Is 25 ohms a universal safe grounding resistance?

No. NFPA public material presents 25 ohms in the specific NEC supplemental-electrode exception for one rod,pipe or plate electrode;it is not a universal safety verdict.

Does low resistance alone prove a safe substation grid?

No. Fault current distribution,clearing time,soil model,geometry and permissible touch and step voltages must also be evaluated under the governing design basis.

Document-controlled grounding test comparison

Documented Ground-Resistance Measurement Comparator

Compare a measured grounding-system resistance or impedance magnitude with a documented project limit for the same test configuration. No universal 1/5/25Ω safety threshold is supplied.

IEEE 81-2025 measurement contextNo parallel-rod estimateNo safety verdict

Documented test comparison

Numerical position

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Measured x

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Documented limit L

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Remaining margin L−x

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Ratio x/L

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Implemented comparison—not an electrode design

margin=L−x
ratio=x/L
x≤L → at or below entered limit; x>L → above entered limit

1Ω=1000mΩ exactly. This is general arithmetic,not an IEEE grounding formula or safety criterion.

NEEDS_LICENSED_SOURCE: the former single-rod formula was labelled IEEE80 and the multi-rod result used undocumented efficiencies0.58/0.42/0.33. Exact geometry/end-effect assumptions and a validated multi-electrode model require the governing licensed standard or engineering method. They were removed rather than guessed.

Measurement and safety standards

IEEE81-2025Active guide for measuring earth resistivity,ground impedance and earth-surface potentials. Its public scope includes safety,methods,instrumentation limitations and distortion factors.

IEEE80-2013Guide for safety in AC substation grounding;Corrigendum1-2015 was incorporated. A rod-resistance estimate alone is not an IEEE80 safety study.

Official IEEE record: IEEE81-2025. Licensed access is required for detailed procedures and equations.

Why 25Ω is not a universal target

Public NFPA NEC development material states that a single rod,pipe or plate electrode is supplemented unless its resistance to earth is25Ω or less;it also specifies minimum spacing for multiple electrodes and notes that larger spacing improves efficiency. That conditional installation rule is not a universal declaration that25Ω is safe for every facility,system or jurisdiction. The former “<5Ω good / <25Ω acceptable / >25Ω too high” output was removed.

Removed assumptions

The former page assumed homogeneous soil,used generic soil-type ranges as design inputs,substituted spacing=2L when blank,ignored conductor/contact resistance and seasonal/layered soil,and replaced an interaction sum with an unrelated efficiency map. It did not evaluate fault current,clearing time,touch/step potentials or measured uncertainty. Use site measurements and the approved design/test procedure.