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Earth continuity, and working out the expected resistanceWhat's the most an extension lead can read?

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Earth continuity

?Continuity asks: is the protective conductor unbroken from the earth pin of the plug all the way to the accessible metal part? You want a complete path with very low resistance.
✋Class I only. A class II appliance has no earth, so there is no continuity test to do — a dedicated PAT tester set to class II won't even offer it. Extension leads get their own treatment, because class I equipment can be plugged into them.

The limits, from Section 10:

EquipmentMaximum
No supply cable0.1 Ω
With a supply cable (including extension leads and trailing sockets)0.1 Ω + R of the cable
3-core lead sets and extension leads0.1 Ω + R
Multiway adaptors and RCD adaptors0.1 Ω
"Supply cable" means the lead from the plug to the appliance — the bit you'd coil up to carry it.

In practice you put one probe on an exposed-conductive-part — the metalwork itself, not the protective conductor connection point such as the earth screw head. On a class I kettle that's the element housing; on a hot plate, the metal casing or trim. Where there are several, test each. Only exposed-conductive-parts need testing.

Flex the cable while you watch the reading. A value that jumps or climbs points to a poor connection. Painted or powder-coated parts can read high — repeat the test to get a better contact (10.5.2, Table 10.1 notes).

Section 10 · 10.5.2 · protective conductor continuity Table 10.1 · the 0.1 Ω rule

earth pin plug socket at the far end protective conductor — must be unbroken line and neutral run alongside tester measures across the whole length R of the cable
Class I only. An extension lead is tested end to end — earth pin of the plug to the earth contact of the socket — so the reading includes the resistance of the whole cable.

Working out the expected resistance

This is the one calculation in the module, and it's worth having cold.

R = (mΩ/m × length) ÷ 1000
Divide by 1000 because the table is in milliohms per metre — exactly like volt drop and mV/A/m

The mΩ/m figure for the cable is given in the question. The worked examples here use these figures:

CSAmΩ/m
1.25 mm²15.6
2.5 mm²8
4 mm²5
16 mm²1.2

A 12 m extension lead in 1.25 mm²

R = (15.6 × 12) ÷ 1000
R = 187.2 ÷ 1000
R = 0.187 Ω

Maximum permitted = 0.1 + 0.187 = 0.287 Ω

So your measured result should fall
between 0.187 Ω and 0.287 Ω

Two more, worked the same way

15 m of 4 mm²
R = (5 × 15) ÷ 1000 = 0.075 Ω
Maximum = 0.075 + 0.1 = 0.175 Ω

75 m of 16 mm²
R = (1.2 × 75) ÷ 1000 = 0.09 Ω
Maximum = 0.09 + 0.1 = 0.19 Ω
You work it out for one conductor, not two. Where a flex is marked "2 × 0.5", that means line and neutral are each 0.5 mm² — it isn't telling you to double anything. Earth continuity is measured along the protective conductor only — earth pin to the metalwork — so R is for that one conductor.
How do you know the CSA on site? It's printed or embossed on the cable. Also on the drum label and the wrapping.
Extension leads have a maximum length too. Table 10.3 (p.75): 12 m in 1.25 mm², 15 m in 1.5 mm². On a lead, polarity is checked by continuity as well (10.10(d), p.75).

Section 10 · Table 10.1 · p.71 · add 0.1 Ω Table 10.3 · extension lead lengths · p.75

12 m extension lead, 1.25 mm² — 15.6 mΩ/m R = (15.6 × 12) ÷ 1000 = 0.187 Ω · maximum = 0.1 + 0.187 = 0.287 Ω 0.187 Ω 0.287 Ω the cable itself + 0.1 Ω allowance below — check your figures PASS FAIL A reading under 0.187 Ω cannot be right for this cable — recheck the length, the CSA, or whether the leads were nulled.
The answer is a band, not a single number: at or above the cable’s own resistance, and no more than 0.1 Ω above it.

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