RCD protection to garage.

[Bobble23]

Hello. I'm afraid I've got a bit of a hole in my knowledge about RCD's so if anyone could reassure me/tell me I'm completely wrong about this I'd appreciate it.

Was called to a customer today to put an extra socket in his garage and found that it's fed by twin 1.5mm with no cpc, buried in conduit, from a 15A breaker on the house's RCD-protected DB. The garage is about 25m away so I think he just about gets away with the 1.5mm volt-drop wise, but obviously I need to sort out the lack of any earth in the garage.

I'm planning on just putting in an earth spike and an RCD garage unit in place of the two MCB's he has in the garage at the minute. My (possibly stupid) question is, would having a separate earthing arrangement at the garage when it's fed from an RCD-protected breaker in the house cause nuisance trips or anything in the house? There are no free non-RCD spaces in the house DB, though I suppose I could Henley block the supply if I had to.

Any other glaring holes in my plan would be gratefully received.

Thanks.

 

[Meggerman]

1. Why do you need to TT garage?
2. What you are proposing is putting two RCD's in series, in the house RCD 30ma?

 

[Bobble23]

1. Need to get some sort of earth into the garage - absolutely nothing there now and thought TT would be easier than running another cable from the house.
2. Yes, I suppose I am, and yes, the house is 30ma.

Thanks.

 

[davesparks]

Needs to TT the garage because there is no earth there at all, apparently.
Twin with no earth hasn't been used for many many years so I doubt the conduit will be in good enough condition to act as a cpc.

Personally I would be advising them to have a new submain run to the garage.

 

[Meggerman]

Sorry, I missed that, I see you said there is no CPC from house. Regarding RCD, you could just put Main Switch DB (Non RCD) in garage as the sub is protected. Would normally be better with RCD at Garage but probably better having that sub on RCD in this case as has no cpc.

 

[davesparks]

Better off replacing the submain though. That conduit is likely to be rusted away by now so not affording suitable protection to the cable.

 

[Bobble23]

Thanks both. And I think that's where I didn't understand how RCD's work. I thought that, as they measure leakage to earth, they'd need some sort of cpc in the circuit to work. I can see now that obviously that leak to earth can go through a washing machine or a dog or a person and doesn't have to be a wire. Presumably that means the proposed earth rod in the garage won't cause any problems as it just provides another potential way for a fault to leak.

 

[Bobble23]

Take your point about replacing the submain though.

 

[davesparks]

Forget the earth rod for a minute.

Is the submain and it's conduit in good condition and safe for continued use?
If the conduit is in good condition then use it as the CPC.
If the submain and conduit are not in good condition then you need to look at the options to replace it.

Based on experience of similar installations Ive developed a picture in my mind which is not very pretty.

 

[davesparks]

Are you an electrician?

I only ask because it seems highly unlikely that an electrician doesn't understand how an RCD works.

 

[GLENNSPARK]

Thanks both. And I think that's where I didn't understand how RCD's work. I thought that, as they measure leakage to earth, they'd need some sort of cpc in the circuit to work. I can see now that obviously that leak to earth can go through a washing machine or a dog or a person and doesn't have to be a wire. Presumably that means the proposed earth rod in the garage won't cause any problems as it just provides another potential way for a fault to leak.

what you have to remember here ...is that if you have 2 30mA RCDs in series...the one likely to let go under fault conditions first (assuming the fault occurs on one of the garage finals)..will be the RCD thats already being subjected to a certain amount of leakage....this is likely to be the RCD top end...

 

[Octopus]

I'd start by doing a Ze at the garage DB and see what the reading was like!

 

[Meggerman]

That would be a sub circuit Zs then.

 

[davesparks]

But there is apparently no cpc so it can't be Zs

 

[Meggerman]

But there is apparently no cpc so it can't be Zs

The conduit may be the cpc... or more like a corroded earth pipe by now