Rated short-circuit capacity
Icn
The highest fault a circuit breaker can break under BS EN 60898-1, shown in the box on its front. The breaker may not be fit for further use after clearing a fault that size. Its has to suit the where it's installed, unless a device further up the supply backs it up.
Using Rated short-circuit capacity in context
Reviewed
A distinction worth remembering
Load current is what equipment draws in normal use. Prospective fault current is the largest current a fault could drive at a given point, and it decides what the protective device there has to be able to break. A device's rated current and its breaking capacity are separate specifications, so a breaker that's right for the load can still be wrong for the fault.
Questions to take further
- How can I revise Rated short-circuit capacity beyond the definition?
- Explain what Rated short-circuit capacity describes, compare it with a related term and use it in a topic question. State any assumptions and what evidence you would need before applying the idea to real work.
Sources
- Professional Electrician: prospective fault current, how to determine values (NICEIC) — 8 July 2018, before the current amendments. Enquiry, measurement or calculation, the live test at the origin, what stays connected, recording 16 kA, and the far-end example.
- BEAMA: consumer unit 16 kA conditional rating application — December 2018, before the current amendments. The 19.6 kA and 16 kA figures from ENA P25, the BS 88-3 test arrangement, interconnected networks, and other upstream devices.
- Kewtech: KT63 Plus multifunction tester instructions (hosted by TLC Direct) — One maker's manual, undated, referring to the 2018 wiring regulations. Its functions, the two loop modes, PSC and PEFC, the value to record, the ramp example, phase rotation, and the BS EN 61557 parts.
- IET: current BS 7671 edition and updates — BS 7671:2018+A4:2026 and the main subject areas updated by Amendment 4.
- IET: inspection and testing FAQs — Inspection, test interpretation and protecting equipment during testing.
Where Rated short-circuit capacity fits in the learning path
The Design Calculations section covers these units:
- Sizing a Cable, Every Factor
- Volt Drop and Zs in Depth
- Fault Current, the Adiabatic Check and Device Choice
- Submains, Three-Phase Boards and Demand
Curriculum mapping: C&G 2365 Unit 305.
See this section in the full curriculum
This section is on the Plus path. Lesson availability depends on content review and your learning path.
Read, practise, return
1. Read it in context
- Prospective Fault CurrentTesting & Inspection
- MCB Types B, C and DInstallation
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
- Inspection & Testing — quiz previewPlus · Testing & Inspection
3. Return to the meaning
After reading or practising, explain the idea in your own words, then check what you remembered.
Recall a related term
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
The largest prospective fault current that a protective device can interrupt safely under its stated conditions. If the available fault current exceeds that rating, the device may fail while trying to clear a short circuit. Selection compares the device rating with the fault current possible at its point in the installation.
Breaking capacity
Read the Breaking capacity entryCompare related terms
Open a meaning to compare its role with Rated short-circuit capacity, or follow its full entry.
Breaking capacity
The largest prospective fault current that a protective device can interrupt safely under its stated conditions. If the available fault current exceeds that rating, the device may fail while trying to clear a short circuit. Selection compares the device rating with the fault current possible at its point in the installation.
Read the Breaking capacity entryService short-circuit capacity — Ics
The largest fault current a circuit breaker can clear and still be fit to stay in service. Under BS EN 60898-1 it's linked to Icn: a 6 kA breaker also has an Ics of 6 kA, but a 10 kA breaker has an Ics of 7.5 kA. It matters where a circuit has to keep working after a big fault, because above Ics the breaker may need replacing.
Read the Service short-circuit capacity entryPFC — Prospective Fault Current
The current that could flow at a point if a fault of negligible impedance occurred there. Assessment considers the relevant short-circuit and earth-fault conditions. Protective devices must have adequate breaking capacity, or a verified coordinated arrangement with upstream protection, for the fault current at their location.
Read the PFC entryMCB — Miniature 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 MCB entryA definition is a start. Using it in a real question is where it sticks.