So I'm installing a line voltage monitor [if not a then similar] to protect a single-phase 240V residential heatpump from self-destructing during windstorm-instigated power flaps. It will trigger a 5-10 minute shutdown period.
Since I've failed at finding such a monitor surplus [and NO, I'm not going to build a one-off when I can buy..] I have the choice of ordering a 240, 120 or 24V model.
Now, normally I'd say "Of course, look at the 240; that's what powers the heatpump..." But that means a separate box, or have 240v lurking in the otherwise LV controls cabinet [no...].
If I looked at 120, yea one side might droop, but in this case... The more I think, the more I like looking at the 24V from the heatpump's own control transformer.
See, this house is fed by its own 7600V in/240/120 out "yard pig" transformer. [It can't be a pole pig with no pole, right?] It's the end of the line on that leg -- fed by a 2000' buried cable from the public road, and there, there's another 3500' until it meets up with other users.
And the heat pump is the only 240v load of note. So the way I see it, the 240/120 & 24v are all going to flap in lockstep, set by the windings ratios; when the 240 drops 15%, so will the 24v, etc.
Is my thinking wrong? Do I really need to be looking directly at the 240 for some reason I'm overlooking?