A disconnected wire is floating - the only thing defining its voltage is leakage through the voltmeter and
leakage in the wiring. 30m is a long cable run, plenty of leakage possible.
You probably need a 1k pull-up resistor on the N/O relay contact to provide stable output when its open?
Thanks for the reply MarkT.
Would it be preferable to have the 1K pull-up at the central board ( ie .. closer to the Arduino where the power supply is ) or at the relay ?
I do not have a positive line at the gate relay, so would have to run a new wire if that is where it would be required.
Also, would you recommend the pull-up in the 'loose' wire be connected to the supply 12V or the regulated 8V ( and I also have a regulated 5V available after that ) ?
I do also have 10K and 100K resistors - would using either of these not reduce current required when the relay does activate and the Ground signal is connected from 'com' to 'NO' ?