ramifications - thoughts

What are the ramifications of grounding the negative of 24 DC and allowing vendors to fire inputs w/ their own 24V + DC source?

the only way that could work is if the vendors source is also tied to ground.
This would create the possibility of a ground fault traveling through two machines.
Is that a good idea???????????

Put isolators on the inputs from the vendors, they could be a relay for slow signals or opto-isolators for higher speed ones. You can get very narrow relays and optos that are not much wider than a standard terminal.

I do this all the time, but have a dedicated 0v connection between...

It is common to tie together commons, so various controllers can share what they have in common (input and output voltage). For 24vdc, they can both float from ground and share a common, as long as they keep the signals connected to your inputs at the correct potential difference. If by vendor, you mean vendor of a part in your panel then sure, share the commons, ground them separately. Grounding a 24vdc system that has been running fine floating is not something I would advise.

I also agree with using relays or optos to maintain isolation for some cases, whole machines or sections, but for intra panel stuff like VFD inputs, you just wire their commons together, and connect PLC outputs to VFD inputs, as many as you need, so why not do this with any other properly referenced 24 vdc signal?

If by vendor, you mean separate machine or major section, then it is common practice to provide dry contacts or solid state relays for hard wired interfacing.

I have done both Grounding and non-grounding of 24VDCneg.
but using a common 24v between to machines and using the one commonly grounded DCneg. i think is really risky

a ground fault can also be a welding ground
do you want a few hundred amps through your control wiring

What BryanG said

I always specify optos or volt free interfacing relays

Another way to handle it if your I/O capability can handle it, is to dedicate an input card exclusively to the separate machine. The card is powered from the remote machine power supply. This can be done without tying the power supplies together (very handy when interfacing machines with different polarity grounds) and does not require any interfacing relays.
When we do this we run Yellow wire from the input card to the terminal strip to flag the external voltage source and add a tag to the panel door to indicate a separate power source in the panel.
I’ve seen this done time and time again at several plant locations with both input an output cards to pass signals back and forth.

HI JerryH that is OK provided the Card is not supplied from the PLC Rack Bus
it is also a huge risk
you Isolate that machine to replace the PLC rack
if the card was 240v or 110 there is a chance of electrical shock

if inputs are shorted it may damage the other machine

sounds great - But only if you know the wiring method.
Personally I believe that is absolutely dangerous

ganutenator said:

What are the ramifications of grounding the negative of 24 DC and allowing vendors to fire inputs w/ their own 24V + DC source?

Click to expand...

It is standard practice with 120 VAC I/O for all power sources to share a common ground.

I don't see anything wrong with it, provided you are using a grounded 0vdc conductor in the same way you would run a conductor that was NOT grounded.

IMHO it is preferable to using a NON grounded conductor.

All the voltage / current circuits istalled properly will return to their respective sources

If you do not ground anything and one wire anywhere gets grounded by a fault, everything goes on as if nothing happened... . If ANOTHER wire gets grounded, good luck finding that