Supplementary bonding in bathrooms, and when you can avoid itWhen can you leave out bathroom bonding?

Unlike main bonding, supplementary bonding can often be avoided. The classic example is a bathroom, meaning any room containing a bath or shower. The fan, light and shaver point are often on the lighting circuit, joined by its CPC back to the MET. The bath taps, radiator and sink taps are on copper pipes that connect to the main water and gas, which should have main bonding downstairs.

Why a bathroom is an area of increased risk

• It's usually a small, confined space, full of things connected to extraneous conductive parts: taps, radiators, baths, shower trays and towel rails.
• People are likely to be undressed, so more skin is exposed and there's more chance of touching, say, a radiator.
• Skin is likely to be wet, so body resistance is much lower.

Imagine an earth fault at the shaver point. Even with main bonding in place, a person touching the shaver point and a tap could still see a potential difference, and they're in a much riskier situation.

One way to check the third condition: the resistance between the MET and each extraneous conductive part is less than 1667 Ω. Measure it with a low-resistance ohmmeter using the wander (long) lead, test method 2.

R ≤ 50 V ÷ IΔn (Reg 415.2.2), the same 50 V figure
as Table 41.5 (Zs × IΔn ≤ 50 V)
30 mA RCD: 50 ÷ 0.03 = 1667 Ω
→ no more than 50 V (the touch voltage limit) can appear between a faulty
  exposed part and an extraneous part before the RCD disconnects.

BS 7671 · Reg 701.415.2 supplementary bonding · p.258 (Section 701 starts p.257) BS 7671 · Table 41.5 max earth fault loop impedance for RCDs (30 mA → 1667 Ω) · p.77