We are wirking for Russian construction project. According to our Russian partner, wiring methos for 24V DC instrument such as transmitter, solenoid valve, gas detectors, etc. shall be 3-wire system (+/- and earthing). As far as I know, the 3-wire method is very old Russian practice and it is not valid any more. Therefore we understand that europian style 2-wire system (+/- only) is sufficient for 24V DC instrument wiring. I appreciate if somebody give me your feddback/experience.

Typically in the US we use two wire (+,-) for these things. The negative (-) is at ground potential so there is no need for a separate earth ground.

In electronics (such as at the circuit board) there are many times when you will have 3 wires, one is at ground potential and one is positive relative to ground and one negative relative to ground. Fo example: +/- 9V which gives you 18V from + to -. but I've never seen this in control wiring.