Question 1

Why is the 80% rule applied to Zs readings?

Accepted Answer

When you test a circuit, the conductors are typically at ambient temperature (around 20°C). Under fault conditions, conductor temperature rises rapidly — up to 70°C for thermoplastic insulation. As temperature increases, so does conductor resistance. The 80% rule accounts for this: if your measured Zs at ambient is no more than 80% of the tabulated maximum, the circuit will still disconnect within the required time even at elevated operating temperatures.

Question 2

My Zs is 1.2 ohms and the table says 1.44 ohms — does it pass?

Accepted Answer

You need to apply the 80% rule. 80% of 1.44 ohms is 1.15 ohms. Your measured reading of 1.2 ohms exceeds 1.15 ohms, so it does not pass using the simplified 80% method. However, you can calculate the exact temperature-corrected value using the conductor operating temperature formula. If the precise calculation still exceeds the limit, the circuit fails and remedial work is required.

Question 3

What Ze should I use if the DNO will not confirm the value?

Accepted Answer

BS 7671 provides default maximum Ze values: 0.35 ohms for TN-C-S (PME), 0.8 ohms for TN-S, and 21 ohms for TT systems. These are worst-case assumptions. If you measure a lower Ze at the intake, you may use the measured value for design purposes, but bear in mind it can increase if the DNO alters the network. Using the default values provides a safer design margin.

Question 4

Why is my measured Zs lower than calculated?

Accepted Answer

Parallel earth paths are the most common cause. Bonded metallic services — gas pipes, water pipes, structural steelwork — provide additional return paths for fault current alongside the CPC. This lowers the overall earth fault loop impedance. While lower Zs is better for disconnection times, you should verify the circuit still passes with bonding isolated, as those parallel paths may not always be present.

Question 5

What is the difference between Zs for Type B and Type C MCBs?

Accepted Answer

Type B MCBs trip magnetically at 3-5 times rated current, while Type C trip at 5-10 times. Because Type C requires a higher fault current to trip within the required time, the maximum permissible Zs is lower. For example, a 32A Type B MCB allows Zs up to 1.44 ohms, whereas a 32A Type C MCB allows only 0.72 ohms. Type C devices need lower impedance circuits to guarantee fast disconnection.

Question 6

What is earth fault loop impedance (Zs) and why does it matter?

Accepted Answer

Zs is the total impedance of the fault current path. If Zs is too high, insufficient current flows to trip the protective device within 0.4s (or 5s for distribution circuits), creating a shock hazard per Regulation 411.4.5.

Question 7

What are the maximum Zs values for a 32A Type B MCB?

Accepted Answer

Maximum Zs is 1.44Ω (Table 41.3 with Cmin 0.95). For testing, measured value at ambient must not exceed 80%: 1.15Ω. Above this, the circuit fails simplified verification per IET Guidance Note 3.

Question 8

How do I calculate Zs from Ze and R1+R2?

Accepted Answer

Zs = Ze + (R1 + R2). Example: Ze = 0.35Ω, R1+R2 = 0.78Ω gives Zs = 1.13Ω. Compare against 80% values from Tables 41.2–41.4.

Question 9

What is the difference between Zs and Ze?

Accepted Answer

Ze is external earth fault loop impedance at the intake (DNO supply). Zs is total including internal circuit conductors: Zs = Ze + (R1 + R2) per Regulation 411.4.5.

MCB Rating (A)	Max Zs (Ω)	80% Value (Ω)
6	7.67	6.14
10	4.6	3.68
16	2.87	2.3
20	2.3	1.84
32	1.44	1.15
40	1.15	0.92
50	0.92	0.74

Max Zs Calculator

How This Calculator Works

Quick Reference — Maximum Zs for Type B MCBs

Quick Lookup by Device

Type B MCBs

Type C MCBs

Type D MCBs

Practical Notes

Frequently Asked Questions