AC Voltage: Wild Reading: phase-neutral & neutral-ground (240V/120V)

Question

I'm getting some unexpected readings when testing a circuit for my wall mounted AC&heat. I'm on standard USA 120V/240V split-phase/three-wire AC power; the specific circuit is 240V.

here's a picture of my wiring terminal:

United States Wiring

When comparing black(phase 1) to red(phase 2) I get the expected 240V. When I compare black or red to ground, I get the expected 120V. When I compare red or black to white(neutral) the value jumps around and does not settle within any recognizable/readable range. It also jumps around the same way when comparing neutral to ground. Why is this? I am using a basic digital multimeter(Mastech MS8268).

I want to wire in some extra fans with a fan speed controller, for a filter-box I am building. The controller is only rated to 120V. So I was thinking I would wire it between phase 1 and neutral, although after getting my readings, I am concerned. Is there a problem? Do I need to use a different circuit? I assume the unit was installed correctly, and it has been running fine, although it gets clogged with dust...

Answer 1

After searching the manual, I can confirm /u/jasen's suspicion of a communication wire, thus explaining the floating value. I probably should have checked the manual first, but I suspect this is similar for all brands.

https://portal.fujitsugeneral.com/files/catalog/files/HFI%20Install%20Guide8.pdf

http://www.fujitsuklime.com/wf-doc/fujitsu-klima-uredjaj-zidni-inverter-asyg09leca-installation-manual.pdf

Answer 2

There is something wrong with your Neutral wire and this is dangerous. You must track down the path of the neutral and fix it before you do anything else. Never use the ground wire for any power return to the breaker box, this is a deadly mistake. The Neutral and ground are to be shorted together at the breaker box but they serve different purposes. All the current must be in the Neutral (although in this 240V split-phase it may be small with most of the current in the Red and Black wires). The ground wire's only other connection is to the metal housing of the fans and provides safety. If the field coil in a fan motor was to become shorted to the motor housing this would prevent someone from being electrocuted. If you were to use the ground wire for your fan power you could cause someone to die. Don't do it.