[VAC] Re: Solution to 12 volt fuse problem

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxxxx>]

A 30 amp fuse is more sensitive to overloads than a 40 amp fuse. Heat
from a loose clip makes the fuse even more sensitive because its a
thermal device with a low melting point metal as the fuse element.

In my experience, large fuses used in mobile applications tend to fail
from vibration and fatigue. Headlight fuses used to fail about every
50,000 miles in my cars and trucks. The surge and operation of the
headlights (incandescent lamps draw a surge about 15 times operating
current at the instant of being turned on) heats the fuse element which
expands. It's constrained by the fuse assembly so bends. Then when the
load is taken off, the fuse element shrinks and straightens the bend.
Eventually the fuse element breaks in the bend. This is a fatigue
failure, not an overload. There's no melting each side of the crack. If
overloaded there would be melting. Sometimes the crack happens where the
fuse element is soldered to the end caps. That you can't see but
sometimes the end cap comes off the fuse. That also is not a sign of
overload. Because there are several different metals in the end cap
(fuse wire allow, solder, and brass cap) there can be corrosion if the
fuse gets wet from condensation and that corrosion can cause an fuse to
open.

So one should always suspect a fuse, when a circuit has no voltage. If
the replacement fuse blows rapidly (in under a year) there might be some
problem in the circuit, such as a wire worn bare by vibration along a
structural member. If a fuse blows in minutes, there's a severe problem
that has to be found and fixed.

Standard electrical safety practice is to choose the fuse size to
protect the wire. E.g. a fuse rating that will keep the wire from
overheating and damaging the insulation. By the National Electrical
Code, #14 copper wire is rated at 15 amps, #12 at 20 amps, and #10 and
30 amps. Smaller fuse sizes are appropriate to protect loads like
motors. In 12 volt applications, we tend to use larger wire to minimize
voltage drop.

Gerald J.