Grounding the Reference Voltage in PLCs


Thread Starter

M. Vafaei

Some PLC suppliers ground the reference voltage in their cabinets, but the other don't. Is there any special reason for that. What do we get by grounding the reference voltage and what do we lose? For a particular project, how can we
conclude that the reference voltage should be grounded or not? Is it related to the process conditions or number of I/O signals or some other things like that?
I would like to know who does and does not, and am not sure of what you mean by "reference
voltage". Assume you mean a voltage generated on an I/O card to sense a contact status.

If it IS grounded, then the reference voltage cannot go above the nominal voltage vs. ground, and if someone touches it, there is reduced hazard. If another live wire touches it, there
is a greater chance that circuit protection will
disconnect things.

If it is NOT grounded, then a live wire can touch
the circuit, and it MIGHT still work. Say 110V hits the ungrounded zero voltage, then a 12 volt reference would become 122V (0+12). The danger is that someone could come along and touch the hotter circuit, and be electrocuted.

I expect it has much to do with the National
Electric Code, Underwriter Laboratory listing, and safety vs. reliability.

For low voltage instruments, one often wants a "single point ground" at the instrument, and
this means no ground anywhere else.
The CSA code rule 10-114 says circuits of less than 50v shall be grounded if supplied by transformers of more than 150 volts to ground. I don't know what the NEC code number is.
Thank you very much for your detail answer. By "reference voltage" I mean the negative polar of the power supply.
It is generally good practice to ground the reference voltage (0VDC). Deciding whether or not it is really needed to ground to 0VDC will depend on the wiring of your I/O system. A system that doesn't have the 0VDC grounded will work fine as long as everything is measured relative to the 0VDC. My experience is to always ground the 0VDC otherwise fault-finding becomes confusing because the 0-xVDC system will be floating with respect the ground.
This isn't a real answer, but the biggest advantage to NOT grounding the PLC's internal voltage reference to the chassis is that this allows YOU to decide yes or no to such grounding. If you want it, either add a jumper to the PLC's power-supply or run a wire to your chassis - don't want it you can do nothing.

Some industries (specifically I've had a lot of oil-n-gas experience) prefer a controlled relationship between control ground and earth ground - say 2000 ohms. This allows locating water or other "ground shorts" easier than if the system is naturally grounded with 0 ohms difference.

- Lynn
Dear Mr. Linse,
Thank you very much for your answer. I just did't get what you ment by "locating water or other..." Could you explain it a little more?

M. Vafaei