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?


3 Answers 3


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.


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


I found this table:


I circled what I suppose are the circuit breakers, given the huge amount of them.

This is the description provided in the document (unfortunately, no diagrams):

Toggle switches are used to manually control the operation of the spacecraft. The LM contains approximately 140 toggle switches in 33 configurations. These configurations were selected to simplify system function and electrical operation. The types of toggle switches include momentary, maintain, and lock actuations, in either double- or triple-throw actions. Electrically, the configurations vary in size from one to eight poles. Toggle switches are of two basic types, AT and LS.

The AT-type switch contains hermetically sealed modules where electrical switching takes place and an environmentally sealed housing where mechanical actuation takes place.

The LS-type switch has electrical and mechanical actuation taking place within a completely hermetically sealed case. Sealing is accomplished by using glass headers, a bellows, and a heliarc-welded case.

I also found these pictures:

Original toggle switch


Original toggle switch - 2


  • S/N 101608306347
  • P/N ME 454-0010-0006
  • L/N N52 1500-069-3

Google search for images: link

Various images: http://www.space1.com/Artifacts/Apollo_Artifacts/CM_Switches/cm_switches.html

Apparently same switches were reused in Shuttle program.

  • 2
    $\begingroup$ Lot of good info here, but flip up/down toggle switches are certainly not a pop in/out type circuit breaker (possibly they are not considered 'controls', being nominally a safety device). Also, the circuit breaker pictured is from the command module, built by North American while the Lunar module was built by Grumman who probably used a similar but not identical part. $\endgroup$ Commented Feb 21, 2022 at 11:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.