Proper Grounding of CNC/PLC Systems

Go to the National Electrical Code concerning multiple grounding. You have to take your grounds back to the main electrical ground, usually at the point of entry into the building. Your problem, which maybe caused by multiple grounds, is that the grounds may produce eddy currents running through the soil between the driven ground rods. Think of having an unbalanced grounding system, where an Electro Motive Force (voltage) is jumping back and forth between your ground rods. We had this problem back when this was the manufacturer's recommended policy for installing their electronics.

I run across an intermittent problem on an old Behringer (over 20years) industrial saw recently.
The machine would cut short pieces usually during night shift, randomly.<>They put up with aproblem for years till they got a new foreman who insisted on fixing the problem.

The problem was lack of filter(sic!) on +24V supplying PLC.\

What was probably deceiving for many who tried to fix the it, was a 3-phase selen rectifier that may have given an impression that ripple on DC, although it was p-p 4V(under load), suggested there was some king of filtering on DC.
That was the first time I saw 3-phase rect on 24V myself.

It probably won't help you, but it was an interesting case- at least for me

we had a problem with a plc instalation where twice a day the coms card would blow fuses and every now and again blow the whole card we found that as the tide came in and out the grounding between the two sights would change givin g like 20V diference in ground potential we tied the two grounds together and haven't had a problem since

I don't have PLC experience, but I do have experience of grounding systems in studio and broadcast installations. I would offer the following suggestions.

Always ground equipment to meet the applicable regulations. Safety is the paramount issue here. This will usually involve everything being bonded back to a central point.

Make sure all your control gear and sensors have good RFI protection and power supplies that are immune to large fluctuations in voltage or mains born interference.

Don't create ground loops in your control system or sensors as this is virtually guaranteed to cause problems. Use isolated interfaces to prevent circulating ground currents. Ordinary RS232 and USB interfaces, for example, have common grounds with the equipment at both ends. Good industrial networking companies can supply isolated interfaces to resolve these problems.

One of the most common problems in my 40+ years around electronics has not been grounding, but spiking from any type of coil when it is de-energized. An inexpensive solution is a resistor/capacitor suppressor and a MOV paralleled across the coil. If it cannot be place across the coil, get as close as possible.

Low level signal lines should be run with twisted pair wires that have at least 10-12 turns per inch.

As to grounding try to utilize a star method with single point grounding on a system.

Multiple grounds tend to create problems.

As a Designer of CNC Machines. I would recommend that all CNC/PLC Machines is to be connected to a seperate Earthing Bus bar . This should be seperated from Welding machines and other High current drawing machine Earth Bus bar. The earthing point should be connected firmly with no loose connection.

Conditioners in the machine does not effect the earthing or trigger trip out until earthing wires of conditiner motor is connected to earth strip inside the machine.

Thank you

Vijay

"like an island" with an earth rod at each machine

I should have stated in addition to the ground loop, what I wrote sounds like it wasn't tied into the loop ............but it looks like any independent grounding may be a bad idea after all. Maybe I misunderstood someone along the way. Anyhow we had one machine serviced today and were told the low voltage power supply in the control box sagged or spiked it's supply to the gains and it reset them to default. Oh well, they could tell us the squirrel ran out of nuts and we wouldn't know any different although things are starting to point more towards the supply side.