Existing split load 17th Install RCD question

John- · Mar 19, 2021

Customer has a large metal caravan with three phase supply feeding 3 metal Consumer units with split load RCD outlets contained within - all circuits originate form a RCD. The Boards are then fed from a TP isolate a few feet away, the earth is disconnected on eth SWA and a rod installed near by. ZS ifs pretty good around 30ohms or so.

There are is no RCD in line with the supply to the boards. These are MK Sentry CUs, and the internal wiring is only single insulated and as i read the OSG p21, there should be an RCD in series with the supply offering protection from internal shorts within the CU - a TT system.

I just checked and it seem that MK do not offer an S type RCD.

They do offer a 100mA RCD. As all final circuits are covered with 30mA RCDs, i was planning to use the 100mA RCD and replace the isolator module in each of these boards with one, thus covering the internal Bus Bar and cabling.

Why is it 100mA and not less mA? I tried to find in the regs but could not see.
Would i still get issues with nuisance tripping seeing as it is not timed?

Do i just cut my losses and install a separate 100mA timed RCD?

Thanks

John

timhoward · Mar 19, 2021

I've certainly seen exactly that (a 100ma RCD in place of isolator) done before. I agree your proposed solution would protect the bus bar and internals to some degree.

Tin hat on - and disclaimer that I'm not very experienced with this scenario - but I don't think this totally solves the issues though - as I believe a metal CU on TT has some exacting requirements that it sounds like you were reading about - it generally sounds as though you aren't able to be meeting them with that board and supply hence the inline RCD question.

The biggest nightmare scenario is the incoming line coming out and touching the metal casing. In your case it sounds as though the 7.6 amps PEFC would flow via the casing to the earth rod forever even with your solution in place. Sorry if I'm misunderstanding your description and have that wrong.

Fitting an upstream inline RCD in metal containment would probably just move the problem unless it can be the other side of where the original supply earthing is ceased (without contravening the special location issues).

Do you have any options at the other end (the incoming supply) e.g. if it isn't TT earthing that end maybe changing the TP fused isolator for a small TP consumer unit with RCBO protection for each outgoing SWA cable.
Again, sorry if your description and my perception of the installation are out of kilter!

suffolkspark · Mar 19, 2021

John- said:
I just checked and it seem that MK do not offer an S type RCD.

They do, the following is the part number, it's a 100A 100mA type A/S, full list on the circpro website.

H6900S

i=p/u · Mar 19, 2021

Doesn't have to be s type .. it's keeping it under 50v touch voltage or fire protection isi? so could probably use 300mA if you wanna at 30 gnomes according to my calculations.

John- · Mar 19, 2021

The attached may help a bit to explain.

suffolkspark · Mar 19, 2021

loz2754 · Mar 19, 2021

Change the TPN isolator for a 4 pole 300mA s type RCD.

timhoward · Mar 19, 2021

John- said:
The attached may help a bit to explain.

Thanks for taking the time to draw that.
I'm here to learn, so would appreciate someone helping me out with these arising questions:
Q1 - does 721.415.1 not imply that an RCD is required before the caravan?
Q2 - is the incoming supply to A, B and C complying with the most stupidly long reg number in the book 531.3.5.3.2.201 as the SWA cores won't be sheathed up to the main switch and it's TT.
Q3 - you said all circuits originate from an RCD so is there in fact anything on the currently non-RCD side of the split load boards A, B and C? If not isn't the proposed extra RCD only protecting a short length of unused bus bar and not anything else?
Q4 - are we agreed that simply changing the main isolator for an S type RCD won't protect against the metal casing of the CU becoming live if the incoming line conductor falls out.

timhoward · Mar 19, 2021

loz2754 said:
Change the TPN isolator for a 4 pole 300mA s type RCD.

Now you've posted this I'm happy I wasn't losing plot, thank you.
No need for questions above to be answered now. (Can't delete the post now)

John- · Mar 19, 2021

loz2754 said:
Change the TPN isolator for a 4 pole 300mA s type RCD.

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

timhoward · Mar 19, 2021

John- said:
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 · Mar 19, 2021

Fault protection should operate before 50 volticles

John- · Mar 19, 2021

i=p/u said:
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- · Mar 20, 2021

John- said:
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 · Mar 20, 2021

John- said:
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)

Existing split load 17th Install RCD question

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RCDs, metal consumer units and TT installations | NAPIT

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