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RE: Lucas Fuses

To: "'Tyson Sherman'" <>, Ronald Olds <>,
Subject: RE: Lucas Fuses
From: "Nunez, Eduardo" <>
Date: Tue, 7 Apr 1998 16:11:10 -0400
Just some info that I've picked up over time.  This is not a lesson on
fuses, electrical system, cables, etc.

Fuses are applied to protect the wiring assembly, not the electrical
load.  This means that the fuses blow to protect the wiring in to the
motor, not the motor itself.

A fuse is characterized by its rated current, which is the maximum
current that the fuse can carry at an ambient temperature of 25C for 100
hours without any interrutions (ie. 'blowing').  A change in the ambient
temperature will cause the fuse to be derated accordingly:  
- At higher temperatures the fuse will respond faster to a given
overload (will open or blow at a lower current)
- At lower temperatures the fuse will respond slower to a given overload
(will open or blow at a higher current)

Most modern (stressing the modern part) automotive fuses have an ambient
derating of 70% at 85C (will hold 70% rated current at 85C ambient
temperature).  These same fuses will also have a specified blow or trip
time at 135% of the rated current at 25C.  For example a Minifuse (the
smallest of the fuses in current use in cars) specifies that the fuse
will operate at 110% current rating for at least 100 hours at 25C and
will open at 135% current rating between 0.75 seconds and 30 minutes at

In most modern (that word again) automotive applications the cable size
(gage) and fuse amp rating for a circuit is based on the expected load,
the expected ambient temperature where the load AND the fuse is located,
the type and thickness of the cable insulation, the number of wires that
are run next to (or in the 'bundle') the target circuit, and other
factors such as proximity to heat sinks or sources.  As a general rule,
the normal expected current through a fuse in any given circuit is 70%
of its rating.  For example, a circuit that normally sees 25 amps
continous at 25C will have a 35A fuse, and a 30A fuse will be expected
to function with a 21A load continously (or 100 hours, whichever comes

Since a fuse is to protect the wiring assembly the cable size (gage) or
that circuit is selected to carry the rated fuse current so that it can
carry any expected overload for the time it takes the fuse to blow.

This is a very simplistic approach to circuit protection, fuse size
selction, and cable gage selction.  There are some texts available for
reference (the Bosch Automotive Handbook being the first to come to
mind) on this subject, but specific application information is usually
internal to the auto companies, and as such is somewhat guarded as it
applies to their specific applications.  An excellent guide to fuses and
circuit protection in general is published by Littelfuse, 'Fuses for
Automotive Applications'.  Any engineer working with electrical circuits
in vehicles will usually have one (and will kill you if you even think
of taking it away!).

Bear in mind that the above information is in reference to most modern
cars.  Cars which have benefited from at least 23 years (from the build
date of my '75B) of electrical evolution and improvements (which is open
to interpretation!).

Please feel free to comment, or add any other information that we could
all benefit from.

Ed Nunez

> -----Original Message-----
> From: Tyson Sherman []
> Sent: Tuesday, April 07, 1998 3:23 PM
> To:   Ronald Olds; MG mailing list
> Subject:      Re: Lucas Fuses
> >From what I've undersood, a lucas 35 is rated at a continuous 17amps
> and will
> blow at 35, so replace it with an american 35.
> --
> Tyson Sherman
> Ronald Olds wrote:
> > I have a question regarding Lucas fuses in an article in the latest
> MG
> > magazine (No. 68 page 25). It stated that a Lucas 35 amp fuse is
> designed
> > to blow at 17 amps. An American type 35 amp would blow at 35 amps.
> Can
> > anyone explain to me why a Lucas fuse wold blow at 17 amps?  Is this
> just a
> > typical example of  fine Lucas technology?
> >
> > Ron Olds

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