Prospective FaultCurrent (kA)

Prospective fault current (PFC), also called prospective short-circuit current (PSCC), is the maximum current that can flow through a circuit under fault conditions — either a short circuit between live conductors or an earth fault. It is calculated by dividing the supply voltage by the total circuit impedance (Zs). PFC determines the minimum breaking capacity that protective devices (MCBs, fuses) must have. If the fault current exceeds the breaking capacity, the device may fail catastrophically rather than safely interrupting the fault.

PFC (prospective fault current) and Icc (rated short-circuit making capacity) refer to the same fundamental concept — the available fault current at a point in the installation. PFC is the quantity you calculate and measure on site; Icc (or Ics) is the rated value stamped on the protective device that must equal or exceed the measured PFC. BS 7671 Regulation 536.4.1 requires that every protective device has a rated short-circuit capacity not less than the prospective fault current at the point of installation.

The 6kA rating covers the vast majority of domestic and commercial final circuits where the total loop impedance is high enough to limit fault current below 6,000A. Most UK domestic TN-C-S installations have Ze around 0.2–0.4Ω, giving PFC of 575–1,150A at the board — far below 6kA. However, at the origin of the installation (the main board), with very low Ze (0.05–0.1Ω), PFC can approach or exceed 2,300A and may reach 6kA+ at the DNO cut-out. Industrial premises near the local transformer can have PFC exceeding 10kA, requiring 10kA or higher rated devices.

Specify 10kA MCBs when the measured or calculated PFC at the distribution board exceeds 6,000A. This typically occurs: in commercial or industrial premises close to the local distribution transformer; in large residential blocks with a short incoming supply cable and low external impedance; at the main distribution board of large installations. A Ze measurement at the origin below 0.04Ω (230V ÷ 6,000A) suggests 10kA MCBs may be required. When in doubt, the 10kA type can always replace a 6kA — it provides extra protection at minimal additional cost.

Prospective short-circuit current is measured using a loop impedance tester set to measure at the line-to-line or line-to-neutral terminals (not line-to-earth). The tester measures the loop impedance and calculates the prospective current. For a single-phase supply, PFC = 230V ÷ Zs; for three-phase, the line-to-line PFC = 400V ÷ Zs. The measured PFC must not exceed the breaking capacity of the installed protective devices. This measurement should be taken at the incoming terminals of each distribution board and recorded on the installation certificate (EIC) or condition report (EICR).

A ring final circuit has a lower loop impedance than a radial circuit because current can flow both ways around the ring. The mid-point of the ring (the point furthest from the board) has the lowest fault current, but the board end has the highest. For ring circuits, the prospective fault current at the board end is typically dominated by the Ze and the incoming supply impedance rather than the cable. The PSCC at the socket-outlet with the lowest impedance (closest to the board) should be checked against the MCB's breaking capacity.