RCD protection to supply tails???

[sparkybird]

Hi everyone and happy easter!

I'm doing a PIR next week in a house converted to flats and the supply from the cut out to my flat is in 16mm t and e, run in walls in the safe zones as far as I can tell.

My take on this is as the original install was not to 17th, then this would warrant a code 4 (not RCD protected, does not comply).

However, if the board was then upgraded to 17th, would the supply require rcd protection?? This seems to be at odds with the requirement not to have one RCD protecting the whole install....?

Your thoughts would be welcome!

SB

 

[Adam_151]

Code four for being in the wall without RCD protection as long as its in safe zones, but as its twin, do check the earth is sufficent...is there a separate earth in addition? and where does the bonding go back to... he subboard in question? oh and what earthing arrangement?

As to the board change, you're not installing the submain and you are not altering the protective device for it, so I'd just leave it and note it in comments on existing installation (no doubt someone will come up and disagree with me )

EDIT:

missed something... is the only protection the DNOs cutout? are they happy with the arrangement, many would take a dim view of long lengths of twin protected by their cutout fuse!

 

[ricicle]

Yes - it seems as if to comply with one regulation you must forego another. Perhaps a deviation but you would have to decide which poses the most danger - losing all circuits in a fault or drilling through a 16mm² T+E.
Or you could channel out the sub-main and refix it surface !!

 

[sparkybird]

thanks for speedy replies (have we all nothing better to do on a damp bank hol? )

Yes, shoud have clarified - the DNO supply is 3 phase and then 60A 1361 switch fuse for each flat. I did the adibatic (or however, you spell it!) calc for the cpc and it's fine. Bonding (gas only as water is all plastic) is local to flat.

Don't think that running supply on surface is an option (here at least, but hadn't thought of that for other sits!).

If all circuits on new DB are rcd protected what would happen if someone put a nail through supply? Would rcd's trip? Both of them? If this did happen I guess at least the risk to person would be a bit less as no load???

Personally I think more risk to have whole install tripping off...

 

[rebuke]

If someone put a nail through the supply cable, the RCDs wouldn't trip, since they're downstream of it - the person would get the full 60+A until the fuse in the switchfuse blew...

 

[Adam_151]

I'm still not 100% on the bonding.. whats the cpc size in 16mm twin? 6mm?

Earthing arrangement is TNS I presume?

How is the gas distributed? a separate pipe from the street for each flat? or one for the entire structure and branches out?

And water? I presume there is a main bond from the suppliers earthing terminal to where it enter building even if all internal pipework is non-metallic?

If all circuits on new DB are rcd protected what would happen if someone put a nail through supply? Would rcd's trip? Both of them? If this did happen I guess at least the risk to person would be a bit less as no load???

No, its upstream of the RCDs, so they wouldn't trip, if they were really unlucky they'd end with with a live nail if they only git the phase, if they hit phase and cpc together, then a big bang, and he'd need a new submain cable, new 1361 fuselink and new pair of trousers

 

[sparkybird]

Adam - you're on fire this morning!

Cpc is 6mm. Bonding 10mm.

I'm doing the PIR this week, so I've only poked around a bit when I was there to do estimate, but gas meter for my flat is sep from rest and appears to be bonded back to my DB, will find out how pipes are run and what the water situation is too.

3 phase is PME.

 

[Adam_151]

As its PME, and the cpc of the twin is forming part of the bond between the incomming electrical service and the gas service, then you have a problem, if you have a look at 544.1.1 and table 54.8 then it lists minimum bond sizes in relation to suppliers neutral conductor, and it does not allow a bond smaller than 10mm where PME conditions apply (its due to circulating currents in a healthy network, together with the risk of a dropped neutral)

It is IIRC a requirement that comes from the supply regs, and under certain conditions DNO might require even bigger bonds than thos recommended in the table! (hence the note to the side of it)

Probably a code two

Oh... and its a dull bank holiday morning and I probably should be doing some DIY, but I can't get in the mood

 

[sparkybird]

Adam - yes I see that now - thanks for putting off the DIY!

I need to check that this is actually the situation (as I only had a fairly quick look before), but would one way to rectify this be to main bond (in 10mm) the flat's gas back to the 'main' MET thus effectively removing the cpc of the t and e from the bonding equation?

SB

 

[ricicle]

and under certain conditions DNO might require even bigger bonds than thos recommended in the table! (hence the note to the side of it)

But if you ring them and ask about it like I did a couple of weeks ago they won't have a clue what you are on about !

 

[sparkybird]

yes, indeed my NIC inspector effectively said 10mm will do!

SB

 

[londonboy]

How about reterminating the 16mm in a small cu beside main head protected by a 50/63a (depending on capacity of 16mm- I havent got my regs indoors) with a 63/80amp 30ma rcd? A type S/time delay rcd would be good to prevent nusience tripping, might be worth a google.

Then theres no way that the 16 can be overloaded, and its protected as its below 50mm from surface.

 

[securespark]

All this talk of dull weather: just drive up to SK7 & enjoy a gloriously warm, sunny morning!

As for the RCD, would a 30mA S type on the submain comply? Food for thought.

 

[londonboy]

As for the RCD, would a 30mA S type on the submain comply? Food for thought

Yeh I just thought of this to protect the actual submain, not the final circuits hence why I suggested a type s. I have had a quick look on google and im not sure they exist! Obviously 100ma rcd doesn't comply, but a standard 30ma would mean lots of trips down to the main head everytime a bulb blows!

 

[sparkybird]

Thanks for suggestions, but I'm not sure that s type (even if available) would comply because of the inconvenience thing (ie one RCD protecting whole install) - made worse in this case as the main cutout is outside of the flat in question!

I've more to investigate as I've not yet carried out PIR but think I would note it as a non compliant at the moment.

Will check out the bonding stuff as suggested by Adam though.

Thanks again folks

SB