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Supply polarity, and the Edison screw lampThe lamp still lights. Is it safe?

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Supply polarity: three tests, or ten

Circuit polarity is proved during continuity, dead. Supply polarity is the live one.

SupplyTests
Single phase3
Three phase10

The Edison screw lamp, and why line-to-neutral will not find it

Guidance Note 3 · 2.6.12 · p.77. The polarity test makes sure every fuse and single-pole control and protective device is in the line conductor, and that the centre contact of a screw lampholder is connected to the line (the book excepts E14 and E27 lampholders to BS EN 60238). This is the clearest example of why polarity matters, and why the centre contact needs a check of its own.

Part of the lamp holderCorrectly connects to
The screw shellNeutral
The centre pin at the bottomLine

Now swap the brown and the blue

Does the lamp light?   Yes. It works perfectly.

So a line-to-neutral test tells you nothing — you have two live
conductors and they are both connected.

What has actually happened: the screw shell is now live,
permanently — the part you touch when you change the lamp.
The centre contact gets its own check

An R1 + R2 test (method 1) does not prove that the centre contact of an ES lampholder is the line. That needs its own check, between the centre contact and the line (Guidance Note 3 · 2.6.12 · p.77; the book excepts E14 and E27 lampholders to BS EN 60238). It is the same lesson as the ring final circuit: one reading on its own does not prove polarity.

Correct screw shell = NEUTRAL centre pin = LINE safe to change a bulb Brown and blue swapped screw shell = LIVE centre pin = neutral ⚠ the bit you touch is live The lamp lights either way — so a line-to-neutral test proves nothing. Only a polarity test finds this.
Both conductors are connected and the circuit works, so every continuity and function check passes. The fault only shows when someone unscrews the bulb.

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