6.0mm earthing conductor to MET on TNS

[Marcus Vaughan]

Hello.
I am carrying out an inspection on a domestic TN-S system. First observation is that I can see 16.0mm earthing conductor fro CU to MET - but then is steps down to 6.0mm connecting to the supply cable.
This should be 16.0mm - shouldn't it? I am going by table 54.7 in the big yellow book.
Sorry if this has been asked before.....

 

[Andy78]

The cable connecting to the supply cable is not in your remit and should be sized by the DNO, assuming it's a legit DNO connection.
The adiabatic equation can be used as an alternative to table 54.7 in any case.

 

[Marcus Vaughan]

Thank you. I did think that it's time to call the DNO....

 

[Andy78]

Thank you. I did think that it's time to call the DNO....

Why ? Chances are it's fine. Does it look like a DIY connection or something ?

 

[Marcus Vaughan]

No it doesn't look DIY actually.
I'll see what Ze's and Z's I get......

 

[westward10]

Not saying a word.

 

[Octopus]

Not saying a word.

but you are thinking and biting your lip!

 

[telectrix]

if the link from the MET to the supply cable is in 5A fuse wire, as long as DNO have done it, it's fine.

 

[Specialist]

if the link from the MET to the supply cable is in 5A fuse wire, as long as DNO have done it, it's fine.

Think 5A is a bit thin Tel, 15A minimum surely!

 

[westward10]

Still not saying a word but how do you use the adiabatic equation when you don't know the fuse size/type at the service head.

 

[Ian1981]

I would use the Adiabatic on the worst case scenario say 100 amp fuse and also allow for other sizes.
Thing is most of the time depending on the actual fault current you need the let through energy from the manufacturer as the time is below the 0.1 second minimum given in bs7671
Often tho the 0.1 second is just used for time if fault current is over the max given in table.

Really the only time I've used the Adiabatic is when your disconnect time is closer to 5 seconds for fuses and then it's often required to use a bigger sized conductor

If anyone's still got the red copy of the 17th edition it has the data in for bs 1361 fuses as these are now replaced in yellow book with bs88

 

[telectrix]

still got red copy??? i still using it.

 

[davesparks]

Hello.
I am carrying out an inspection on a domestic TN-S system. First observation is that I can see 16.0mm earthing conductor fro CU to MET - but then is steps down to 6.0mm connecting to the supply cable.
This should be 16.0mm - shouldn't it? I am going by table 54.7 in the big yellow book.
Sorry if this has been asked before.....

Minimum required size of an earthing conductor is 6mm.
You can't just fail it on the basis of table 54.7 because the designer may have calculated the size using the a diabetic equation.

 

[westward10]

Mine did that "diabetic" bit earlier

 

[Marcus Vaughan]

Thanks for the responses. It's always good to get the reassurances.
Everything tested just fine - I was just initially alarmed when I saw it step down from 16mm to 6mm at the MET.
Every day is a school day!