UK Multiple Domestic Distribution Units - RCD Topology

[Switchdoctor]

Cascaded Distribution Units.

Given how cheap populated distribution boards are; i've started to use them for almost every major junction in my home:
Domestic Main Distribution Unit >10mm swa > Garage Distribution Unit > 6mm swa >Spa Distribution Unit
4mm swa > Outdoor Distribution Unit
Each unit has an RCD as well as a bank of MCBs
I'm planning ahead to install an Electric Vehicle charge-point downstream of the Garage unit.

My questions are:
Is there any issue with endlessly cascading RCDs?
Is there any best topology (e.g. star) as opposed to the tree structure i'm using?

 

[nicebutdim]

You'll have no selectivity between RCDs. It's not an ideal arrangement as there's no guarantee which might trip should a fault to earth occur. As it's your own home you may be comfortable enough checking each inline RCD. Given that each submain is run in armoured cable, I'd have been inclined to avoid upstream RCD protection where it isn't required.

 

[pc1966]

As above, and a few other comments:

• With a generator you need an independent means of earthing as under loss of supply you have absolutely no guarantee that the supply earth will remain. This must not be a water or gas pipe, even though they must be bonded (if conductive, not plastic water obviously) so a rod or similar should be included!
• It is not clear why you have an up-front 100A RCD, if the supply is TT (i.e. depends on an earth rod as no supplier earth) then you should have this with your planned style of installation, but then it should be a 100mA selective (delay) type so you have some degree of selectivity with down-stream RCD
• Having mains-only outlets is a good plan, also some indicator lamp so you know when to come back off the generator?
• With the sub-main cables in SWA you don't need them RCD protected (generally speaking) so they could be fed off a MCB and that should be as large as safe for the SWA current carrying limit, etc, to give you better selectivity with down-stream MCB/RCBO
• Given that RCBO are quite cheap (but not usually thrown in with budget CU) you would be better to look at using them for final circuits which need the 30mA RCD protection against shock.
• These days you should have an SPD at the incoming CU
• Remember that SWA must be terminated with suitable glands so the armour is earthed, even if you use 3C so separate CPC in copper.

 

[Switchdoctor]

As above, and a few other comments:

• With a generator you need an independent means of earthing as under loss of supply you have absolutely no guarantee that the supply earth will remain. This must not be a water or gas pipe, even though they must be bonded (if conductive, not plastic water obviously) so a rod or similar should be included!
• It is not clear why you have an up-front 100A RCD, if the supply is TT (i.e. depends on an earth rod as no supplier earth) then you should have this with your planned style of installation, but then it should be a 100mA selective (delay) type so you have some degree of selectivity with down-stream RCD
• Having mains-only outlets is a good plan, also some indicator lamp so you know when to come back off the generator?
• With the sub-main cables in SWA you don't need them RCD protected (generally speaking) so they could be fed off a MCB and that should be as large as safe for the SWA current carrying limit, etc, to give you better selectivity with down-stream MCB/RCBO
• Given that RCBO are quite cheap (but not usually thrown in with budget CU) you would be better to look at using them for final circuits which need the 30mA RCD protection against shock.
• These days you should have an SPD at the incoming CU
• Remember that SWA must be terminated with suitable glands so the armour is earthed, even if you use 3C so separate CPC in copper.

Fantastic, clear advice - Thanks

Especially re earths, i'll ensure i drive another earth-spike (maybe near the garage) and test all my earth continuity.

I must admit that not all my SWA is earthed at both ends - time to spend some time in my garden retro-fitting.

Currently on amazon searching 100ma selective delay RCDs.

Follow on question: How long do copper earth spikes last? I have one that must be 40+ years old. Since i'm going to drive another one in by the garage, should i think about replacing the original anyway?

 

[timhoward]

Main immediate issue is this one:

Looking at the size of that installation it's not going to be hard to gather 20ma leakage. If that one starts nuisance tripping the generator is going to be kicking in a lot.

The short answer is yes, you should avoid stacking RCDs for the reasons explained above.

Currently on amazon searching 100ma selective delay RCDs.

Respectfully, you should find a friendly sparks and get some guidance. This is not a trivial installation.

 

[Switchdoctor]

Main immediate issue is this one:
View attachment 108170

Looking at the size of that installation it's not going to be hard to gather 20ma leakage. If that one starts nuisance tripping the generator is going to be kicking in a lot.

The short answer is yes, you should avoid stacking RCDs for the reasons explained above.

Respectfully, you should find a friendly sparks and get some guidance. This is not a trivial installation.

Your advice is amazing - i wish i had your years of experience. I did indeed spend 2 days chasing random trips befor changing out the 100a RCD at the root of my tree. I'll check my earths and swa bonding and maybe take some of the RCDs out of the chain once i can gurantee the earthing.

