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...!!"
- 1. Everybody - design, engineering, operations - recognized the differences, as stated by MP; and great debates were held when a CB was proposed as a 'switch'.
- 2. The CSM was designed and engineered before the operational (flight) procedures were developed and written. Also, the CSM was a very complex machine, and had more systems (or subsystems) than an aircraft, especially new systems ( or subsystems). I had flown 10 different fighter-types, and systems-wise, the CSM was a real wake up call!! In general, each system was designed and manufactured by a different subcontractor. The LM was generally the same, and followed the CSM by a couple years or so. (Incidentally, my experience with the CSM began with AS-204 in March 66 during the very difficult days of initial testing of the Block I spacecraft.)
- 3. As the initial procedures (checklists) for Block I were being developed, we were also participating in the SC-012 tests at Downey, in great detail. There were no simulators at that time, thus had nothing but the actual s/c (spacecraft) to develop and verify operational procedures. To integrate the procedures of all of the systems, we - and the engineering folks - found that the use of CBs was absolutely necessary in many cases so as to develop efficient and effective checklists. Also, to add to the challenge, the s/c test procedures had to be written from scratch, and rewritten, to conform to cockpit and pilot operations (vice engineering development procedures) -- in many cases, an engineering test of a system (or subsystem) was quite different from an operational procedure. 'We' (the pilots) insisted that the ground tests be conducted with flight procedures and that the s/c operator during the tests had to be a qualified pilot, who then wrote a report on each test. There were many, many glitches and problems during these early days, some hardware and software, some procedural, some both.
- 4. Everybody did try to minimize the use of CBs, recognizing the difference with switches. Circuitry was also a contributor to the eventual use of either, or both.
- 5. Many other factors contributed to this (CBs vs. switches), but were generally secondary. Among these (in random order):
- The s/c had to be operated in hard suits.
- Malfunction and emergency procedures had to be developed along with normal procedures, thus some of these set up a CB as prime.
- Failures of components, subsystems, and systems during test, qual, etc., and corrective actions, resulted in one being favored over the other; e.g., a procedural change might solve an engineering problem, or vice versa.
- Procedures across several s/c had to be standardized as much as possible to facilitate training and flight operations. Remember that the crew had to train and fly the CSM, CM, LM/Descent, LM/Ascent, S-V/II, S-IVB - and even a/c (aircraft) such as the LLTV, T-38, T-33. Although secondary, this was kept in mind in developing the s/c procedures.
- Although not a major factor in the CB vs. switch issue, the simulators had to operate just as the spacecraft -- a major challenge since the simulators were software representations of hardware. The hardware actually came first; however, once a procedure was verified in hardware test, the simulator had to follow. (Even before A-9, the simulator would just not function properly until about a month before flight).
- Volumes of documents were written for pre-flight test procedures -- these all had to conform to flight procedures.
- In all of these cases, a small change - say, one step in a checklist - could be a major factor in proper operation. Therefore, once a decision was made on CB vs. switch, it was very difficult to change. And with the pressure of the program, decisions had to be made early and quickly. Hindsight might say that one or the other would have been better for something; but again the overall system was so complex that people were reluctant to make a change if not absolutely necessary; time was of the essence, but of course safety was first.
- There were a limited number of switch locations in the cabins (CSM and LM); and in some cases of early design, and early procedure development, and unforeseen by design or engineering, a CB just had to be used; and for one reason or another, a switch, although the better choice, was not an option.
- And of course, the number of times a function was to be used was indeed a major consideration -- if many times, the switch won, if only a few, the CB. But again this had to be incorporated into a very complex integration of systems, subsystems, component operations and flight procedures.
- 'Mode control' vs. 'function control' was another factor; i.e., the number of options provided to the crew determined just how many devices (CBs or switches) would be available to operate. When the Block II arrived, a conscious decision was made to go with function control and give the crew as many options as practical. This drove the use of additional devices to operate the systems, i.e., CBs.
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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