Prospective Fault Current ($I_(pf)$) is the maximum current that would flow in the event of a dead short-circuit or earth fault of negligible impedance. If the prospective fault current exceeds the rated breaking capacity of a circuit breaker or distribution board, the device can explode or weld its contacts, causing catastrophic arc flash and fire.

The Two Components of Fault Current

Prospective Fault Current ($I_(pf)$) is determined by taking the higher of two separate measurements:

  • Prospective Short-Circuit Current (PSCC): Current flowing between live conductors (Line to Neutral on single-phase, or Line to Line on three-phase supplies). Measured impedance: $Z_(LN)$ or $Z_(LL)$.
  • Prospective Earth Fault Current (PEFC): Current flowing between Line and Earth. Measured impedance: $Z_e$ or $Z_s$.

Golden Rule for Documentation:

Ipf = Maximum of (PSCC, PEFC)

On domestic TN-C-S (PME) supplies, PSCC and PEFC are typically almost identical because Neutral and Earth are combined at the cut-out. On TN-S systems, PSCC is usually higher than PEFC. On 3-phase supplies, phase-to-phase PSCC is approximately $√3 ×$ (approx 1.73×) or 2× the single-phase line-to-neutral reading.

Breaking Capacity Ratings (Icn vs Ics vs Icu)

Regulation 434.5.1 mandates that every protective device must have a rated breaking capacity not less than the maximum prospective fault current at its point of installation:

  • Domestic Standard (BS EN 60898): Typically 6 kA (6,000 A). Standard consumer unit devices are rated at 6 kA.
  • Commercial Standard: Typically 10 kA to 16 kA.
  • Industrial Switchgear (BS EN 60947-2): Molded Case Circuit Breakers (MCCBs) rated from 25 kA to 100 kA.

The Annex ZA 16 kA Conditional Rating

In UK domestic installations close to a street substation, measured fault current can occasionally exceed 6 kA (e.g. 7.5 kA).

Under BS EN 61439-3 Annex ZA, a domestic consumer unit fitted with 6 kA MCBs is accepted for prospective fault currents up to 16 kA, provided the assembly is protected by an upstream BS 1361 / BS 88-3 100 A service fusewhich limits energy let-through ($I^2t$).