Earth fault loop impedance calculation vs. measurement

Few areas of electrical safety are viewed as ambiguously as loop impedance. Many companies rely solely on calculations, and even the standards contain sections that support this approach.

In our guide, we explain why you should still measure the loop impedance in your installations - and not rely on calculation alone.

Calculating vs. measuring fault loop impedance

In the planning phase of an electrical installation, the earth fault loop impedance is usually determined using design software. This includes factors such as cable conductance, lengths, cross-sections, contact resistances, and transformer parameters. Based on this data, the disconnection conditions can be planned in theory.

However, if you’ve ever been involved in a construction project - even in your own home - you know that changes during execution are almost inevitable. In electrical installations, even small changes in cable routing, lengths, or contact resistances can cause deviations in actual values. Sometimes, it only takes a single careless action: incorrect torque, damaged conductor or insulation.

Measuring loop impedance with a calibrated tester provides a practical verification after the work is completed. Performing an initial test using a loop impedance tester offers several advantages:

• Comparison of calculated and measured fault loop impedance
• Early detection of errors during the construction phase
• Proof of subcontractor performance during handover
• Standards-compliant initial testing with documented proof of disconnection compliance
• Detection of aging effects in the system (via regular electrical inspections)

Only through practical measurement are we truly able to assess the quality of completed work - and, when dealing with subcontractors, confirm acceptance or request corrections. Moreover, proper documentation of the initial test, including loop impedance measurement, prepares us for legal or regulatory disputes, where proof may be required by grid operators, authorities, or property owners.

The right loop impedance tester for industrial installations

That’s why we recommend high-current fault loop impedance testers (300 A test current) from the MZC series—for reliable and accurate results even in demanding industrial settings.