8.2 Zs Limits, Disconnection Times & the 0.8 Rule

Disconnection Times (BS 7671 Chapter 41)

The reason Zs matters is disconnection time. When an earth fault occurs, the protective device must operate quickly enough to limit the duration of dangerous touch voltage on exposed-conductive-parts. BS 7671 Chapter 41 sets the limits.

For TN systems with a nominal voltage to earth Uo = 230 V:

Regulation 411.3.2.2 applies the 0.4 s maximum disconnection time to two categories of final circuit, and candidates routinely remember only the first:

• a final circuit not exceeding 63 A with one or more socket-outlets; and
• a final circuit not exceeding 32 A supplying only fixed connected current-using equipment.

Regulation 411.3.2.3 then applies 5 s to distribution circuits and to any final circuit that falls outside those two categories — for example a 40 A fixed-equipment circuit, which exceeds the 32 A fixed-equipment threshold but is not a socket-outlet circuit.

The longer 5 s allowance recognises that fixed equipment presents a lower touch-voltage risk than something a person is holding.

Exam shorthand: 0.4 s for socket-outlet final circuits ≤ 63 A and fixed-equipment final circuits ≤ 32 A; 5 s for distribution circuits and everything else. Learn both thresholds — a question that gives you a 40 A fixed-equipment final circuit is testing exactly this distinction.

The protective device can only meet its disconnection time if the fault current is high enough — and fault current is dictated by the loop: I = Uo / Zs. The lower Zs, the higher the fault current, the faster the device operates. BS 7671 therefore publishes maximum Zs values for each device and rating.

Maximum Zs Tables (41.2 / 41.3 / 41.4)

BS 7671 Chapter 41 publishes maximum Zs in four tables, and they are split by protective device and disconnection time, not by earthing system:

So for an MCB you always use Table 41.3, whether the system is TN-S or TN-C-S; the earthing system changes the value of Ze you measure, not which table you read.

Every tabulated value is calculated at the maximum conductor operating temperature (70 °C for thermoplastic insulated CPCs run with thermoplastic phase conductors) with Cmin = 0.95 applied to the supply voltage. Cmin is the minimum voltage factor that ensures the calculation is conservative against supply voltage dips.

Type B MCB reference values

For Type B MCBs to BS EN 60898, 230 V, Cmin = 0.95, tabulated at 70 °C, the maximum Zs values and the corresponding on-site (×0.8) values are:

These are the values you will read off in the exam. Learn the pattern: doubling the rating roughly halves Zs.

The 0.8 Rule (GN3 10th Edition)

There is a catch. The tabulated values assume the conductors are at 70 °C (their maximum operating temperature under load). When you test on site, the installation is cold — typically around 10 °C ambient — because you test before the circuit has carried load for any length of time. Cold copper has a lower resistance, so a cold measurement that looks satisfactory might fail once the conductors warm up under load.

GN3 10th Edition therefore gives on-site maximum values = 0.80 × the tabulated value. The 0.80 factor approximates the resistance change from 70 °C down to 10 °C for copper.

Pass criterion

Measured Zs ≤ 0.80 × tabulated maximum Zs → PASS.

If measured Zs falls between 0.80× and 1.00× of the tabulated value, the simple rule cannot confirm compliance; you must apply the full temperature adjustment method of Appendix 3 (Regulation 411.4.203) to determine whether the circuit is satisfactory.

In practice, the 0.8 rule lets you make a clear pass/fail decision on site without doing the full temperature calculation for most circuits.

Type C and Type D MCBs

The Zs limit of a Type C MCB is half that of the equivalent Type B. A Type D is one-quarter. This is because Type C and D devices need a higher fault current to trip magnetically (they tolerate higher inrush), so the loop must be lower to push enough current.

This is exam-critical. A question may give you a Type B value and ask for the Type C or D limit — halve it, or quarter it.

Worked Example 1 — 32 A Type B MCB, PASS

A 32 A Type B MCB final socket circuit. Measured Zs at the furthest socket = 0.95 Ω.

• Tabulated max Zs (Table 41.3, circuit-breakers) = 1.37 Ω — that is 0.95 × 230 ÷ (5 × 32 A).
• On-site max (×0.8) = 1.37 × 0.80 = 1.10 Ω.
• 0.95 Ω ≤ 1.10 Ω → PASS.
• Disconnection time: a Type B 32 A MCB at this fault current disconnects well within 0.4 s (the 0.4 s requirement for a socket-outlet final circuit up to 63 A is met).

Worked Example 2 — 32 A Type C MCB, FAIL

Same circuit, but protected by a Type C 32 A MCB. Measured Zs = 0.95 Ω.

• Tabulated max Zs for Type C = 1.37 × 0.5 = 0.685 Ω.
• On-site max (×0.8) = 0.685 × 0.80 = 0.55 Ω.
• 0.95 Ω > 0.55 Ω → FAIL.
• The circuit would need a lower loop (shorter run, larger CPC, or a different device) to comply.

Notice the same measured Zs (0.95 Ω) passes on a Type B but fails on a Type C — the device type, not just the measured value, drives the verdict.

TT Systems — Brief Note

For TT systems the loop is completed through the earth electrode, so Ze is generally too high for overcurrent devices alone. BS 7671 Table 41.5 applies and protection is RCD-based:

Ra × IΔn ≤ 50 V (BS 7671 Reg 411.5.3), i.e. Zs ≤ 50 / IΔn

For a 30 mA RCD (IΔn = 0.03 A), Table 41.5 gives a maximum Zs of 1667 Ω — the standard TT limit, derived from the 50 V touch-voltage limit divided by IΔn (50 ÷ 0.03 ≈ 1667 Ω). Regulation 411.5.3 also carries a note that a value of RA exceeding 200 Ω may not be stable, so 200 Ω is the practical ceiling to design and verify to, whatever the RCD rating. Full earth electrode testing is covered in Chapter 9.

Worked pass/fail decision table

Chapter 8.2 summary

• 0.4 s for final circuits ≤63 A; 5 s for distribution and fixed-equipment circuits.
• Tabulated max Zs is at 70 °C with Cmin = 0.95 — Table 41.2 (fuses, 0.4 s), Table 41.3 (circuit-breakers), Table 41.4 (fuses, 5 s), Table 41.5 (RCDs).
• On-site pass: measured Zs ≤ 0.80 × tabulated.
• Type C = half Type B; Type D = quarter Type B — memorise.