Thanks for the run down on your theory. It makes sense. Also sounds like
one single fuse to protect many devices is foolish. In that scenario it
clearly is only to protect the wiring.
On the issue of circuit breakers (especially those that are thermal
sensitive and reset themselves when cool) I have this story. An
acquaintance had a problem where the lights on the car would go out after a
55 minute drive. His nightly drive home from work was 60 minutes. Each
night for a few weeks he'd have the lights go out on an unlit country road.
His wife would get out with a flashlight and walk in front the last mile or
so. Drove them nuts of course. By the time they got to the house on a
gravel road the lights would suddenly come back on. I was not aware there
were breakers of this sort on a car (80-something Mustang) and couldn't help
them. I think a fuse would have been a better idea here.
Peter S
----- Original Message -----
From: <DANMAS@aol.com>
To: <alfapete@pacbell.net>; <spitfires@autox.team.net>
Sent: Wednesday, July 21, 1999 6:51 PM
Subject: Re: Spit fuses
>
> In a message dated 7/21/99 1:22:25 PM Eastern Daylight Time,
> alfapete@pacbell.net writes:
>
> > Certainly the first function is to protect the wiring. I don't dispute
> > that. But devices such as a wiper motor which requires a fairly high
> > current draw / high amp fuse has internal motor windings which are
smaller
> > than the wiring going to it. If the wiper is stalled in one position
(by
> > snow or something placed above the wipers) the amp draw goes up until
> > someting gives. Wiring, with its length will disipate heat up before
> > finally melting. The windings in the motor will cook and you ruin it -
> > potentionaly faster than damaging the wire. If you put in a fuse which
is
> > rated higher than normal you certainly could/will ruin the motor before
the
> > wiring has any ill effects. IMHO.
>
> Peter,
>
> I understand what you're saying, but I must still respectfully disagree
with
> you. The wiper motors in our Triumphs only pull 1.2 amps under normal
usage.
> Even if the current is doubled under locked rotor conditions, it would
still
> only be 2.4 amps, and I think the motor windings can safely carry that for
a
> reasonably long period of time.
>
> Larger motors that can be damaged by locked rotor current are usually
> protected against damage from long term operation in this condition by
> thermal breakers rather than overcurrent trip devices. That is, they are
> tripped on high winding temperature rather than over current. There is a
good
> reason for this. Most motors will, from time to time, experience brief
> moments of locked rotor current during the course of normal operation. For
> example, a temporary jam on a conveyor belt. Hopefully, the jam will break
up
> by itself, and the operation can continue uninterupted. If not, the
thermal
> breakers will trip after a period of time before any damage is done to the
> motor. If over current devices were to trip the motor on overcurrent, it
> would be an instantaneous trip, and it would be a big nuisance having to
> reset the breaker or install a new fuse unnecessarily for a temporary
jammed
> condition that would have cleared by itself if left alone.
>
> Taking the wipers and snow example you gave, you can see where this would
be
> a nuisance also. If you turned on the wipers, hoping it was a soft snow
and
> not hard ice, the wiper motor would instantly blow the fuse, and you would
be
> faced not only with having to clear the snow to get the wipers freed up,
you
> would also have the task of getting the snow and ice cleared from the hood
so
> you could open it and replace the fuse.
>
> Another thing to consider is how you would need to fuse it. If you put a
two
> amp fuse on it, it would have to be the only thing on that fuse. As
presently
> configured (Triumphs), the fuse feeding the wiper motor also has about 15
> amps of other loads on it - heater fan, gauges, turn signals, etc. A two
amp
> fuse just wouldn't carry the normal load of the wipers and ANY thing else.
>
> You may be right, the wiper motor may burn up if left on in a stuck
condition
> for an extended period of time, but I'm sure it would take much longer
than
> the time the average driver would leave the switch on if the wiper blades
> were not moving. If a driver leaves the switch on that long, as far as I'm
> concerned that is the same as putting aluminum foil in the fuse holder to
> replace a blown fuse. Not something to design for. I know in the times
I've
> had wiper blades stuck from ice and snow (I grew up in Indiana, where
winters
> were a bit harsher than here in Tennessee), no damage was done to the
motor
> even after several tries, hoping the wipers would break free on their own
and
> I wouldn't have to get out and remove the ice.
>
> There are some situations where the fuse protects not only the wiring, but
> the device as well, but these cases are few and far between. I don't know
of
> any in a car of the era we're interested in. Power windows might be an
> example in a modern car, but there the makers use torque switches (I
think -
> anyone know for sure?) to shut off the motors when the window travel is
> obstructed. Having to replace a fuse everytime one of the kids hung out
the
> window as it was going up would be a pain (and you certainly don't want
the
> motors powerful enough to cut the kid in two!).
>
> My purpose in posting what I did was not to correct you, but to get people
> thinking about how to use fuses when installing aftermarket devices. If
you
> are protecting the device with a fuse, you will put the fuse as close to
the
> device as you can. If you are protecting the wiring, you will put the fuse
as
> close to the power source as you can, which is as far away from the device
as
> you can get. Installing a radio with the fuse located right at the radio
is
> almost as bad as not having a fuse at all. The power feed to the radio is
> completely unprotected. It's far, far more likely that you will have a
> problem with the wire insulation chaffing and exposing the bare wire to
the
> car body than it is that you will have a problem inside the radio.
Especially
> if the person doing the installation is not experienced in wiring
techniques,
> as most of us are not.
>
> Regards,
>
> Dan Masters,
> Alcoa, TN
>
> '71 TR6---------3000mile/year driver, fully restored
> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion -
see:
> http://members.aol.com/danmas/
> '74 MGBGT---3000mile/year driver, original condition - slated for a V8
soon
> '68 MGBGT---organ donor for the '74
>
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