5,000
V
2,500
V
2.5
A
50
V
50
50
5,000
V
2,500
V
2.5
A
50
V
50
50
An earthing (grounding) system is the backbone of electrical safety in any installation. By connecting exposed conductive parts of equipment to the general mass of the earth, an earthing system ensures that fault voltages are safely diverted, protective devices operate reliably, and human beings are protected from dangerous electric shocks. The design of an earthing system must satisfy both functional requirements (equipment protection) and safety requirements (human protection), and is governed by standards including IEC 60364, IEEE Std 80, and BS 7430.
The Earthing System Calculator evaluates the most critical parameters of an earthing design: the Ground Potential Rise (GPR), touch voltage, the current that would flow through a human body, and compliance with permissible touch voltage limits. These calculations are essential for substations, industrial plants, wind farms, solar installations, and any facility where earth fault currents are significant.
Ground Potential Rise (GPR)
When an earth fault occurs, fault current flows into the grounding system and through the soil to remote earth. This raises the local ground potential above remote earth potential — a phenomenon called Ground Potential Rise. GPR is simply the product of the fault current and the grounding system resistance: GPR = I × R. A high GPR can be hazardous to people and equipment connected to the grounding system, including telecommunications cables and data lines entering the facility.
Touch Voltage
Touch voltage is the potential difference between a grounded metallic object (that a person might touch) and the point on the earth surface where the person is standing. In a simplified model, touch voltage is approximately 50% of the GPR for a person standing 1 meter from the grounded structure. In detailed calculations per IEEE Std 80, the exact touch voltage depends on the grounding grid geometry, soil resistivity, and surface layer resistivity.
Permissible Touch Voltage Limits
The permissible touch voltage depends on the fault clearing time. IEC 60479 and IEC 60364-4-41 specify that for fault clearance within 0.2 seconds, the limit is 50 V; for 0.2–0.5 s, 75 V; for 0.5–1.0 s, 90 V. After 1 second, the 50 V limit applies regardless. These limits are based on the threshold of ventricular fibrillation — the most dangerous cardiac effect of electric shock.
Safety Compliance Factor
The safety compliance factor in this calculator is the ratio of actual touch voltage to the permissible limit. A value below 1.0 indicates a safe design. A value above 1.0 means the touch voltage exceeds safe limits and the earthing system must be improved by adding more electrodes, using equalization conductors, or applying surface treatment (crushed rock) to increase footwear resistance.
Proper earthing system design requires professional expertise and site-specific soil resistivity measurements (Wenner 4-pin method). This calculator provides a first-order assessment to guide design decisions.
The calculator uses standard IEEE Std 80 and IEC 60364 formulas:
GPR: Values above 430 V (the conventional safety limit for telecommunications) require special measures for connected services. Touch Voltage: Must be below the time-dependent limit. Body Current: Below 10 mA is generally safe; 30–100 mA can cause ventricular fibrillation. Safety Compliance Factor: A value below 1.0 confirms the design is compliant. Values between 1.0 and 2.0 require design improvements. Values above 2.0 indicate a serious hazard requiring immediate redesign.
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A 5000 A fault with 2 Ω grounding creates a 10 kV GPR and 5000 V touch voltage — vastly exceeding the 75 V limit. This design is extremely hazardous and requires a comprehensive grounding grid with much lower resistance (typically <0.5 Ω for high-fault-current substations).
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A 200 A fault with 0.5 Ω grounding produces exactly 50 V touch voltage, right at the 50 V limit for fast clearing. Any increase in fault current or grounding resistance would require improvement. Consider additional ground rods or a grounding ring conductor.
The terms are used interchangeably in most contexts. 'Earthing' is preferred in British/IEC standards (IEC 60364, BS 7671), while 'grounding' is used in American standards (NEC, IEEE). Both refer to the connection of electrical systems and equipment to the general mass of the earth to ensure safety and proper operation.
IEC 60364 defines three earthing system types: TN (neutral connected to earth at the source, exposed parts connected to neutral); TT (neutral connected to earth at source, exposed parts connected to independent earth electrode); IT (no direct connection between live parts and earth, exposed parts earthed). Each has different protection requirements and fault characteristics.
The most common method is the Wenner four-pin method, where four equally spaced electrodes are driven into the ground and a known current is passed between the outer two while voltage is measured between the inner two. Soil resistivity ρ = 2πaR, where 'a' is the electrode spacing and R is the measured resistance. Multiple measurements at different spacings reveal resistivity variation with depth.
Requirements vary by application: General installations: ≤ 10 Ω (NEC) or ≤ 4 Ω (many IEC countries). Substations: typically ≤ 0.5–1 Ω. Communication towers: ≤ 5 Ω. However, the absolute resistance value is less important than ensuring safe touch and step voltages — a 10 Ω resistance with low fault current may be safer than 0.1 Ω with very high fault current.
Step voltage is the potential difference between two points on the earth surface separated by a typical stride (about 1 meter), which a person walking might bridge with their feet. Touch voltage occurs when a person touches a grounded structure with their hand while standing on the ground. Step voltage is generally lower than touch voltage but affects the full body current path (foot to foot vs. hand to foot), and both must be evaluated in substation design.
The danger of electric shock depends on both current magnitude and duration. Ventricular fibrillation — the most lethal cardiac effect — is governed by the total energy delivered to the heart. A higher touch voltage is permissible for a shorter time because less total energy reaches the heart. IEC 60479 provides the scientific basis for these time-voltage curves based on human physiological data.
Applying a 100–150 mm layer of crushed rock (gravel) with high resistivity (typically 3000–10,000 Ω·m) over the earthing grid area significantly increases the contact resistance between a person's feet and the ground. This reduces the current through the body even if the touch voltage remains unchanged, effectively improving safety compliance without reducing the grounding resistance itself.
Equipotential bonding connects all exposed conductive parts (water pipes, gas pipes, structural steel, metallic cable sheaths) to the earthing system. During a fault, this ensures all metal surfaces rise to the same potential — eliminating the potential difference that would cause a dangerous shock. Main equipotential bonding is required by IEC 60364-4-41 and virtually all national wiring regulations.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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