Undersize earth conductor

[Stevesplatto]

Carrying out an EICR and some minor works on a client's flat.
This is a grnd floor flat in a converted victorian house.
All the services go into the lobby of the basement flat.
Water, gas and electrical meters are all there. The bonding is a bit flaky but easily rectified.
Bonding, main protective earth from the supply and to the fuseboard all connect to a MET adjacent to the meter in the basement.
There is a TN-S supply BS1361 > meter > switch fuse with 40a HRC fuse > 10mm T&E (3 metres ish above) > Wylex consumer unit (in good condition unfortunately)

Problem:
The cpc of the 10mm T&E supplying the flat is only 4mm and the MET is in the basement.
In effect the main earth conductor is 4mm but the Main protective conductors are 10mm so no matter how I try to make a case for the 4mm (haven't got all the figures i need for the adiabatic yet) it is effectively down grading the equipotential zone..

The way I understand it is that no matter what the adiabatic equation tells me I need a minimum of 10mm to maintain the equipotential zone.

Somebody please give me a straw to grasp at there's a massive concrete slab between the basement and the flat.
Thanks
Steve

 

[Deleted member 9648]

TN-S will require minimum 6.0mm main bonds,not 10mm. I assume you mean that main bonding within the flat will not be of the required size back to the MET. Your call as far as the EICR is concerned,if there is no signs of thermal damage to existing bonding,and the cpc in the sub calculates as adequate then it would be a code 3 only I feel.

 

[Stevesplatto]

TN-S will require minimum 6.0mm main bonds,not 10mm. I assume you mean that main bonding within the flat will not be of the required size back to the MET. Your call as far as the EICR is concerned,if there is no signs of thermal damage to existing bonding,and the cpc in the sub calculates as adequate then it would be a code 3 only I feel.

Thanks for the response:
My point is that the main earth in the sub is 4mm terminating at the MET in the basement were the protectives are and the earth bar in the fuseboard in the flat.

Yes I've see a table saying 6mm protective for a 10mm conductor. However as the main earth is 4mm isnt that effectively down grading the equipotential bonding?

Whatever happens I'm thinking I've got no choice but to C2 the main earth on the sub because if inadequate x sectional area...

He wont be happy..

Thanks Steve

 

[Deleted member 9648]

Thanks for the response:
My point is that the main earth in the sub is 4mm terminating at the MET in the basement were the protectives are and the earth bar in the fuseboard in the flat.

Yes I've see a table saying 6mm protective for a 10mm conductor. However as the main earth is 4mm isnt that effectively down grading the equipotential bonding?It is downgrading the EPB in the flat....but as already stated,if it's been like that since god knows when and there is no sign of thermal damage then it's hardly 'potentially dangerous'. Still a code 3 for me but would be interesting to see what others think.

Whatever happens I'm thinking I've got no choice but to C2 the main earth on the sub because if inadequate x sectional area...

He wont be happy..

Thanks Steve

.......

 

[Geoffsd]

You're looking at it the wrong way round.
Because there is a 10mm bond does not mean you need a 20mm earth.

There is a 40A fuse and 10mm live conductors with 4mm earth.
That's an oven circuit round here lately.

If the 4mm is ok when you do the adiabatic then you only need a 2mm bond (but 6mm is the minimum allowed).
The fact that someone has fitted 10mm doesn't matter.

Do the adiabatic backwards and see what PEFC and Zs it comes to.

 

[Farmelectrics]

what did the adiabatic equation give you

 

[davesparks]

You're looking at it the wrong way round.
Because there is a 10mm bond does not mean you need a 20mm earth.

There is a 40A fuse and 10mm live conductors with 4mm earth.
That's an oven circuit round here lately.

If the 4mm is ok when you do the adiabatic then you only need a 2mm bond (but 6mm is the minimum allowed).
The fact that someone has fitted 10mm doesn't matter.

Do the adiabatic backwards and see what PEFC and Zs it comes to.

Wtf? Main bonds are sized in relation to the size of the incoming supply, the required csa does not reduce throughout the installation.

Main bonds must be the same size all the way back to the MET

And the minimum size allowed is 6mm for a main bond.

 

[Geoffsd]

You seem to like shouting at me, don't you?

Wtf? Main bonds are sized in relation to the size of the incoming supply

It's TNS. they're sized in relation to the main earthing conductor.
Minimum of 6mm as I wrote.

Main bonds must be the same size all the way back to the MET

And the minimum size allowed is 6mm for a main bond.

Post #5

(but 6mm is the minimum allowed).

 

[Deleted member 9648]

Wtf? Main bonds are sized in relation to the size of the incoming supply, the required csa does not reduce throughout the installation.

Main bonds must be the same size all the way back to the MET

And the minimum size allowed is 6mm for a main bond.

Glad I'm not the only one confused by Geoffd's post.

Edit...Geoff

I believe the OP is thinking that the 4mm cpc in the sub is effectively the main bond back to the MET and is therefore of reduced CSA.

 

[Geoffsd]

I thought the 4mm was the earthing conductor and the OP was worried because it was smaller than the main bond.

If 4mm is satisfactory as the earthing conductor - which we don't know yet - then only a 2mm bond is required but 6mm being the minimum allowed.

 

[7029 dave]

Main earth conductor should be the same csa as neutral conductor.

 

[davesparks]

Main earth conductor should be the same csa as neutral conductor.

Go on then, give us a regulation number for that bit of nonsense!

 

[7029 dave]

Go on then, give us a regulation number for that bit of nonsense!

Not contracting anymore, but 16th iee regs section 547-02-01 table 54h, cant of changed.?

 

[richy3333]

Not contracting anymore, but 16th iee regs section 547-02-01 table 54h, cant of changed.?

Or can it...
We are on 17th Ed now and nearly amendment 3. Its what 17th says that counts. Also, TNS has been mentioned...

 

[Geoffsd]

but 16th iee regs section 547-02-01 table 54h, cant of changed.?

No, that's Bonding Conductors for PME - 54.8 now.

Earthing conductors are sized with the adiabatic equation.

 

[davesparks]

Not contracting anymore, but 16th iee regs section 547-02-01 table 54h, cant of changed.?

Under the 16 th and 17th the selection from the table is
Live of 1mm to 16mm is equal size cpc
Live of 25 or 35 is 16mm cpc
Live of over 35mm is cpc >\= 1/2 size of live

Or it can be calculated by the adiabatic.

The commonly accepted size for a 100A service is 25mm tails with 16mm earth

 

[Geoffsd]

That's 543.1.4 Table 54.7 when too lazy to do sums .

Otherwise 543.1.3 - then bonding conductors are half of that, except PME.

 

[Deleted member 9648]

I thought the 4mm was the earthing conductor and the OP was worried because it was smaller than the main bond.

If 4mm is satisfactory as the earthing conductor - which we don't know yet - then only a 2mm bond is required but 6mm being the minimum allowed.

But you are taking the cpc for the distribution circuit as the earthing conductor. It is the main earthing conductor at the intake which will determine the size of the main bonding conductors.

 

[Geoffsd]

Ok, but even that is determined by the adiabatic regardless of what is actually fitted.
So, we would need to know the DNO fuse rating and the Ze.

There is a 10mm bond fitted anyway.

However, it is the 40A distribution circuit CPC that the OP is worried about.

 

[Engineer54]

The main earthing conductor on a TN-S system is the conductor between the cable sheath on a PILC cable or the earth conductor within a split concentric cable to the MET. From the MET they will be earthing conductors or CPC's and main bonding conductors...