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Prospective fault current, and breaking capacityPSCC or PEFC. Which do you record?

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The question at the end: PSCC or PEFC. Which do you record? The answer is in the notes below.

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Prospective fault current: the three abbreviations

Prospective fault current is the current that would flow in the event of a fault — and it matters because every protective device has a limit on what it can interrupt without being destroyed.

AbbreviationWhat it isFault between
PEFCProspective earth fault currentLine and earth
PSCCProspective short-circuit currentLine and neutral
IpfWhat the schedule calls itThe greater of the two
Three abbreviations, two tests, one number to write down they measure between different pairs of conductors PEFC prospective earth fault current line earth A the fault PSCC prospective short-circuit current line neutral A the fault Ipf — what the schedule calls it the greater of the two and that is the figure you record it appears twice on the schedule — page 1 and page 2 — and every device has a limit on what it can interrupt without being destroyed
Two measurements between two different pairs of conductors, and one figure that goes on the paperwork. PEFC is line to earth, PSCC is line to neutral, and Ipf is simply whichever came out higher. You record the greater because the device has to be able to interrupt the worst case, not the average one.
On the schedule it appears twice. Ipf is on page 1 and again on page 2, just above Ze. Fill in both.
✋The sheet wants kA. Your meter reads amps. A measurement of 223.3 A goes on the sheet as 0.223 kA. Some instruments toggle; the basic ones do not.

Breaking capacity: ICN and ICS

Guidance Note 3 · 2.6.16 has a table of short-circuit capacities, and the same table appears in the On-Site Guide. This is the one everybody asks about and nobody reads the facing column of.

MarkingWhat it means
IcnThe maximum fault current the device can interrupt safely — but it may be destroyed doing it
IcsWhat it can interrupt and still be usable afterwards, without replacement

How they relate

A device marked Icn = 10 kA

Ics might be stated as 7.5 kA, or as a percentage —
some devices state Ics at 50% of Icn.

So it will interrupt 10 kA once. Below Ics it survives to work again.

Typical capacities, from the table:

DeviceBreaking capacity
Rewirable fuseUp to 4 kA, depending on designation (an S1A only 1 kA)
BS 1361 Type 2 — the main cutout fuse33 kA
Circuit breakers and RCBOs6 kA, 10 kA, up to 25 kA

Using it: a worked judgement

Ipf at that point          = about 7.3 kA  (the measured maximum)
Device on the board               = Type B 32 A RCBO, 6 kA

6 kA < 7.3 kA  ->  not suitable on its own. You need the next rating
up, 10 kA, unless a device on the supply side gives back-up
protection (Reg 434.5.1) — domestic consumer units carry a
conditional rating with the cut-out fuse.
✋This is not about tripping faster. The device's breaking capacity must be at least the prospective fault current where it is installed. If it isn't, the device is not suitable: choose one with a higher rating.
Go by the device, not the book. The table is guidance; the figure stamped on the device by the manufacturer is what governs. Read the marking: type letter, current rating, Icn, and Ics where it is given.

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