TNS max Ze 0.8 ohms WHY?

[silver surfer]

Hi folks.

Can someone clear this one up please.

Exisitng TNS system to a domestic, max permissable Ze is 0.8 ohms.

So if there was a fault immediately after the cutout head 230v/0.8 ohm a fault current of 287 amps would flow, if the PD was 100 amp BS88-2 it would take 100 seconds to operate (BRB p296).

Cant be right!

 

[telectrix]

looks right to me. bear in mind that 0.8 is the max. allowable. in reality, it's generally much lower.

 

[silver surfer]

Yes i agree in reality its much lower, but aint a distribution circuit supposed to a 5 second disconnection time?

 

[telectrix]

but we're talking DNOs here. they make up the rules as they go. and if they can't abide by the rules, they change them anyway.

 

[silver surfer]

The penny dropped as soon as i hit the "post reply" key.
Cheers.

 

[oldtimer]

0.8 sounds like a earth clamp on to a old metal sheathed mains cable if so call the DNO for a proper earth I had 0.7 when I surveyed a job and 0.9 when I was doing it plus when all was finished the guy down stairs was in the same boat but his improved 10 fold as he had 2 6mm bonds onto the communial gas and water pipes ie he was borrowing a good earth from my customer

 

[Chr!s]

but we're talking DNOs here. they make up the rules as they go. and if they can't abide by the rules, they change them anyway.

Their installation within the customers premises needs to meet the requirements of BS7671.

 

[i=p/u]

And before that esqwar.. Before you do any job, do you need to know the ze??

 

[telectrix]

And before that esqwar.. Before you do any job, do you need to know the ze??

if you don't measure the Ze, how do you know if you've got a good earthing system?

 

[i=p/u]

But what if it is new build and you cant get definate readings,

 

[oldtimer]

Too many what ifs here thats me unsubscibing from this post

 

[GaryM]

But what if it is new build and you cant get definate readings,

If there's a live supply then what's stopping you measuring Ze (or Zdb) with or without bonding conductors connected (in the case of an in-service installation which can't be isolated)?

 

[Des 56]

Existing TNS system to a domestic, max permissable Ze is 0.8 ohms.



The question relates to the earth loop

Lets not forget Double insulation

 

[GaryM]

Double insulation?

 

[Des 56]

The thread seemed to be saying the Supplier had to abide by Bs 7671 in the property

Their installation within the customers premises needs to meet the requirements of BS7671.

It was a possible reason why it may not bother them and their equipment

 

[MarkieSparkie]

But what if it is new build and you cant get definate readings,

You could contact the DNO and enquire, but they will almost certainly give you their maximum value (e.g. 0.8 Ohms for TN-S). Most electrical designers faced with a new build installation, base their design on the worst case i.e. maximum allowable Ze value anyway.

 

[Deleted member 26818]

Hi folks.

Can someone clear this one up please.

Exisitng TNS system to a domestic, max permissable Ze is 0.8 ohms.

So if there was a fault immediately after the cutout head 230v/0.8 ohm a fault current of 287 amps would flow, if the PD was 100 amp BS88-2 it would take 100 seconds to operate (BRB p296).

Cant be right!

You wouldn't use one of those types of fuses at that rating.