We are working 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. Also as far as we checked, we have never found out such code&standard requirement of "3-wire".