Prospective Fault Current
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Full course section in Plus
Two figures live under this heading. is what would flow in a fault between line and neutral; prospective earth fault is what would flow between line and earth. The greater of the two is the one recorded, because the device has to cope with the worst case, not the average one.
The number matters for one reason: breaking capacity. A protective device that cannot interrupt the available at its terminals does not fail gracefully. Measurements are taken at the origin and at distribution boards, and the value falls the further you travel from the supply, because the conductors' own is doing the limiting.
On a single-phase supply, the three-phase line-to-line case is conventionally taken as twice the measured line-to-neutral value — a rule of thumb that exists because you cannot measure a fault that has not happened. Backup protection from the supplier's cut-out fuse is part of the picture too, and is why some boards are acceptable on supplies with a high prospective .
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Common questions
- Can I try Prospective Fault Current free on this page?
- This guide is free to read, but there is no public quiz specifically for this topic on this page. Browse public practice samples for other subjects, or create an account for the learning path. The full course section requires Plus.
- Where does Prospective Fault Current sit in the City & Guilds scheme?
- maps to Unit 304 / 2391. On TradeTraining it lives in the Testing & Inspection section as bite-size lessons with practice questions.
- Do I need Prospective Fault Current for the 18th Edition exam?
- Yes — the C&G 2382-26 exam is set against the BS 7671, and this topic is part of that ground. The exam is open book, so practising fast navigation to the right regulation matters as much as recall.
- What is the difference between PSCC and PEFC?
- is the prospective short-circuit current, measured between line and neutral. PEFC is the prospective earth fault , measured between line and earth. The higher of the two is recorded as the prospective fault current for the installation.
- Why does prospective fault current fall further from the origin?
- Because every metre of conductor adds to the fault path. The origin sits closest to the source and therefore sees the highest available fault in the installation.
Explore key terms
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PFCProspective Fault Current
The highest current that could flow at a given point if a fault of negligible impedance occurred there — whether between live conductors or from live to earth. Every protective device must be able to break the prospective fault current at its position without failing, which is why PFC is measured or calculated at the origin of every installation.
Read the full PFC glossary entryPSCCProspective Short-Circuit Current
The highest current that would flow in a short circuit between live conductors — line to neutral, or line to line on three-phase supplies. Together with the prospective earth fault current it makes up the prospective fault current, and the larger of the two is what switchgear breaking capacity is checked against.
Read the full PSCC glossary entryOvercurrent
Any current above the value a circuit is designed to carry. It comes in two flavours with different time scales: overload, where a healthy circuit is simply asked for too much for too long, and fault current, where a short circuit drives an enormous current that must be broken in milliseconds. Protective devices are chosen to handle both.
Read the full Overcurrent glossary entryMCBMiniature Circuit Breaker
An automatic switch that turns a circuit off when too much current flows, whether a slow overload or a sudden short circuit. It has replaced the rewirable fuse in modern boards. Each one is chosen by its current rating, such as 6 A for lights or 32 A for sockets, and by its type: B for most home circuits, C or D where equipment draws a big surge as it starts.
Read the full MCB glossary entry