RCD discrimination - 10mA?

[nothanks]

I've had to connect an external pump controller (for a well) to a house circuit that's on a 30mA RCD. I'd prefer to have some discrimination but don't want a delayed RCD on the house circuit, any ideas? I wondered whether there is a fast-acting 10mA RCD available that I could use for the controller but I suspect the 30mA RCD would probably trip as well. If not, and nobody can suggest another solution, I'll leave it as it is, or (possibly) use an isolating transformer.

[ARW]

I agree both or either would trip. You are probably not allowed a time delay RCD on the house.

New dedicated circuit with it's own RCD?

[nothanks]

A new circuit would be the "correct" answer but it's not really viable - a large house with awkward runs and the need to drop down a plastered wall. Having just looked at the price of 1kVA isolating transformers I don't think they will be an option either.

[tabbypurr]

You could just live with them being on the same RCD. Add an isolator if concerned in case the outdoor wiring goes wet, so you can continue using the indoor before solving it.

NT

[nothanks]

Yes, that's the fallback plan

[Harry Bloomfield]

[e-mail] presented the following explanation :

It would need to isolate both poles, a double pole isolator.

[John Rumm]

If you are cascading RCDs then the only reliable way to ensure discrimination is with a time delay type upstream.

One way to avoid that would be to split the supply before the upstream RCD and install a separate CU, and feed the pump controller from a circuit on that.

Indeed that is the problem - stick 50mA of leakage on the end of the circuit and either one or both would trip. Most will react within two mains cycles given enough trip current - so its harder to make a "fast" one.

Here I split the incoming supply after a stand-alone main switch, and have two CUs - one for the house, and one for anything outside. That way regardless of what happens on on the outside circuits, problems can't com back to affect the house CU.

[nothanks]

I've done that (with a split CU) at the other end of the house, which feeds: the workshop, sheds, kitchen, car charger, utility area and some other stuff. From there is a sub-main to a central CU with radials, rings and light circuits for the main part of the house; the well is connected, via a switched FCU, to one of these radials. It would be a pain to get a dedicated circuit to where the SWA starts, whereas now it's just a cable through the wall from the FCU to an adaptable box. I think I'll have to live with it being on the one RCD for now (I may change the MCB to be an RCBO to reduce the impact of any trip) and when/if I next need to get the floorboards up I'll think about a new circuit.

Now to confess my sins: as a short term solution I've exported the earth about 45m to the well pump controller - an earth rod will be my next purchase from SFix!

>

[John Rumm]

I take it the house CU is all RCD in one way or another?

Any chance of introducing a non RCD split that you could then add a RCBO to?

(I did that for a kitchen circuit on my 16th edition style TT board, when I was getting nuisance trips on the 30mA RCD side. Moved one circuit to the 100mA time delayed side, and put in a single module RCBO with 30mA trip just for the kitchen)

What earthing system is in use in the house currently?

[nothanks]

It's on a journey in that direction. It will be a mix of RCDs and RCBOs, plus a couple of non-RCD-protected buried SWA circuits to sheds with their own CUs.

That would still need me to run a dedicated cable - it may be the eventual solution but it's not viable at the moment.

TN-C-S

[John Rumm]

So non RCD head end of the circuit, exported earth just for the sub main, and isolated TT earth at the pump end seems like the ultimate goal. For the mo you will just have to risk the nuisance trip and try and make sure stuff stays dry at the pump end.

[nothanks]

Yes, we're in violent agreement ;-) In practice, the pump supply may be used for other things in the future, such as decorative lights, so I would either feed it from an RCBO at the head end or add an RCD at the load end of the SWA (if I can find a suitable weatherproof and lockable box at a sensible price).