Maximum Zs values: BS 7671 Table 41.3 for Type B, C and D breakers
In short: The maximum earth fault loop impedance (Zs) for a circuit breaker is the value in BS 7671 Table 41.3 for its type and rating, already corrected by Cmin 0.95: for example 1.37 ohm for a B32, 2.73 ohm for a B16, 7.28 ohm for a B6, and 1.37 ohm for a C16. A measured Zs must be below that figure, and below about 80 percent of it when measured on a cold installation.
Key points
- Table 41.3 values already include Cmin 0.95: B32 1.37 ohm, B16 2.73 ohm, B6 7.28 ohm, C32 0.68 ohm, C16 1.37 ohm.
- A Type C limit is half the Type B limit for the same rating; Type D is a quarter.
- Compare a cold measurement against 80 percent of the table value, or temperature-correct it.
- For MCBs the 0.4 s and 5 s limits are the same figure; only fuses differ.
- Where a 30 mA RCD provides fault protection the limit is 1667 ohm, so an RCBO often rescues a long circuit.
Every circuit on an EIC or EICR carries a measured Zs, and every one of them is judged against a maximum. The maximum is not a rule of thumb: it is the loop impedance at which the fault current is just enough to trip the protective device within the time BS 7671 demands. Get the table wrong and a circuit that will not disconnect in time gets signed off. This guide sets out the Table 41.3 figures for the breakers you fit every week, where they come from, and how to use them on site.
Table 41.3: maximum Zs for circuit breakers at 0.4 seconds
These are the BS 7671 (2018 with amendments) values for MCBs and RCBOs to BS EN 60898 and BS EN 61009, for a 230 V supply, with the Cmin factor of 0.95 already applied. They are the numbers to write against each circuit.
| Rating | Type B | Type C | Type D |
|---|---|---|---|
| 6 A | 7.28 Ω | 3.64 Ω | 1.82 Ω |
| 10 A | 4.37 Ω | 2.19 Ω | 1.09 Ω |
| 16 A | 2.73 Ω | 1.37 Ω | 0.68 Ω |
| 20 A | 2.19 Ω | 1.09 Ω | 0.55 Ω |
| 25 A | 1.75 Ω | 0.87 Ω | 0.44 Ω |
| 32 A | 1.37 Ω | 0.68 Ω | 0.34 Ω |
| 40 A | 1.09 Ω | 0.55 Ω | 0.27 Ω |
| 50 A | 0.87 Ω | 0.44 Ω | 0.22 Ω |
| 63 A | 0.69 Ω | 0.35 Ω | 0.17 Ω |
The max Zs checker gives the same figures for any device and will compare a Ze plus R1+R2 reading against them.
Where the numbers come from
A Type B breaker trips magnetically at 5 times its rating, a Type C at 10 times and a Type D at 20 times. For the device to clear the fault instantly the fault current must reach that threshold, and the fault current is the supply voltage divided by the loop impedance. So the maximum Zs is simply:
Zs max = (U0 × Cmin) ÷ (trip multiple × In) = (230 × 0.95) ÷ (5 × 32) = 1.37 Ω for a B32
Cmin is the factor for the supply voltage sitting at the bottom of its permitted tolerance, 0.95, which the 18th Edition built into the tables. That is why a B32 that used to be quoted at 1.44 Ω under earlier editions is now 1.37 Ω. The same arithmetic explains the pattern in the table: double the trip multiple and the limit halves.
Why 0.4 seconds, and why the 5 second column does not help
Final circuits up to 63 A in a TN system must disconnect within 0.4 seconds under earth fault, because a person could be touching the faulty equipment. Distribution circuits and circuits over 63 A are allowed 5 seconds. For fuses that gives two genuinely different Zs limits. For circuit breakers it does not: once the fault current passes the magnetic threshold the device opens in a few milliseconds, well inside either time, so the same figure applies to both columns. On a TT installation the disconnection times are 0.2 and 1 second, but in practice a TT circuit relies on an RCD for fault protection and the RCD sets the limit.
The 0.8 rule for measured values
Table 41.3 assumes the conductors are at their normal operating temperature, because that is when the loop impedance is highest and the fault current lowest. You measure Zs on a cold, unloaded installation, so the reading will rise once the circuit is working. Guidance Note 3 gives two ways to allow for that: apply the correction factors for conductor temperature, or use the rule of thumb that a measured value should not exceed 80 percent of the tabulated maximum. For a B32 that means a cold reading under 1.10 Ω. A reading between 80 and 100 percent of the limit is a pass on the day and a fail in August; treat it as a circuit to look at.
Measured Zs against Ze plus R1+R2
The two ways of arriving at Zs should agree. The external loop impedance at the origin plus the resistance of the line and protective conductors of the circuit, Ze + (R1+R2), is what the measured Zs at the far end ought to be. If your loop tester says 1.5 Ω on a circuit where Ze is 0.31 Ω and R1+R2 is 0.41 Ω, something is wrong: a parallel earth path, a poor connection, a mislabelled circuit or a tester on the wrong range. The R1+R2 and expected Zs calculator works out what a circuit should read from its conductor sizes and length.
When a circuit fails the maximum
- Check the reading. Retest at the same point, check the tester lead, and confirm the circuit is the one you think it is.
- Check the connections. A high Zs on one circuit with the rest fine is usually a loose or corroded terminal on the CPC somewhere along the run.
- Look at the design. A long run in 1.5 mm² or a small CPC in an old cable may simply have too much resistance for a Type B device of that rating.
- Consider the protective device. A 30 mA RCD or RCBO provides fault protection with a maximum Zs of 1667 Ω (Table 41.5), which is why an RCBO is the standard fix for a long outbuilding supply or a TT installation. It does not remove the need for the overcurrent device to be right for the cable.
On an EICR a circuit over its maximum Zs is coded C2 at least, because the disconnection time is not met and the fault protection is potentially inadequate. On a new installation it does not get signed. SparkCerts runs this check as you type each circuit's reading, against the device you entered, so the flag appears on site rather than in the assessor's office.
Common questions
What is the maximum Zs for a B32?
1.37 ohm at 0.4 seconds, from BS 7671 Table 41.3 with the Cmin factor of 0.95 already applied. A reading measured on a cold installation should be under about 1.10 ohm, which is 80 percent of the limit, to allow for the conductors warming up in service.
Why is the maximum Zs lower for a Type C breaker?
A Type C device needs 10 times its rated current to trip magnetically, against 5 times for a Type B, so the fault current must be twice as high and the loop impedance half as much. A C32 is limited to 0.68 ohm where a B32 allows 1.37 ohm.
Does the 5 second disconnection time give a higher Zs for MCBs?
No. Once the fault current reaches the magnetic trip threshold an MCB clears in well under 0.1 seconds, so the same Zs figure satisfies both the 0.4 second and 5 second requirements. The two-column difference only matters for fuses.
What is the 0.8 rule?
Table 41.3 assumes the conductors are at their operating temperature. A reading taken on a cold installation will rise when the circuit is loaded, so Guidance Note 3 advises comparing the measured value against 80 percent of the tabulated maximum, or correcting it for temperature.
What if the measured Zs is too high?
The circuit does not meet the disconnection time and cannot be signed off as it stands. Check the reading and the connections first, then look at the cable size, the run length and the protective device; a 30 mA RCD or RCBO provides fault protection with a much higher permitted Zs, which is often the practical fix on a long circuit.
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