Achieving selectivity from 30mA?

[HappyHippyDad]

My customer has some garden pole lights up his driveway, they occasionally trip one of the RCD's in his dual RCD baord (once every 1-2 years). I will fix the problem but they are in such a poor position that I feel it may happen again in a year or 2.

They are simply plugged in to a socket in the garage. I would like to fit a 10mA or 20mA RCD socket or similar plug. Is there such a thing?

I have considered fitting an RCBO in the main CU but that would still disconnect the garage which houses his freezers as the pole lights are supplied from the garage. At present he knows when it trips as half the house goes. If it is on it's own RCBO he may not notice it has tripped.

 

[SJD]

You can get 10mA RCD sockets:

http://www.powerbreaker.co.uk/fluidcms/files/files/PB-Brochure-Oct%2016_Part16.pdf

 

[pc1966]

Unless the upstream one is a delay (S-type) you are not guaranteed selectivity, but if lucky those powerbreaker ones might trip on a slight fault quickly enough.

Could you safely put the garage feed on a MCB instead and then use some RCBO in the garage CU so the freezer is not subject to a common feed trip yet all final circuits are protected?

I guess that comes down to the type of cable and route that feeds the garage.

 

[HappyHippyDad]

Unless the upstream one is a delay (S-type) you are not guaranteed selectivity, but if lucky those powerbreaker ones might trip on a slight fault quickly enough.

Could you safely put the garage feed on a MCB instead and then use some RCBO in the garage CU so the freezer is not subject to a common feed trip yet all final circuits are protected?

I guess that comes down to the type of cable and route that feeds the garage.

I new someone would say that and I should have preempted it in the OP .
Its 6mm twin and earth coming from the main CU and then it just disappears! I cant see any junction box on the outside and no SWA so I must assume it needs 30mA protection.
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You can get 10mA RCD sockets:

http://www.powerbreaker.co.uk/fluidcms/files/files/PB-Brochure-Oct%2016_Part16.pdf

Only 4 to choose from!
Thanks SJD, I thought they existed but struggles to find them. It may or may not offer some discrimination in this case but it's worth a go.

 

[Julie.]

Basically No!

It can be selective, but won't discriminate.

If the "Fault" is a gradually deteriorating insulation, 1mA one day, the 2mA … etc, then yes the downstream 10mA one will go first.

However this is a very rare fault type - mostly either a proper short (nail etc) or due to moisture ingress, a flashover occurs - these both result in many amps flowing - even if only for a short duration in the case of water.

When the current is a few times the setting or rating (10mA/30mA) then the RCD will trip as soon as it can, and whilst the 10mA MAY go first (not always) the other RCD is on its way so both operate.

Two instantaneous type devices will not work in series unless they are of considerably different ratings - 100A fuse above a 10A fuse for example - where the lower one chops off the current before it gets to a high enough value to operate the bigger one.

There must be a time delay on the upstream one in order to always prevent both operating together.

 

[davesparks]

You won't achieve selectivity with non time delayed RCDs, it will still be apparently random as to whether a 30mA or 10mA goes first depending on the type of fault.

Do you have any idea what the actual fault is that is causing the tripping?

Is there any way to get a separate circuit there to feed the outside lights?

Or is there a way to extend a different existing power circuit to the garage to feed the freezers?
I'm thinking that if the freezers could be put on to a different existing circuit then the garage power cir uit could be put onto an RCBO without fear of defrosting if it trips.
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If the "Fault" is a gradually deteriorating insulation, 1mA one day, the 2mA … etc, then yes the downstream 10mA one will go first.

Not necessarily, the upstream 30mA RCD is likely to have some normal earth leakage from other circuits and so may be biased to trip first in this type of fault.

 

[pc1966]

How much current for the lights? Could you put in an isolating transformer and RCD the secondary?

Basically you earth one side of the secondary as the new 'neutral', and it goes with the other secondary 'live' through the RCD and out to the lights.
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Or even go 12V bulbs and SELV setup using existing cables/fittings?

 

[Julie.]

[automerge]1588683795[/automerge]


Not necessarily, the upstream 30mA RCD is likely to have some normal earth leakage from other circuits and so may be biased to trip first in this type of fault.

Agree, lots of ifs buts and maybes, however that would need around 20mA of leakage within half the electrical system, so yes, possible but unlikely.
SELV/PELV or isolating transformer would be best.

 

[HappyHippyDad]

You won't achieve selectivity with non time delayed RCDs, it will still be apparently random as to whether a 30mA or 10mA goes first depending on the type of fault.

Do you have any idea what the actual fault is that is causing the tripping?

Is there any way to get a separate circuit there to feed the outside lights?

Or is there a way to extend a different existing power circuit to the garage to feed the freezers?
I'm thinking that if the freezers could be put on to a different existing circuit then the garage power cir uit could be put onto an RCBO without fear of defrosting if it trips.
[automerge]1588683795[/automerge]


Not necessarily, the upstream 30mA RCD is likely to have some normal earth leakage from other circuits and so may be biased to trip first in this type of fault.

If the fault is the same as the time before and the time before that it will be ants coming up through the bottom of the light. The last time I put all the connections into an IP box filled with magic gel so i'm unsure how they will have got in this time but will update when i go there.

I have suggested a separate feed to the lights but that would be a pretty big cost due to the layout of everything. It would be the same for getting a different power source to the freezers.

I did consider simply putting the garage circuit (this houses the freezers and suspect lights) on to an RCBO. The owner does check the garage 'most' days but it would be sods law it trips on the day he doesn't check.

I didn't think the 10mA socket would offer an acceptable amount of discrimination but I wasn't sure. I have explained this to the customer but they prefer to try this first. If this doesn't work then they want the RCBO option.

I realise I am focusing more on the tripping options rather than the fault itself but this fault originally happened every few months before I became the customers electrician. I then fixed it (cleared ants and trying to seal) and it was OK for a year or 2. Next time (approx 3-4 years ago) I used Ip boxes filled with magic gel and it appears they have got in again, hence having a think about limiting disruption 'when' it happens again.

 

[davesparks]

If the fault is the same as the time before and the time before that it will be ants coming up through the bottom of the light. The last time I put all the connections into an IP box filled with magic gel so i'm unsure how they will have got in this time but will update when i go there.

I'm not sure that magic gel will stop things like ants from getting where they want to go, I've used it but never been 100% convinced that it's as good as people seem to think it is.

I'd consider joint boxes that they can't get in to, and/or some good old fashioned R391 compound (aka dog turd)

Of course it could be that another, completely different, fault has occurred.

It sounds like these outside lights have been there for a few years now so it could be that they have deteriorated to the point they let water in. Outside equipment doesn't last forever, especially in the most exposed locations.
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How much current for the lights? Could you put in an isolating transformer and RCD the secondary?

Basically you earth one side of the secondary as the new 'neutral', and it goes with the other secondary 'live' through the RCD and out to the lights.
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Or even go 12V bulbs and SELV setup using existing cables/fittings?

Volt drop will be a killer at ELV, especially 12V.

 

[pc1966]

Volt drop will be a killer at ELV, especially 12V.

Yes, but it depends on the set-up. If they can do with only a couple of 3W LED bulbs then it might be OK depending on current cable and length OK. It is one possible route to look at as it might be significantly cheaper than an isolation transformer and RCD.