Silly Question?

[hightower]

Hmm, think I'm having a moment here, but having a bit play with adiabatic, and say a TN-S system has a Ze of 0.8, the fault current would then be 287.5A which would not break a BS 88-3 100A supply fuse within 5 seconds.

I know I'm missing something stupid here, but can't for the life of me see what it is....

 

[telectrix]

you are correct.but....... the actual Ze will likely come down a bit from 0.8, due to the bonding being reconnected. most TN-S systems are well below 0.8in reality.the 0.8 is a suggested max. value.

 

[hightower]

you are correct.but....... the actual Ze will likely come down a bit from 0.8, due to the bonding being reconnected. most TN-S systems are well below 0.8in reality.the 0.8 is a suggested max. value.

Doesn't it the supply need to disconnect in a maximum of 5 seconds though? Which would result in (for a 100A BS 88-3) a current of 580A in a fault, 230 / 580 = 0.397 ohms. Why are Ze's allowed to be 0.8 on TN-S if 0.8 won't blow that fuse in 5s?

 

[Bobster]

Remember the DNO fuse doesn't fall under BS7671

 

[hightower]

So, (and correct me if I'm way off, just trying to get my head around it), using the adiabatic for an main earthing cable in a TN-S with a Ze of 0.8 would be as follows:

Ipf = 230/0.8 = 287.5A

100A BS 88-3 would take around 200 seconds to disconnect with a fault current of 287.5A

K from the table would be 143

S = √287.5(2) * 200 / 143

S = 28.42mm(2)

I know you can use the table and select a 16mm(2) earthing conductor, but surely according to the adiabatic a cable of 16mm(2) wouldn't be able to take a fault current of 287.5A for 200 seconds. If I was to transpose the formula, a 16mm(2) would only be able to take 287.5A for a duration of 63 seconds.

 

[hightower]

Remember the DNO fuse doesn't fall under BS7671

How would you calculate a main earthing conductor size then using the adiabatic? For example, it's a new install, you don't know the Ze but it's TN-S and the DNO have said "it'll be no more than 0.8".

 

[Bobster]

How would you calculate a main earthing conductor size then using the adiabatic? For example, it's a new install, you don't know the Ze but it's TN-S and the DNO have said "it'll be no more than 0.8".

Your main earthing conductor is not there to protect the DNO's cable or the tails from the meter. This is why your tails are double insulated.

Edit: your maths is correct though.

 

[hightower]

Your main earthing conductor is not there to protect the DNO's cable or the tails from the meter. This is why your tails are double insulated.

Edit: your maths is correct though.

So for a main earthing conductor is it really just a case of "we've got 25mm tails so 16mm earthing conductor" from the table in the BYB?

 

[Bobster]

It was reg 543.1.4 on the BGB.

But yes, in a nutshell.

 

[hightower]

It was reg 543.1.4 on the BGB.

But yes, in a nutshell.

Thanks, I'm at work so don't have my BYB with me to check through the regulations - will have a look when I get back from college tonight.

 

[Simonslimline]

The Distribution Network operators don't work to BS7671. It's The Electricity Safety, Quality & Continuity Regulations.

@ Hightower - I will PM you back tomorrow mate, when i am not so tired.

 

[davesparks]

If you look at UKPN's specifications and design guides it does mention that their equipment fitted on a consumers premises should comply with the 5s disconnection time of 7671.
There's a copy buried in my laptop somewhere, I'll see if I can find it.