Thread: Fuses
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Old 22nd Mar 2014, 12:11 pm   #42
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Join Date: Mar 2014
Location: Wolverhampton, West Midlands, UK.
Posts: 153
Default Re: Fuses

One more quick thought,

the type of voltage across the fuse is obviously different with AC and DC.
So the AC voltage is following a sine wave + whatever transients are on it.
The fuse element is heated with the combination of V & A so this is varying on an AC supply. The clearance time is dependant upon the arc quench with a drop in voltage this is helped (depending where on the cycle the fault occurs).
Fault current is dependant upon impedance of supply and VD.<<how much current can you draw from a fuse board..a fuse is a piece of wire a CB is two contacts.

On DC the fault has a clearance time dependant upon Arc and distance gap of the fuse element. There is no change in supply voltage except possible fast release of current from supply caps.

The fuse rating is different on different fuse types cartridge fuse 1.5 times rated value. The clearance time is dependant upon fault resistance path.
If you have a "Bad" earth then the clearance time will be slower. You need enough current to flow as fast as possible to make the fuse element operate.
This is known as ADS(automatic disconnection of supply)
A fuse not exceeding its operation point could allow extreme damage. heating of both earth cables and supply cables possible shock from elevated voltage on a chassis. Waiting for the fuse to blow..

Slow blow for things like magnetisation current of a transformer. (Surge at turn on..)
Quick blow for electronics or supplies with low current rating that will not stand high fault current..

Over rating of fuses to allow for surge like magnetisation current is a bad idea..when its value would exceed well above operational current.

Just for fun..


Last edited by Tubeglow; 22nd Mar 2014 at 12:26 pm.
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