Earthing Value

10 ohms?

<1 ohm

The smaller the better, but a value ≤ 4 Ω is acceptable. Make sure that the earthing of your PLC and the instrument screen/shield have an earthing system separated from the structural (equipotential) and electrical earth. So do not connect to the earthing of the panel because this has to be structural earth. All this systems should only at the real earthing point (rod or grid) come together. Star principle.

Can you please tell us that the value ≤ 4 Ω is recommended by BS,IEE.If yes can you give us the relevant pages Please.

I suspect the earthing method and location is equally important. Frequently in the product documentation I have read, the earthing (grounding) conductor has a minimum size that can be greater than the circuit or signal conductors---necessary for proper control of the low-level signals which are protected by it. The source and type of interference or noise needs to be considered, particularly if it is a nearby power conductor.

I had a very vexing problem in a PLC panel with 4-20mA analog signal cables and a small variable frequency drive (VFD) for the equipment's conveyor motor. The electrical noise and control errors were reduced when the earthing conductor for the motor was moved to the VFD's output earthing terminal instead of the control panel's nearby earthing terminal, but were eliminated only when I replaced the power conductors with a shielded power cable and similarly earthed the shield in the VFD. I had already taken the precautions of running the cables in separated wireways and separate conduits, etc.

JMM