Accebtable earth resistance values

[AKassem]

Dear All ,

1) What is the maximum acceptable earth (ground) resistance for intrinsically safe IS instruments
and for non intrinsically safe instruments NIS??

2) what is the maximum earth resistance for grounding a PLC and DCS sytems according to standards ?
as i know that for other electrical equipment (panel body , motors , junction boxes ,... ) the limit is around 5 ohm

can any body refer to IEC or IEEE standards for these values ?
Thank you for your help

 

[7anoter4]

See: NEC [NFPA-70]Art.504.50