Prospective Fault Current (PFC) Calculator

Calculate Prospective Short Circuit Current (PSCC) and Prospective Earth Fault Current (PEFC) according to BS 7671 Regulation 612.11. Determine protective device breaking capacity requirements.

Prospective Fault Current Calculator

Calculate Prospective Short Circuit Current (PSCC) and Prospective Earth Fault Current (PEFC) according to BS 7671 Regulation 612.11. Results help determine required breaking capacity of protective devices.

Typical Ze Values (for reference)

TN-S (Separate neutral and earth)

Typical: 0.8Ω

Range: 0.35Ω - 0.8Ω

Cable sheath used as protective conductor

TN-C-S (PME - Protective Multiple Earthing)

Typical: 0.35Ω

Range: 0.1Ω - 0.35Ω

Combined neutral and earth (PEN) from supplier

TT (Local earth electrode)

Typical: 21Ω

Range: 5Ω - 200Ω

Highly variable depending on earth electrode resistance

Understanding Prospective Fault Current

Prospective Fault Current (PFC) is the maximum current that would flow in the event of a fault with negligible impedance. It's a critical safety parameter that must be determined to ensure protective devices can safely interrupt fault currents without damage or danger. BS 7671 Regulation 612.11 requires PFC to be measured, calculated, or determined for all installations.

PSCC

Prospective Short Circuit Current is the maximum current that would flow during a line-to-neutral (single-phase) or line-to-line (three-phase) short circuit. Calculated as PSCC = U₀ / Ze.

PEFC

Prospective Earth Fault Current is the maximum current that would flow during a line-to-earth fault. Calculated as PEFC = U₀ / Zs, where Zs = Ze + (R₁ + R₂).

PFC (Maximum)

The Prospective Fault Current is taken as the higher value of PSCC or PEFC. This determines the minimum breaking capacity (Icn) required for protective devices.

BS 7671 Requirements

Regulation 612.11: Prospective fault current shall be measured, calculated or determined by another method at the origin and at other relevant points of the installation
Regulation 536.1: Every protective device shall have adequate breaking capacity. A device with inadequate breaking capacity may be used if backed up by another device with the necessary breaking capacity
Verification: PFC must be verified during initial verification (Regulation 611.3) and periodic inspection and testing
Documentation: PFC values must be recorded on Electrical Installation Certificates (EIC) and Electrical Installation Condition Reports (EICR)

Protective Device Breaking Capacity (Icn)

The breaking capacity (Icn rating) of a protective device is its ability to safely interrupt fault currents. The device's breaking capacity must exceed the prospective fault current at its point of installation.

Device Type	Breaking Capacity (Icn)	Typical Application	Standard
Standard MCB	6kA (6,000A)	Domestic installations	BS EN 60898
Enhanced MCB	10kA (10,000A)	Commercial installations	BS EN 60898
MCCB	25kA - 50kA	Industrial installations	BS EN 60947-2
BS 88 Fuse	80kA+	Main switchgear	BS 88

Important Safety Considerations

⚠️ Always Verify: PFC should be measured during testing, not just calculated. Calculated values may differ from actual site conditions
⚠️ Margin of Safety: Select devices with breaking capacity exceeding PFC by at least 10% for safety margin
⚠️ Back-Up Protection: Devices with inadequate breaking capacity may be used if backed up by a suitable device upstream (Regulation 536.1)
⚠️ TT Systems: In TT systems, RCD protection is mandatory. PEFC is limited by earth electrode resistance
⚠️ Distribution Networks: PFC can change if supply authority makes network modifications. Periodic testing is essential

Measurement vs Calculation

Measurement (Preferred)

✓ Most accurate method
✓ Uses PFC test instrument
✓ Accounts for all parallel paths
✓ Reflects actual site conditions
✓ Required for certification

Calculation

• Useful for design stage
• Based on measured Ze
• May not account for all factors
• Should be verified by testing
• Can be conservative or optimistic

Example Calculation

Given:

• System: TN-C-S (PME)
• Ze: 0.35Ω (typical PME value)
• Circuit R₁+R₂: 0.65Ω (32A ring, 80m of 2.5/1.5mm²)
• Voltage: 230V (single-phase)

Calculations:

• PSCC = 230V / 0.35Ω = 657A
• Zs = 0.35Ω + 0.65Ω = 1.0Ω
• PEFC = 230V / 1.0Ω = 230A
• PFC = max(657A, 230A) = 657A

Device Selection:

Standard 6kA MCB is suitable (6,000A > 657A)

Three-Phase Considerations

For three-phase systems, the prospective fault current can be higher during three-phase faults:

• Single-Phase PSCC: Line-to-neutral fault = U₀ / Ze (where U₀ = 230V)
• Three-Phase PSCC: Line-to-line-to-line fault ≈ √3 × single-phase PSCC
• Line-to-Line Voltage: 400V between phases (√3 × 230V)
• Device Selection: Ensure MCCBs are rated for three-phase fault currents