RCD - all MCBs off (no L) + no N-output from RCD connected (no N) = still trips instantly

Not sure I follow that bit... what do you mean 3rd earth tail connected? (we are talking about an RCD and not a ELCB I take it?)

What type of CU is it? i.e. if its a split load unit, does it have a dedicated N busbar for the RCD?

knackered RCD would be a prime suspect.

Are you sure there is no possibility of for example one of the circuits from the RCD being connected back to a N wire that returns to the main CU?

What happens if you turn off this secondary RCD, does the first one still fail to reset?

Probbably. Could you swap over the RCDs to prove this?

Try disconnecting the earth flying lead. However, I suspect that your faulty RCD premise may be correct, unless there is a serious wiring fault with the house. But this worked before, didn't it?

If disconnecting the lead works, then make sure the RCD is tested with an RCD tester at your earliest opportunity. This does not just mean pressing the 'T' button, although you should do that as well.

Christian.

Hi,

Try disconnecting live from the RCD too, even a spiders web can make them trip so check for these around the terminals as well.

Also try it with the earth tail connected or disconnected, maybe it's a safety feature for earth rod/TT installations.

cheers, Pete.

Is it definitely a (current operated) RCD, or perhaps an old style voltage operated ELCB (earth leakage circuit breaker)? If it's a voltage operated ELCB it would trip if the earth voltage rose above a certain level.

In article , dorothy writes

Pardon me asking but to *what* is the Earth tail connected to on an RCD?...

It's an MEM RCD - they used a 3rd tail, and was just 4yrs old. o I think the 3rd tail ones are electronic? o Most others are usual L+N only, and more electromechanical

One RCD for the CU - so not a split-load.

MEM technical support this morning agreed. NICEIC this afternoon who came in to check things it is dud too, he's seen the MEM 3-wire ones fail like this before.

MEM RCD tech support: o If the RCD with no L & N load (just E connected) still will not reset o Then the fault is probably with the RCD internally o Confirmation is by removing the E connection allowing trip to reset o Removing the E connection turns off the RCD component, making it an isolator

Sure enough, removing the E connection stopped it tripping. I guess their duty cycles are quite small - 100-250 perhaps.

No - the secondary CU is connected to the Main Switch after the meter.

Yes - indeed disconnecting it from the main switch had no effect.

Went a bit further: o MK 63A, 80A or 100A RCD cost too much o Spare Newlec 40A 30ma too weedy, 5am emergency light failed

So picked up a new MK Split-Load CU, 100A-Incomer, 63A-30ma-RCD. o Just £47+VAT from Newey & Eyre (on special, 80A is >£97+VAT) o Non-RCD - Cooker, Lights Up/Down/Kitchen, Kitchen Power/Heating o On-RCD -- Immersion (I know), Fridge/Freezer, Alarm, Dryer

Routed cables neatly, oversleeved older cables in new PVC.

All working fine. o Brought back each circuit one at a time - testing as we went o New CU was a bit smaller, so eased wiring

Circuits to add back to on-RCD: o Living-room ring - 24" of butyl to first point on that one

---- bigger than 4mm^2, 6-10mm^2! o Upstairs ring - all new PVC T&E ring, one 4mm^2 other 2.5mm^2

---- I suspect 4mm^2 radial extended into a ring by 2.5mm^2 (perhaps wrongly)

---- to disconnect - 8ft butyl run in wall separating two bedrooms

The new MK CUs are a bit flimsy compared to the old Sentry units, they were much thicker plastic - however it all goes together ok. Still, I like the MK plug screw fasteners.

All back to normal. I want an RCBO on the kitchen re wet hands & disabled person, worth the downside risk re cooker/microwave/fan/washer heating elements. MK seem to have brought back their 1-mod-wide RCBOs.

Icing on the cake would be a 10ma RCD in 1-module RCBO format, that is needed for a new radial for medical equipment vs an inline version.

Thanks for the reply and help.

that doesnt rule out all sorts of unsavouries though... and if N connection is connected to E... I wouldnt be surprised at anything much. Its remarkable what can actually work!

NT

I found it interesting too.

In answering, some general notes: o Supply is not TT - 2-cable incomer, L+N, u/ground, paper/tar/lead o Worked fine for years - minor nuisance tripping 1 month ago

I notice MEM do 3-wire RCDs (mine), Merlin Gerin do 3-wire RCBOs.

Supposedly: o 3-wire are electronic, 2-wire more electromechanical o 3-wire detect other faults by linking to Earth

That Earth runs 12" to the incoming supply-fuse 100A-block: o Earth via incomer armour (TN-S), or

---- which is earthed at the substation, like incomer neutral o Earth via incomer neutral (TN-C-S)

---- which is of course the same neutral the RCD sees

I'm not sure which I have: o There is an earthing block on the side of the supply-fuse block

---- that may internally be connected to the Neutral o From that earthing block an earth runs down to the incoming armour

---- that may simply be earth cross-bonding, usual earthing clamp

Better tell Bush the tar exuding will not suit USA refineries.

So replacing the RCD (via split-load CU, cheaper than an RCD) worked. Odd failure of an RCD - I thought they just stuck on (not ideal).

Check your earth-neutral voltage is not high -- some RCD's with an earth tail check for live-neutral reversal by monitoring the earth-neutral voltage and trip if it gets too high. The Merlin Gerin ones (MK used to rebadge them some years back) do this, but I wasn't aware MEM's do.

Q: How do I test Earth-Neutral voltage?

The replacement RCD is 2-wire, so doesn't monitor E-N voltage.

Many thanks for all the replies.

Another circuit up today, GET-connector to get it to reach the CU. No elec factors do the TLC-one, oblong with cable-strain relief.

Last circuit & removal of the a long suspect 8ft butyl tomorrow.

Hi,

Should have a test button on it, which simulates the effect of an earth leak.

They work by passing L and N through two opposing coils, so any current imbalance creates a magnetic field and it trips.

I'd have thought the test button just allows 30mA through one coil but not the other, eg using a resistor between L on the load side and N on the supply side.

cheers, Pete.