RCD spurs that trip faster than the MCB RCD

[opps]

Hi all,

I had to swing by a mate's mum's house last night to sort out a problem with the CU RCD tripping.

The house has a garage on the side of the property. It soon became obvious that the garage spurs off the ground floor ring.

The garage only has one double gang socket (with the florescent light spurring off it).

I traced the fault to an old freezer in the garage.

I would like to minimise the risk of anything in the garage tripping the ground floor ring in the future.

Is it possible to purchase a fused RCD spur that is able to trip faster than the RCD in the CU?

 

[ericmark]

I have a 10 mA RCD socket, but time is the same. Also pressing the test button would trip the 100 mA one it was fed from.

 

[JohnW2]

I would like to minimise the risk of anything in the garage tripping the ground floor ring in the future.

Fair enough, but isn't that likely to be a 'once in a blue moon' occurrence?

Is it possible to purchase a fused RCD spur that is able to trip faster than the RCD in the CU?

As has been implied, you might be able to get 10 mA ones, but even one of them would not be guaranteed to trip before a 30 mA one upstream, particularly if a fault results in a very high leakage current.

Kind Regards, John

 

[opps]

Thanks guys, yeah, I guess it is unlikely to happen again. I was just trying to mitigate the possibility.

 

[Harry Bloomfield]

The only way, it to have it on it's own circuit and own rcd at the consumer unit.

 

[opps]

The only way, it to have it on it's own circuit and own rcd at the consumer unit.

Agreed, but I don't think that they want me rip up floors to make that happen.

 

[JohnW2]

The only way, it to have it on it's own circuit and own rcd at the consumer unit.

Indeed (probably an RCBO) - but, as the OP seems to agree, I personally would not regard the hassle of doing that to be justified, particularly to 'guard against' the slight inconvenience that might otherwise arise 'once in a blue moon'.

Kind Regards, John

 

[SFK]

You could fit a mains failure alarm in the garage like this (not that cheap though)

Mains Power Failure Alarm 3

Be informed of a Power cut with an External Siren being activated, ideal when you are working outside or in different buildings

www.ultrasecuredirect.com

 

[JohnW2]

You could fit a mains failure alarm in the garage like this (not that cheap though)

I don't think that would help the OP - what he is/was wanting was a means of preventing a fault in the garage tripping the house RCD.

Kind Regards, John

 

[SFK]

John, good point - I was over thinking them loosing the food in the fridge.

 

[SUNRAY]

The only way I've got anywhere near this has been to use a 3phase RCD and run through twice which makes it 7.5mA however that will only be of any real benefit if it's a low current leakage as the tripping times will be similar as John has already mentioned.

 

[JohnW2]

The only way I've got anywhere near this has been to use a 3phase RCD and run through twice which makes it 7.5mA however that will only be of any real benefit if it's a low current leakage as the tripping times will be similar as John has already mentioned.

Quite. If, as is quite likely with L-E faults, there is a fault current (hence 'residual current'/'imbalance') of 'very many amps' (rather than a relatively 'tiny' current flowing through human beings) that is such an 'enormous' residual current in relation to even a 0.030 A (i..e. 30 mA) RCD, that I think all bets are off as to whether it is likely to trip more slowly than a 0.01 A (or even your 0.0075 A) one.

It's much the same as with over-current devices when there are very large fault currents - I've certainly seen a 3A downstream fuse survive when an upstream 32A MCB has operated.

Kind Regards, John

 

[ericmark]

I have 14 RCBO's and only trips have been genuine faults, but last house 2 RCD's and yes tripped with no known fault, I assume a spike.

Going all RCBO is reasonable, but after that it's a lot of work for little gain.

 

[JohnW2]

I have 14 RCBO's and only trips have been genuine faults, but last house 2 RCD's and yes tripped with no known fault, I assume a spike.

As often discussed, experiences clearly vary.

I have, and have had for over 30 years, lived with about 10 RCDs (plus a few RCBOs), but in all those years have had no, or virtually no, trips other than due to genuine faults (or, more commonly, my allowing N and E to touch when working on a SP-isolated circuit ). Having had 'all RCBOs' would therefore not have benefitted me at all in terms of trips - and, in terms of fault-finding, I don't recall it ever having required appreciable effort/time on my part to determine which circuit had been responsible for a genuine RCD trip (the culprit circuit commonly bring pretty obvious).

The bottom line of all that is that I can't see that having had 'all RCBOs' would have significantly benefitted me personally in any way, although it would have represented a fairly considerable financial cost, particularly in the past.

However, as said, experiences of others will undoubtedly differ from this.

Kind Regards, John

 

[ericmark]

Out of interest I put my clamp-on around my incoming tails, and as a total it was over 20 mA over the 14 circuits, the DC was 9 mA also over what one wants, to measure each one I would need to remove cover, and not really worried just interested, but we are allowed 9 mA per 30 mA RCD/RCBO and clearly my home now would exceed the limit if only using two RCD/RCBO on the board.

Only measured as got a new clamp on and was trying it out, the DC was hard to zero, so could be well out, but AC reasonably steady.

In this house only genuine faults water into socket when roof leaked have tripped the RCBO's, but when it did, I was able to simply leave that circuit turned off, with no worry about neutral to earth leakage causing other circuits to trip.