Isolated ground for instrument and computers

[bahram]

According the IEEE80 standard there is only one grounding system in substations and power stations. and all electrical devices as well as computers and electronic devices and metalic parts are to be connected to the same ground system. The reason is very clear, but in some industrial plant such as oil industries, I found that it is instructed that the grounding of instruments and electronic devices shall be separated from electrical grounds, to prevent any noise in control systems.
I would be appreciated to get more information about this matter. and have a clear standard specification.

 

[jbartos]

Suggestions:
1. There are some basic grounding categories:
1a. System ground
1b. Equipment ground
2. There are grounding quality categories pertaining to the equipment ground:
2a. Noisy ground, i.e. ground that may contain electrical noise.
2b. Quiet ground, i.e. ground that does not contain electrical noise above a predetermined noise level.
References:
1. IEEE Std 142-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, (Green Book)
2. IEEE Std 1100-1992 IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment (Emerald Book)(Revised recently).