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Maximum Zs: Tables 41.2, 41.3 and 41.4Which table gives the maximum Zs for your device?

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Every protective device has a maximum permitted Zs — the highest loop impedance it can still tolerate and disconnect in time. The values are tabulated in Chapter 41, and which table you use depends on the device and the circuit type. This is a classic trap.

Table	Devices	Circuits	Time
41.2	Fuses — BS 88-2, BS 88-3, BS 3036, BS 1362	Final, within 411.3.2.2	0.4 s
41.3	Circuit breakers — BS EN 60898, RCBOs	Final and distribution	0.4 s & 5 s
41.4	Fuses — same list as 41.2	Distribution, and final circuits under 411.3.2.3	5 s

✋41.2 and 41.4 cover the same fuses. 41.2 is fuses at 0.4 s; 41.4 is fuses at 5 s, for distribution circuits and for final circuits under 411.3.2.3. This exact question catches people out, by sending them to the wrong table. Highlight "0.4 s" and "5 s" in the two headings. Table 41.4 (5 s) is for distribution circuits and for final circuits under 411.3.2.3.

Worked example: "What is the tabulated earth fault loop impedance for a 63 A BS 88-2 (gG) fuse supplying a distribution circuit?" → distribution means 5 s means Table 41.4 → 63 A row → 0.78 Ω.

The ABC trap in Table 41.3

✋Table 41.3 labels its circuit-breaker column groups a, b, c — but those letters mean type B, type C and type D. So column a = type B, b = type C, c = type D. Read the row heading, not the letter, or you'll price a type C answer for a type B question.

Values all from Table 41.3:

Device	Max Zs (tabulated)
Type B, 16 A	2.73 Ω
Type B, 50 A	0.87 Ω
Type C, 32 A	0.68 Ω

≡The four points to memorise.
1. Every circuit has a Zs.
2. Every circuit has an overcurrent protective device (an OCPD) — not every circuit needs an RCD, but every circuit needs overcurrent protection.
3. Every OCPD has a maximum Zs.
4. If the circuit's Zs does not exceed that maximum, ADS will be achieved.

Table 41.2 · fuses, 0.4 s Table 41.3 · circuit breakers (a = type B, b = type C, c = type D) Table 41.4 · fuses, 5 s (distribution circuits, and final circuits under 411.3.2.3)

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