Loop Impedance Measurement
Short answer: Loop impedance measurement measures the total impedance of the fault current path that will occur in the event of a fault. This value determines whether the protective device (fuse/circuit breaker) will trip quickly enough in the event of a fault.
Electrical safety depends on the protective device interrupting the circuit quickly enough in the event of a fault. This requires the impedance of the fault loop to be sufficiently low; otherwise, the fault current remains small and the protection trips too late.
High loop impedance prevents the current from reaching a level that would trip the protective device in time during a phase-to-ground fault. The result is metal surfaces remaining under voltage for extended periods, posing a risk of fire and electric shock.
What is involved in loop impedance measurement?
- Phase-to-ground (Zs) and phase-to-neutral loop impedance measurement.
- Calculation of expected fault current.
- Compatibility assessment with the opening characteristic of the protective device.
- Comparison of measurement results with standard opening times.
- Measurement at panel and line end points.
The rule Zs ≤ U/Ia
The primary purpose of loop impedance measurement is to verify a simple condition: the measured fault loop impedance (Zs) must be low enough to cause the protective device to trip within the required time. This is roughly expressed by the relation Zs ≤ U/Ia, where U is the circuit voltage and Ia is the current required for the protective device to trip within a defined time (e.g., 0.4 seconds). If the impedance is below this limit, the current will rise sufficiently at the time of the fault, and the fuse/breaker will trip in time. If it is above the limit, the cable cross-section, connection quality, or protective device selection must be re-evaluated. This measurement is particularly critical at end-of-line (farthest) points.
Legal Basis and Accurate Assessment
Loop impedance measurement is one of the fundamental tests in the periodic inspection of electrical installations; the results are documented in a TÜRKAK accredited report together with the protection coordination.
Frequently Asked Questions
Why is loop impedance important?
For the protective device to trip quickly enough in the event of a malfunction, the impedance must be low.
What does high impedance mean?
The fault current remains low, and the protection system trips late; the cable cross-section and connections should be examined.
Where is it critical to measure?
The impedance is highest, especially at the furthest (last) point of the line.
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