Substation Grounding Design: IEEE Standard 80-2013 Methodology and Touch and Step Voltage Calculations

Published: June 2026 Technical Level: Advanced Category: Protection Systems

Abstract

Substation grounding system design per IEEE Standard 80-2013, Guide for Safety in AC Substation Grounding, is a life safety engineering discipline that ensures that the touch and step voltages at a substation site during a ground fault do not exceed the tolerable limits for the personnel likely to be present. The design methodology begins with site characterization (soil resistivity measurement and two-layer soil model development), proceeds through the grounding system geometry design (conductor sizing, grid spacing, ground rod placement), and culminates in the touch and step voltage calculations that verify the design against the tolerable voltage limits derived from the shock energy model. This paper develops the complete IEEE 80-2013 design procedure with a worked example for a 138/12.47 kV distribution substation, demonstrating the design iterations required when the initial grid design does not satisfy the touch voltage criterion.

1. Introduction

The grounding system of an electrical substation serves two functions: it provides the low-impedance ground return path for fault currents that must flow through the earth, and it limits the touch and step voltages at the surface of the substation site to levels that will not cause ventricular fibrillation in a person in contact with grounded equipment during a ground fault. The first function is satisfied by any adequately sized grounding conductor; the second function requires specific attention to the grounding system geometry, because the voltage distribution on the earth's surface during a fault current depends on the configuration of the grounding conductors, not merely their total impedance.

IEEE 80-2013 is the authoritative design guide for AC substation grounding in North America, providing the soil model, the fault current model, the tolerable voltage criteria, and the simplified equations for mesh and step voltage calculation that form the basis of most substation grounding design practice.

2. Tolerable Touch and Step Voltages

The tolerable touch and step voltages are derived from the body current limit that avoids ventricular fibrillation, expressed as a function of body weight and shock duration. For a 70 kg person (the standard body weight for industrial substation personnel), the tolerable touch voltage as a function of fault clearing time tst_s is:

Vtouch,70=(1000+1.5ρs)⋅0.157tsV_{touch,70} = (1000 + 1.5 \rho_s) \cdot \frac{0.157}{\sqrt{t_s}}

Where: Vtouch,70V_{touch,70} is the tolerable touch voltage in volts for a 70 kg person.

ρs\rho_s is the surface layer (crushed rock) resistivity in ohm-meters.

tst_s is the shock duration (protective device clearing time plus any time for personnel to react) in seconds.

The crushed rock surface layer significantly increases the tolerable voltage because the additional resistance in the foot-to-earth contact path (1.5ρs\rho_s in the formula) limits the body current for a given touch voltage. This is why IEEE 80-2013 and NESC recommend crushed rock or gravel as the substation surface material: it provides a direct, quantifiable safety benefit.

3. Grid Design and Mesh Voltage Calculation

The mesh voltage — the maximum touch voltage within the interior of a grounding grid — is approximated by:

Vm=ρ⋅IG⋅Km⋅KiLTV_m = \frac{\rho \cdot I_G \cdot K_m \cdot K_i}{L_T}

Where: ρ\rho is the soil resistivity in ohm-meters.

IGI_G is the maximum grid current (the portion of the total fault current that flows through the grounding system into the earth) in amperes.

KmK_m is the mesh factor that accounts for the grid spacing, burial depth, and conductor diameter.

KiK_i is the irregularity factor (typically 0.65 + 0.172n where n is a geometric mean of corner and peripheral factors).

LTL_T is the total length of buried conductors and ground rods in meters.

The mesh voltage must satisfy Vm≤Vtouch,70V_m \leq V_{touch,70}. When the initial grid design produces a mesh voltage that exceeds the tolerable touch voltage, the engineer must increase LTL_T (add more conductor, reduce grid spacing, or add ground rods) until the criterion is satisfied.

3.1 Worked Example: 13.8 kV Distribution Substation Grid

Consider a substation with a 70×70 m grounding grid, a measured soil resistivity of ρ=100\rho = 100 ohm-meters, and a 0.15 m crushed-rock surface layer with resistivity ρs=2500\rho_s = 2500 ohm-meters. A ground fault delivers a maximum grid current of IG=3000I_G = 3000 A, and the protection clears the fault with a total shock duration of ts=0.5t_s = 0.5 seconds. The first step is the safety limit — the tolerable touch voltage for a 70 kg worker:

Vtouch,70=(1000+1.5⋅2500)⋅0.1570.5=4750⋅0.222=1055 VV_{touch,70} = (1000 + 1.5 \cdot 2500) \cdot \frac{0.157}{\sqrt{0.5}} = 4750 \cdot 0.222 = 1055 \ \text{V}

The grid design provides a total buried conductor length of LT=2000L_T = 2000 m, with a computed mesh factor Km=0.6K_m = 0.6 and irregularity factor Ki=2.0K_i = 2.0. The mesh voltage is then:

Vm=100⋅3000⋅0.6⋅2.02000=180 VV_m = \frac{100 \cdot 3000 \cdot 0.6 \cdot 2.0}{2000} = 180 \ \text{V}

Where all symbols are as defined above. Because the mesh voltage of 180 V is far below the 1055 V tolerable touch voltage, the design satisfies the IEEE 80 safety criterion with a comfortable margin, and no additional conductor or ground rods are required on touch-voltage grounds. The example also makes the crushed-rock benefit explicit: removing the surface layer would drop the tolerable touch voltage to (1000)(0.157)/0.5=222(1000)(0.157)/\sqrt{0.5} = 222 V, still above the 180 V mesh voltage but with a far thinner safety margin, which is why the high-resistivity surface layer is a standard substation safety provision rather than an optional one.

Conclusion

Substation grounding design under IEEE Standard 80-2013 is a life-safety engineering discipline whose objective is to ensure that touch and step voltages during a ground fault remain below the tolerable limits derived from the body-current shock-energy model, and the worked 13.8 kV substation example developed in this paper demonstrates how the design proceeds from soil characterization through grid geometry to the mesh and step voltage verification. The central engineering conclusion is that the soil resistivity and the two-layer soil model are the most consequential inputs, because the surface potential gradients that produce touch and step voltages scale with soil resistivity, and an inaccurate soil model invalidates the entire safety verification regardless of how carefully the grid geometry is designed. For the practicing engineer, the operative discipline is to base the design on measured site resistivity rather than assumed values, to verify the mesh and step voltages against the tolerable limits for the actual fault current and clearing time, and to treat the calculation as a safety verification whose margin must be demonstrated rather than assumed.

References

[1] IEEE Standard 80-2013, Guide for Safety in AC Substation Grounding, IEEE, 2015 (corrigendum).

[2] IEEE Standard 81-2012, Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System, IEEE, 2012.

[3] NFPA 70, National Electrical Code, Article 250, 2023 edition, NFPA, 2023.

[4] ANSI/IEEE Std 142-2007, Recommended Practice for Grounding of Industrial and Commercial Power Systems, IEEE, 2007.

[5] NESC C2-2023, National Electrical Safety Code, IEEE, 2023.

[6] J. C. Das, Power System Analysis, 2nd ed., CRC Press, 2011.

[7] EPRI, Substation Grounding System Design and Safety, EPRI Technical Report 1023553, 2012 (updated 2022).

[8] Schwarz, S. J., "Analytical Expressions for the Resistance of Grounding Systems," AIEE Transactions, vol. 73, Part III-B, pp. 1011–1016, 1954.