Assistance with my installation

[Poacher3536]

I’m a trainee electrician and would really appreciate the benefit of someone’s skill and experience on the following:

Our house is a TN-C-S (100A fuse) with a main DB (100mA RCD).

This DB feeds a sub DB in a garage outbuilding via a 6mm SWA with all 3 conductors used, by a 63A MCB. The armour is also earthed for mechanical protection.

The sub DB (30mA RCD) is also a TT system with 10mm earth cable to electrode. (I’ve tested the electrode and it’s at 83 Ohms so well inside the 200 Ohms limit.

The sub board feeds the following in garage and outside:

Outside

32A hot tub via 6mm SWA

16A power radial

6A lighting

Inside garage:
32A ring

6A lighting

I’ve checked all cable runs for voltage drop and current capacity, all fall within BS7671.

The issues I’m unsure of is:

1.The sub DB is taking the PME from main DB, whilst also benefiting from an earth electrode. AFAIK having 2 x systems of earthing may conflict and lead to main DB RCD tripping, and not the sub board, but I could be wrong?

2.The 6mm SWA has a current capacity of 53A (ref method D), with a 32A hot tub and diversity applied to both power and lighting I’m coming in at 36.92A + 32A = 68.92A. In reality the ring circuit in garage is rarely used above a freezer and occasional drill etc. Certainly nothing with a load that would require a ring final circuit for a new instal.

My initial thoughts were to change the ring final to a radial, with diversity applied will be 13A therefore bringing the total to 60.9A but this is still too much. The next choice would be to upgrade the feed SWA to 10mm giving me 71A.

What’s your options on this? And the best way to make things compliment, both for SWA size and earthing issue?

Thanks in advance.

 

[buzzlightyear]

I have it in writing from 3 DNOs (every one that I have contacted so far) that they have no special requirements regarding earthing arrangements at outbuildings

how many faults per year on pme lost neutral very little the power lines are monitored ,you have got more chances of being kissed on the lips of a giraffe .

 

[mainline]

how many faults per year on pme lost neutral very little the power lines are monitored ,you have got more chances of being kissed on the lips of a giraffe .

One death would be one too many.

 

[buzzlightyear]

One death would be one too many

its like throw a frying pan in the air in a crowd which unlucky one get it first .

 

[davesparks]

We do have to remember though that bs7671 forbids the use of pme earthing for some types of installations.

Yes it is prohibited for certain installations, but not garden sheds.

 

[mainline]

Yes it is prohibited for certain installations, but not garden sheds.

That would depend on what the shed is made of.

 

[davesparks]

Because a diyer/trainee reading that statement may well believe that they can export pme to any outbuilding without any special requirements, which isn't true.

They, or anyone else, cannot 'export' PME to any outbuilding. They can however extend an PME equipotential zone to any outbuilding (except those falling under specific circumstances)

 

[mainline]

They, or anyone else, cannot 'export' PME to any outbuilding. They can however extend an PME equipotential zone to any outbuilding (except those falling under specific circumstances)

There’s no regulation that says you can’t export a PME earth to another building.

 

[Pretty Mouth]

There’s no regulation that says you can’t export a PME earth to another building.

I think a lot of the confusion surrounding this comes from the word 'export'. It only crops up in these conversations, and doesn't seem to exist in any set of regulations that I'm aware of.

What does it mean? Does it mean extending the separated earth, from the MET, to the outbuilding (generally acceptable), or does it mean extending the combined neutral/earth to the outbuilding (not acceptable)? DNOs almost certainly will tell you that you can't do the latter.

 

[pc1966]

What is of interest is whether there is selectivity between the two RCDs. Why is there a 100mA upfront on a TN-C-S?

I was wondering about that.

Certainly you could use a 100mA delay RCD along with something like a 50A D-curve MCB to get better selectivity with down-stream OCPD based faults and/or still meeting disconnection times on the higher resistance SWA armour to save cost going 3C.

I suspect it might just have been TT originally and in recent years the DNO made it TN-C-S?

 

[Dartlec]

how many faults per year on pme lost neutral very little the power lines are monitored ,you have got more chances of being kissed on the lips of a giraffe .

I'd like to know this too - wonder if anyone has collated such events via FOI request or similar. Since it is regularly raised as a potential risk in guides etc, I'd like to know just how often it happens so that I can make a well informed risk assessment on certain types of job.

(For avoidance of doubt, I'm talking about faults on PME, not being kissed by a giraffe )

 

[davesparks]

There’s no regulation that says you can’t export a PME earth to another building.

No there isn't, however there is the definition of the word 'export'

 

[timhoward]

I'd like to know this too - wonder if anyone has collated such events via FOI request or similar. Since it is regularly raised as a potential risk in guides etc, I'd like to know just how often it happens so that I can make a well informed risk assessment on certain types of job.

Available data getting a bit old now....

Interestingly someone tried an FOI request in Nov 2021 and HSE never responded, leading to the ICO upholding a complaint.
I can't find the results though.

 

[mainline]

No there isn't, however there is the definition of the word 'export'

I think that it has to be a given in that “Export to an outbuilding” means beyond the building and its equipotential zone.

 

[default]

I looked at a few of these posts then skipped to the end. Take no notice of the majority of these as they just seem as though they like to argue rather than knowing what there doing. Exporting a TNCS is fine as long as you can prove and are happy it's OK. A 100mA RCD can trip before the 30mA at x5, it shouldn't but it can. To stop this you can install a separate isolator at the mains so it doesn't come from the board in the house then run the SWA to the shed from that. Your running a sub main not a finale circuit so it does not need protecting by an RCD. If you still want the shed to be TT then terminate the SWA into a plastic adaptable box then run the correct sized earth from the stake to the shed DB.

 

[mainline]

default said : " Take no notice of the majority of these as they just seem as though they like to argue rather than knowing what there doing. "

But do you know what you're doing? Because I think what you're suggesting wouldn't be compliant.