ZE PFC Calculator
Quickly calculate maximum Prospective Fault Current (PFC) from external earth fault loop impedance (Ze) under BS 7671 requirements. Supports single-phase (230 V) and three-phase (400 V) UK supply systems with instant risk assessment.
ZE PFC Calculator
How to Use ZE PFC Calculator
Determining the prospective fault current (PFC) is critical for verifying that protective devices can clear faults safely without presenting a fire or blast hazard. Here is how to use the calculator in a practical UK installation testing workflow:
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1Select the supply system configuration. Specify whether the system is a standard UK single-phase supply (230 V nominal line-to-neutral/earth) or a commercial three-phase supply (400 V line-to-line).
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2Input the measured external loop impedance (Ze). Enter the measured Ze value obtained at the origin of the installation (e.g., at the main intake or consumer unit) in Ohms.
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3Run the calculation. Click the Calculate button to compute the maximum short-circuit current.
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4Review the PFC outputs. Inspect the calculated prospective fault current displayed in both Amperes (A) and kiloamperes (kA).
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5Verify protective device capacities. Use the risk classification and fault current levels to ensure that all installed circuit breakers (e.g., BS EN 60898 MCBs) and fuses have a breaking rating (like 6 kA or 10 kA) that exceeds the calculated PFC.
How to Calculate ZE PFC
The Prospective Fault Current (PFC) represents the highest current that could theoretically flow through the electrical installation under zero-impedance short-circuit conditions. It is derived using Ohm's Law by dividing the nominal supply voltage by the external path impedance.
PFC Calculation Formulas
Depending on the supply type of the UK electrical installation, the calculations are structured as follows:
Single Phase System Formula
Three Phase System Formula
Where:
- PFC = Prospective Fault Current (A)
- Ze = External Earth Fault Loop Impedance (Ω)
- 230 V = Nominal single-phase line-to-neutral/earth voltage in the UK
- 400 V = Nominal three-phase line-to-line voltage in the UK
Worked Examples
Example 1: Single Phase Domestic Intake
Consider a domestic TN-C-S installation where the measured external impedance Ze at the consumer unit is 0.35 Ω:
Result: The prospective fault current is 657.14 A. Since 657.14 A is below 1,000 A (1 kA), the risk classification is Low. Any standard domestic protective device rated at 6 kA or 10 kA is fully compliant.
Example 2: Three Phase Commercial System
Consider a light industrial facility with a three-phase supply and a measured Ze of 0.08 Ω near the main distribution board:
Result: The prospective fault current is 5000 A (5.00 kA). This falls exactly into the Moderate risk classification. High-capacity commercial switchgear rated at 6 kA, 10 kA, or higher must be installed to safely clear any fault condition.
Interpretation of Fault Current Levels
Electricians must evaluate the calculated PFC against equipment ratings. Under BS 7671, protective devices must be rated to safely handle the prospective short-circuit current at their point of installation. For example, standard domestic consumer units are usually supplied with 6 kA protective devices, which are adequate for Ze values above 0.04 Ω on single-phase systems.
ZE PFC Calculation Chart
This table maps common measured Ze impedance values to their corresponding theoretical prospective fault currents for both UK single-phase and three-phase systems.
| Measured Ze (Ω) | Single Phase PFC (A) | Three Phase PFC (A) | Risk Assessment (Single / Three) |
|---|---|---|---|
| 0.05 Ω | 4,600.0 A (4.60 kA) | 8,000.0 A (8.00 kA) | Moderate / High |
| 0.08 Ω | 2,875.0 A (2.88 kA) | 5,000.0 A (5.00 kA) | Moderate / Moderate |
| 0.10 Ω | 2,300.0 A (2.30 kA) | 4,000.0 A (4.00 kA) | Moderate / Moderate |
| 0.15 Ω | 1,533.3 A (1.53 kA) | 2,666.7 A (2.67 kA) | Moderate / Moderate |
| 0.20 Ω | 1,150.0 A (1.15 kA) | 2,000.0 A (2.00 kA) | Moderate / Moderate |
| 0.35 Ω | 657.1 A (0.66 kA) | 1,142.9 A (1.14 kA) | Low / Moderate |
| 0.50 Ω | 460.0 A (0.46 kA) | 800.0 A (0.80 kA) | Low / Low |
| 0.80 Ω | 287.5 A (0.29 kA) | 500.0 A (0.50 kA) | Low / Low |
| 1.00 Ω | 230.0 A (0.23 kA) | 400.0 A (0.40 kA) | Low / Low |
Note: Values shown in this chart are calculated using nominal voltages of 230 V (single-phase) and 400 V (three-phase) in accordance with BS 7671. Always consult your testing device results and the manufacturer instructions of the protective equipment installed on the supply lines.
ZE PFC Calculator Frequently Asked Questions
Ze is the external earth fault loop impedance measured at the origin of an electrical installation, such as the main intake or consumer unit. It represents the impedance of the supply path outside the consumer's premises, including the utility transformer windings, the line conductors, and the earth return path back to the source.
PFC, or Prospective Fault Current, is the highest electrical current that could theoretically flow in an installation during a short circuit or earth fault. It is critical for selecting protective devices with appropriate short-circuit breaking capacities, ensuring they can safely interrupt fault currents without failing catastrophically.
PFC is calculated using Ohm's Law: PFC = Uo ÷ Ze, where Uo represents the nominal line-to-earth voltage (230 V in standard UK single-phase systems) and Ze is the external earth fault loop impedance. For three-phase supplies, the nominal line voltage of 400 V is used to determine the maximum prospective fault current level.
Fault current limits are important because they dictate the minimal breaking capacity of equipment like circuit breakers and consumer units. If a protective device is rated lower than the actual prospective fault current (PFC), it may fail to clear the fault safely, resulting in arcing, explosion, fire, or severe equipment damage.
A good Ze value is one that is low enough to permit sufficient fault current to quickly trip protective devices. Under standard UK utility guidelines, the maximum Ze is 0.35 Ω for TN-C-S systems and 0.80 Ω for TN-S systems. For TT installations, the local ground electrode resistance should ideally be kept below 100 Ω.
Ze is the external loop impedance measured strictly at the origin of the installation with parallel earth paths isolated. Zs is the total loop impedance measured at the furthest point of a final circuit. Zs includes the external path (Ze) plus the resistance of the line and circuit protective conductors (R1 + R2), modeled as Zs = Ze + (R1 + R2).
Yes, Ze must be measured live because the loop impedance test relies on drawing a small test current from the active mains supply. Because it is a live test, electricians must exercise extreme caution, use appropriate personal protective equipment (PPE), and ensure all main bonding conductors are safely reconnected immediately after testing.
Yes, BS 7671 requires the prospective fault current (both short-circuit current and earth fault current) to be determined at the origin and other relevant points of the installation. This can be achieved either by direct measurement using a loop tester or by calculating the value based on the measured external impedance Ze and transformer data.