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This article claims that Apollo ground support equipment used 65 V for electrical power, and may have been involved with the cause of the Apollo 13 explosion:

The oxygen tanks had originally been designed to run off the 28 volt DC power of the Command and Service modules. However, in 1965 the tanks were ordered to be refitted to also run off the 65 volt DC ground power at Kennedy Space Center. All components were upgraded to accept 65 volts except the heater thermostatic switches, which were overlooked.

Why was 65 V used in general for ground support equipment? Not only is it different from the voltage of the spacecraft, it also seems like quite an arbitrary number.

This question is part of a series honoring the 50th anniversary of Apollo 13, "NASA's finest hour".

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    $\begingroup$ Is the question asking "why did KSC GSE run on 65 volts" or "why was the decision made to use the 65 volt GSE power to supply the cryo tank heaters"? $\endgroup$ – Organic Marble Apr 11 '20 at 17:40
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    $\begingroup$ @OrganicMarble: The former. Sorry about the confusion. $\endgroup$ – DrSheldon Apr 11 '20 at 18:21
  • $\begingroup$ Thanks for the clarification. $\endgroup$ – Organic Marble Apr 11 '20 at 18:26
  • $\begingroup$ Related - Scott Manley at youtube.com/watch?v=eO19LTJZM6c $\endgroup$ – Criggie Apr 12 '20 at 2:07
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    $\begingroup$ 65V might be related to 110/120V mains, being rectified using a single diode. $\endgroup$ – asdfex Apr 12 '20 at 12:01
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I found two independent articles claiming the increase from 28 to 65 volts was implemented in order to shorten the time needed to pressurize the Service Module's tanks:

Apollo 13 Flight Journal:

The heaters inside the Apollo oxygen tank were designed to operate in flight using 28-volt direct current power generated by the fuel cells. However, at Kennedy Space Center, the ground support equipment utilized 65-volt DC power. The higher voltage was applied to provide more power and pressurize the tanks more rapidly in the preparation for spaceflight.

Glynn S. Lunney Apollo 13 Oral History:

Back in 1966, the voltage for the heater was increased from 28 volts to 65 volts in order to aid in faster pressurization.

I didn't find any specific reason why 65 volts specifically was used, and it's not clear to me whether 65V was already in use for other systems at Kennedy.

If 65V was used only (or at least originally) for this application I would guess that 65V and the consequent power dissipation level was the highest tolerable by all the components of the tank heating and pressurization system that the engineers looked at, or by straightforward substitution of higher-rated components -- except, as we now know, for the thermostatic switches.

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    $\begingroup$ as we now know Ouch. $\endgroup$ – chrylis -cautiouslyoptimistic- Apr 12 '20 at 2:55
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    $\begingroup$ As we all know, the best voltage choice would have been .... 42 $\endgroup$ – Carl Witthoft Apr 12 '20 at 13:32

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