Neither NFPA 30 nor NFPA 77 specifies a 25-ohm resistance threshold for metal-to-metal bonding and grounding circuits, and neither document has a history of endorsing 25 ohms for equipment and piping bonds. Instead, the confusion regarding “25 ohms” typically stems from a crossover with a completely different code:
Where Does the 25-Ohm Figure Come From?
- The National Electrical Code (NEC / NFPA 70): The 25-ohm rule is famously embedded in the NEC (specifically under the requirements for earth electrode grounding resistance, such as a single rod or pipe electrode under old or simplified provisions). If a single ground rod does not naturally achieve 25 ohms or less to earth, the NEC historically required a supplemental electrode.
- The API 2003 Cross-Over: While API RP 2003 (Protection Against Ignitions Arising Out of Static, Stray, and Lightning) and historical industrial guidelines focus tightly on low resistance for static mitigation, they sometimes reference earth/soil grounding grid resistances or specific non-critical applications where higher thresholds might be discussed, but they do not set a metal-to-metal bonding target at 25 ohms.
What NFPA 30 and NFPA 77 Actually Require
- NFPA 77: Consistently emphasizes that for all-metal continuous ground paths, the resistance should ideally not exceed 10 ohms. Auditors treat a jump toward higher resistance (let alone 25 ohms or higher on a metal path) as an indicator of a failing mechanical connection, paint buildup, or severe internal corrosion. For general static dissipation to earth (like grounding grids), values up to 1 megohm ($1 \times 10^6$ ohms) are often referenced as sufficient to bleed off static charges, provided you aren’t dealing with high-energy metal-to-metal spark gaps.
- NFPA 30: Focuses primarily on the mandate that systems must be effectively bonded and grounded to prevent static spark accumulations during Class I liquid transfers, deferring to the engineering principles outlined in NFPA 77 and API 2003 for the actual electrical continuity numbers.
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