About

In electrical installations, the Earth Fault Loop Impedance (Z_s) is a critical safety parameter. It determines whether a circuit breaker will trip fast enough (usually within 0.4s for TN systems) during a fault to ground. If the impedance is too high, the fault current will be too low to trigger the magnetic trip of the MCB, creating a fire hazard or shock risk.

This tool calculates the total impedance by combining the external impedance (Z_e) with the resistance of the Phase and Earth conductors (R₁ + R₂). It accounts for temperature rise during faults (using a factor of 1.2) and conductor material (Copper or Aluminum). The result is automatically validated against standard "Max Zs" tables for Type B, C, and D breakers according to IEC/BS standards.

Formulas

The formula for Total Loop Impedance is:

Z_s = Z_e + [ L × (r₁ + r₂)1000 × C_t ]

Where:

Z_e: External earth loop impedance (from supply).
L: Length of circuit (meters).
r₁, r₂: Resistance per meter of Phase/Earth (mΩ/m).
C_t: Temperature correction factor (typically 1.2 for operating temp).

Reference Data

Breaker Type	Current (In)	Max Zs (0.4s) @ 230V	Magnetic Trip
Type B	6A	7.28 Ω	3 - 5 x In
Type B	10A	4.37 Ω	3 - 5 x In
Type B	32A	1.37 Ω	3 - 5 x In
Type C	6A	3.64 Ω	5 - 10 x In
Type C	10A	2.19 Ω	5 - 10 x In
Type C	32A	0.68 Ω	5 - 10 x In
Type D	6A	1.82 Ω	10 - 20 x In
Type D	32A	0.34 Ω	10 - 20 x In

Frequently Asked Questions

If Zs exceeds the maximum limit, the breaker may not trip within the required safety time (0.4s or 5s). You must either increase the cable cross-section (to lower resistance), reduce the circuit length, change the breaker curve (e.g., C to B), or install an RCD (Residual Current Device).

Ze (External Impedance) depends on your transformer and grid connection. It is measured at the distribution board. Typical values are 0.35 Ω for TN-C-S systems and 0.8 Ω for TN-S systems, but you should measure it with a tester.

Phase-Zero is a term used in some regions (like Eastern Europe/Russia) equivalent to the Earth Fault Loop. It refers to the complete circuit path from the transformer phase, through the fault, and back via the neutral/earth (zero) conductor.

Standard resistance is given at 20°C. However, when a circuit is loaded or faulting, the cable heats up. As temperature rises, resistance increases. The factor of 1.2 adjusts the calculation to reflect "operating temperature" (approx 70°C) for a worst-case safety scenario.