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From Apollo Lunar Surface Journal: "112:56:28 Aldrin: Houston, Tranquility. Do you have a way of showing the configuration of the engine arm circuit breaker? Over. (Pause) The reason I'm asking is because the end of it appears to be broken off. I think we can push it back in again. I'm not sure we could pull it out if we pushed it in, though. Over. [Aldrin - "There weren't guards over any of the circuit breakers. One got pushed in and one got broken off. So I must have pushed one in and broke one off (with his PLSS prior to doffing)."]"

The broken tip of the CB and the pen:

enter image description here

Image source: (http://www.collectspace.com/review/apollo11_circuitbreaker02-lg.jpg)

LMP circuit breaker panel:

enter image description here

Image source: https://www.hq.nasa.gov/alsj/a12/LM6-co04.jpg

How were circuit breakers in LM operated? Push for "ON", pull for "OFF"? Or push for "ON" and push again for "OFF"? If it's the former (which would explain why Aldrin said he couldn't "pull it out" once pushed in), wouldn't it be awkward to grab it with two fingers whilst wearing gloves? What material was the tip of the CB made of so it snapped so easily?

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Cannot identify the specific model, but they would be ancestors of these that work by popping out under load or manual trip and push back in to reset/turn on.

They are widely used in aviation because they are compact and relatively easy to operate by feel. They generally take very little force to pop out, since that is spring assisted.

Because they produce heat in operation and need to provide electrical insulation they are made from thermoset plastics like Bakelite and derivatives so often brittle compared to things like ABS. Design loads would be those for single finger operation while being light enough for aviation use.

As per the quote they were left with a broken off plastic surface of unclear state that they could push in with the pen but would have no real means of getting back out again, and if it was already in and set then they potentially needed to adjust sequence of operation for launch.

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  • $\begingroup$ Link should be fixed $\endgroup$ – GremlinWranger Nov 21 at 13:09
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They worked like standard aircraft types.

enter image description here

Pushed in = circuit closed

Popped out = circuit opened

The "knob" was made of non-conductive plastic (don't know the exact type)

Circuit breakers, are thermal-mechanical in nature. Bimetallic elements, with one metal expanding more under heat than the other, pop the breaker open. This also enables them to be reset, albeit only after they have cooled down.

Picture and quote from the good writeup here: http://www.cadmus.ca/Circuit%20Breakers.htm

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