In a message dated 7/21/99 12:29:09 AM Eastern Daylight Time,
> Bill wrote:....... If you have a large resistive short, the wire may
> > go before the 35A fuse. It is always a good idea to use the smallest
> > that you have no problems with. Fuses are to protect the wire.
> Fuses are to protect the device as well.
I have to disagree with you on that. Except in rare cases, the fuses cannot
protect the devices, they are only there to protect the wiring.
What happens if you get a short to ground on the wire feeding a device? You
have ground on both sides of the device, and nothing happens to the device
itself - there is no current flow through the device. What happens if you
short the feed wire to +12 volts? Nothing, there is already 12 volts there.
What if the device itself becomes shorted? Nothing happens to the device
itself, it is already ruined, but if the fuse doesn't blow, there may well be
a fire from burning insulation.
Take a radio for example. A high powered unit can draw as much as 10 amps or
more, and must be fused accordingly. Hwever, it is entirely possible to get a
short inside the radio that draws only 9 amps, which is enough to set the
wiring inside the radio on fire, but not blow the fuse. In this case, a
correctly sized fuse provides no protection at all to the radio.
Bill is right, the function, and the only function, of the fuse is to protect
As an interesting aside, the National Electric Code, which governs the
installation of all commercial and home wiring installations in this country,
and provides criteria for wire and fuse sizing, is published by the National
Fire Protection Association rather than by an Electrical Engineering society
as you might think. That's because wire and fuse sizing is based on
protecting the wire, not the end device, as a fire protection measure.
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74