4.2 Prospective Fault Current (PFC/PSCC) & Polarity

Understanding Prospective Fault Current (PFC)

Prospective Fault Current (PFC) is the absolute maximum electrical current that could theoretically flow through an electrical installation in the event of a short circuit or earth fault of negligible impedance at a given point. Determining PFC is a mandatory requirement of BS 7671 Part 6 initial verification and periodic inspection.

The primary engineering reason for determining PFC is to ensure that all protective devices—including Miniature Circuit Breakers (MCBs), Residual Current Breakers with Overcurrent protection (RCBOs), Moulded Case Circuit Breakers (MCCBs), and cartridge fuses—possess a short-circuit breaking capacity ($I_{cn}$ or $I_{cu}$) that equals or exceeds the maximum prospective fault current at their point of installation.

If a fault occurs and the resulting fault current exceeds the rated breaking capacity of the circuit breaker, the breaker will fail catastrophically. The severe arc energy can melt contacts, vaporize internal components, explode the casing, cause severe arc-flash injuries to personnel, and initiate catastrophic electrical fires without successfully clearing the fault.

Distinguishing Between PSCC and PEFC

Prospective Fault Current is an overarching parameter that incorporates two distinct fault conditions:

1. Prospective Short Circuit Current (PSCC)

PSCC is the maximum current that would flow during a fault directly between current-carrying live conductors. In single-phase systems, PSCC occurs between Line and Neutral ($L-N$). In three-phase systems, PSCC occurs between two line conductors ($L1-L2$, $L2-L3$, $L1-L3$) or simultaneously across all three lines ($L1-L2-L3$). Because line and neutral conductors are fed directly from low-impedance transformer windings, PSCC is typically the highest fault current in single-phase and three-phase installations.

2. Prospective Earth Fault Current (PEFC)

PEFC is the maximum current that would flow during a fault between a Line conductor and Protective Earth ($L-E$). The magnitude of PEFC is dictated by the Earth Fault Loop Impedance ($Z_s$ or $Z_e$). In TN-C-S (PME) installations, where the supply earth is combined with the neutral conductor, PEFC can be virtually identical to PSCC. In TT systems, PEFC is very low due to the resistance of the earth electrode.

Measuring and Recording PFC

PFC is measured at the origin of the installation (main distribution board) using a dedicated fault current meter or multi-function installation tester complying with BS EN 61557.

Measurement Procedure

1. Set the instrument to the PFC / Loop test function.
2. Measure PSCC between Line and Neutral at the main incomer.
3. Measure PEFC between Line and Main Earthing Terminal (MET).
4. In three-phase installations, measure single-phase PSCC and double the resulting value (or multiply by 1.732 for symmetrical 3-phase faults) to account for phase-to-phase fault currents.

Rule for Certificate Recording

Under BS 7671, the inspector must record the higher value of either PSCC or PEFC as the official Prospective Fault Current on the Electrical Installation Certificate (EIC) or Condition Report (EICR). For example, if measured PSCC is 2.2 kA and PEFC is 1.8 kA, the recorded PFC for the installation is 2.2 kA.

The recorded PFC must then be verified against the breaking capacity of protective devices. Standard domestic BS EN 60898 MCBs are typically rated at 6 kA ($6000,A$), while commercial MCBs are rated at 10 kA, and industrial MCCBs/HRC fuses are rated up to 25 kA – 80 kA.

Live Polarity Verification Procedures

While dead polarity testing is conducted during initial verification, BS 7671 mandates Live Polarity Verification once the installation is energized to provide absolute confirmation that the incoming supply and all outgoing sub-circuits are correctly wired.

Hazards of Reversed Live Polarity

If incoming supply conductors are reversed by the supply authority or during meter installation, the Neutral busbar becomes energized at 230V while the Line busbar sits at 0V. Consequently, every single-pole MCB and light switch in the installation will be switching the Neutral conductor.

When a user switches off a light or appliance, the device stops operating because the current path is broken. However, the internal element, wiring, and lamp contacts remain permanently live at 230V relative to earth. Anyone attempting routine maintenance, cleaning, or lamp replacement will touch live components, resulting in severe or fatal electric shock.

Live Polarity Test Method

1. Using an Approved Voltage Indicator (AVI) compliant with HSE GS38, verify voltage at the main switch:
   • Line to Neutral: Approx 230V AC.
   • Line to Earth: Approx 230V AC.
   • Neutral to Earth: Approx 0V AC (typically <2V).
2. At every distribution board, verify that single-pole MCBs are fed from the Line busbar.
3. At all socket outlets, use an approved socket tester or meter probes to confirm correct termination: Line to right-hand terminal, Neutral to left-hand terminal, Earth to top terminal.
4. Confirm center contacts of Edison Screw (ES) lampholders are connected to switched Line conductors.