nuckolls.bob(at)aeroelect
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 Posted: Wed Jan 12, 2011 5:23 am Post subject: Circuit protection for relay/contactor trigger circuits. |
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At 05:24 PM 1/11/2011, you wrote:
| Quote: | Bob and all,
I notice in Z-10/8 (as an example) that some trigger circuits are
protected and some are not.
Starter contactor trigger - shows 7A protection.
E-Bus alternate feed relay trigger - shows no protection (just 22AWG)
Battery contactor trigger - shows no protection (just 22AWG)
Brownout battery relay (triggered from start switch) - shows no
protection (just 20AWG)
I know the Z-figures are not to be taken as anything other than
architecture guidance, but is there a rule of thumb on protecting
some of my relay-closing switch circuits?
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Architecture drawings do not discount the
elegant use of circuit protection. Yes,
the rule of thumb is:
Any wire of significant length (6" or more
in the TC aircraft world) at risk for
damage due to faults in the system should
be protected from burning.
A review of Part 23
http://www.aeroelectric.com/Reference_Docs/FAA/Part23_electrical_A.pdf
and in particular paragraph 23.1367
and Chapter 11 of AC43.13
http://www.aeroelectric.com/Reference_Docs/FAA/AC43.13-1B_Ch11_Electrical.pdf
might be useful. But be aware that not all
words carved into FAA tablets of stone
are possible or practical. For example,
the words in paragraph 11-48 of AC43.13
are pretty much carved in peanut butter.
There is no such thing as a "rating" for
a wire's ability to carry current. There
is a WIDE band of operating conditions for
wires that do not necessarily put them
at risk for failure.
Circuit protection in the form of fuses and
breakers cannot be depended upon to "open
before any component downstream can overheat
and generate smoke or fire." Fuses and breakers
protect wires from hard faults (LOTS OF CURRENT)
and nothing more.
See:
http://aeroelectric.com/Pictures/Wire/22AWG_20A.pdf
This 22AWG wire in the foreground has been
subjected to 20 amps of current flow until
temperatures stabilized. Note that the wire's
insulation has 38 degrees C headroom before
bumping it's published performance limits of
150C. Yet we commonly protect 22AWG wires at
5A. This is NOT because 22AWG wire is RATED
at 5A. The legacy design decision for protecting
22AWG wire at 5A is a very conservative decision
based on temperature rises in bundles, system
efficiency goals (voltage drops) and tolerance
for special cases where the wire might need to
pass through a hot-spot on the airframe.
This is why your exercise in wire sizing did
not yield any exciting pop-up revelations.
As a general rule, wires are much more tolerant
of abuse than the table we commonly apply to
new system design
http://aeroelectric.com/Pictures/Wire/Wire_Table.jpg
. . . these "ratings" are based on a paltry
10C temperature rise.
Now, armed with this simple-idea, run the
traps on what kind of fault would cause the
contactor control wires you've cited to become
exposed to high fault currents?
When these wires are used to pull down on
the grounds side of a relay, contactor or
most other devices, there are no conditions
of system failure that will subject these
wires to unlimited fault currents.
The most common failure in wiring is an
open circuit. The next most common failure
is a fault to ground. Faulting a supply wire
to ground exposes it to the potential fault
current of a battery . . . perhaps 1000 amps
or more. Faulting a ground-to-close control
wire to ground exposes it only to the current
it takes to close the contactor. Hence, no
protection is called for in these lines.
Bob . . .
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