John Rumm wrote in message ...
dorothy wrote:
RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected
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?)
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
o MCBs all OFF -- no L connection to house
o RCD-N-output disconnected -- no N connection to house
What type of CU is it? i.e. if its a split load unit, does it have a
dedicated N busbar for the RCD?
One RCD for the CU - so not a split-load.
RCD still trips instantly.
I found this after:
o RCD would not reset - all MCBs off
o RCD would not reset - all MCBs off & all appliances dbl-pole off / unplugged
o RCD would not reset - all MCBs off & most cables L+N disconnected
o RCD would not reset - all MCBs off & RCD N-output to N-bar disconnected
So I'm sort of running out of options.
I don't understand how it can trip with no L connected or N connected.
knackered RCD would be a prime suspect.
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.
The meter also powers a separate RCD for outside lights, skt, shed.
So I'm powering fridges & a few backup items off that for now.
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?
No - the secondary CU is connected to the Main Switch after the meter.
What happens if you turn off this secondary RCD, does the first one
still fail to reset?
Yes - indeed disconnecting it from the main switch had no effect.
Q: Does this sound like a faulty RCD - only 7-10yrs old?
Probbably. Could you swap over the RCDs to prove this?
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.
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