Prospective Fault Current (PFC), also known as Prospective Short-Circuit Current (PSCC), is a critical parameter in electrical system design. It represents the maximum current that could flow during a short circuit at a given point in the installation.

Correctly assessing PFC is essential for safe selection of protective devices and compliance with UK wiring standards under BSI BS 7671.

What Is Prospective Fault Current?

PFC is the highest current that can flow when there is negligible impedance between live conductors or between live and earth. It is influenced by:

• Supply transformer capacity
• Network impedance
• Cable length and conductor size
• Earthing system type

At the origin of an installation, PFC is typically highest and reduces further downstream due to increased impedance.

Why PFC Matters

Every circuit breaker has a rated breaking capacity, expressed in kA. This rating must be equal to or greater than the maximum PFC at its point of installation.

If the fault current exceeds the device’s breaking capacity, catastrophic failure may occur. This could result in equipment damage, arc flash hazards, or fire.

Common domestic MCB breaking capacities in the UK include:

• 6kA
• 10kA

Commercial and industrial boards may require 25kA or higher depending on the supply characteristics.

Measurement of PFC

During initial verification, electricians measure PFC using a multifunction tester. The tester measures:

• Line-to-neutral fault current
• Line-to-earth fault current

The higher value is recorded as the PFC for that point.

It is essential to measure at the origin and at the furthest point of each circuit to confirm protective adequacy.

Relationship Between PFC and Zs

Earth fault loop impedance (Zs) directly influences fault current magnitude. Lower impedance results in higher fault current. Designers must ensure that:

• Disconnection times comply with regulatory limits
• Protective devices operate within required time frames

Accurate Zs calculations and measurements ensure safe fault clearance.

Design Considerations

When upgrading consumer units or installing distribution boards, electricians must:

• Confirm supply characteristics from the DNO
• Verify breaking capacity of devices
• Consider upstream protective discrimination
• Account for potential network upgrades increasing fault current

Failure to reassess PFC during system modifications can create hidden safety risks.

Conclusion

Prospective Fault Current assessment is fundamental in UK electrical design. Proper calculation and measurement ensure protective devices can safely interrupt faults, maintaining system integrity and compliance.