RCD just wont hold!

[datadiffusion]

Hi all,

Installed a new fuse board today, following a full inspection in which all was clear (no insulation problems, rings all OK, etc...). Its a 100ma / 30ma split load and there are no probs with the 100ma side of things. And just to be sure, the 30ma is wired in parallel not series - there is a communal main switch.

But, if you have ANY circuit switched on before the 30mA RCD is energised, it trips straight away, except for the single socket in the basement. The other circuits are 3 rings and a cooker.

But, if you reset whilst circuits are off, you can slowly turn on circuit by circuit and have had kettles boiling on 2 rings at once and the 11kw cooker on overdrive - so not a load / wiring problem. But plug my double insulated Henry into *any* (ring) circuit and it will instantly TRIP.

Have checked all the obvious stuff, its very odd. Any ideas would be appreciated though, thanks!

 

[andy]

each circuit couldbea leakin X amount. all together is enough to trip. and why do you have the cooker on the 30mA? it should be on 100

 

[datadiffusion]

Just what I was told to do I guess - not a problem to move as it a modular board. Actually, slight edit, I dont believe it was tripping when only the cooker was on.

Thing is I understand about lots of small leakages adding up, but it appears to be almost the reverse happening?

 

[andy]

Just what I was told to do I guess - not a problem to move as it a modular board. Actually, slight edit, I dont believe it was tripping when only the cooker was on.

Thing is I understand about lots of small leakages adding up, but it appears to be almost the reverse happening?

unplug everything. does it still trip?

yes - wiring problem

no - appliance problem

 

[datadiffusion]

Just what I was told to do I guess - not a problem to move as it a modular board. Actually, slight edit, I dont believe it was tripping when only the cooker was on.

Thing is I understand about lots of small leakages adding up, but it appears to be almost the reverse happening?

unplug everything. does it still trip?

yes - wiring problem

no - appliance problem

That would be the classic solution - but it doesnt appear to affect it. It is possible to have everything plugged in to all the circuits and not trip, until some random time, when it suddenly does. So, you switch the circuits off. Then you switch them on one by one - but the trip then blows instantly whichever one you choose. The only way to reset is by turning off all the circuits, turning the circuits back on one by one, and only then resetting the trip.

 

[supersparks]

or something even simpler.

the 30mA rcd is a duff

have had a wylex one recently

 

[theshogun]

If it is just the henry tripping the circuit check that the plug dose not have the neutral connected to the earth pin,have come across this with diy people connecting plugs before(the old saying well it worked before you rewired and put these fancy trips in).

 

[datadiffusion]

Not a bad idea, but the henry has a sealed plug. Very odd that it trips it instantly on any of the circuits though, when they dont seem to mind (otherwise) being loaded.

Maybe this time it is a faulty RCD rather than a silly mistake...!

Thanks for your help!

Edit: That said (about sealed plugs) I have found moulded IEC leads when PAT testing that have had no earth continuity and L-N reversed.

 

[Damocles]

It sounds as though you are saying it trips only when you plug in henry. dodgy henry.

It is possible for a RCD to hold on a level of leakage which would be too great to allow it to be reset.

 

[fireman22]

if it is tripping on all circuits is sounds as if a neutral is connected onto the wrong neutral bar, check each one in turn.

 

[datadiffusion]

Neutrals are all ok - have checked for *all* usual problems!

The more I think about it, the more it has to be a faulty RCD, as there are very few ways that a fault could be spread over 3 ring mains. Although, its an MK RCD, just wondering if N-E fault would not actually trip the device until a load was attempted to be drawn (the 'any circuit' causes trip wierdness would in fact make sense then)

And even then the circuits were clear when meggered - though of course the N-E fault could be in a plug.

It is possible for a RCD to hold on a level of leakage which would be too great to allow it to be reset.

This is also something to be considered!

thanks again - this is the first time I have ever had an installation baffle me in this way!

 

[HDRW]

The more I think about it, the more it has to be a faulty RCD, as there are very few ways that a fault could be spread over 3 ring mains. Although, its an MK RCD, just wondering if N-E fault would not actually trip the device until a load was attempted to be drawn (the 'any circuit' causes trip wierdness would in fact make sense then)

Yes indeed - think of what an RCD is doing - comparing the current in the L & N conductors, and tripping if there's an imbalance. If there's nothing switched on, the current in both will be zero, which is balanced!

As soon as you turn something on and some current flows down the L and back up the E (even if it's not a full short but a high resistance, so most goes up the N with only a little up the E) there will be an imbalance between L & N, and if it's enough 30mA or whatever), it'll cause the trip. I think I'd try an insulation test between N & E...

Cheers,

Howard

 

[SwindonSpark]

datadiffusion wrote:
following a full inspection in which all was clear (no insulation problems, rings all OK, etc...).

Did you do the inspection and test yourself or did you get a sparks to do it for you?

Do you have a schedule of test results to refer to regarding the insulation resistance test between each conductor?

 

[fireman22]

swap the rcd`s over if its a fault somehow on the circuits it will probably trip the 100mA rcd too, or the 30mA rcd may start to trip on the other side,maybe.????
if you have (prob not) another rcd you could swap it

 

[fireman22]

just remember i had afault like this on a TT supply, is yours? it turn out to be external problem and local elec board had to fix it loose connection if i remember on pole or junction i think