Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
What it is
A distribution board takes an incoming supply and splits it into a number of protected final circuits. In larger installations, a main board feeds sub-boards via sub-mains, branching power out to where it's needed.
How it's arranged
The incoming supply arrives at a main switch, runs along busbars, and is tapped off through protective devices (breakers, RCBOs) to each circuit. A sub-main is a larger cable feeding a downstream board, which then distributes further.
Protection is coordinated so a fault trips the device closest to it (discrimination), ideally without tripping everything upstream. A clear, accurate board schedule tells you what each device controls.
Why it matters
Understanding the hierarchy — supply → main → sub-main → sub-board → final circuit — lets you localise a fault to the right level. Hot busbars/connections are a fire risk; mislabelled boards are a safety hazard; poor discrimination takes out more than it should.
Safety first
Boards are high-energy — isolate and prove dead, and treat hot busbars/connections as an urgent fire/arc-flash risk. Licensed work.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Busbar or board connection running hot
A busbar joint, incomer, or board connection runs hot / discoloured — a high-energy fire-risk fault on a distribution board that must be addressed quickly.
Sub-main keeps tripping the main board
A sub-board's incoming protective device (or the main feeding it) trips — taking out everything downstream — and you need to tell overload from a fault on the sub-main or sub-board.
Distribution board circuits mislabelled / wrong breaker isolates wrong circuit
Switching off a labelled breaker doesn't isolate the expected circuit (or isolates the wrong one) — a schedule/labelling problem that's a real safety hazard for anyone relying on it.
Related definitions
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
RCBO
Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.
Isolator / disconnector
A switch whose job is safe isolation — visibly and securely disconnecting a circuit for work.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.