I have been deluded by my simple mind into thinking that a 5-amp contact
breaker for dc circuits would break at 5 amps regardless of whether it was
rated at 12 or 24 amps. Not so. Having installed 11 of them from a motor
trader's catalogue, with the great advantage that they were exchangeable
with spade-type motor fuses, I was warned that because they were deemed
suitable also for 24-volt circuits they might not break at the right
amperage. I was incredulous - amps are amps.... relative to wire thickness
that's what makes the heat that breaks the fuse or clicks the breaker,
surely. No; they broke contact at twice the rated current, by which time my
trial variable-length of hotwire was glowing brightly. Had to resort to good
'ole fuses and write off 110.00. Still don't understand the physics. Ah.,
well....
----- Original Message -----
From: Fred Fillinger <fillinger@ameritech.net>
Subject: Re: amps
> Very well stated, Tony. Picking fuse/breaker ratings with respect to
> the device's nominal current draw may not allow for inrush current.
> More pertinent to fast-acting fuses verses breakers, and often true of
> motors and avionics -- on the test bench I've seen some spirited
> inrush on things like transponders or avionics with DC-DC converters
> for displays.
>
> OTOH, if wire size selection has been conservative, I'd use caution in
> placing too high a fuse/breaker rating on something that's really
> expensive. Sometimes an avionics mfr. will specify even 16 ga. be
> used on the 14V line, even though the box draws, say, 1.3 A. But if
> the fault inside were to kick the draw to just below a 10-amp
> fuse/breaker, you could be slowly cooking component$ inside that are
> not initially defective and frying traces off a PCB board. 5-amp here
> would make me feel better in this example, even less if the mfr. says
> so (e.g., 3-amp for the King KT-76/78 xponder that draws 1.8 A max,
> 1.1 standby, but just maybe not a fast-acting).
>
> Regards,
> Fred F., A063
>
> Tony Krzyzewski wrote:
> >
> > The thing to remember is that the fuse protects the wiring, not the
device
> > on the end.
> >
> > 1/ Work out the current draw of the device.
> > 2/ Select a wire size that can handle the current
> > 3/ Select the protection appropriate to the wire size.
> >
> > For the Europa use the following current ratings...
> >
> > Wire size Maximum Current
> > 24 4
> > 22 5
> > 20 7
> > 18 9
> > 16 11
> > 14 14
> > 12 19
> > 10 26
> >
> > Use a fuse equal to or lower than the maximum current rating. eg a 3A
fuse
> > would protect any wire size of 24AWG or larger diameter.
> >
> > The basic concept is that the fuse should blow before the wire.
> >
> > The table above is for 'short' wire runs. With the possible exception of
> > position lights on the end of the wing tips there are no long wire runs
in
> > the Europa. For the position lights it would probably pay to go to the
next
> > larger wire size than is required. I am running mine with 18AWG.
> >
> > Hope this helps
> >
> > Tony
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