Fuse ratings

wallyanker · May 3, 2012

Hi Guys

Just about to start a project on a new build. When i signed for all the paperwork on handover, it was described that the unit has a three phase 63a supply to an isolator, it is then our responsibility from there.

Get to the site this morning, the isolator is installed no problems but each phase has a 125a fuse. Is this normal as the fuse "upstream" is only 63a?

Thanks
Pete

jumpin jax · May 3, 2012

not very discriminating is it, but still protected by 63 amp. might as well have links put in instead. Have you physically seen the 125 amp fuses or is that what the holder says on it?

wallyanker · May 3, 2012

Seen them this morning as I put a temp board in for the builders and dry liners. Just thought that they should be the same size or smaller than the 63a. Is there a reg against this or just bad practise?

jumpin jax · May 3, 2012

Why has the supply got another set of fuses and not entering a DB isolator or is access to the primary fuses not available and if so this is not good.

wallyanker · May 3, 2012

Our unit is within a health centre and access to the supply 3 phase 63a is not accessible by us, unless we speak to the health centre manager to gain access to the main board. Our unit has something very similar to the below picture, 125a marked on each fuse inside the box.

jumpin jax · May 3, 2012

What are you using the supply for? what load are you anticipating? could you replace the fuse links for lower rating ie40a or 50a even?

spark 68 · May 3, 2012

I have seen this type of thing before, where it is just used as a switch (Isolator, non fused), as one of the other posters has said, with copper links in place of the fuses.

If you think about discrimination, you would need to fuse down by half again, which would effectively reduce the current capacity of your supply significantly.

Nothing against the regs here as the supply is (or should be) already fused down to the requirement of the supply cable, and presumably you will be feeding a DB from this.

wallyanker · May 3, 2012

The unit will be fitted out to be a pharmacy, so not that heavily loaded. My point is, should the original installer have put in the same size, if not smaller fuses in the 3 phase box above to resolve discrimination? Is there any regs that state this? We have had all kinds of issues with the contractors who built this building, not fire painting steel supporting columns, etc, so need everything to be in order before we commence our fit out works.

jumpin jax · May 3, 2012

Iwould gain access to the 63a fuses, pull them, remove the switch fuse, put it on e bay, install panel board with isolator, replace fuses, job done.

spark 68 · May 3, 2012

My point is, should the original installer have put in the same size, if not smaller fuses in the 3 phase box above to resolve discrimination?

Why ?

Putting the same size fuses in it would not discriminate, and to put smaller ones in it they would need to be half of your 63 A OCPD feeding it.

Is this a final circuit ?, or is it just the Isolator ?, and is it going to be feeding a DB ?

wallyanker · May 3, 2012

It will feed a db that will then supply power and lighting circuits within the pharmacy unit.

spark 68 · May 3, 2012

It will feed a db that will then supply power and lighting circuits within the pharmacy unit.

So no problem then, as your final circuits will be fused/OCP'd at the DB and provide some discrimination, it is a bit overkill, but ok nevertheless, as said it is just essentially an Isolator, and the fuses could be replaced with suitable copper links.