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Proving dead: where the probes actually goTop or bottom of the main switch?

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A favourite question. The rule is the same on every board: test on the load (busbar) side of the main switch, whichever end that is on the board in front of you, or in the busbar chamber.

SystemWhere you testWhy
Single phaseLoad (busbar) side of the main switch — usually the bottom on a consumer unit, where the supply lands on topThe busbar links all the breakers and RCDs, so the load side of the main switch proves every circuit
Three phaseLoad (busbar) side of the main switch, or the busbar chamberWhich end the supply comes in varies between boards, so find the busbar side on the board in front of you
✋At the board is not enough. Go to the point of utilisation as well — the furthest point — and prove it dead there too, because of borrowed neutrals. A neutral taken from another circuit can leave your circuit live at a point while the board reads dead.

On larger three-phase installations there may be an open busbar chamber above the board carrying the line conductors. That gets safely isolated too.

SINGLE PHASE · EXAMPLE supply in at the TOP MAIN SWITCH TEST AT THE BOTTOM the load side the busbar links the bottom of every breaker THREE PHASE · EXAMPLE supply in at the BOTTOM busbar chamber, if fitted MAIN SWITCH TEST AT THE TOP or the busbar chamber supply comes in here
The side you test is the side the busbar is on, and which end that is depends on the board, so look before you probe. On this single-phase example the supply lands on top and you test underneath; on this three-phase example the supply comes in underneath and you test on top. If there is an open busbar chamber above the board, that is live too and gets isolated with everything else.

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