Thread: Fuses
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Old 4th Oct 2009, 6:20 pm   #29
jamesinnewcastl
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Join Date: Jul 2009
Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Default Re: Fuses

Fuses and MCBs only protect the wiring 'upstream' of them in order to avoid fires and damage (who wants to rewire everything?) - they are only intended to perform that function. Your equipment can go fry as far as the power distribution cares, that's all the way up your plug-top fuse!

Take a mains radio with a 3 Amp fuse for example, lets imagine that it develops a fault that causes it to draw 2.8A rather than the normal 0.4A. While your radio is busy burning furiously, the wiring up to and including the fuse is happily (and safely) operating normally within all its design limits. The fuse didn't protect your radio and your electric bill is growing larger.

The 'safety' gap between 'normal' operating current and fuse popping current is always difficult to assess absolutely as there are usually surges associated with switch on (e.g. charging capacitors) and even normal running (e.g. spinning up disc drives). The difference between the normal minimum and maximum operating currents is one reason for many fuse and MCB variants.

If you know your equipment will draw 20A for 2ms after switch on, then settle to 0.4A, the only fuse that is guarenteed not to nuisence trip is 20.1A. But the prospect of this fuse supporting a fault current of 18A may be a little unnerving! So you would fit a slow-blo 1A fuse (for example). Better to fit an MCB though since fuse wire heating and cooling (through the current spike) causes the fuse wire metal to expand and contract, thus causing metal fatigue and accelerating failure.

It's a hard job providing really reliable 'protection'.

Jim
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