(COUGARNFW)
shared these priceless pearls of wisdom:

-Nobody looks it up. The 4522 lamp is usually used in a swing-down lamp. The
-swing down lamp has a motor to drive the lamp up or down.

Yes, well, when you look it up, you might want to use the right number. The
original poster said 4552, not 4522. If you had a clue about things, you would
know that no 172 ever made had a swing-lamp setup. That died with the C-170
series. Google on 4552 and see your error.


-
-The lamp is 12 volt, 250 watt, something the guesser at 9 amps (if for a 24
-volt plane) would have realized if the system voltage of the plane had been
-given, so the current draw for the lamp along is a nominal 20 amps.

No, the lamp is 28 volt, 250 watt, which calculates to a skosh below 9 amps.
The guesser didn't guess, he calculated.


-
-If the motor circuit is not opened by the switch/relay built into the light,
-the combination of the lamp current and the motor current will pop the breaker
-every time, in a short time.

There is no motor.

-
-Yes, check the amps with a current meter, and please do not let the motor run
-long in the test or it will burn out.

a. There is no motor.

b. If you check the current with an ammeter (sorry, John), you will blow the
fuse in the ammeter when the failure mode occurs. As somebody who knew what
they were saying commented, the best way is with a second lamp of the same
rating in series with the wire right at the breaker. Under "normal" conditions,
both lamps will light equally bright. Jiggle/wiggle/tap/thump the wire along
its whole length, and when the wing mounted light goes dark and the test light
brightens up, you've found the location of the sharp edge that is cutting
through the wire's insulation.

Having said that, in general it is easier to pull the old wire out and pull a
new wire into place, being sure that when you pull the new wire that it doesn't
go over the same sharp edge that killed the old wire. Yes, this is called
shotgun troubleshooting, but in a situation like this it can result in reduced
labor costs and headscratching to find the exact point along the wire where it
is being inadvertently grounded.

No, a loose ground AT THE LAMP will cause the wire and aluminum at the lamp to
get hot, but it won't pop the breaker. However, a loose or corroded connection
at the BREAKER will cause the breaker to heat up (I-squared R loss) and pop.
You might check the connections at the breaker before you go off on a tangent.

Jim
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com