I WILL find a spark. I studied electrical engineering many years ago and fitted machine tools in factories for many years but never got my sparks ticket.

The layout IS maybe over-complex; as stated in the original post; this is mainly due to me spending too much time on ebay and using multiple CUs as cheap junction boxes rather than doing a formal plan with a spark. At least the 'box with din rail' format will allow me to re-work without major re-installation.

 

[nicebutdim]

One quick point to clarify...

I took that 100A RCD as being just that - rated to carry 100A, but most likely only 30ma where detection of leakage to earth is concerned. It's this the case or should the drawing read 100mA?

 

[Switchdoctor]

One quick point to clarify...

I took that 100A RCD as being just that - rated to carry 100A, but most likely only 30ma where detection of leakage to earth is concerned. It's this the case or should the drawing read 100mA?

They're all box-standard 30ma types. Only the rated current changes.
I made the drawing to stick near the main panel to at least give someone a clue as to what's going on in case i ever fall under a bus. It's just a standard 3 bed uk bungalow but i think i may have over engineered things vis current and protection. I needed reassurance that it was basically safe to chain RCDs. Answer = yes but to watch my earths and get a pro to check it.
Thanks you you and all for the interest.

 

[James]

That’s one heck of a complex install for a 3 bed bungalow.

 

[timhoward]

I needed reassurance that it was basically safe to chain RCDs. Answer = yes but to watch my earths and get a pro to check it.

My thinking was that a pro might help you slightly simplify it a bit.
@pc1966 mentioned this earlier - it looks as though the submains don't in fact require RCD protection but this needs to be confirmed by inspection and testing.
It also looks as though the Garage CU has a spare non-protected way that could be used to feed the Spa CU.
You'd need an RCBO for the service socket (or move that socket to share a breaker on the top row)
My guess is that it wouldn't take too much to confirm that some subtle changes could remove the stacked RCDs.

If you ever sell this property, warn the poor sparks quoting for an EICR that you have a small cul-de-sac's worth of electrics packed into this 3 bed bungalow!

PS You might consider changing the garage consumer unit for a much smaller Fusebox RCBO board as only 3 circuits are being used in a 12 way board!

 

[Switchdoctor]

My thinking was that a pro might help you slightly simplify it a bit.
@pc1966 mentioned this earlier - it looks as though the submains don't in fact require RCD protection but this needs to be confirmed by inspection and testing.
It also looks as though the Garage CU has a spare non-protected way that could be used to feed the Spa CU.
You'd need an RCBO for the service socket (or move that socket to share a breaker on the top row)
My guess is that it wouldn't take too much to confirm that some subtle changes could remove the stacked RCDs.

If you ever sell this property, warn the poor sparks quoting for an EICR that you have a small cul-de-sac's worth of electrics packed into this 3 bed bungalow!

PS You might consider changing the garage consumer unit for a much smaller Fusebox RCBO board as only 3 circuits are being used in a 12 way board!

Thanks for the interest.

I'm going to comission a spark to re-dress the SWA glands and once i've confirmed the SWA earths i'll consider replacing some of the remote RCDs with straight DP switches since they'll 'just slip in'. I fully linked in the internal earth core of each SWA cable so i know i have good earth continuity to each CU but I didn't pay attention to the outer armour at every end.

As per earlier advice, since the house only has a two-wire feed, i'm going to replace the earth spikes anyway.

I will seek the spark's advice on how to cascade the RCDs though. I'd still like the garage to trip 1st if i cut a power lead trimming the hedge rather than cutting the whole house.

The 12 way board in the garage and 34 way for the house was the result of a perfect storm of a little knowledge, early retirement, an ebay account and that "it's only $2 more" moment.
I wanted the main house board to contain my own smartmeter as i couldn't integrate with the supplier-provided one and i wanted to allow for future generator / off-grid solar switching along with EV charging in the garage.
So yes, very over-engineered.

 

[MJPD29]

What is this absolute mayhem? God knows what rcd will trip on a fault it’s like a lottery. God helps whoever buys this house after you and decides to do some electrical work

 

[nicebutdim]

What is this absolute mayhem? God knows what rcd will trip on a fault it’s like a lottery. God helps whoever buys this house after you and decides to do some electrical work

Diplomatic as always

 

[Switchdoctor]

What is this absolute mayhem? God knows what rcd will trip on a fault it’s like a lottery. God helps whoever buys this house after you and decides to do some electrical work

LMAO - love it... at least they have a nice diagram to follow!

 

[MJPD29]

On the bright side you could probably suffer multiple rcd failures before your house has no rcd protection