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5 ohms or less some instruments like loadcell required 1 ohm or less
As per IEEE, NEC and NFPA standards, the grounding resistance for sensitive equipment should be 5 ohms or less.
there are a lot of details for this subject of path for grounding: the type of instrument , location and the zone where the instrument is located
as perANSI/ISA-RP12.06.01-2003 Formerly ANSI/ISA-RP12.06.01-1995 (R2002) Recommended Practice for Wiring Methods for Hazardous (Classified) Locations Instrumentation Part 1: Intrinsic Safety
"All grounding path connections should be secure, permanent, visible, and accessible. The groundingpath resistance from the farthest barrier to the grounding electrode should not exceed 1 ohm. "
