Fault Level (Prospective Short-Circuit Current): What It Is & Why It Matters
Protection · 7 July 2026 · For contractors and project engineers
The fault level is the current that would flow if a short circuit happened at a given point in the installation. It's an unglamorous number that quietly governs two safety-critical things: whether your switchgear can safely break the fault, and whether your cables survive it. Underestimate it and equipment can fail violently; overestimate it and you overspecify everything.
What Fault Level Means
The prospective short-circuit current (PSCC) at a point is the maximum current that would flow in a bolted (zero-impedance) fault there. It's set by the impedance of everything between the source and that point — the supply network, the distribution transformer, and the cables in between. Low impedance means a big fault current; add cable length and impedance and the fault current falls.
The Two Headline Uses
- Equipment breaking capacity. Every protective device has a rated breaking capacity (Iₘₕ / Iₘₛ). It must exceed the PSCC where the device sits — otherwise the device may not clear the fault safely.
- Cable withstand. The cable and its CPC must survive the fault energy let through before the device operates — the adiabatic check, which needs the fault current and the disconnection time.
It Falls With Distance — So Every Board Is Different
Fault level is highest at the origin, near the transformer, and decreases along each cable run. That means the switchboard at the intake and a distribution board three floors up have different fault levels, and each must be assessed on its own figure — not a single number borrowed from the intake.
How It's Calculated
Fault studies (to BS EN 60909) sum the source and cable impedances to the point of interest and compute the resulting current. Two subtleties matter. First, you need both the three-phase fault (usually the largest, sizing equipment) and the line-to-earth fault (which governs earth-fault protection). Second, you need both the maximum fault (for equipment rating) and the minimum fault (to confirm protection still operates fast enough at the far end of a long run).
The Traps
- Using the DNO's headline fault level everywhere instead of calculating it board by board.
- Ignoring the contribution from large motors, which feed extra current into a nearby fault for the first few cycles.
- Checking only the maximum fault and forgetting the minimum-fault check for automatic disconnection.
What This Means for Your Design
Fault level ties several things together: it sets equipment ratings, feeds the adiabatic cable check, underpins discrimination, and even drives mechanical forces like cable cleat spacing. Our ProDesign calculation packs compute the fault levels at each board and carry them through to the protection and cable checks — one model, one consistent set of numbers.