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.

 

[mainline]

Maybe but to export it you would have to do exactly that. The fact that you could have a 'special location' downstream in some cases may not be relevant. I could have a bathroom in this downstream location but could still employ the TN-C-S derived earth.

The way I see it is this :
if you have a tnc-s /pme earthing system, and it is taken to an outbuilding that is out of the equipotential zone of the property then you would be exporting it.

If the building is within that zone and no extraneous, then you would be utilising the cpc.

 

[timhoward]

I think a lot of the confusion surrounding this comes from the word 'export'.

Thank you. The word "export" makes me cringe every time!

The way I see it is this :
if you have a tnc-s /pme earthing system, and it is taken to an outbuilding that is out of the equipotential zone of the property then you would be exporting it.

The way I see it is that we are excellent at over-thinking this topic.

-We are creating a distribution circuit.
-Normal fault protection rules apply
-Normal RCD selectivity rules apply
-Whatever the earthing is at the origin will be the earthing for the distribution circuit unless we deliberately change it.
-There might be stuff to bond the other end, in which case normal bonding rules apply including the size of the CPC that supports it (noting new AM2 reg about bonding conductor sizes in a separate building)
-As with all installation work, special location rules can apply

In my experience most of the time this all amounts to "carry on".
And don't let the OpenPen doom angels deter you! I'd bet it's more likely the never-tested RCD will fail on a TT setup than an OpenPEN fault happening to that very house.

 

[westward10]

@mainline can you say where in BS7671 they reference equipotential zone and then further support your statement.

 

[mainline]

@mainline can you say where in BS7671 they reference equipotential zone and then further support your statement.

As said, it's how I differentiate between the word "utilise" and “export”.

The word "export " is recognised and used by all the schemes, and everyone understands the meaning.

 

[westward10]

Equipotential zone is what you need to clarify.

 

[mainline]

Equipotential zone is what you need to clarify.

You want me to give explain what equipotential zones are ? And the risk of step potential ?

Just because its the way i differentiate between extending, utilising and exporting is not to your liking, then maybe you can explain it.

 

[westward10]

Just wondered as that term hasn't been used in BS7671 for many a year.

 

[mainline]

Just wondered as that term hasn't been used in BS7671 for many a year.

Do you mean The term “equipotential zone” hasn't been used ? Does that mean Equipotential zones no longer exist, I thought you create an equipotential zone when you equipotential bond which I think there maybe a mention of in bs7671.

 

[default]

Do you mean The term “equipotential zone” hasn't been used ? Does that mean Equipotential zones no longer exist

Do you mean The term “equipotential zone” hasn't been used ? Does that mean Equipotential zones no longer exist

Wow an argument about wording on this forum. Will wonders never cease.

 

[mainline]

Wow an argument about wording on this forum. Will wonders never cease.

I'll refer to the #40 post

 

[default]

I'll refer to the #40 post

Exactly