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

RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected
o MCBs all OFF -- no L connection to house
o RCD-N-output disconnected -- no N connection to house

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.

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.

Q: Does this sound like a faulty RCD - only 7-10yrs old?

No call-out available tonight :-( *blink* *blink*
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John Rumm
 
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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?)

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?

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.

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?

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

Q: Does this sound like a faulty RCD - only 7-10yrs old?


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

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
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| John Rumm - john(at)internode(dot)co(dot)uk |
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Christian McArdle
 
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RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected

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.


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Mike Clarke
 
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In article , dorothy
wrote:

RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected o MCBs all


[Snip]

Q: Does this sound like a faulty RCD - only 7-10yrs old?


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.

--
Mike Clarke


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tony sayer
 
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In article , dorothy
writes
RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected


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


--
Tony Sayer

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dorothy
 
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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.
  #8   Report Post  
N. Thornton
 
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"Christian McArdle" wrote in message . net...
RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected


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?


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
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dorothy
 
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tony sayer wrote in message ...
In article , dorothy
writes
RCD is MEM 100A 30ma RCD - *with 3rd earth tail* connected


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


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).
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