Prospective Fault Current
Testing & Inspection · C&G 2365 Unit 304 / 2391 · Premium section
Two figures live under this heading. is what would flow in a fault between line and neutral; prospective earth fault current 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 current 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 impedance 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 current.
Common questions
- Is there a free Prospective Fault Current quiz?
- sits in the Testing & Inspection section, which is part of TradeTraining Premium — but the app is free to start, sample questions are open to everyone, and the free sections cover the fundamentals first.
- Where does Prospective Fault Current sit in the City & Guilds scheme?
- maps to C&G 2365 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 current 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 current, measured between line and earth. The higher of the two is recorded as the for the installation.
- Why does prospective fault current fall further from the origin?
- Because every metre of conductor adds impedance to the fault path. The origin sits closest to the source and therefore sees the highest available fault current in the installation.