IEEE Std 80 utilizes Charles Dalziel's empirical formulas for the threshold of ventricular fibrillation. The human heart can tolerate a higher shock current if the duration is short, but the threshold drops exponentially as duration increases. The standard divides populations by average body weight:

50 kg Body Weight: Assumes a lower threshold for fibrillation (conservative, typical default):

70 kg Body Weight: Used where the design environment warrants a higher weight limit assumption:

During a fault, if the current through the body $I_{\text{body}} = E_{\text{touch}} / (1000 + 1.5\rho_s)$ exceeds $I_B$, fibrillation is statistically probable. Thus, designs are sized to keep body currents strictly below this curve.

Crushed rock possesses a high resistivity (typically $2500 - 3000\,\Omega\cdot m$ even when wet). When an operator stands on gravel, the rock layer acts as an insulator, introducing a high contact resistance in series with the feet. Each foot is modeled as a circular plate of radius $0.08\,\text{m}$, giving a ground resistance of $3\rho_s$ per foot.

Because the gravel thickness $h_s$ is finite and the native soil below has lower resistivity $\rho$, the electric field penetrates into the soil, reducing the effective resistance. To account for this, the surface layer derating factor $C_s$ is calculated using the reflection factor $K$:

This derates the apparent resistivity of the rock. The total resistance of the body path for touch voltage is $R_B + 1.5 C_s \rho_s$, where $1.5$ represents two feet in parallel ($3/2 = 1.5$).

Ground faults are asymmetrical transients. At the instant of contact, the current contains an AC symmetrical component and a decaying DC offset component. The rate of decay of the DC component is governed by the subtransmission/generator system's reactive-to-resistive impedance ratio, or System X/R ratio.

The time constant of decay $T_a$ is defined as:

To ensure that the ground conductors do not melt under the thermal stress of this asymmetrical peak and that human safety margins are maintained during the initial cycles of the fault, the symmetrical fault current is multiplied by the Decrement Factor $D_f$:

If the system X/R ratio is high (e.g., 30 near generating stations) and the fault duration is short (e.g. 0.1s), $D_f$ can exceed $1.3$, significantly increasing the effective design current $I_G = I_{\text{sym}} \times D_f$.

For power-frequency grounding faults (50/60 Hz), an earthing system acts primarily as a static resistance $R_g$. However, a lightning strike is a transient surge with high-frequency harmonics equivalent to 100 kHz or higher. At these frequencies, the copper grid's self-inductance ($L$) becomes dominant.

The transient impedance $Z_g(f)$ of the grid scales according to:

Due to propagation delays along conductors, current cannot reach the extremities of a large grid during the transient wavefront. This limits the grid's "effective active radius" to:

Where $t_r$ is the wavefront rise time (e.g. 1.2 $\mu$s). Thus, increasing the grid's physical dimensions beyond $r_{\text{eff}}$ does not reduce surge impedance; instead, short, direct vertical ground rods or low-inductance radial strips must be installed close to surge arresters.

Solid rock has an extremely high soil resistivity ($\rho > 2000\,\Omega\cdot m$), which makes achieving touch/step safety limits difficult. A standard shallow mesh grid is insufficient. The following engineering measures are applied:

• Deep Boreholes: Deep vertical boreholes (up to 30m) are drilled to tap into moist, lower-resistivity water tables.
• Ground Enhancement Materials (GEM): Native rocky backfill around rods is replaced with conductive material such as Bentonite clay, Marconite, or chemical gel backfills, which artificially expand the electrical boundary of the conductor.
• Counterpoise/Radial Wires: Horizontal radial strips are extended outward from the substation perimeter into areas with softer or deeper soil layers.

Changing the spacing of grid conductors has different effects on the grid's overall resistance and local safety voltages:

Grid Resistance ($R_g$): The overall grounding resistance is primarily determined by the total area of the grid ($A$) and the total length of the conductors ($L_T$). Decreasing the mesh spacing from 10m to 5m increases the total length of the conductor, which slightly reduces $R_g$ (typically by less than 5%), but this is an inefficient way to lower the overall ground resistance.

Mesh/Touch Voltage ($E_m$): In contrast, touch voltage is a local safety parameters. Shrinking the mesh spacing creates a denser conductor layout. This reduces the electric fields in the ground between conductors, smoothing out the surface potential profile and significantly lowering the actual touch voltage $E_m$ at the mesh center.

Under fault conditions, current leaves the grid and enters the surrounding earth. Conductor elements at the perimeters and corners of the grid have fewer neighboring conductors to share the current discharge. This leads to current crowding at the outer perimeter, resulting in high current densities in the soil at the grid corners.

This localized current crowding creates steep voltage gradients at the edges of the grid. Consequently, the step potential ($E_s$) is highest just outside the perimeter grid loop, and the mesh touch potential ($E_m$) is highest in the corner mesh compartments. To mitigate this risk, ground grids are designed with closely spaced outer conductors and vertical ground rods driven at the grid corners.

A common misconception is that achieving an extremely low grounding resistance (e.g., $< 1\,\Omega$) guarantees safety. If a substation experiences a massive 20 kA fault, even a ground resistance of $0.5\,\Omega$ results in a Ground Potential Rise (GPR) of:

The substation grid and all connected metal enclosures jump to 10,000 V relative to remote earth. If an operator touches a metal enclosure while standing on the ground, they are exposed to a shock hazard unless Equipotential Bonding is established.

By bonding all metallic structures, fences, operating handles, and concrete reinforcing bars to the ground grid, everything rises to the same electrical potential during a fault. Because there is no potential difference ($\Delta V \approx 0$) across the operator's body, no current flows, and safety is maintained regardless of the GPR magnitude.

The two standards are applied to different earthing scenarios and have different sizing philosophies: