Fault Loop Impedance Calculator
Calculate and check earth fault loop impedance Zs, maximum Zs, PEFC and Ze + R1+R2 for protective-device disconnection checks.
⚠️ Safety note: This calculator is a practical estimating and checking tool. Always verify against your local wiring regulations, manufacturer trip curves, measured test results and approved inspection procedure. Use it with the Short Circuit Current Calculator, Breaker Size Calculator, Voltage Drop Calculator and Motor Cable Size Calculator.
🛡️ Earth fault loop path: source → protective device → line conductor → fault → CPC/earth → transformer return
Line-to-Earth Voltage U0
Measured Zs
Protective Device
Breaker / Fuse Rating
Custom Trip Current Ia
Safety Factor on Max Zs
For MCB quick checks, this uses common magnetic trip multipliers: B = 5×In, C = 10×In, D = 20×In. Use custom Ia when your regulation table or manufacturer data gives a different value.
Presets:
External Loop Impedance Ze
R1 + R2 at 20°C
Temperature Factor
U0
Protective Device
Rating / Custom Ia
Estimated Zs = Ze + (R1 + R2 × temperature factor). This is useful before testing or when checking a circuit design from conductor resistance values.
Presets:
Voltage U0
Loop Impedance Zs
Required Trip Current Ia
Target Margin
Use this tab for quick Ohm’s-law checks: fault current from measured loop impedance, or maximum Zs from required disconnection current.
Presets:
📐 Formula Reference
Fault Current
I_fault = U0 ÷ Zs
Maximum Zs
Zs_max = U0 ÷ Ia
Calculated Zs
Zs = Ze + (R1 + R2)
Design Margin
Margin = (Zs_limit - Zs) ÷ Zs_limit
📋 Quick Reference
MCB Trip Multipliers
Type B5 × In
Type C10 × In
Type D20 × In
Common Terms
Zeexternal loop
R1line conductor
R2CPC / earth
Check Result
Lower Zshigher fault current
Higher Zsslower trip risk
High PEFCcheck kA rating
📚 Field Notes
Zs is a disconnection checkThe protective device must see enough fault current to trip within the required time. That is why Zs is compared with a maximum allowed value.
PEFC also affects breaker ratingHigh prospective fault current means the breaker or fuse interrupting capacity must be high enough for the installation point.
Use real tables for complianceMCB multipliers are useful for quick checks, but final inspection should follow your local standard and manufacturer data.
Measured values can differLong cables, conductor temperature, loose terminals, supply conditions and parallel earth paths can affect real test readings.
What is a Fault Loop Impedance Calculator?
A fault loop impedance calculator checks whether the measured or calculated earth fault loop impedance is low enough for a protective device to disconnect during an earth fault. It also estimates prospective earth fault current using I = U0 / Zs.
How do you calculate earth fault loop impedance?
For a design estimate, earth fault loop impedance can be calculated as external loop impedance plus the circuit line and protective conductor resistance. In simple form, Zs = Ze + R1 + R2. Temperature correction may be applied to conductor resistance for a practical design check.
Why is maximum Zs important?
Maximum Zs is important because a higher loop impedance gives lower fault current. If the fault current is too low, the MCB, fuse or breaker may not disconnect within the required time.
❓ Frequently Asked Questions
Zs is earth fault loop impedance. It is the total impedance of the fault-current path from the supply through the line conductor, the fault, the protective conductor and back to the source.
Use PEFC = U0 / Zs. For example, with 230V and 0.5Ω loop impedance, the prospective earth fault current is 460A.
If Zs is too high, fault current may be too low to operate the protective device quickly. This can create a shock or fire hazard.
Ze is the external earth fault loop impedance at the origin of the installation. Zs is the total loop impedance at the circuit point, including Ze plus the circuit conductors.
R1 + R2 is the resistance of the line conductor plus the circuit protective conductor for a final circuit. It is commonly used with Ze to estimate Zs.
Yes, for quick estimation this calculator uses Type B = 5×In, Type C = 10×In and Type D = 20×In magnetic trip assumptions. For final approval use regulation tables and manufacturer curves.
No. RCD protection improves shock protection, but loop impedance and continuity still need to be verified according to the applicable installation rules.
A reduced design limit such as 80% is often used as a practical margin for conductor temperature, supply variation and measurement uncertainty. Follow your local method.
High PEFC means the protective device must have enough breaking capacity, such as 6kA, 10kA or higher depending on the installation point.