Prospective Fault Current (Ipf) is the maximum prospective current that would flow in the event of an electrical short circuit of negligible impedance. Under BS 7671 Regulation 434.5.1 and Regulation 643.7.3.201, every protective device must be capable of interrupting this prospective current without explosive destruction, contact welding, or sustained arc flash.
The Two Components of Fault Current (PSCC vs PEFC)
Under BS 7671 Regulation 313.1 and Regulation 643.7.3.201, the Prospective Fault Current (Ipf) recorded on electrical installation certificates (EICs) and condition reports (EICRs) is determined by taking the higher of two distinct measurements at the origin and at distribution boards:
- Prospective Short-Circuit Current (PSCC): Current flowing between live conductors (Line to Neutral on single-phase supplies, or Line to Line / 3-phase symmetrical on three-phase supplies). Measured impedance: ZLN or ZLL.
- Prospective Earth Fault Current (PEFC): Current flowing between a Line conductor and Earth (protective conductor). Driven by loop impedance Ze or Zs.
Regulatory Recording Rule (Reg 643.7.3.201 / GN3):
Ipf = Maximum of (PSCC, PEFC)
On TN-C-S (PME) supplies, PSCC and PEFC are virtually identical because Neutral and Earth are combined at the service cut-out head. On TN-S supplies, PSCC is typically higher because the sheath/armour earth path has higher impedance than the neutral conductor. On TT supplies, PEFC is minuscule (e.g. 5 A to 100 A) due to earth electrode resistance, making PSCC the governing value for breaking capacity.
Three-Phase PSCC: The 2× Multiplier Explained
When testing a three-phase distribution board with a single-phase loop tester, electricians measure Line-to-Neutral impedance and short-circuit current. However, a symmetrical three-phase bolted short circuit produces significantly higher fault levels:
| Fault Type | Circuit Impedance Path | Formula | Ratio to Single-Phase L−N |
|---|---|---|---|
| Single-Phase Line to Neutral (L−N) | Line winding + Line conductor + Neutral conductor + Neutral return | IL−N = U0 / (Zph + Zn) | 1.0 × (Baseline) |
| Phase to Phase (L−L) | Two phase windings + Two line conductors (Driven by √3 × U0 = 400 V) | IL−L = (√3 × U0) / (2 × Zph) | ≈ 1.73 × IL−N |
| 3-Phase Symmetrical Fault | Neutral is not in the path (Neutral point is virtual ground). Impedance is Zph only! | I3φ = U0 / Zph | ≈ 2.0 × IL−N (assuming Zph ≈ Zn) |
Authoritative Rule of Thumb: At a three-phase supply origin or sub-main distribution board,3-phase symmetrical fault current is approximately 2 × the single-phase Line-to-Neutral PSCC. Electricians must never multiply by √3 (1.73) for three-phase breaking capacity verification, as 1.73 only applies to phase-to-phase faults and understates the maximum fault level!
Breaking Capacity Ratings: BS EN 60898-1 vs BS EN 60947-2
Protective devices must be capable of breaking prospective fault currents. The standards distinguish between household installations and industrial switchgear:
| Parameter | BS EN 60898-1 (Household / MCBs) | BS EN 60947-2 (Industrial / MCCBs & ACBs) |
|---|---|---|
| User Category | Uninstructed persons (domestic / commercial) | Instructed / skilled persons (industrial plant) |
| Primary Rating | Icn (Rated short-circuit capacity): e.g. 6,000 A (6 kA) or 10,000 A (10 kA). | Icu (Rated ultimate breaking capacity): e.g. 25 kA, 50 kA, 100 kA. |
| Post-Fault Serviceability | Ics (Service breaking capacity): The breaker must remain serviceable and calibrate correctly after clearing fault. (For 6 kA, Ics = 100% Icn). | Ics is declared as a percentage of Icu (25%, 50%, 75%, or 100%). After interrupting Icu, the device is safe but may require replacement. |
The Annex ZB / ZA 16 kA Conditional Rating
In UK urban properties located adjacent to a local distribution substation, measured single-phase fault currents frequently exceed 6 kA (e.g. 8 kA to 12 kA). Under a literal reading of Regulation 434.5.1, standard 6 kA domestic consumer units would fail.
However, Regulation 434.5.1 explicitly permits backup protection (cascading), where an upstream device limits let-through energy to within the breaking capability of downstream devices.
BS EN 61439-3 UK National Condition (Annex ZB / formerly ZA):
A single-phase domestic distribution board (consumer unit) conforming to BS EN 61439-3 and fitted with 6 kA BS EN 60898-1 circuit breakers is conditionally rated at 16 kA, provided the assembly is protected by an upstream service cut-out fuse conforming to BS 1361 (or BS 88-3) Type II rated ≤ 100 A.
The HRC service fuse severely curtails peak prospective current and limits let-through energy (I²t) to a level the 6 kA MCBs can clear safely.Note: This conditional rating is valid strictly for single-phase BS EN 61439-3 consumer units backed by a 100 A cut-out fuse. It does not apply to three-phase commercial distribution boards.