The purpose of calculating ( Z_s ) is governed by a fundamental safety principle: Ohm’s Law. When a fault occurs, the fault current (( I_f )) is determined by the supply voltage (( U_0 )) divided by the loop impedance (( Z_s )). For a protective device (such as a circuit breaker or fuse) to clear the fault safely, it must trip within a prescribed time (typically 0.4 seconds for final circuits). This requires that the fault current be high enough to operate the device instantaneously. If ( Z_s ) is too high, the fault current will be too low, and the protection may not operate, leaving dangerous voltages present on exposed metal parts.
Completing the circuit back to the neutral point. 3. Fundamental Formulas
We calculate $Z_s$ to prove that the resistance is low enough to allow enough current to flow to trip the breaker instantly. fault loop impedance calculation
This value represents the resistance of the supply network upstream of your installation.
The "loop" is the complete circuit formed during a fault, starting from the source and returning to it. It typically consists of: The purpose of calculating ( Z_s ) is
Consider a final lighting circuit protected by a 6A Type B circuit breaker, with a supply voltage of 230V. The external impedance ( Z_{source} ) from the utility is 0.35 ohms. The phase conductor (2.5 mm² copper) and earth conductor (1.5 mm² copper) run 40 meters. The resistance per meter for 2.5 mm² is 0.00741 ohms/m, and for 1.5 mm² is 0.0121 ohms/m.
This guide covers the theory, the mathematical formulas, the step-by-step calculation process, and the practical considerations for verifying these values on-site. This requires that the fault current be high
Where:
Design a circuit for a single-phase water heater.