Ground Grid Step & Touch Potential Analysis Using CDEGS or IEEE Std 80 Methods

Industry Applications

Power substations, wind turbine foundations, telecom towers, rail traction systems

Key Standards

IEEE Std 80-2013, IEC 61936-1, ENA TS 43-24, CIGRE TB 656

Typical Scale

Substation grids: 50 m × 50 m to 200 m × 200 m; conductor: 70 mm² bare copper or 120 mm² ACSR

Fatal Threshold

≈100 mA AC through chest for >0.1 s — corresponding to ~1,500 V touch potential on wet soil

⚠️ Why It Matters

1

Inadequate ground grid design

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2

High current density near grid conductors

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3

Excessive voltage gradients in surface soil

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4

Electric shock hazard to personnel or animals

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5

Non-compliance with OSHA/IEEE safety limits

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6

Catastrophic injury, fatality, or regulatory shutdown

📘 Definition

Step potential is the voltage difference between two points on the earth’s surface 1 meter apart (typically heel-to-heel), while touch potential is the voltage between a grounded metallic structure (e.g., substation fence or transformer tank) and a point on the earth’s surface 1 meter away, measured at hand height (1.5 m). Both arise from non-uniform current dispersion through soil during ground fault or lightning events and are critical safety metrics in grounding system design per IEEE Std 80-2013. Their magnitudes depend on soil resistivity, grid geometry, fault current magnitude/duration, and conductor burial depth.

🎨 Concept Diagram

AI-generated illustration for visual understanding

💡 Engineering Insight

Never assume a 'dense grid' solves everything — overly tight meshes (<1.5 m) in high-resistivity soil can *increase* touch potential at equipment pads by concentrating current near surface conductors. Always prioritize conductor depth and surface layer resistivity over mesh fineness. The most cost-effective safety improvement is often a properly specified 3,000 Ω·m gravel layer — not more copper.

📖 Detailed Explanation

Step and touch potentials originate from Ohm’s Law applied across soil: when fault current enters the ground, it spreads outward, creating voltage gradients (V = I × R) in the soil volume. Since human feet occupy two points in this gradient field, a voltage appears across the body — potentially driving lethal current. IEEE Std 80 provides analytical formulas to estimate these voltages based on uniform soil models and simplified grid geometries, assuming steady-state DC-like conditions.

CDEGS (Current Distribution Electromagnetic Interference Grounding Software) replaces those simplifications with rigorous finite-element modeling: it accounts for multi-layered soil, buried structures (pipes, rebar), overhead shield wires, and frequency-dependent effects. Its ‘SESSCAD’ and ‘MALZ’ modules solve Maxwell’s equations numerically, revealing hot spots missed by IEEE 80 — especially near corners, fences, or cable trenches where current crowding occurs.

Advanced practice requires coupling electromagnetic transient analysis (e.g., ATP-EMTP) with CDEGS to model time-domain behavior: lightning surges induce high-frequency skin effects and inductive coupling that elevate peak step voltages beyond DC-based predictions. Modern designs also integrate corrosion-aware conductor sizing (per IEEE Std 80-2013 Table 11), thermal withstand validation (using I²t curves), and GIS-linked risk mapping for maintenance prioritization — turning grounding from a compliance checkbox into an integrated reliability asset.

🔄 Engineering Workflow

Step 1

Step 1: Site-specific soil resistivity profiling (Wenner 4-pin test + layered modeling)

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Step 2

Step 2: Determine worst-case fault current (I_f), duration (t), and source impedance from protection coordination study

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Step 3

Step 3: Preliminary grid layout (mesh size, depth, conductor size) per IEEE 80 Annex A or CDEGS ‘GRID’ module

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Step 4

Step 4: Compute Ground Potential Rise (GPR), step & touch voltages using analytical (IEEE 80) or numerical (CDEGS) methods

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Step 5

Step 5: Iteratively refine design: adjust mesh, add rods/gravel, verify against human body tolerances (IEC 60479-1 curves)

