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Re: Electrical systems, Fuses

To: british-cars@encore.com
Subject: Re: Electrical systems, Fuses
From: garnett@theory.TN.CORNELL.EDU (Roger Garnett)
Date: Fri, 25 Jan 1991 11:20:36 EST
>From John Taylor:
} One question about fuses - the handbook specifies fuses which run at 17 amps 
} continuously (give or take a bit) but blow at about twice this current.  I 
have
} yet to see a fuse which shows both values, so should I be using "standard" 17
} or 35 amp fuses?  At the moment I assume the rating on ordinary fuses is the 
} blowing current (so I have 35 A fuses)

This spec sounds a bit odd, but possible if the 2x rating is the
_surge_ rating, not the continous rating. 

Seeing as how my degree and a lot of work is in _electronics_, 
I suppose I should answer this with some facts.

Standard fuse ratings reflect their _continous_ load carrying capibility.
The continous rating is what circuits are usually referenced by-
ie: a circuit which normally carries up to 18 Amps might be designated as
a 20 Amp, which gives 10% overhead, and would use a 20 Amp fuse.

There are some variations in reaction time, to allow for surges, etc.
And the blow time of ANY fuse is related to the % of overload & time.
There are fast acting, and slow blow fuses, for critical circuits and high
inductive surge circuits, respectivly. (A computer circuit might want a
fast blow, while a motor circuit might warrent a slow-blow)
Some spec examples: 

LittelFuse 3AG Fast acting- Blow time:
                                110%- 4 hr minimum.
                                135%- 1 hr Maximim
                                200%-10 sec. *Max.*

Littelfuse 3AG Slo Blo :
                                110%- 4 hr min.
                                135%- 1 hr max
                                200%- 5 sec. *min.* 
Note that the ratings overlap, and the real difference occurs in high overload
conditions. There are different reaction time/current curves available.

It sounds like the moke should have a 17 Amp fuse. Most auto fuses
are standard fast acting fuses, usually rated at 32 Volts.
(Don't use 125/250V fuses in your car- they are designed differently)
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