Signal earthing to control system

to combine or not to combine , this is a problem. In one project , DCS vendor Honeywell requested that the signal earth of DCS system should be seperated from plant electrical
earthing system. They said electrical earth maybe disturb instrument signals to DCS. But our electrical engineer suggested that combined earth was better and seperated earth may make system worse. Anyone has experience of control system earthing?
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snipped-for-privacy@gmail.com wrote:

Single-point earthing avoids ground loops (should that be earth loops? I get dizzy thinking about that) The earthing leads should carry no current common to the instruments and the power equipment. In sever cases, instrument amplifiers (or isolation amplifiers) may be helpful.
Jerry
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snipped-for-privacy@gmail.com wrote:

Mine are always earthed to one point, and if several power supplies are used I tie the neutrals together
But my DCS's are cobbled together expansions and bits added to machines rather than something designed from the ground up
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