Question 1

My PFC reads 10kA but the board is only rated 6kA — is it safe?

Accepted Answer

No, it is not safe. The prospective fault current exceeds the breaking capacity of the consumer unit. This must be resolved before energising the installation. Common solutions include fitting a BS 88-3 HRC fuse upstream (which limits the let-through energy), upgrading to a consumer unit with a higher fault rating, or requesting the DNO assess whether supply impedance can be increased.

Question 2

Why is my measured PFC much lower than the DNO's declared value?

Accepted Answer

The DNO declares the maximum prospective fault current at the cut-out, which represents the worst-case scenario (e.g. after a transformer tap change or network reinforcement). Your measurement reflects the actual impedance at the time of testing, which is usually higher due to cable lengths, connections, and the current state of the network. Always design to the DNO's declared value as it may be reached in future.

Question 3

Do I measure line-to-neutral or line-to-earth for PFC?

Accepted Answer

You should measure both. The line-to-neutral measurement gives the prospective short-circuit current (PSCC), and the line-to-earth measurement gives the prospective earth fault current (PEFC). Record the higher of the two values, which is typically the L-N reading since the neutral path usually has lower impedance than the earth path. Both must be within the breaking capacity of the protective devices.

Question 4

How does cable length affect fault current?

Accepted Answer

Every metre of cable adds resistance to the fault loop, which reduces the prospective fault current. Extending meter tails from 3m to 10m can reduce PFC by 1-2kA depending on cable size. This is why PFC is highest at the origin (closest to the transformer) and decreases along the length of each circuit. You must verify PFC at the origin to confirm device breaking capacity is adequate.

Question 5

What is the difference between PSCC and PEFC?

Accepted Answer

PSCC (Prospective Short-Circuit Current) is measured line-to-neutral and represents the maximum current that would flow during a short circuit between live conductors. PEFC (Prospective Earth Fault Current) is measured line-to-earth and represents the maximum current during a fault between line and earth. PSCC is typically higher because the neutral conductor usually has lower impedance than the earth path.

Question 6

What is prospective fault current (PFC) and why must it be measured?

Accepted Answer

PFC is the maximum current during a short circuit. Regulation 612.11 requires it measured at every installation origin. Every device must have breaking capacity ≥ PFC per Regulation 434.5.1.

Question 7

What is the maximum PFC for a domestic consumer unit?

Accepted Answer

Most CUs to BS EN 61439-3 are rated 16kA conditional — but only with upstream BS 88-3 HRC fuse. Without it, the board's inherent 6kA or 10kA rating applies.

Question 8

How do I calculate fault current from Ze?

Accepted Answer

PFC = U0 ÷ Ze. Example: Ze of 0.35Ω gives PFC = 230 ÷ 0.35 = 657A (0.66kA) per Regulation 434.5.1.

Question 9

What happens if PFC exceeds the consumer unit breaking capacity?

Accepted Answer

The device may fail to safely interrupt a fault — causing arc flash, fire, or explosion. Solutions: fit upstream HRC fuse, upgrade CU, or request DNO assessment.

Ze (Ω)	PFC (A)	PFC (kA)
0.1	2300	2.3
0.2	1150	1.15
0.35	657	0.66
0.5	460	0.46
0.8	288	0.29

Fault Current Calculator

How This Calculator Works

Quick Reference — Typical PFC by Loop Impedance

Practical Notes

Frequently Asked Questions