63ma rcd in consumer box trips - external 30ma doesnt RAIN!!

[greeny129]

Ok so i have a conundrum i'm hoping you can help with. My issue starts and finish's with heavy rain, sometimes when it rains my main rcd in consumer box trips (not all the time, just sometimes), I feel this is to do with the outdoor spur (maybe a nick in a cable - although it is armored...) as when i disconnected this i didn't see any issues (a few weeks loads of rain etc). I have now reconnected it and Kalamazoo rain tonight and the power has died.

So that's the background, here's a little bit more info - the outside unit is connected to a 25a, 30ma rcd (ip67 outdoor housing), i have replaced RCD last week so can exclude that as an issue, the earth is passed through, when the power trips only the rcd in the indoor consumer unit trips, not the outdoor one and this is the perplexing bit - as if it is the outdoor spur, why isn't the outdoor rcd tripping?.

My initial thoughts is that it is as the earth is passed through so detected at source first?, as such should i disconnect the earth pass through and create a seperate earth (unsure how to do this, but will google it!)?.

Any thoughts/suggestions on trouble shooting or other input greatly appreciated.

Thanks

 

[RF Lighting]

It means the fault is after the inside RCD and before the outside one.

An RCD will only detect a fault on it's outgoing (load) side.*







*in theory anyway although sometimes upstream faults have been known to cause strange problems.

 

[greeny129]

Thats why im confused - as when outside disconneted (pre outdoor rcd) all was ok - even during heavier rain than i have now, but now reconnected and with new rcd i have the issue again - trouble shooting wise whats best route?

 

[JohnW2]

It means the fault is after the inside RCD and before the outside one.

It might mean that but, as I'm sure you would agree, even if the fault is downstream of both RCDs (which, despite the title of this thread, are probably both 30mA ones), it's essentially pot luck as to which one (or both) will operate.

Kind Regards, John

 

[RF Lighting]

Ah hadn't really thought of that.

It's possible the upstream device already has leakage on it from other circuits making its tripping threshold lower, maybe it is more sensitive than the downstream device, maybe the new RCD simply doesn't work, or like you say it's just pot luck as to which device trips.

 

[JohnW2]

Ah hadn't really thought of that. ... It's possible the upstream device already has leakage on it from other circuits making its tripping threshold lower, maybe it is more sensitive than the downstream device, maybe the new RCD simply doesn't work, or like you say it's just pot luck as to which device trips.

Indeed. Any of those possibilities - or, indeed, your original suggestion (although that seems somewhat less likely, given that the tripping goes away if the outdoor circuit, including the downstream RCD, is disconnected).

Kind Regards, John

 

[greeny129]

It means the fault is after the inside RCD and before the outside one.

It might mean that but, as I'm sure you would agree, even if the fault is downstream of both RCDs (which, despite the title of this thread, are probably both 30mA ones), it's essentially pot luck as to which one (or both) will operate.

Kind Regards, John

quite right - consumer unit is 30ma trip, as is the outside one - think i'll down rate the outside one at the weekend and see if this re-occurs at the next heavy rains - will update in a few weeks to confirm if resolved.

Also re last night once I had tripped the outside one manually the power remained on despite the rain getting heavier as such confident this is an outside circuit problem.

Thanks for the thoughts, greatly appreciated

 

[greeny129]

actually just thinking about this.....
-can i use a 16 Amp 10mA RCD on a 32a circuit?
-If i do down rate the outside i expect this would pick the drops up quicker than the 30ma one as has lower tolerances, is this a fair assumption?

Thanks

 

[Taylortwocities]

NO, it will not help!

If you have a 10mA RCD is series with a 30mA RCD the lower rated RCD may still not trip first, if the nature of the leakage current is greater than 30mA. The first RCD to trip will be the one that has a faster trip characteristic, it is a function of time, not current!

We use time delayed RCDs to solve these issues, but only in certain configurations.

This problem needs examination using and RCD tester (to check the RCDs are working in spec) and an insulation resistance tester to determine the reason for the fault. You will not sort it by trial and waiting for the next rain storm.

 

[greeny129]

Thanks - just saved me 20quid (although now wondering where the point is having an rcd on an external socket or unit...) - i'll check all the exterior cables out on the next sunny day as is definitely the external circuit (still raining heavy all night and very damp/drissly this morning without issue).

Will provide a heads up when done

 

[Taylortwocities]

(although now wondering where the point is having an rcd on an external socket or unit...)

There isn't any point if the circuit is already RCD protected!

Do i understand that you have a single RCD in your main consumer unit that serves all circuits? That is not a great design, if so.

 

[greeny129]

(although now wondering where the point is having an rcd on an external socket or unit...)

There isn't any point if the circuit is already RCD protected!

Do i understand that you have a single RCD in your main consumer unit that serves all circuits? That is not a great design, if so.

It protects the sockets only (down up and outside), I was surprised there wasn't a separate one for outside also as agree that this is a poor thought process on the part of whoever set this up!

 

[JohnW2]

quite right - consumer unit is 30ma trip, as is the outside one - think i'll down rate the outside one at the weekend and see if this re-occurs at the next heavy rains - will update in a few weeks to confirm if resolved.

As TTC has said, that is not the answer. Quite apart from the fact that it might not work (the 30mA one may still trip before a 10mA one), it's the totally wrong approach. If, as seems probable, you have a fault (rain-related or whatever) in the outdoor circuit, you need to investigate, identify and rectify that fault - not just make changes in an attempt to reduce the inconvenience caused by an unrectified fault being there!

Kind Regards, John

 

[greeny129]

quite right - consumer unit is 30ma trip, as is the outside one - think i'll down rate the outside one at the weekend and see if this re-occurs at the next heavy rains - will update in a few weeks to confirm if resolved.

As TTC has said, that is not the answer. Quite apart from the fact that it might not work (the 30mA one may still trip before a 10mA one), it's the totally wrong approach. If, as seems probable, you have a fault (rain-related or whatever) in the outdoor circuit, you need to investigate, identify and rectify that fault - not just make changes in an attempt to reduce the inconvenience caused by an unrectified fault being there!

Kind Regards, John

see response @ 08:30

 

[greeny129]

Really appreciate all the support with this guys - I'm open to all options you can give - and knowledge you may have.

Thanks