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.
Safety first
Sub-mains carry significant current. Isolate and prove dead; a sub-main fault can be high-energy. Licensed electrical 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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Downstream fault on a sub-board circuit
Most likelyA short/earth fault on one of the sub-board's circuits trips the sub-board incomer or the main.
- 2
Sub-board overloaded
#2Total sub-board load exceeds the sub-main/incomer rating, tripping after running.
- 3
Sub-main cable fault
#3Damage, moisture, or rodent damage to the sub-main cable itself.
- 4
Discrimination / coordination issue
Least likelyProtective devices not coordinated, so an upstream device trips before the downstream one.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Note when it trips (instantly vs after load) and isolate the sub-board's circuits, then restore to localise.
Sub-main holds with the sub-board circuits isolated.
Restore circuits one at a time — the one that drops it has the fault.
Trips with all sub-board circuits off — suspect the sub-main cable or overload.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Sub-main keeps tripping
→ step 2 - 2decision
Does the feed hold with the sub-board circuits isolated?
Yes→ step 3No→ step 4 - 3decision
Reintroducing circuits, does one drop the feed?
Yes→ step 5No→ step 6 - 4result
Trips with all off — test the sub-main cable / check overload.
- 5result
That circuit has the fault — rectify it.
- 6result
No single circuit — sub-board overloaded; review the load.
Common mistakes apprentices make
- Resetting the feed repeatedly into a downstream fault.
- Not isolating the sub-board to separate the cable from the load.
- Overlooking a discrimination/coordination problem.
- Upsizing the incomer instead of finding the fault.
When to stop & escalate
Sub-main cable faults and protection coordination are licensed (and sometimes design) work. A genuinely overloaded sub-board needs a load/sizing review before any rating change.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
MCB (circuit breaker) keeps tripping
A circuit breaker trips repeatedly — instantly on reset, or after a load runs for a while — and you need to tell a short from an overload from a faulty breaker.
Three-phase distribution board badly unbalanced
One phase of a three-phase board runs much hotter / higher current than the others — nuisance tripping on that phase, a hot neutral, or a warm phase conductor at the board.
No power to a shed or outbuilding
A shed or outbuilding has lost power — points at the sub-circuit/sub-board feeding it, the submain, a tripped protective device, or moisture.
Learn the theory
How the gear and circuits behind this fault actually work.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
Discrimination (selectivity)
Arranging protection so only the device nearest a fault trips — not the whole board above it.
Maximum demand
The realistic peak load a supply has to carry — the figure that sizes the mains, the main switch and the protection.