Fuses in general

Okay, neither my flight manual nor maintenance manual say what kind of
fuses I should be using between the batteries and instruments. The
printing on one looks to be T500L250V, which I understand to mean 0.5
amps, slow-blow, 250 Volts. (It's 5x20mm.)

I know that to protect my wiring I need something in the 2 to 3 amp
range, but slow-blow or fast-blow? Does it matter? In choosing the
fuse, do I consider only the wiring to the instrument, or do I
consider the instrument itself? (If it draws 300 mA peak, I imagine
the fuse must be at least 600 mA.)

I need to replace the blown T500L250V and get some spares for the
others -- I just don't know what to order.

~ted/2NO (software guy)

Borget B400 uses quick acting 0.5amp fuse type "M205" (meaning 5 x20mm)
5x20mm 250Vac
Borgelt B500 uses the same type in 1 amp
any fuse manufacturers 5x20mm fast acting fuse would be acceptable in the
proper value
these should be easy to find wil almost any electronics supplier
tim
Please visit the Wings & Wheels website at www.wingsandwheels.com


"Tuno" wrote in message
...
Okay, neither my flight manual nor maintenance manual say what kind of
fuses I should be using between the batteries and instruments. The
printing on one looks to be T500L250V, which I understand to mean 0.5
amps, slow-blow, 250 Volts. (It's 5x20mm.)

I know that to protect my wiring I need something in the 2 to 3 amp
range, but slow-blow or fast-blow? Does it matter? In choosing the
fuse, do I consider only the wiring to the instrument, or do I
consider the instrument itself? (If it draws 300 mA peak, I imagine
the fuse must be at least 600 mA.)

I need to replace the blown T500L250V and get some spares for the
others -- I just don't know what to order.

~ted/2NO (software guy)

Ted:

You've gotten a good answer from Tim about the proper fuse to use, but
you asked another good question that went unanswered and that is all
too often overlooked. That is, what does the fuse protect? The
answer is, as you suggested, the wiring. AC43.13 has wiring size
tables for common A/C wires and the size fuse needed to protect them.
It is a table worth having available somewhere in your shop. By
referring to that table you can know what size fuse will safely
protect the wire (gauge and run) it is connected to. The other
question you asked -- does the fuse protect the instrument -- is more
problematic. Generally speaking it does not because the instrument
has an internal fuse to protect it. Not all do, of course, and some
fuses are so difficult to access and replace that you'll wonder if
you're better off putting a low value fuse before the instrument.
These are individual decisions based on the instrument you're
installing. Fred

Fred LaSor
SoaringNV
Minden, NV

Fred had some really great thoughts. One thing to add.

Fast blow versus slow blow? Slow blow seems like the easy way to
helping to prevent blown fuses. Slow blow (slo-blo) fuses were
created to handle devices with high "inrush" currents. That is, when
you turn on this type of device, there is a high initial current draw
for a short period (maybe 1-2 seconds) which settles down to a lower
level. Simplistic examples are motors and incadesent light bulbs.

Do you have any devices in your glider that need a slo-blo? Highly
doubtful. I don't know of any "modern" (last 10+ years) avionics/
instruments that need slo-blo. I don't use any. Check the manual on
all your devices to be sure.

Finally, as to using one fuse total or using one fuse per
device...this is a question in which you will get equal proponents on
each side. I prefer one fuse directly at the battery. I run two
batteries.

My $0.02.

- John DeRosa

On Dec 9, 12:11*pm, ContestID67 wrote:
Finally, as to using one fuse total or using one fuse per
device...this is a question in which you will get equal proponents on
each side. *I prefer one fuse directly at the battery. *I run two
batteries.

I don't there is a choice to either fuse at the battery or to fuse at
the instruments. There must be a fuse at, or in, each battery or you
risk an in flight fire. The battery fuse (or circuit breaker) should
be large enough to support all equipment but no larger than required
to do that.

The advantage of also having separate fuses for each item, or groups,
of avionics is that a short at the panel or an internal avionics
short, will only blow the local fuse and leave other equipment still
working. This of course requires all fuses to be properly rated.

