front RCD at the house (not S-type).

I tested everything before leaving (still need to go back to finish the job but it’s in Sussex and I am London so can’t go back until January). All was fine in the cabin and got good readings on the supply cable for IR between all conductors, no continuity between conductors, good R1+R2. Energised everything, lights on, power on - all ok.

The next day I get a call saying the power has tripped out in the house at the incoming RCD. My first thought was a N-E fault on the sub-main but was confused as all had tested ok before I left. The main switch in the cabin was off, and after a few more days of nuisance tripping multiple times a day, I explained how to disconnect N and E from the DB (bloke is competent, just not a spark) and this seems to have cleared the fault for now.

It would be good to hear if anyone can elucidate further or has had a similar issue before? Does the fact the up front RCD is not time delayed make any difference?

Many thanks.

it might be seeing an earth fault might help to add a second earth stake to shed

 

[Lucien Nunes]

Improving the Ra won't clear the fault, but it does appear to have helped reveal an existing one. This was a month ago, I wonder if @Toblerone has any further findings to report back?

 

[Toblerone]

Hi guys,

I returned a couple of weeks ago and began where I had left off. I had asked the customer on the phone if the DB in the new garden office was turned off at the Main Switch and he had told me it was. I found this was not the case, and upon further testing discovered a N-E fault on the socket radial between the last two outlets. This must have been tracking back through the single-pole RCBO (I had put these in before realising double pole was required on TT) and closed Main Switch and tripping the up front RCD in the house.

He had run the cables for this circuit himself and it turned out the carpenter had caught the cable when nailing the cladding onto the outside wall (absolving me of any blame). We ran in a new leg, changed the RCBOs to double pole and fitted a 100mA S-Type in the house. Problem solved and no further issues reported.

I really appreciate all the responses and advice on here, but it turned out in the end to be a fairly simple fault to find and rectify. A good learning experience nonetheless!

 

[buzzlightyear]

Maybe wonder full pc 1966 or LN with is wonderfull diagrams showing apprentice how it should be done regarding TT to the outer building regarding lowing a ra.