Question Apollo 13 Oddity - 65 volts?

[Zatnikitelman]

One thing that has struck me as slightly odd about Apollo 13 is why the thermostat switch had to be designed for two voltages, 28 from the spacecraft and 65 from the launchpad. Why? Why wasn't the launchpad designed to just output the same 28 volts the spacecraft would be using? From an engineering standpoint, it seems it would be better to test and condition the flight equipment under flight-conditions.

 

[Urwumpe]

One thing that has struck me as slightly odd about Apollo 13 is why the thermostat switch had to be designed for two voltages, 28 from the spacecraft and 65 from the launchpad. Why? Why wasn't the launchpad designed to just output the same 28 volts the spacecraft would be using? From an engineering standpoint, it seems it would be better to test and condition the flight equipment under flight-conditions.

Blame NASA. They changed the GSE voltage for the tanks to 65 volts in 1965, for letting the tanks pressurize faster before launch (by heating the cold oxygen), after the CSM was already in development for 28 V.