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Using Circuit Breakers as Switches

Copyright © 2005 by Eric M. Jones.
Last revised 29 November 2005.


During our 1990 review of the preparations for the Apollo 11 LM launch at 121:46:02, I asked Neil and Buzz about the philosophy of using circuit breakers in some cases and switches in others.

Aldrin - "Some things were circuit breakers only, like the TV didn't have a switch, it had a circuit breaker. Other things had sort of a double protection. You had a circuit breaker that sort of armed the circuit, and then some switches that were in that same circuit, too."

Armstrong - "This is not unlike the same situation you have in your house. Some things, when you have the circuit breaker On, you have power to the receptacle, for example. On the other hand, your oven probably won't turn on until the switch is on. And it's also backed by a circuit breaker behind it."

Aldrin - "But you don't walk in the house in the evening after being out to dinner and go over and turn your circuit breakers on, using them on and off. 'Cause you conduct daily activities."

Armstrong - "We probably used the circuit breakers as switches sometimes because the system was designed to minimize the number of switches that you had to have, yet added the protection."

Aldrin - "Gave you good redundancy."

Armstrong - "And, then, sometimes we pulled the circuit breaker to prevent a failure from getting you into trouble by having an inadvertent actuation of something due to a switch failure or some other failure."

In August 2005, Journal Contributor Mike Polizsuk, who works for the U.S. Navy as a civilian engineer, most recently for the Naval Air Systems Command at Pax River in the Program Office for the F/A-18 Hornet fighter jet where he was responsible for engineering oversight of the Hornet's structural and mechanical systems, wrote:

One thing I note continuously throughout Apollo is the way they cycled circuit breakers, often as a matter of routine operations. In my experience with aircraft systems, circuit breakers are considered as protective devices that should not be used as a switch to turn systems on and off. While it may be acceptable for maintainers to open breakers to make a system safe for maintenance, it is not standard practice for pilots to cycle breakers. We would only put it into a checklist reluctantly, and only if no other way exists to turn off a system. Since the circuit breaker plunger is held in mechanically, repeatedly cycling it can wear it out. For a system or component that is required to be turned on and off, a Power switch should be used, with a circuit breaker also in the circuit for protection."
Dave Scott, e-mail reply - "Mike Poliszuk (MP) is correct in the functions of CBs (Circuit Breakers) vs. switches in aircraft as well as the reluctant inclusion of CBs in checklists. He is also quite astute to notice the Apollo procedures, and actually the use-of-CB philosophy. As I recall, the evolution of CB use was as follows: Well, that's the gist of my recollection, off the top. Those early days were exciting times....but once the program got going, it all worked like a charm; I don't recall any major problem being contributed to the use of a CB rather than a switch -- so I guess the decisions were correct...!!"

Armstrong, e-mailed comment on Dave's discussion - "I think Dave recalls it very accurately. We did not like to use CBs as switches, but we already had hundreds of switches and, for operations that were completed just a few times, we accepted the practicality of operating the CBs to energize or de-energize circuits."


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