TNS max Ze 0.8 ohms WHY?

[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.