Calculate Prospective Short Circuit Current (PSCC) and Prospective Earth Fault Current (PEFC) according to BS 7671 Regulation 612.11. Determine protective device breaking capacity requirements.
Calculate Prospective Short Circuit Current (PSCC) and Prospective Earth Fault Current (PEFC) according to BS 7671 Regulation 612.11. Results help determine required breaking capacity of protective devices.
Typical: 0.8Ω
Range: 0.35Ω - 0.8Ω
Cable sheath used as protective conductor
Typical: 0.35Ω
Range: 0.1Ω - 0.35Ω
Combined neutral and earth (PEN) from supplier
Typical: 21Ω
Range: 5Ω - 200Ω
Highly variable depending on earth electrode resistance
Prospective Fault Current (PFC) is the maximum current that would flow in the event of a fault with negligible impedance. It's a critical safety parameter that must be determined to ensure protective devices can safely interrupt fault currents without damage or danger. BS 7671 Regulation 612.11 requires PFC to be measured, calculated, or determined for all installations.
Prospective Short Circuit Current is the maximum current that would flow during a line-to-neutral (single-phase) or line-to-line (three-phase) short circuit. Calculated as PSCC = U₀ / Ze.
Prospective Earth Fault Current is the maximum current that would flow during a line-to-earth fault. Calculated as PEFC = U₀ / Zs, where Zs = Ze + (R₁ + R₂).
The Prospective Fault Current is taken as the higher value of PSCC or PEFC. This determines the minimum breaking capacity (Icn) required for protective devices.
The breaking capacity (Icn rating) of a protective device is its ability to safely interrupt fault currents. The device's breaking capacity must exceed the prospective fault current at its point of installation.
| Device Type | Breaking Capacity (Icn) | Typical Application | Standard |
|---|---|---|---|
| Standard MCB | 6kA (6,000A) | Domestic installations | BS EN 60898 |
| Enhanced MCB | 10kA (10,000A) | Commercial installations | BS EN 60898 |
| MCCB | 25kA - 50kA | Industrial installations | BS EN 60947-2 |
| BS 88 Fuse | 80kA+ | Main switchgear | BS 88 |
Given:
Calculations:
Device Selection:
Standard 6kA MCB is suitable (6,000A > 657A)
For three-phase systems, the prospective fault current can be higher during three-phase faults:
In-depth technical reference covering fault loop physics, testing methodology, breaking capacity standards, and the regulatory requirements UK electricians must understand for BS 7671 compliance.
The magnitude of any fault current is governed by Ohm's Law for AC circuits:Ipf = U0 / Zs
Ipf
Prospective Fault Current in Amperes - the current that would flow if no protective device interrupted it
U0
Nominal voltage to earth: 230V for single-phase, 400V line-to-line for three-phase
Zs
Total fault loop impedance - includes supply transformer, DNO cables, meter tails, and installation wiring
Critical insight: Because voltage is fixed (~230V), fault current is inversely proportional to impedance. Lower impedance = higher fault current. This is why PFC is always highest at the origin of an installation, before cable resistance dampens it.
BS 7671 distinguishes between two specific fault conditions. Both must be assessed, and the higher value is recorded as the PFC for the installation.
The overcurrent resulting from a fault between live conductors:
Test: L-N measurement
The overcurrent resulting from a fault between a live conductor and earth:
Test: L-E measurement
⚠️ Record the HIGHER value as the installation PFC
This ensures protective devices are rated for the absolute worst-case fault scenario.
| System | Typical Ze | PSCC vs PEFC | Practical Impact |
|---|---|---|---|
| TN-S | 0.35-0.8Ω | PSCC often higher than PEFC (cable sheath/armour has higher resistance than neutral) | Both tests required - don't assume L-E is higher |
| TN-C-S (PME) | 0.1-0.35Ω | PSCC and PEFC typically identical (PEN conductor serves both returns) | Low impedance = high fault currents; check breaking capacity carefully |
| TT | 10-200Ω+ | PSCC is high (metal L-N path); PEFC is very low (earth electrode resistance) | Still verify PSCC for MCB rating! RCDs handle earth faults, but L-N faults can be kA range |
TT system trap: Don't record low PEFC (e.g., 20A) as your PFC. The PSCC between Line and Neutral can still be thousands of amperes if the transformer is close - that's what determines MCB breaking capacity.
✓ Use "High Current" mode
At the origin (upstream of RCDs), High Current mode provides more accurate low-impedance readings with less electrical noise
CAT III/IV rated instrument
Test instruments must be rated to CAT III 300V/600V minimum for supply intake testing
Null test leads first
Lead resistance (0.05-0.10Ω) must be zeroed out - see common mistakes section
Testing three-phase supplies involves 400V measurements. Not all loop testers are rated for line-to-line connection. These methods allow safe estimation using single-phase readings.
IET On-Site Guide recommended method using standard single-phase tester.
