Earth Cabling by Meter + CU: Regs + Size ?

[JackSpratt]

I am putting up a modern consumer unit (to replace one of four 'fuse boxes') and have the elec board coming to disconnect while I move tails across. Also connected to the meter are other existing 'fuse boxes' which supply quite separate circuits in the house. The earth cable is around 4mm in all of the existing wiring around the meter. So my question is; will I need to upgrade all visible earthing before they do this work (regulations wise, I know it needs to be done) ?

And should this be 16mm or 10mm ?

Many thanks

 

[Spark123]

All earthing and bonding must be adequate before any alteration or addition is carried out. If you live in England or Wales, replacing a consumer unit is notifiable to your local area building control under Part P of the building regs.
What sort of supply do you have, TN-S, TN-CS, TT? What size is the main equipotential bonding? What size meter tails?
Do you own any test equipment?

 

[JackSpratt]

The supply is TN-S. Gas, water and elec pipes all come in at the same point with elec meter only just a couple of feet away. The equipotential bonding, and earth cables to the existing and the 'to be replaced' box are all around 4mm in size (ie way below the 10mm). The meter tails are 25mm.

I have a digital multimeter and a Steinel LED voltage indicator.

..and thanks for the help

 

[JackSpratt]

...were you going to add something following an answer ?

 

[Spark123]

You don't have the adequate test gear to commision a new consumer unit. A 100 TN-S service with 25mm² meter tails requires an earthing conductor of 16mm², a main CPC of 16mm² and main equipotential bonding conductors of 10mm².

 

[JohnD]

the supplier might not care, but if they do object, you will feel silly for not upgrading the earthing (16mm) and main bonding (10mm) first, and having identified an unsatisfactory installation, they can demand a certificate before they'll come back.

Best approach is to do it first.

Provide an 8-way earth block near the service head, with enough 16mm G&Y for them to connect to, and take the 16mm to your CU(s) and the 10mm to your gas and water services from there.

If I were you I would also provide a 100A DP isolator in the tails between the meter and the CUs or Henley block. It will save time later.

You can look up the part P stuff yourself.

edited: bah too slow

 

[GaryMo]

A 100 TN-S service with 25mm² meter tails requires an earthing conductor of 16mm²

What does it work out to using the adiabatic equation?
I would look myself but haven't got BS7671 handy but will have a look at some point today.

I assume main fuse is a BS1361?

 

[JackSpratt]

JohnD - Many thanks for your suggestions and help. You have answered my concerns with clear and helpful advice. I have now requested that the power company fit a 100A DP Isolator switch instead of a disconnect and reconnect. I will follow your guidance on the earth bonding cable sizes, and have 16mm (for the supply to earth block) available when the switch is installed. The bonding upgrade will also be carried out before the power company visits.

 

[Spark123]

A 100 TN-S service with 25mm² meter tails requires an earthing conductor of 16mm²

What does it work out to using the adiabatic equation?
I would look myself but haven't got BS7671 handy but will have a look at some point today.

I assume main fuse is a BS1361?

Stick with table 54G, it's a lot easier
Unless it is possible to measure the prospective earth fault current it may be difficult to calculate, I suppose you could calculate it for all extremeties.

 

[luminaire]

Stick with table 54G, it's a lot easier

Why isn't table 54G in the OSG?

 

[Spark123]

Maybe they run out of space?
On the other hand they did put some nice pictures of the different supply types (up to 100A) and the required size of the earthing conductors/main CPC/MEBs. Theres also a bit of an explaination at the start of section 4 for MEBs.

 

[Adam_151]

A 100 TN-S service with 25mm² meter tails requires an earthing conductor of 16mm²

What does it work out to using the adiabatic equation?
I would look myself but haven't got BS7671 handy but will have a look at some point today.

I assume main fuse is a BS1361?

Can work out as low as 4mm² is the Ze is low enough!

0.1sec - 1800A = 3.98mm²
0.2sec - 1400A = 4.37mm²
0.4sec - 1200A = 5.31mm²
5sec - 630A = 9.85mm²

Would never fit 4mm² though

 

[JackSpratt]

Can I ask how this might affect the situation ? (eager to understand how all this might play a part)

 

[Spark123]

It won't effect your situation, better stick with the 16mm² earthing conductor / main CPC and 10mm² Main Equipotential Bonding conductors. What was said above is just banter between ourselves and (slap wrists) would ideally be in its own topic.

 

[JackSpratt]

ok, thought it might be ! thanks everyone for the tips.