Earth Fault Loop Impedance Calculator
Will the breaker trip in time? Check Zs for a design, a dead test or a live test.
Data: Clause numbers from AS/NZS 3000:2018. MCB limits are calculated from Uo / Ia, not copied from Table 8.1. Fuse limits: edition not confirmed. About standard editions
For reference only. Always verify with the current published standard and consult a licensed electrical engineer for critical installations.
Table basis
Live limit Zs = Uo / Ia, with Uo = 230 V and Ia = 4 x In (Type B), 7.5 x In (Type C) or 12.5 x In (Type D). The dead-test and design limit for the circuit alone (R1 + R2) is 0.8 x the live limit, because at least 80% of Uo is assumed to reach the protective device. The other 20% is left for the supply. So the dead-test limit can never be higher than the live limit.
Check against the current edition of AS/NZS 3000 Tables 8.1 and 8.2. Some references use 5, 10 and 20 x In for Types B, C and D, which gives lower limits. HRC fuse limits come from a table we have not verified.
Subcircuit Resistance (Design Check)
Set to the earth fitted in flat TPS of this size (Nexans Olex data). Change it if your cable differs.
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About This Earth Fault Loop Impedance Calculator
This calculator estimates earth fault loop impedance and checks whether your protection device will trip within the required disconnection time. Built for Australian electricians verifying fault protection compliance.
Use it when designing circuits or checking expected Zs values before testing on site. Enter your cable details and protection device, and compare the result against the maximum allowable impedance.
Maximum Zs for MCBs is calculated as Uo / Ia with Ia at 4, 7.5 or 12.5 times the rating for Types B, C and D. The dead-test limit for the circuit alone is 80% of that. Check the results against Tables 8.1 and 8.2 in the current edition of AS/NZS 3000 and confirm with on-site loop impedance testing.
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What Earth Fault Loop Impedance Is
When an active conductor faults to earth, current has to complete a full loop to flow: out along the active conductor to the fault, back through the earthing system and the supply transformer’s earth connection, then through the transformer winding and back down the active conductor to the point of fault. Earth fault loop impedance (Zs) is the total impedance of that entire path.
Zs matters because it sets how much fault current flows. Ohm’s law does the rest: fault current equals supply voltage divided by Zs. A low Zs means a high fault current, which trips an MCB or blows a fuse fast. A high Zs starves the fault of current, and the protection device may not trip in time (or at all, in a severe case), leaving a live fault on exposed metalwork.
That is why AS/NZS 3000 sets maximum disconnection times for automatic protection: 0.4 seconds for final sub-circuits up to 32A on a TN system, and 5 seconds for distribution circuits and sub-mains. Every combination of protection device and rating has a corresponding maximum Zs. Keep the loop impedance under that figure and the device is guaranteed to trip inside the time limit.
Zs Values And Where They Come From
Maximum Zs is not one number. It depends on the protective device type, its curve, and its rating. A Type B MCB trips instantaneously between 3–5 times its rated current, a Type C between 5–10 times, and a Type D between 10–20 times. The higher the instantaneous trip multiplier, the more fault current is needed, and the lower the maximum Zs has to be to guarantee it. HRC fuses follow their own published time-current curves rather than a fixed multiplier.
In practice there are two ways to get a maximum Zs figure for a specific device and rating. Manufacturers publish trip curves showing the current needed to open the device within a given time. Read the current at the 0.4s or 5s mark, then work back to impedance using I = V / Zs. Alternatively, AS/NZS 3000 Table 8.1 (live test, total loop impedance) and Table 8.2 (dead test, subcircuit resistance) publish the maximum values directly for common MCB types and HRC fuses, so you do not have to read a curve on site. This calculator does not copy those tables. It works the live limit out as 230 V divided by the trip current (4, 7.5 or 12.5 times the rating for Types B, C and D), and allows 80% of that for the circuit alone on a dead test. For a 20A Type B MCB that gives 2.87 ohms live and 2.29 ohms dead. Every device type and rating has its own figure, so confirm the exact value for your device against the current edition of the standard or the manufacturer’s data rather than assuming it is the same as a similar-looking breaker.
Worked Example
Take a 20A Type B MCB protecting a power circuit run in 2.5mm² TPS (2.5mm² earth), 30m from the board to the furthest outlet. A common GPO circuit. Each 2.5mm² copper conductor is 7.41 ohms per km at 20 °C (Nexans Olex data). At 75 °C that rises by a factor of (234.5 + 75) / (234.5 + 20) = 1.216, so active plus earth works out to about 0.0180 ohms per metre.
