The 10 ohm figure is a static-dissipation threshold, not a safety ground: copper bonding paths on metal plant items are typically required to measure 10 ohms or less, and stainless-steel or other metals 25 ohms or less, per widely cited static-grounding practice [S3].
That 10/25 ohm envelope is several orders of magnitude looser than what protective bonding and equipment grounding demand on the same metal item, where 1 ohm or less to the equipment grounding conductor is the rule for U.S. factories under ANSI/ESD S20.20, and roughly 4 ohms applies in many Chinese installations [S5]. On U.K. COMAH sites the measured resistance between two coupled flanges must not exceed 10^6 ohms when used as a static-earth path, illustrating how the "10 ohm" habit migrates across regions and standards [S8].
Where the 10 ohm number actually comes from
The 10 ohm bonding limit on metal plant items traces back to static-electric control, not power-system safety: copper conductor resistance of 10 ohms maximum, and 25 ohms for stainless steel or other metals, is the commonly cited practical envelope for dissipating electrostatic charge from plant hardware such as drums, IBCs, and metal process vessels [S3]. The logic is charge relaxation: a path resistance on the order of 10^7 ohms already drains a static charge fast enough to prevent incendive discharge, so 10 ohms is conservative headroom, not a derived minimum [S3].
For the same metal items, the protective bonding numbers come from entirely different documents: 30 CFR Parts 55, 56 and 57 (Sections 12025-12028) require frame grounding and testing of the grounding components at metal/nonmetal mines, with a target of 5 ohms or less to earth for the grounding electrode itself [S2]. NFPA's NEC Article 100 defines bonding as "connected to establish electrical continuity and conductivity" between metal parts likely to become energised, a power-fault requirement, not a static requirement [S7].
Comparing the three thresholds side by side
The same bolted metal item can face three different resistance limits depending on which hazard is being addressed, and the differences are not subtle. For static-electric dissipation on copper paths, 10 ohms is the commonly cited practical limit, and 25 ohms for stainless steel [S3]. For U.S. factory equipment grounding under ANSI/ESD S20.20, the requirement drops to under 1.0 ohm to the equipment grounding conductor (typically the building protective earth), measured with an AC impedance meter rather than a DC multimeter [S5].
Earth-electrode targets on mining sites run 5 ohms or less for the grounding electrode at metal/nonmetal plants under MSHA's preferred practice, distinct from frame-bonding resistance between equipment parts [S2]. Bonding of metal parts on a fixed luminaire is expected to read under 0.020 ohms when measured end-to-end, an even tighter continuity number for the same kind of metal-to-metal joint [S6]. U.K. apprentice guidance for BS 7671 main protective bonding of water and gas services commonly references around 0.05 ohms as the working test figure, again for safety bonding, not static [S4]. The result is a layered ladder: ~0.02-0.05 ohms for hardwired safety bonding of small accessories, 1 ohm for factory equipment grounds, 5 ohms for mine earth electrodes, 10-25 ohms for static dissipation, and up to 10^6 ohms still acceptable for dedicated static-earthing couplings on COMAH hazard zones [S6][S4][S5][S2][S3][S8].
When 10 ohms is correct, and when it is not
If the metal plant item is being protected against static charge accumulation during transfer of flammable liquids, powders, or solvents, specifying a bonding path that measures 10 ohms or less end-to-end on copper (25 ohms on stainless) is consistent with common static-grounding practice [S3]. A reading above that, especially on stainless hardware where the higher base resistance of the conductor and clamp stack-up is well known, signals a loose or contaminated joint that needs re-terminating, not just a passed test.
If the same item is also a power-system bonded part, or sits in a hazardous area requiring equipotential bonding, the 10 ohm envelope is not tight enough: 1.0 ohm to the equipment grounding conductor under ANSI/ESD S20.20, and 4 ohms or less as the common Chinese site practice for the same measurement, apply instead [S5]. On U.K. hazardous-area sites under COMAH, dedicated static-earthing couplings between metal plant items must be measured at 10^6 ohms or less and re-tested at defined intervals, a separate verification from the power-system bond [S8]. Specifying only the 10 ohm number on metal plant items that cross both worlds is a frequent source of inspector pushback.
