What Should Rating of Switchfuse Be?

[acvc]

OK sorry I'm being really impatient and possibly thick here, but I've just done this for the first time today .....


if a switchfuse is used on tails over 3m long to protect the DNO fuse, should the fuse rating in the switchfuse be less than the DNO main fuse? If not why not? I think I'm missing the point here??????

 

[topquark]

OK sorry I'm being really impatient and possibly thick here, but I've just done this for the first time today .....


if a switchfuse is used on tails over 3m long to protect the DNO fuse, should the fuse rating in the switchfuse be less than the DNO main fuse? If not why not? I think I'm missing the point here??????

The switch fuse is to protect the run of cable, not to "protect" the DNO fuse. The DNO suggest that there fuse is only to protect a maximum tail run of 3M, anything over that and you should provide your own protection. The fuse rating should be less than or equal to the DNO fuse for common sense reasons (protecting the DNO fuse, there I've said it!).

 

[acvc]

Thanks Topquark, but if the two fuses are of the same rating e.g. both 80A or 100A, then which will blow first? Shouldn't there be some discrimination?

 

[1shortcircuit]

What size fuse is currently fitted by the DNO and what size cable will you be protecting?

What is the max demand of the installation that is being supplied?

:thumbsup

 

[acvc]

7m of 16mm single-core tails, clipped direct, (backwards from C/U) to 80A switchfuse, to DP isolator to meter to 80A main cut out fuse. New mains head +PME+isolator being fitted by DNO tomorrow to this spec.

 

[acvc]

Is this issue of discrimination between a switchfuse and main cut-out fuse a non issue?

If they are both e.g. 80A and both HRC, will the fuse closest to the point of fault blow first?

 

[Guitarist]

You will only achieve discrimination if you make your fuse size significantly lower than that provided by the DNO, and this may not then be adequate for your installation needs.
As has already been said, protect your installation with the correct size fuse, just as long as it is not larger than the DNO one.

 

[telectrix]

there should be no discrimination as DNO are an equal opportunities employer. :devil2:

 

[PEG]

ALL fuses are created equal,but some are more equal than others.......:yinyang:

 

[topquark]

If you want to provide some discrimination then put a lower rated fuse in (in correleation with the demand). DNO around here only will offer 60A or 100A now. So use a 100A DNO fuse with maybe an 80A in the switch fuse.

With fuses discrimination is difficult (at best) to achieve with equal fuse sizes. It would be "touch and go" as to which would blow first as they are not exact beasts. Even with the same rating from the same manufacturer.

 

[1shortcircuit]

Knowing the max demand of the installation will help you considerably and help you to determine the correct size fuse. Max you can go is 80 which is equal to the DNO but if you could prove max demand will be considerably less then you may be able to prove that a 60A will do the job comfortably.

What installation is it?

I thought that a new install would have been 100A fuse with 25mm tails???

 

[telectrix]

it could be argued that a better solution would be to fit a MCB instead of a fuse. as a rule, the MCB would operate well before the DNO fuse, irrespective of discrimination in ratings.

 

[Guitarist]

it could be argued that a better solution would be to fit a MCB instead of a fuse. as a rule, the MCB would operate well before the DNO fuse, irrespective of discrimination in ratings.

That does indeed seems a better solution.

 

[Mark_Burgess]

it could be argued that a better solution would be to fit a MCB instead of a fuse. as a rule, the MCB would operate well before the DNO fuse, irrespective of discrimination in ratings.

Thats the route I'd go down.

 

[telectrix]

and it's resettable as well. imagine trying to get hold of a 60/80A BS1361 at 2.00a.m.