Fault Loop Impedance Calculator

Enter Circuit Data

System Voltage Uo (V)

External Impedance Ze (Ω)

Additional Source Impedance (Ω)

Line Resistance R1 (Ω)

CPC Resistance R2 (Ω)

Temperature Factor

Line Reactance X1 (Ω)

CPC Reactance X2 (Ω)

Fault Voltage Correction Factor

Load Current (A)

Protective Device Type

Device Rating In (A)

Target Disconnection Time (s)

Example Data Table

Voltage (V)	Ze (Ω)	R1 (Ω)	R2 (Ω)	X1 + X2 (Ω)	Device	In (A)	Calculated Zs (Ω)	Fault Current (A)
230	0.35	0.18	0.12	0.04	Type B MCB	32	0.7109	323.54
230	0.28	0.12	0.08	0.03	Type C MCB	20	0.5508	417.56
230	0.45	0.26	0.19	0.05	BS 88 Fuse	16	1.0412	220.90

These rows are illustrative. Measured impedances, conductor temperatures, and protective device data will change the final result.

Formula Used

1. Temperature adjusted loop resistance
R_loop = (R1 + R2) × Temperature Factor

2. Total loop reactance
X_loop = X1 + X2

3. Calculated fault loop impedance
Zs = √[(Ze + Source Impedance + R_loop)² + X_loop²]

4. Prospective earth fault current
I_f = (Uo × Fault Correction Factor) ÷ Zs

5. Simplified device trip current
I_trip = Device Multiplier × In

6. Simplified maximum allowable loop impedance
Z_max = (Uo × Fault Correction Factor) ÷ I_trip

7. Margin
Margin = Z_max − Zs

8. Touch voltage estimate
V_touch ≈ I_f × (R2 × Temperature Factor)

Device multipliers here are simplified engineering assumptions for quick comparison. Always verify tripping behavior against the exact device curve and governing installation rules.

How to Use This Calculator

Enter the circuit voltage for the phase-to-earth fault path.
Add measured or estimated external impedance Ze.
Include any extra upstream source impedance if known.
Enter R1 and R2 values for the final circuit path.
Adjust the temperature factor if conductor heating is expected.
Provide estimated reactance values for line and CPC conductors.
Select the protective device type and enter its current rating.
Set the target disconnection time and correction factor.
Press the calculate button to view Zs, fault current, margin, and voltage estimates.
Use the export buttons to save a CSV or PDF summary.

Frequently Asked Questions

1. What is fault loop impedance?

Fault loop impedance is the total impedance of the earth fault current path. It includes the supply, line conductor, protective conductor, and source effects.

2. Why does lower Zs matter?

Lower Zs usually allows higher fault current. Higher fault current often helps the protective device operate faster during an earth fault.

3. Why is temperature factor included?

Conductor resistance rises with temperature. The factor helps model warmer operating conditions instead of relying only on cold resistance values.

4. Does this replace on-site testing?

No. It is a planning and learning tool. Actual verification still needs proper testing, inspection, and compliance checks under the relevant rules.

5. Why do I enter reactance?

Reactance can influence the total fault path, especially on longer circuits or systems with noticeable inductive effects. It improves the estimate.

6. What does the device multiplier mean?

It is a simplified way to estimate the current needed for rapid device operation. Different device families can need different multiples of rated current.

7. Can I use measured Ze and calculated R1+R2 together?

Yes, that is common in design reviews. Just keep units consistent and confirm that your measured and estimated values reflect the same fault path.

8. What does the margin result show?

Margin compares calculated Zs with the simplified maximum allowable value. A positive margin means the estimate is within the selected limit.