Meter tails

Wilko · Feb 28, 2017

Deleted

Wilko · Feb 28, 2017

I think the 3m is an historic answer to our question. There needs to be a limit so DNO fuse will melt when required, and with TNCS and 25mm tails I think technically it could be a lot longer than 3m. For example 88-2 80A fuse needs 400A to melt in 5 sec, and this is a fault loop of no more than 0.575 Ohms at 70C. DNO quote 0.35 Ohms as max for TNCS so 0.575-0.35 = 0.225 Ohms left for tails. So assuming 25+16 is 0.875+1.4x0.8 = 2 mOhms per metre. And so tails 100m long would still provide ADS, unless I've messed it up (always possible). This was just out of interest and it's DNO call anyway.

Midwest · Feb 28, 2017

Des 56 said:
If there were a 100 amp fuse in the cut out and we had to account for discrimination,we would end up putting a 2 sizes lower fuse (63A )to have any chance

The client could become unhappy losing a fair portion of their capacity for no apparent benefit if that was the case

So the point of putting a 100a switch fuse after a 100amp cut out fuse is?

sparksburnout · Feb 28, 2017

Midwest said:
So the point of putting a 100a switch fuse after a 100amp cut out fuse is?

I think the point, is that the DNO have no control over what may be installed beyond the meter. Some numpty might come along and run a bit of bell wire to a DB for all they know. In the case of a switch fuse 4M from the cutout then under a low impedance fault then obviously both devices are going to go, even if it is as low as 63A. They are trying to put the responsibility on the installer to make sure the cables beyond their equipment are adequately protected, which their fuse may not?