RCD location for TTd outbuilding

[JohnW2]

The only reason I can see where the RCBO needs fitting in the out building is to comply with 314.1 so that lights would not be lost with a fault with power but this is hardly going to be a problem with a green house.

As I said, "just a couple of sockets". No lights in the greenhouse (unless one plugs one into one of the sockets ).

Kind Regards, John

 

[JohnW2]

Bernard-like high neutral potential in the house,

I agree the current from neutral into a ground rod from a Neutral - Earth fault will be at most an amp or two, but if the "rod" is very low impedance it is effectively a path in parallel with the network neutral to the sub-station.

It's certainly 'a rod', and I doubt very much whether it's more than a single 4' job, so I don't think there is any risk of a very low impedance.

I would opt for a two pole RCD in the TT zone for that reason.

I accept that there's a small case for a double-pole device (and, as you will have seen, I'm going to check what we are actually dealing with). However, given that you are definitely someone who understands the underlying principles, why do you say "...in the TT zone"? What is the difference between inserting a two-pole RCD in the L&N current paths in one location and inserting it into the same current paths a number (even a lot) of metres away?

Kind Regards, John

 

[JohnW2]

Just having a head scratch here. Its an RCBO, correct.?

That's what I was told - but to be honest I didn't look. I will make a phone call and interrogate him!

OK, I think that this has got a lot simpler. Albeit I've only been able to speak to his t'other half, it's pretty clear that this "RCBO" is no RCBO! It was originally described to me with words something like "a thing which is an RCD and a breaker", and I jumped to the (incorrect) conclusion that this meant it was an RCBO. My apologies. It is now clear that this 'thing' is actually a mini-CU which contains a (presumably 2-pole) RCD and a B20.

So, given that we are dealing with a double pole device (which doesn't have/require a functional earth), does anyone (other than ricicle) think that the electrician was correct in 'condemning' the arrangement as unsafe and totally unacceptable?

Kind Regards, John

 

[ericmark]

I would not consider fitting the RCD in the green house with just two sockets it would be in the house every time. Reasons are:-
1) Dryer in house less chance of damp causing the device to stick.
2) Can use a passive type so power is restored after a power cut.
3) No need to go out to green house after electric storm to see if tripped.
I can not think of a single advantage of fitting in green house.

 

[JohnW2]

I would not consider fitting the RCD in the green house with just two sockets it would be in the house every time. Reasons are:- ... I can not think of a single advantage of fitting in green house.

Thank you. I was starting to wonder whether I was going mad!

Kind Regards, John

 

[bernardgreen]

In the TT zone for convenience of resetting.

With the RCD /RCBO at the feed end of a long length of SWA the RCD would be more sensitive to Live to Earth faults at the remote end and less sensitive to Neutral to Earth faults at the remote end.

Capacitive "leakage" from Live to Earth in the cable will reduce the amount of Live to Earth fault leakage needed at the remote end to trip the RCD. It would be counter to Neutral to Earth fault leakage current at the remote end.

 

[JohnW2]

In the TT zone for convenience of resetting.

Fair enough. I conceded the 'convenience issue' in my OP, but that is very different from "unsafe and totally unacceptable". In practice, I suspect that my friend might spend his whole remaining lifetime without ever having to reset the RCD, so I don't see it as a major 'convenience' issue!

With the RCD /RCBO at the feed end of a long length of SWA the RCD would be more sensitive to Live to Earth faults at the remote end and less sensitive to Neutral to Earth faults at the remote end. ... Capacitive "leakage" from Live to Earth in the cable will reduce the amount of Live to Earth fault leakage needed at the remote end to trip the RCD. It would be counter to Neutral to Earth fault leakage current at the remote end.

I think that's really scraping the barrel a bit (it's not a very long run of SWA) but, in any event, in terms of safety it's primarily L-E 'faults' (or L-E currents through people) that we're most concerned about, so those (I imagine very small) effects work in the direction of enhanced safety.

Have you any idea what sort of capacitance/metre one has between a cores, and/or between core and armour in SWA (his looks as if it's probably 6mm², possibly 10mm²)?

Kind Regards, John

 

[ericmark]

I think that's really scraping the barrel a bit

I would agree. I was told in South Africa with a few thousand miles they did have a problem and had to use DC but from house to greenhouse even with HF I would not be too worried about those distances. Balanced feeders and correctly terminating with impedance matching does not really come into a greenhouse supply.

I still think the guy was told as an apprentice when ELCB-v were in use that it should not be in the house and he has never updated his skills.

 

[EFLImpudence]

Isn't all this a bit over-technical for the original question?

Surely, the installation is no different than supplying outbuildings in numerous threads.

An RCD in the outbuilding is always recommended 'for convenience'.
No one has ever suggested that were it at the house end would be 'unsafe and totally unacceptable'.

 

[JohnW2]

I still think the guy was told as an apprentice when ELCB-v were in use that it should not be in the house and he has never updated his skills.

That's obviously a possibility. I didn't enquire about the age of the electrician, but I will when I next have an opportunity. I suppose that, statistically speaking, it's more likely than an electrician working today was still 'in short trusers' when VO-ELCB's were in use!

Kind Regards, John

 

[JohnW2]

Isn't all this a bit over-technical for the original question?

Quite so - it never occurred to me that the discussion would be anything like it has been.....

An RCD in the outbuilding is always recommended 'for convenience'. No one has ever suggested that were it at the house end would be 'unsafe and totally unacceptable'.

Exactly. I started this thread simply because I wanted 'reassurance' myself before I gave the reassurance (which I was sure was justified) to my friend, who was somewhat frightened by what the electrician said. I thought that a few people would simply agree with me (that the electrician was talking nonsense), and that would be the end of it. As it turns out, you and eric are the only two who have explicitly said that you agree with me, and at least one person has said that he agrees with the electrician!

Kind Regards, John