Existing split load 17th Install RCD question

[timhoward]

Even if the supply cabjes are meters tails to the double poll switch?

You've got two different things going on here which makes this situation complicated.
1 - the standard problem of metal CU and TT. Most agree best practise is a front-end RCD. The 17th edition required it in most circumstances, and the 18th just has a higher spec for the supply (belt and braced secured and double insulated etc. ) but doesn't require it.

2 - the fact that you are almost certainly dealing with a special location, whether it be a caravan supply or a mobile home supply. Whichever it is there are regs in both sections requiring an RCD in the final circuit supplying it.

Put it all together - the best place for an RCD for both 1 and 2 is where loz2754 said which is also where section 7 says there should be one (protecting a final circuit supplying the mobile home or park home / protecting the supply to the caravan)

The only fly in the ointment is that section 7 says the supply RCD should be 30ma which would leave no discrimination. As there are downstream RCDs for everything and it's a distribution circuit I'd be inclined to stick with the 300ma suggestion unless anyone has a better idea.

 

[i=p/u]

Fault protection should operate before 50 volticles

 

[John-]

Fault protection should operate before 50 volticles

Agree. The ZS is pretty good for a TT as there is an all year round high water table. So do i ho for s timed 100mA, timed 300mA or just a 300 mA?

if i went for a standard 300mA snd <50v it would still give us over a 100ohms comfort zone even if it doubled to 60ohms.
Thanks all.

 

[John-]

Agree. The ZS is pretty good for a TT as there is an all year round high water table. So do i ho for s timed 100mA, timed 300mA or just a 300 mA?

if i went for a standard 300mA snd <50v it would still give us over a 100ohms comfort zone even if it doubled to 60ohms.
Thanks all.

One last point...

i found this really good article last night that summarises the key points.

https://professional-electrician.com/technical/metal-consumer-units-in-installations-forming-part-of-a-tt-earthing-system/.

 

[timhoward]

One last point...

i found this really good article last night that summarises the key points.

https://professional-electrician.com/technical/metal-consumer-units-in-installations-forming-part-of-a-tt-earthing-system/.

That's a very good article. Thanks.
I'd found a very similar article (I thought it was same one at first) and the same conclusions are reached but it includes a bit of history too:

RCDs, metal consumer units and TT installations | NAPIT

NAPIT's Bill Allan answers another common contractor query. Contractors sometimes ask whether it's necessary to install an RCD in a TT installation before a metal consumer unit (i.e. a 'front end' RCD

professional-electrician.com

I personally think an exclusion allowing plastic containment for RCD enclosures would help matters a great deal. I notice that DNO's seem to be allowed to to fit plastic contained isolators....
(I wonder how many fires have ever started in an RCD enclosure versus electric shock deaths on TT earthed installations)

 

[i=p/u]

So not needed at all if if final circuits 30mA protected and meet the installation methods mentioned .. learning day

 

[John-]

So not needed at all if if final circuits 30mA protected and meet the installation methods mentioned .. learning day

Unfortunately whilst i have RCDs and RCBOs in these boards, bus bars are not insulated and interconnecting cables are standard MK single insulation, so i will need to fit an RCD in series with the supply.

 

[Deleted account]

I still fitted And would continue to fit plastic enclosures for up front RCDs In Domestic TT systems. normally a 100mA s type.

possibly this is a deviation from the regs, but if it is I’m making the installation just as safe if not safer.

 

[loz2754]

I still fitted And would continue to fit plastic enclosures for up front RCDs In Domestic TT systems. normally a 100mA s type.

possibly this is a deviation from the regs, but if it is I’m making the installation just as safe if not safer.

I do the same. I do not classify a simple enclosure as "similar switchgear".