slow blow circuit breakers

[GeorgeC]

I was poking around under the panel and saw that, while most of my CBs are 7274s, one of them is a 2A 7277, a slow blow.

In my extensive internet "research" [1], it seems that slow blow CBs are used to ride out inrush transients, like for flap/gear motors or incandescent bulbs.

Thing is, my plane doesn't have flap or gear motors. In fact, pulling that breaker doesn't seem to do anything.

So, why would this have been installed in the first place? I haven't traced the wires yet.

From left to right, I have
CB #1 - 10A - gtr 225
CB #2 - 5A - gtx 335
CB #3 - 5A - ???
CB #4 - 1A - insight g2 engine monitor
CB #5 - 2A slow blow - ???
CB #6 - 1A - plane power voltage regulator

fuse #1 - 2A - aileron trim
fuse #2 - 15A (one label says 15, one label says 10...) - the entire avionics bus appears to be downstream of this fuse
fuse #3 - 20A fuse (label says 10) - position lights
fuse #4 - 5A - ???
fuse #5 - 30A - landing light
fuse #6 - 10A - strobe

[1] https://www.euroga.org/forums/maintenance-avionics/8808-which-klixon-circuit-breaker-to-use

 

[Ryan Klems]

7274-2 (there are a couple different -#'s that specify different terminal styles) and 7277 are essentially identical, in fact this is a quote from Sensata's product pages "The 7277 and 7274-2 series circuit breakers are physically and electrically identical, with the exception that the 7277 has wider calibration limits.". The 7274 is Mil-spec and the 7277 is commercial grade, with larger tolerances. Breakers typically have 2 different types of trip mechanisms, one that works magnetically via the massive inrush current that occurs in a dead short to trip very quickly and the other is essentially heat based that is a slower trip mechanism, which allows short term excursions beyond their rated capacity, which I suppose is more "slo-blo"-esque. I would guess that someone used a 7277 because that's what they had (they are usually cheaper than the 7274's).

 

[GeorgeC]

I would guess that someone used a 7277 because that's what they had (they are usually cheaper than the 7274's).

That had crossed my mind.