cartridge v. wire fuse?

[WhinYett]

There is an fault in the mains supply to the electric heaters in the community hall for which I am responsible. The heaters are fed from an off-peak meter via a time switch. This switch is not working. This morning an electrician diagnosed a possible fault with time switch which he replaced. However, the replacement is not working either (it's not a new switch but one he retained from a previous job.)

Several years ago something similar occurred but I could only remember vaguely that it concerned a fuse which is in the power line to the time switch. I went and had as look at the fuse myself. I was surprised to find that the cartridge fuse had been by-passed by a piece of fuse wire. No sign of either having blown - I didn't disconnect to check the cartridge independently. Then I noticed that moulded onto the fuse holder was a message saying "wire fuse should not be used". What could be the problem?? The system is 30 years old or more. The fuse may not have anything to do with the fault but there is something suspicious there. Any suggestions TIA Frank

[fred]

In article , WhinYett writes

Simply put, the fuseholder is not designed to safely interrupt its rated fault current with a wire fuse. In a cartridge fuse the wire element will be surrounded by an inert filler and be rated to contain any arc flash. Under high fault conditions a wire fuse may either coat the fuse holder with evaporated metal leading to ongoing conduction or expose it to excess heat, resulting in damage. A wire rated fuse holder will have some mechanism to contain the heat/flash/metal.

No suggestions as to the nature of the fault, you'll need to trace it with a multimeter if you feel safe to do so. Volts and then continuity with the power safely disconnected.

[Andrew Gabriel]

A BS1361 HRC fuse is designed to safely contain the explosion as the fuse blows on a fault current. The carrier and fuse way for the fuse are not designed to contain the exploding fuse wire and resulting arc. Burning metal may spill out and ignite nearby materials, and/or the arc may contact some other metalwork, and cause a short circuit of the supply.

A BS3038 rewirable fuse holder is designed to contain the exploding fuse wire and stop the arc spreading to any other metalwork.

Another point is that the energy let through a rewirable fuse before it blows can be significantly more than an HRC fuse because it has nothing to quickly quench the arc. In some circumstances, this can cause live to earth shorts to take too long to blow the fuse, and cause overheating/damage of the cable - 2.5mm² ring circuits with old T&E cable which has only a 1mm² CPC is an example of this. (This is not an issue if there is also an RCD protecting the circuit.)

I would guess the breaking capacity of a BS1361 HRC fuse is higher than that of exposed bare fuse wire, which might mean that in some cases of a short circuit where a heafty supply is available, the bare wire fuse will fail to interrupt the supply, and the next fuse back up will blow, and/or the circuit wire will blow, which might start a fire somewhere.

[Brian Gaff]

So these are presumably storage heaters then. I've seen older types of these where the elements just die for no apparent reason, or the thermostats, erm dont and cook the elements with a similar result in the long run.

Brian

[Windmill]

My recollection of the fusing curves found long ago in the main public library (probably in a book which had taken the curves from the then-current IEE regulations, or perhaps it was in the regs themselves) is that rewirable fuses generally blow a little sooner than the newer cartridge fuses. It surprised me, until I began to think about the irritation of having to pay higher prices for cartridge fuses.