Ground-Fault Calculator

Most electrical injuries never involve phase-to-phase shorts. They involve a live conductor meeting earth through a broken cable, a damp wall or a person - the ground fault, which does its damage in the half-second before anything clears.

Ground-Fault Calculator estimates the fault current and the touch voltage for a line-to-earth fault, then judges whether the RCD or breaker protecting the circuit would actually clear it in time.

It is a field-aid. Always verify with an instrument (loop tester) and the code in force.

• Phase voltage: Line-to-earth voltage input.
• Source impedance: From transformer kVA and %Z.
• Loop impedance: Phase plus earth conductor path.
• Fault resistance: Contact resistance at the fault.
• Touch voltage: Hazard voltage computed directly.
• RCD check: Trips in time or not, with the reason.
• Breaker check: Minimum earth-fault current rule.
• Export: Copy or download as text, CSV or JSON.

1. Set the phase voltage (230 V, 240 V, 120 V...).
2. Enter the transformer kVA and %Z for the source.
3. Enter the loop cable length and impedance.
4. Optional: set the fault contact resistance.
5. Pick the RCD rating (or none).
6. Read the fault current, touch voltage and clearance verdict.

Example 1 — Bolted fault. 230 V, 30 m loop at 8 m-ohm/1000 m: about 190 A faults; a 32 A breaker clears it, a 30 mA RCD trips instantly.

Example 2 — High-resistance contact. Same circuit with a 10 ohm fault: 22 A fault, touch voltage near 220 V near the source side of the contact - still hazardous but too low for the breaker, so the RCD is essential.

Example 3 — Poor earth. Loop impedance doubled by a long earth run: fault current halves, an RCD stays near its clear boundary - time to improve the earth.

• Hazard visible: Touch voltage shown against 50 V.
• Clearance judged: RCD and breaker verdicts computed.
• Loop-aware: Cable and earth path included.
• Field practical: Simple inputs, immediate answer.
• Private: No uploads, no accounts, no logging.