Measurement method drives the reading you get
The same 10 ohm static-bonding joint can read anywhere from 0.5 ohms to over 20 ohms depending purely on how it is measured, which is why several standards explicitly call out the instrument. ANSI/ESD S20.20 guidance is that the under-1 ohm equipment-ground impedance must be taken with an AC impedance meter, not a DC multimeter; a DC measurement on a live floor picks up power-system noise and tends to give unstable or inflated readings, and only works if every piece of equipment on the line is powered down [S5].
For the static 10 ohm / 25 ohm check, a standard low-voltage ohmmeter or a dedicated static-ground continuity tester is normally sufficient, since the requirement is for a low-impedance path to a verified earth point, not for fault-current capability [S3]. MSHA's mine-side verification also distinguishes between the ground-electrode resistance to earth (5 ohms or less preferred, measured with a fall-of-potential or clamp-on ground tester) and the resistance of the equipment grounding conductor run itself, which must simply be continuous and capable of carrying fault current [S2]. Picking the wrong test method for the wrong number is the most common way a "passing" 10 ohm static bond turns into a failing 1 ohm protective bond on the same bolted stack-up.
Standards and codes that govern the same joint
No single document publishes "10 ohms" as a universal metal-plant-item bonding limit; the number is a cross-reference of several standards each governing a different hazard. Static grounding on metal plant items is treated by industry practice, with the 10 ohm copper / 25 ohm stainless envelope cited as the typical practical maximum, not as a normative test value [S3].
Power-system bonding of the same metal item is covered by NFPA 70 NEC Article 100, which defines bonding as establishing electrical continuity and conductivity between metal parts likely to become energised, with the actual resistance targets set by product standards and verification test sheets [S7]. Mining-site frame grounding and earth electrodes fall under 30 CFR Parts 55, 56 and 57, Sections 12025-12028, with 5 ohms or less to earth preferred at metal/nonmetal operations [S2]. On European hazardous-area sites, the COMAH-specific earthing measurement between two coupled flanges of 10^6 ohms or less is the controlling number for static-earth verification on metal couplings [S8]. Equipment-grounding practice for electronics manufacturing is set by ANSI/ESD S20.20 at under 1.0 ohm to the equipment grounding conductor, with under 4 ohms cited as the common Chinese site value for the same reading [S5].
Specifying metal plant items against all three hazards
A practical specification on a metal plant item that crosses all three hazards reads as a layered test, not a single number: verify the static-bonding path measures 10 ohms or less on copper hardware (25 ohms if stainless) with a standard low-voltage ohmmeter, verify the equipment grounding conductor to that same item measures under 1.0 ohm (U.S. factory) or under 4 ohms (typical Chinese site) with an AC impedance meter, and on hazardous-area couplings verify the static-earthing path between flanges measures 10^6 ohms or less on the interval the site safety report requires [S3][S5][S8]. Engineers buying metal material for process vessels or metal powder handling skids should write those three numbers into the inspection test plan rather than leaving "grounded" as a single checkbox.
For a metal plant item that also functions as a structural or electrically continuous part of a larger assembly, the relevant reference on how mechanical joint hardware and conductive spacers interact with bonding is the limit switch and limit switch hardware spec practice, which uses continuity rather than resistance as the pass criterion on the bolted stack-up. Adjacent to that, a decade resistance box is the typical shop-floor reference used to confirm that the ohmmeter or impedance meter itself is reading correctly before signing off a bonding test, and where a concrete batching plant is the host site, the same bonding numbers apply to metal aggregate hoppers, cement silos, and water-supply piping just as they do to process skids.
Trackable signals to watch over the next revision cycle: any update to ANSI/ESD S20.20 that shifts the under-1.0 ohm equipment-ground rule to a national-electric-code-based value, which several community contributors flagged as likely for the next release [S5]; and any harmonisation of the 10 ohm / 25 ohm static-bonding envelope into a single IEC-based document, since today the 10/25 ohm pair is cited as common practice rather than as a normative IEC 60079 or NFPA test value [S3][S8].
See also our earlier report, MCC enclosure ratings for underground mining: 2026 specification guide.