Fuses in general

OK Andy, I'll bite. Lighting loads are generally considered to be
resistive and not inductive. While not perhaps true in some examples,
for the subject of this thread I think the nit was picked correctly.
For example, if you connect 16 amps of lighting load to a 20 amp fast
fuse (or 'Quick Operating' magnetically enhanced circuit breaker) you
will have a reliable circuit. If you connect a 16 amp capacity core
and coil transformer to that same fuse, or if you connect 16 amps of
old style HID lighting with the reactive ballasts acting as inductors,
you will pop that fast fuse most of the time in an AC circuit. I think
that if in an AC circuit the contact closure is timed exactly as the
sine wave crosses the 0 volt line there is sometimes sufficient lag to
slow the inrush enough to keep the fuse from popping. That's my theory
anyway.

If you still don't buy what I am saying, I'm perfectly happy to
concede the point. However, the original post implied that motors and
incandescent light bulbs require slow-blow fuses. (Many) motors do and
incandescent light bulbs do not, so if you are designing an electrical
system and the only "Inductive" loads are incandescent it would be
fine to use a fast blow fuse in order to protect electronic gear.

I guess this brings up another point, if there is more than one fuse
certainly the battery fuse or CB could be a slow blow and the fuses at
the gear would want to be fast blow in most cases.

On Dec 10, 11:39*am, Andy wrote:
On Dec 10, 8:44*am, Andy wrote:

Sorry wrong nit to pick. *The resistance of an incandescent lamp
filament is temperature dependent. *Anyone that has used lamps as
current control devices is well aware of this.

I suppose I should have completed the information. *The resistance is
low when the filament is cold. *There is a large inrush current which
reduces as soon as the filament gets hot.


Andy

My glider has multiple batteries with a switch to change between them
during flight. To, hopefully, eliminate loss of logger data during the
brief interruption of power during the switching process there is a large
capacitor to ensure continuity of supply to selected instruments during
switching. Although there is a resistor to limit the inrush current to
the capacitor this type of circuit needs a slow-blow fuse. (Factory wired
in last two years.)

This looks like a major clue that changes everything. (Well, almost
everything.)

My ASG29 also has two main batteries and a dial switch on the panel
that selects which battery is providing power to all of the
instruments. I wonder if the transition from B1 to B2 is popping the
(fast-blow) fuse on the back of the B40. And it would explain why
there was a T500L250V fuse in the panel fuse holder; I understand that
the "T" means slow-blow ("T"imed?), where an "F" would indicate fast-
blow.

2NO

On Dec 11, 9:00*am, Tuno wrote:
I wonder if the transition from B1 to B2 is popping the
(fast-blow) fuse on the back of the B40.


I have the same factory standard rotary power switch and an unmodifed
B40 that is always left on and selected to external power. My B40
fuse has never blown. (There are no external capacitors in my glider
nor do I need any).


Andy

On 10 Dec, 22:53, brianDG303 wrote:
OK Andy, I'll bite. Lighting loads are generally considered to be
resistive and not inductive. While not perhaps true in some examples,
for the subject of this thread I think the nit was picked correctly.
For example, if you connect 16 amps of lighting load to a 20 amp fast
fuse (or 'Quick Operating' magnetically enhanced circuit breaker) you
will have a reliable circuit. If you connect a 16 amp capacity core
and coil transformer to that same fuse, or if you connect 16 amps of
old style HID lighting with the reactive ballasts acting as inductors,
you will pop that fast fuse most of the time in an AC circuit. I think
that if in an AC circuit the contact closure is timed exactly as the
sine wave crosses the 0 volt line there is sometimes sufficient lag to
slow the inrush enough to keep the fuse from popping. That's my theory
anyway.

If you still don't buy what I am saying, I'm perfectly happy to
concede the point. However, the original post implied that motors and
incandescent light bulbs require slow-blow fuses. (Many) motors do and
incandescent light bulbs do not, so if you are designing an electrical
system and the only "Inductive" loads are incandescent it would be
fine to use a fast blow fuse in order to protect electronic gear.

I guess this brings up another point, if there is more than one fuse
certainly the battery fuse or CB could be a slow blow and the fuses at
the gear would want to be fast blow in most cases.

On Dec 10, 11:39*am, Andy wrote:



On Dec 10, 8:44*am, Andy wrote:

Sorry wrong nit to pick. *The resistance of an incandescent lamp
filament is temperature dependent. *Anyone that has used lamps as
current control devices is well aware of this.

I suppose I should have completed the information. *The resistance is
low when the filament is cold. *There is a large inrush current which
reduces as soon as the filament gets hot.

I too vaguely recalled that the initial current in a cold
incandescent bulb might be rather high. I cant however
remember precisely. It is though easy to find out
if you have a bulb and a multimeter. Just measure the
resistance when the bulb is cold and check
the expected current with V = IR. Well I = V / R.

This article
http://en.allexperts.com/q/Physics-1...lamp-surge.htm
states that the initial current is 10 times the
running current for a domestic light bulb.
Bulbs used in gliders are likely to run cooler
so that they can be tougher to better resist
vibration, and so the initial vs final current
ratio will likely be lower.

The other aspect to this is of course that the bulb
does warm up very quickly and so even a fast blow fuse
may well be unable to respond quickly enough to
fail 'inappropriately'.