In the U S, 240 volt residential is a 'split phase' or center tapped transformer. IF there is a good, solid GROUND to the secondary panel, you will have 120 volts from either line to ground. The 'neutral' is grounded at the service entrance. (meter can) That's not to say you can get 120 volts power from one line to ground, that would be illegal. And would cancel your fire insurance if ever found. Be so advised. But for trouble shooting, both Delta and Wye connections at the transformer are referenced to a 'neutral' which is usually grounded both at the pole pig and at the service entrance. Read the line to ground for both lines. They should be very close to the same. If there is a wide difference, there is likely a loose connection somewhere.
24 volts is common for industrial instrumentation. As is 240 volts. In the early days of IBM desktops, the power supply had a switch for 120 volt or 240 volt ranging. Since the negative was (and likely still is) grounded to the frame, two supplies seriesed would need to be insulated from each other. I have done this many times over the years. How is not important, it's a technical stunt that involves electronic expertise. The relevant point is that it is an industry 'standard' that low voltage supplies be grounded on the negative side. But that ground is for safety reasons and misuse can (and does, often!) lead to ground loops. As Murphy says, ground loops go from bad to worse. There is no good, or even acceptable, ground loop in instrumentation. The end result is that some supplies will have either direct or ranging connectors for low or high voltage. That should be a reference in the advertisment for a particular supply.
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