When To Fit Switch Fuse/Isolator

[nlbhoy]

I've had to run a 25mm 3core swa from meter (outside) to DB about 8m away, do I need to fit
a switch fuse or isolator switch in meter box?

Or does this only apply to meter tails more than 3m?

Thanks in advance

 

[davesparks]

you dont need a fuse isolator switch..fuse isolator switch needed just when you have under 25mm cable..in every home dno supply its 100a who its suitable for 25mm swa or 25mm tails..if you run a 16mm swa you have to have fuse isolator switch after meter with the fuse suitable for this size of conductors..in your situation its not necessary..for isolator switch its two different situation..if the meter its already fix you don't need to have isolator switch..if the meter its not fix yet and you waiting for suppliers to come and fix again its on two diferent situation..if you call EDF they not asking you for isoator switch .they dont mind if you have or dont have switch isolator..if you call British Gas they fixed their own switch isolator after meter where you fix your swa or tails..

The DNO specific the maximum length of conductor which may be connected to their service fuse, for any cable longer than that a distribution circuit needs to be installed. The size of the cable is irrelevant, it is the length which matters. This is backed up in the regulations by the requirement that cables not protected by a suitable fuse should not be longer than 3m (the service fuse is not there to protect consumers cables and must not be treated as such)

Apologies if I have misunderstood your post at all, but it is very difficult to understand. Could you please make the effort to write in coherent English please.

 

[Midwest]

davesparks or others may be able answer a query I have on the sizing of fuses in such situations.

Lets say the DNO fuse is a 1361 Type 2 100amp, and the switch fuse would be say a Wylex with BS 88-2 fuse (I think) with 25mm tails, total load would not exceed 80amp.

What would be the correct size of fuse to put in the customers isolator, 80amp or 100amp. I only ask, because I have asked previously, and been advised to use either arguments being;

select 100amp cos, you will not be able to predict which would go first in fault, and would deprive installation of 100amp supply should that be required retrospectively or

80amp cos that would provide discrimination with the DNO fuse.

 

[davesparks]

davesparks or others may be able answer a query I have on the sizing of fuses in such situations.

Lets say the DNO fuse is a 1361 Type 2 100amp, and the switch fuse would be say a Wylex with BS 88-2 fuse (I think) with 25mm tails, total load would not exceed 80amp.

What would be the correct size of fuse to put in the customers isolator, 80amp or 100amp. I only ask, because I have asked previously, and been advised to use either arguments being;

select 100amp cos, you will not be able to predict which would go first in fault, and would deprive installation of 100amp supply should that be required retrospectively or

80amp cos that would provide discrimination with the DNO fuse.

For the purpose of the installation you can ignore the service fuse as it is not yours and cannot be used to provide protection to any part of the installation.
This does lead to the ridiculous situation that you can end up with two fuses of the same rating in series, but from a theoretical/regs point of view there is only the consumers fuse there.
If you wanted to discriminate fully with the DNO fuse then I believe you'd need to be installing no more than a 40A fuse downstream of it, but may be wrong on that.
However the service fuse does have different characteristics to the HRC you would be fitting in a switchfuse (unless it's a KMF type switchfuse which uses service fuses)

The bottom line is that you should design your distribution circuit in its own right as you would any other circuit in your installation. So establish your design current, then select your ocpd rating to suit.

The last time I did this in a house was for a small bungalow which I ran a 63A submain.

 

[Midwest]

Thanks davesparks, for explaining that for me.