340A is well past the 60–100A a 20A Type B MCB needs to see to trip instantaneously, so this circuit clears the 0.4-second requirement with plenty of margin. If the run were 150m instead of 30m, R1+R2 would climb to 2.70 ohms, over the 2.29 ohm limit. That circuit fails: increase the cable size, shorten the run, or change the protection. Watch the earth size too. A 6mm² TPS has a 2.5mm² earth, so its R1+R2 is about 70% higher than a calculation that assumes a 6mm² earth.
Testing vs Calculating
A calculated Zs, like the worked example above, is a design-stage estimate. It uses standard cable resistance values at a reference temperature and assumes sound connections throughout the loop. It is what you use before the cable is even pulled, to confirm a proposed cable size and protection device combination will comply.
A fault loop impedance test is the on-site verification step, done with a calibrated loop impedance tester after the circuit is installed and energised. It measures the actual loop, including joint resistance, connector contact resistance, and the real supply impedance at that point in the network: all the things a calculation cannot fully account for. This is the figure that goes on the test certificate, because it reflects what is actually installed rather than what was designed.
Use the calculator above at design stage to check a cable and device combination before you commit to it. Run the actual fault loop impedance test once the circuit is complete, and compare that reading against the same maximum Zs. If it is close to the calculated figure, the installation matches the design. If it is significantly higher, something in the real loop (a loose connection, an undersized earth, a longer run than planned) is adding impedance the calculation did not predict.
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Questions
- What is earth fault loop impedance?
- Earth fault loop impedance (Zs) is the total impedance of the fault current path from the active conductor through the fault to the earth return and back to the supply transformer. It determines whether enough fault current flows to trip the protection device within the required disconnection time.
- What is the 0.4 second rule?
- AS/NZS 3000 requires final sub-circuits rated up to 32A on a TN system to automatically disconnect within 0.4 seconds of an earth fault, so a fault does not stay live long enough to be dangerous. Distribution circuits and sub-mains get a more relaxed 5-second limit, since they are less likely to be handled directly. The loop impedance has to be low enough that fault current trips the protection device inside that window.
- What is a good earth fault loop impedance reading?
- A good reading is comfortably under the maximum Zs for your protection device and rating, with margin left over. As a rule of thumb, aim for well under 80% of the maximum so ageing joints and temperature effects do not push it over the line later. The maximum itself gets tighter as device rating goes up: on this calculator a 20A Type B MCB allows up to 2.87 ohms, but a 32A Type B MCB only allows 1.79 ohms (Uo / 4 x In, check against Table 8.1 in your copy of AS/NZS 3000). There is no single universal figure, so check the maximum for your specific device and rating, not a rule of thumb.
- How do I measure earth fault loop impedance on site?
- Run a fault loop impedance test with a loop impedance tester connected between active and earth at the furthest point of the circuit, then compare the reading against the maximum Zs for your protection device and rating. This calculator estimates the expected value first, so you know roughly what to expect before you test.
- What is the maximum Zs for a 20A Type B MCB?
- This calculator works it out as Zs = Uo / Ia = 230 / (4 x 20) = 2.87 ohms for a live test. For a dead test (R1+R2 of the circuit alone) it allows 80% of that, 2.29 ohms, leaving the rest for the supply. Some references base Type B on 5 x In instead, which gives 2.3 ohms live. Check both figures against Tables 8.1 and 8.2 in the current edition of AS/NZS 3000 before you rely on them.
- What is the difference between live and dead earth fault loop testing?
- A dead test (AS/NZS 3000 Clause 8.3.9.2.3) measures the resistance of the earth fault loop with the supply disconnected, using a low-current ohmmeter. A live test (Clause 8.3.9.2.2) measures actual loop impedance with the circuit energised, including the supply transformer impedance. The live test gives a more accurate real-world value but carries shock risk. Both methods are accepted for verification. This calculator estimates the expected loop impedance to compare against your site measurements.
- What does a high earth fault loop impedance reading mean?
- A high Zs reading means the fault current path has too much resistance for the protection device to trip within the required time. Common causes include long cable runs, undersized earth conductors, loose connections, or corroded joints. If Zs exceeds the maximum for your MCB or RCD, the circuit is non-compliant with AS/NZS 3000 and poses a shock risk. You may need to increase the earth conductor size, shorten the run, or install an RCD which trips at only 30mA regardless of loop impedance. Use this calculator to identify which part of the loop contributes the most impedance.