Earth Fault Loop Impedance Calculator
Check measured Zs against the maximum allowed for your breaker.
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
Why Zs is worth checking, not just recording
Earth fault loop impedance testing is a standard part of most test sheets, but it's easy to treat as a box-ticking measurement rather than something you actually check against a threshold. A measured Zs number on its own doesn't tell you anything - it only means something once it's compared against the maximum allowed for the specific breaker protecting that circuit.
This calculator does that comparison directly: your measured Zs against the tabulated maximum for that exact breaker type and rating, straight from AS/NZS 3000:2018 Table 8.1.
Worked examples
Standard socket circuit, Type B
A Type B 32A breaker on a general power circuit, tested with a loop tester giving a cold reading. This is the most common combination on a domestic test sheet, and the one worth having the max Zs threshold memorised or on hand for, since it comes up on almost every job.
Borderline reading on an older circuit
An older circuit with a longer run and a measured Zs that's close to the tabulated limit. Even a technical pass is worth a second look here, since a reading that's right on the edge today leaves very little room if the circuit ever gets extended or the connection degrades over time.
Working backwards from the breaker
Before you've even tested, knowing the max Zs and minimum trip current for the breaker you're about to install tells you what to expect from the reading, and gives you a sanity check on whether the circuit design is realistic for the run length involved.
Frequently asked questions
Why is there a max Zs at all?
Automatic disconnection on a fault relies on enough current flowing to trip the breaker fast. Zs is the loop impedance a fault current has to push through - the higher it is, the less fault current flows, and past a certain point the breaker won't trip quickly enough. Max Zs is the ceiling that guarantees a fast enough trip for that breaker type and rating.
Does MCB type (B/C/D) change the max Zs?
Yes, significantly. Different curve types trip at different multiples of their rated current, which changes how much fault current is actually needed for a fast trip - and therefore how much loop impedance the circuit can tolerate before that current requirement isn't met.
What does prospective fault current tell me beyond pass/fail?
It's the actual fault current your measured Zs would produce at the supply voltage - useful context alongside the pass/fail result, since a circuit that only just passes is worth a second look even if it's technically compliant, especially if you're about to add load to that circuit.
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Recording Zs results for a CoC? CertMate captures your test results on site and generates the certificate before you leave the job.
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