Earthing the body of instrument

[NASEEM QAMAR]

Dear Experts,

I have a general query with respect to instrument grounding for following scenario:

My instruments are installed on a skid. The skid is earthed from two different points. The analog single pair cable is also using a shield which is grounded from single side. However, the body of the instrument is not externally earthed. My question is that is it necessary to provide external earthing to the body of instrument in this case? Please reply with reference to IEC or equivalent standard.

Regards,
Naseem Qamar

 

[Roy Matson]

Grounding of the instruments is not normally required as it will be grounded through the process piping. If it were something like a transmitter on a plastic tank I would ground it either through through it's conduit or a seperate wire to some well grounded point.
You could possibly connect the shield to the transmitter body in that case but only if you can gaurantee it's not going to ground out in some other fashon. My preference is to just cut and tape the shield and provide a seperate ground wire.
Regards
Roy