Mixed 24V and 120V

[Guest matthmelt]

Greetings, In reworking a piece of equipment, I have run into a dilemma: 24VDC and 120VAC. The motor starters are NEMA, 120VAC. The existing solenoid valves are 120VAC. ALL of the indicator lamps are 120VAC. The process controllers require 120VAC supplies. The safety controller (Pilz) is 24VDC. Without getting really elaborate with the PLC/Pilz/separate/interposing relay setup, the simpliest solution seemed to be 120V for outputs (solenoids, contactors, and indicator lamps) and 24V for limit switches and pushbuttons. There will be a couple of switches and process controllers that have 120V and 24V on them. Does anyone have tips on how to deal with this, as far as maintenance goes? I can just see trying to jumper a wire or change something and cross 120V to 24V...even though the wire color is different, accidents happen. Any thoughts? Thanks, Matt

[comeng]

Having been through these a number of times (on old systems you have know idea where the plant wring goes I hate backfeeds on 110 systems) I would definitely use all 24V on the plc and use interposing relays or cabur cards http://www.cabur.it/catalogo/elettronica/e...XCR81-XCR82.pdf so that the 120v can be kept separate from the plc and then use the pilz to enable both the 120 and 24 if necessary. I also prefer 24v lamps straight from the plc but sometimes you have to use what you have. That way if all your systems are the same then the maintenance crew know to keep any strange voltages away from the plc. Phil

[TConnolly]

I frequently do systems that have 24 V inputs and mixed voltage 24/120V outputs. I recommend that if you do that you divide the outputs into two groups on two different cards. The big problem with interposing relays is your safety PLC. I do not recommend mixing generice ice cube relays with your safety PLC. You may as well toss the safety PLC out if you do that. Using safety relays as interposing relays for the safety PLC can get really expensive and if you are at that point then it will probably be cheaper to replace the solenoids etc.