Human Safety Projects
Enhanced methodology for IEEE Std 80 step-and-touch voltage grounding design
Not All Soil Resistivity Projects Are Equal
When People's Lives Depend on the Data
Soil resistivity data gathered for general purposes — cathodic protection surveys, corrosion assessments, general site characterization — can tolerate a single traverse with moderate spacing density. But when that data will be used to design a grounding system where the performance directly determines whether a person survives a fault event, the methodology must be held to a higher standard.
IEEE 81-2025 §7.4 and IEEE Std 80 together define the enhanced requirements for human safety projects. These requirements are not optional — they are the professional standard of care for any engineer designing a grounding system for a substation, generating station, or other facility where step-and-touch voltages are a safety concern.
E&S position: A single traverse with coarse spacing density is not acceptable for any project where the soil model will be used for IEEE Std 80 grounding design. The additional cost of a second traverse and denser spacing is negligible compared to the liability of a grounding system that fails to protect personnel.
General Characterization vs. Human Safety Projects
| Requirement | General Characterization | Human Safety (IEEE Std 80) |
|---|---|---|
| Number of traverses | One traverse acceptable | Minimum two traverses in different orientations (IEEE 81-2025 §7.4.3) |
| Spacing ratio | Warning if > 1.5× (not enforced) | Hard requirement: each spacing ≤ 1.5× previous (Annex B) |
| Maximum probe spacing | Project-dependent | Must be ≥ diagonal of proposed grounding grid (IEEE 81-2025 §7.4.5) |
| Season documentation | Recommended | Required deliverable: date, season, ambient temperature, soil moisture (§7.4.1) |
| Job Safety Analysis (JSA) | Recommended | Required before beginning any field measurement (§7.5) |
| Meter type | Any four-terminal instrument | High-output (200–400 V), frequency-selective, polarity-reversing (§5) |
| Field curve plotting | Recommended | Required: plot apparent resistivity vs. spacing in the field before leaving the site (§7.5) |
| Post-construction validation | Not required | Current Injection Testing (CIT) strongly recommended to validate touch and step voltage performance |
| Report compliance statement | Not required | Report must include IEEE 81-2025 §7.4 compliance statement and list all deviations |
IEEE 81-2025 §7.4.5 — Table 5
Minimum Maximum Probe Spacing by Grid Size
The maximum probe spacing determines the maximum depth of investigation. For grounding grid design, the maximum spacing must be sufficient to characterize the deep soil layer that governs the grid resistance. IEEE 81-2025 Table 5 provides minimum recommended maximum spacings based on the grid diagonal and the estimated resistivity ratio.
| Grid Diagonal | Resistivity Ratio | Min. Max. Spacing |
|---|---|---|
| 30 m | ρ₂/ρ₁ < 5 | 45 m |
| 30 m | ρ₂/ρ₁ ≥ 5 | 60 m |
| 100 m | ρ₂/ρ₁ < 5 | 150 m |
| 100 m | ρ₂/ρ₁ ≥ 5 | 200 m |
| 300 m | ρ₂/ρ₁ < 5 | 450 m |
| 300 m | ρ₂/ρ₁ ≥ 5 | 600 m |
Source: Adapted from IEEE 81-2025 §7.4.5 Table 5. Minimum spacing = 1.5 × grid diagonal for ρ₂/ρ₁ < 5; 2.0 × for ρ₂/ρ₁ ≥ 5.
Post-Construction Validation
Current Injection Testing (CIT)
Even the best soil model is a prediction. The only way to know with certainty that a grounding system meets the IEEE Std 80 touch and step voltage limits is to measure those voltages directly after the grid is installed. Current Injection Testing (CIT) does exactly that — it injects a known current into the installed grid and measures the resulting surface voltage gradients.
Inject a Known Current
After the grounding grid is installed, inject a known AC current (typically 10–100 A at a frequency away from 60 Hz) between the grid and a remote current electrode.
Measure Touch and Step Voltages
Using a high-impedance voltmeter, measure the voltage between the grid surface and points 1 m away (touch voltage) and between two points 1 m apart on the surface (step voltage).
Scale to Fault Current
Scale the measured voltages to the design fault current to determine the actual touch and step voltages that would occur during a fault. Compare to the IEEE Std 80 tolerable limits.
Document and Report
Document the measured and scaled voltages, the injection current, the frequency, the ambient conditions, and the comparison to IEEE Std 80 limits. This report is the definitive validation of the grounding system performance.
E&S Grounding Solutions performs Current Injection Testing as a standard deliverable on all human safety grounding projects. Contact us to discuss CIT requirements for your facility.
STRATIFY™ Human Safety Mode
When you select "Human Safety / IEEE Std 80" as your project type in the STRATIFY™ calculator, it automatically enforces all the requirements described on this page: minimum two traverses, 1.5× spacing ratio, minimum maximum spacing validation, mandatory weather documentation, and a JSA reminder on the report cover page.
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