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Fuse or Fusible Link Question

 
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nuckollsr(at)cox.net
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PostPosted: Fri Jan 13, 2006 5:38 am    Post subject: Fuse or Fusible Link Question Reply with quote

At 09:21 PM 1/12/2006 -0600, you wrote:

Quote:

<billschlatterer(at)sbcglobal.net>
Bob you replied earlier:

>(2) Im confused about the wire between the connection terminal for the
>main bus and the DC power master switch. Why have a wire with a
>fusible link coming off the bus connection terminal when you could just
>have the wire hooked up to the bus with a standard fuse like the other
>circuits? I know Im missing something...

I you choose to use a fuse block in conjunction with an alternator
control having a crowbar ov protection system, you'll want a circuit
breaker on the panel which is an EXTENSION of the bus from the fuse
block to the breaker. The time constant of fuses is MUCH too fast to
run upstream of a breaker during the crowbar event . . . hence
the fusible link.

Bob . . .
In a previous post, I understood you to say that you could normally
substitute an appropriate fuse, 7a in this case for a 22g fusible link. I
take it from the above that there are instances where you want something
slower than a fuse which is the purpose here for the fusible link and you
would not want to run from the CB directly to a 7a fused slot. True? If so,
are there other instances to be aware of?

You also make the comment specifically as it applies to crowbar OV
protection systems and I see it with the crowbar on the 6/1/05 version of
Z13 using a ford regulator. I assume but am not sure if this also applies
as well if using the B&C LR3 regulator with internal OV protection shown on
earlier versions? ?? (I "was" fusing this one also)

Yes, the LR-series regulators INTEND to open a 5A breaker
during an OV event. A 5A breaker is slower than a 20A fuse
in a fault clearing event, hence the attractiveness of
the fusible link

-------------------------------
Here's the post you're referring to:

At 07:24 PM 11/27/2005 -0600, you wrote:

Quote:

<billschlatterer(at)sbcglobal.net>

Bob, I understand that we size the fuse to the wire but I am a little
confused about when and where one would use a fusible link. In the Z11
figure, you show the E-Bus Alt Feed with a 7a fuse off the main battery bus
using 16g wire through a 1-3 switch and 16g unprotected wire to the E-Bus.
Now, in Z13, you show the same thing with a fusible link between the E-bus
Alt Feed switch and E-Bus.

Questions:

In Z11, are we using the 7a fuse to protect the switch since 16g wire could
take a 12.5 amp fuse or is it size related to the design load on the E-bus?

Keep in mind that the Z-drawings are illustrations of architecture
and minute details such as wire and fuse sizing may (probably will)
take some consideration and adjustment to meed your design goals.

In the case you cite above I have no way to predict the distance (i.e.
length of wire) between battery bus and the e-bus. While limited in
total performance by the 7A fuse (my personal upper limit for always
hot wire feeders), voltage drop and source-impedance looking back at
the battery can be reduced by making the feeder something larger than
the 7A fuse would have suggested.

Quote:
In Z11, why aren't we protecting the wire after the switch since it is hot
when the master is on and it's a pretty long wire?

Longer than 6" (or some other extraordinarily protected length
of your choice)? Then protect it too. Perhaps a fuse slot on the
e-bus would be appropriate for protection of a long feeder between
e-bus switch and the e-bus. Alternatively, you could deduce that
this wire is all inside the cockpit, well routed, well protected from
mechanical faults to ground and decide it's okay as-shown.
Quote:
In Z13, why are we using a 20g fusible link (protects at 7 amps?) on the
wire after the E-Bus Alt Feed switch but not in the Z11 figure?


(I see that I didn't answer this adequately - the EXTRA
protection in the e-bus alternate feed path is for the
case where the e-bus is the SOURCE. Faults on the e-bus
alternate feed wire can be sourced from either end. One
could use one of the e-bus fuses [7A] to compliment the
7A fuse at the other end . . . but it uses up a slot on the
bus. The fusible link seemed like a good player in this
slot)

This is an example of a design goal decision based on the
considerations above. Given the very protected environment
this wire traverses, perhaps the fusible link is satisfactory.
In fact, it probably is . . . the fusible link offers adequate
protection for the rest of the wires in a bundle if the link is
forced open . . . but it's going to be smoky in the cockpit.
Probability is very low so the fusible link saves a fuse slot
for more useful things, offers real (if not exciting) protection
from a fault that is a very low risk event.
Quote:
In the Z13 case, would it be acceptable to run the alt e-bus feed to a tab
on the e-bus with a 7a fuse instead of using a fusible link to the bus stud?
Assumes available tab positions.

---------------------------------------------
You'll note that in this thread we were discussing
the feeder from battery bus to e-bus through the alternate
feed switch. Further, a fusible link is suggested ONLY
at the e-bus end of the alternate feed path (as shown in
Figure Z-13/Cool where the SOURCE that opens this protection
is a powered-up main bus. Note that feeds from the battery
bus are always FAST protection like CB (okay) or fuse (really
fast and my personal preference).

When I started discussions on fusible links several years
ago, I had but two applications in mind. Protection of ammeter
feeders from a shunt and protection of a BUS EXTENSION of
a fuseblock to a 5A breaker in a crowbar ov protection system.

The attractiveness and general substitutability of the fusible
link has morphed into new areas which are much better served
by fuses. In view of the I(squared)*T characteristics of
the ANL current limiters, fusible links seemed practical for
b-lead feeders in small alternators like the Rotax (Figure
Z-16).

The practical use of these devices is rare and limited to
specific locations cited in the Z-figures. The only really
attractive application is bus extension from a fuseblock
to a 5A crowbar breaker. If you have a breaker panel in
lieu of fuseblocks, then the fusible link is obviously not
needed. Virtually ALL other potential applications for
fusible links are well served with fast fuses.

Please don't treat fusible links as plug-n-play substitutes
for any other kind of circuit protection. Using a 7A fuse
on both ends of the alternate feed path is very attractive
from a speed-of-operation in process of protecting the path
from ground faults.
Bob . . .


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