I've never blown a fuse in flight in nearly 30 years of flying
gliders, but I have blown fuses when working on the glider on the
ground. For that reason I chose not to give up any panel space for
fuses. They are all behind the panel and can be reached by raising
one side of the canopy (ASW 28).

Andy

Another consideration on fusing batteries is that some battery
connectors allow you to plug one battery into another when they are on
the charging shelf. The results are VERY messy and occasionally
dangerous. If your batteries are ndividually fused the fuse will go
before the wires melt and swell the battery case. Well worth the
small cost of installing a fuse on each battery.

Fred LaSor
SoaringNV
Minden, NV

This is a timely thread for me...
I've been given the onerous task of maintaining the radios in our club's
6 gliders. This winter we are going to wire all 6 the same. The plan is to
have a fuse at the battery, a main switch on the front panel that switches
everything on and off and when off ties the battery to the charge
connector. We also will move the charge connectors to the front of each
ship. We are loosing a lot of batteries due to user stupidity or laziness.
If they stay deeply discharged during the week, they lose most of their
capacity.

I am a proponent of a fuse at the battery. I am afraid that if the power
wires got shorted somehow, they could either start a fire or create enough
smoke to cause an accident. I have 2 ships that use a metal hose clamp to
hold the battery down. I personally think that is a bad idea, as if the
battery shifted, it would short to the hose clamp.

Two questions:
1. What can I use instead of the hose clamps and where do I get it?
2. Any comments on our plan?

Brian Bange
At 19:11 09 December 2008, ContestID67 wrote:
Fred had some really great thoughts. One thing to add.

Fast blow versus slow blow? Slow blow seems like the easy way to
helping to prevent blown fuses. Slow blow (slo-blo) fuses were
created to handle devices with high "inrush" currents. That is, when
you turn on this type of device, there is a high initial current draw
for a short period (maybe 1-2 seconds) which settles down to a lower
level. Simplistic examples are motors and incadesent light bulbs.

Do you have any devices in your glider that need a slo-blo? Highly
doubtful. I don't know of any "modern" (last 10+ years) avionics/
instruments that need slo-blo. I don't use any. Check the manual on
all your devices to be sure.

Finally, as to using one fuse total or using one fuse per
device...this is a question in which you will get equal proponents on
each side. I prefer one fuse directly at the battery. I run two
batteries.

My $0.02.

- John DeRosa

First, a nit-pick; incandescent lamps are resistive and not inductive
and inrush isn't a common component. But low voltage lighting systems
with a core and coil transformer is exactly what will pop a fast fuse.

I would use Anderson Powerpole connectors and strap the batteries down
with good-sized nylon ties, you can get quite large ones if needed. Or
you could heat-shrink some parts of the hose clamp. Also, it sounds
like the battery connectors you are using leave the connections
exposed, if so an insulated connector is suggested. If using crimp
connectors use Panduit brand or maybe AMP and NEVER use the cheap ones
that you see everywhere.

Anderson powerpoles will allow two batteries to be connected, but it
will be + to + and - to - and that might be less entertaining. I guess
if the batteries are at the same voltage nothing would happen.
Anderson uses a small roll pin to join two connectors together, I
really don't like the pins and let a little glue wick between them
instead.

My opinion is that low current fuses on the panel (or behind it) are
more likely to cause problems than prevent them. I use just one fuse
per battery circuit. I prefer a 10 amp automotive blade fuse mounted
at the battery. If it might be of interest, I've prepared a short
treatise on glider battery wiring and fusing including a picture which
is accessed at this page:

http://www.wingrigger.com/wingrigger5_005.htm

Steve Koerner
GW

Brian:

Sounds like a good plan. I like the Anderson powerpole connectors
mainly because they have enough friction during connection to wipe the
surface clean of any oxidation, so give a good connection. I don't
use the roll pin to hold the two terminals together -- just squirt a
little hot melt into the matching indents. And yes, the automotive
blade fuses are an excellent installation on one of the battery
terminals. As for holding batteries down, I agree a metal hose clamp
is an accident waiting to happen. What's wrong with a short piece of
bungee cord?

Fred LaSor
SoaringNV
Minden, NV