Highest L-N = 3.0kA
Three-phase PFC = 3.0 × 2 = 6.0kA
Conservative estimate with good safety margin
For industrial installations where the doubling rule might unnecessarily condemn equipment.
Ipf(3ph) = IL-L(measured) ÷ 0.87
The 0.87 factor (≈√3/2) accounts for vector relationship in balanced three-phase faults. Guidance Note 3 method.
A common question: "PFC measured at 8kA but the consumer unit has 6kA MCBs - is this safe?" The answer lies in BS EN 61439-3 Annex ZB.
Domestic consumer units can be rated at 16kA conditional, allowing 6kA MCBs to be used with fault currents up to 16kA, provided:
The BS 88-3 service fuse is extremely fast-acting at high fault currents. In a 10kA fault, the fuse operates so quickly that its energy let-through (I²t) is within what the 6kA MCB can withstand without damage.
In these cases, MCBs must be individually rated for full PFC
Fault current drops rapidly through conductor resistance. Even a few meters of cable can significantly reduce PFC at a distribution board - this is why measuring at sub-boards often allows lower-rated (cheaper) breakers to be used.
16mm²
1.15 mΩ/m
25mm²
0.727 mΩ/m
35mm²
0.524 mΩ/m
50mm²
0.387 mΩ/m
Remember: Total loop = Line + Neutral/Earth (double the run length)
At DNO Intake:
After 5m of 25mm² Tails:
Result: Just 5 meters of tails reduced PFC by 5kA. With 10m of tails (20m conductor path), PFC drops to 7.8kA.
This explains why faults at socket outlets are rarely a breaking capacity issue - the resistance of 2.5mm² T&E decimates fault current within meters of the consumer unit.
DNOs declare standard maximum fault levels to cover future network reinforcements. These are design worst-case values, not what you'll typically measure.
Use these values if measurement is impossible (e.g., design stage for new build)
Urban hazard: In dense city areas, substations may be in building basements. With extremely short service cables, Ze can approach 0.01-0.02Ω, genuinely reaching 16kA+. Even residential blocks in these areas may need industrial-grade switchgear.
Many older properties were connected via TT or TN-S systems with high impedance. When DNOs upgrade networks to PME (TN-C-S), impedance drops dramatically - potentially making legacy breakers dangerous.
Original (TN-S)
Ze = 0.8Ω
PFC = 287A
BS 3871 M3 (3kA) ✓ Safe
After PME Conversion
Ze = 0.2Ω
PFC = 1,150A
M3 (3kA) ✓ Still safe
New Substation 50m Away
Ze = 0.05Ω
PFC = 4,600A
M3 (3kA) ✗ DANGEROUS
Why recording PFC on EICRs matters: It tracks degradation of safety margins over time. An inspector seeing PFC increase from 1.5kA to 3.2kA across inspections can identify that the BS 3871 M4.5 breakers are now operating near their limit - before a fault proves it catastrophically.
Some inspectors measure 1.2kA but record "16kA" because "that's what the DNO says." This creates a contradiction if you're justifying keeping 3kA/4.5kA legacy breakers - the certificate states PFC (16kA) exceeds device rating (3kA), which technically triggers a C2 (Potential Danger) code.
Best practice: Record the measured value. Note DNO declared maximum in observations if significantly higher, warning that future upgrades could affect compliance.
Test lead resistance (typically 0.05-0.10Ω) gets added to your reading if not zeroed. This makes measured Ze appear higher than reality → calculated PFC appears lower than reality.
This is a "fail-to-danger" error - you think you're safe when you're not.
"No-Trip" (low current) loop settings are for testing downstream of RCDs to avoid nuisance tripping. At the origin (upstream of RCDs), "High Current" mode gives cleaner, more accurate low-impedance readings with less electrical noise.
Especially in TT systems, electricians often only test L-E and record the low value. The PSCC (L-N) could be significantly higher and is what determines whether the MCB can safely interrupt a short-circuit between live conductors.
Always test both L-N and L-E, record the higher value.
When inspecting older installations, you may encounter obsolete BS 3871 MCBs. Their "M" ratings correspond to these breaking capacities:
| M Rating | Breaking Capacity | Status |
|---|---|---|
| M1 | 1kA (1,000A) | Often inadequate today |
| M1.5 | 1.5kA (1,500A) | Often inadequate today |
| M3 | 3kA (3,000A) | Verify against measured PFC |
| M4.5 | 4.5kA (4,500A) | Verify against measured PFC |
| M6 | 6kA (6,000A) | Usually acceptable |
| M9 | 9kA (9,000A) | Usually acceptable |
EICR coding: If measured PFC exceeds the M-rating of installed BS 3871 MCBs, this is typically a C2 (Potentially Dangerous) code requiring urgent attention. The consumer unit should be upgraded to modern BS EN 60898 devices with appropriate breaking capacity.
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