Window heat system c/b trips

[maro12]

Hello All

We have a/c 737-700 with window L3 c/b tripping in flight. Resistance of heating element ok, termal switch function ok, wiring ok.
I heard that in this case replacement of c/b helps, as current draw is close to c/b treshold (3 amp c/b) and this treshold goes down with time. Anyone experienced this issue?

Regards!

[mspinst]

When the window was replaced last, was the window code and resistance value matched to the terminal strip near the window heat control unit as called out per the M/M. Possibly overheating the window and popping the C/B? Marty from MSP

[maro12]

Hi,

Window L3 has nothing to do with terminal boards as it is in case of window 1 and 2, it is simple circuit.
I just wanted to know if anyone has similar experience with cb.

Thanks anyway,
Rgds.

[Timthom]

A weak circuit breaker is certainly one possible cause.

Another one that I've found was a heating element that was broken but still making contact until the aircraft was pressurized. When I pressurized the aircraft to 4 PSID, the resistance tests, which had been good before pressurization, were out of limits. Window change fixed the problem.

[Dunny]

Cant remember which one it is, but there's a CB that regularly gets hit ( and tripped) by the tech log, when its put back in its stowage.

[maro12]

Thanks for reply!

I replaced c/b a/c departed yesterday and I will wait for feedback if fault has been corrected. I will let You know.

Timthom It is good to know, I did not check resistance with a/c pressurized, if fault will still be there it will be first thing I will do

Thanks again,
Regards!

[maro12]

Unfortunately CB change did not help.

Regards!

[sr79]

do you mean L4 window because L3 is not heated therefore, it does not have a breaker to pop

[maro12]

Sorry You wrong, maybe on some a/c L3 and R3 window is not heated (I suppose it might be an option). On older a/c still with windows 4 and 5 window 3 heating circuit is hooked up parralel to 4 and 5 window circuit (common CB), on a/c without windows 4 and 5 (or post SB - which put blanks instead of 4 and 5 windows) it is single line with separate CB, thermal switch and heating element. L3 CB is tripping though (maybe check eff. for different a/c or airline, not everyone take this option, just checked manual for quite big well known airline, which by your nickname I suppose You do maintenance for, and they do not have it installed).

Regards.

[sr79]

maybe it is an option,i didn't know i work for a major carrier and i guess we dont have that option you learn something new everyday

[sr79]

and NO I AM NOT WRONG if the L3 WINDOW is not heated it does not have a breaker i just asked if you meant the L4 window

[maro12]

I agree, if window no.3 is not heated, there is no circuit breaker for this purpose

Anyway Thanks for reply,
Best Regards!

[fatapple]

From Boeing FTD 30-09001,Boeing gave the explain that the circuit maight be experiencing degradation of the window heat power terminal at the circuit breaker. And Boeing suggest to check the insulation on CB terminal lug.

[fatapple]

We've found same fault as FTD-30-09001 described on one of our NG aircrafts.

[maro12]

That is very interesting! Thank You for that info, really appreciate it!

Regards!