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Step 6

Step 6: Fabricate and install per IEEE Std 80 Sec. 10 & NFPA 70 Article 250

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Step 7

Step 7: Field verification: fall-of-potential resistance test + step/touch voltage measurement during simulated fault (if feasible)

📋 Decision Guide

Rock/Field Condition	Recommended Design Action
High-resistivity soil (ρ > 3,000 Ω·m) with limited area	Install deep-driven ground rods (≥15 m) tied radially to grid perimeter; use conductive backfill (bentonite/carbon mix); avoid reliance on horizontal conductors alone.
Low-resistivity soil (ρ < 100 Ω·m) but high fault current (>40 kA)	Optimize grid mesh spacing ≤ 3 m; add outer ring conductor; verify touch potential at all metallic structures using CDEGS ‘MALZ’ module.
Substation adjacent to public access (e.g., fence line, road)	Install 0.3-m-thick, 3,000+ Ω·m gravel layer extending ≥1 m beyond fence; calculate touch potential at fence posts using IEEE 80 Eq. 32; ensure GPR < 2,000 V.

📊 Key Properties & Parameters

Soil Resistivity (ρ)

10–10,000 Ω·m (clay: 10–100 Ω·m; sand: 100–1,000 Ω·m; granite bedrock: 1,000–10,000 Ω·m)

The inherent resistance of a unit cube of soil to current flow, measured in ohm-meters (Ω·m).

⚡ Engineering Impact:

Directly controls ground grid size, conductor spacing, and required depth — low ρ allows compact grids; high ρ demands extensive counterpoise or chemical treatment.

Fault Current (I_f)

5–63 kA (distribution substations: 5–20 kA; EHV transmission switchyards: 31.5–63 kA)

The maximum RMS symmetrical short-circuit current expected at the grounding location during a worst-case phase-to-ground fault.

⚡ Engineering Impact:

Scales step/touch voltages linearly — doubling I_f doubles hazardous potentials unless grid geometry or soil treatment compensates.

Grid Depth (h)

0.3–0.7 m (standard: 0.5 m; deeper burial reduces surface gradients but increases excavation cost)

Vertical burial depth of the main grounding grid conductors below grade, excluding surface gravel layer.

⚡ Engineering Impact:

Deeper burial significantly lowers touch potential by increasing separation between energized conductor and surface contact points — but has diminishing returns beyond 0.6 m.

Gravel Layer Resistivity (ρ_s)

3,000–10,000 Ω·m (dry, clean crushed limestone: ~5,000 Ω·m)

Surface layer resistivity of crushed rock (typically 10–30 cm thick) placed over the grid to increase contact resistance for personnel.

⚡ Engineering Impact:

Raises effective body resistance during fault, reducing shock current — mandatory for compliance; ineffective if saturated or contaminated.

📐 Key Formulas

Maximum Allowable Touch Voltage (IEEE 80)

E_touch = (1000 + 1.5 × ρ_s) / √t

Permissible touch voltage limit (V) for a person standing on surface gravel of resistivity ρ_s (Ω·m) for fault duration t (seconds).

Variables:

Symbol	Name	Unit	Description
E_touch	Maximum Allowable Touch Voltage	V	Permissible touch voltage limit for a person standing on surface gravel
ρ_s	Surface Layer Resistivity	Ω·m	Resistivity of the surface gravel layer
t	Fault Duration	s	Duration of the ground fault in seconds

Typical Ranges:

Distribution substation (t = 0.37 s)

650–950 V

EHV switchyard (t = 0.15 s)

1,600–2,200 V

⚠️ Must be ≥ calculated touch voltage; typically designed to 80–90% of limit for margin.

🔧 Open Calculator

Ground Potential Rise (GPR)

GPR = I_f × R_g

Voltage rise of the entire grounding system relative to remote earth during fault.

Variables:

Symbol	Name	Unit	Description
GPR	Ground Potential Rise	V	Voltage rise of the entire grounding system relative to remote earth during fault
I_f	Fault Current	A	Current flowing into the grounding system during a fault
R_g	Ground Resistance	Ω	Resistance of the grounding system to remote earth