Main switch / incomer not making on all phases
A board's main switch or incomer isn't passing all phases — downstream gets partial supply (single-phasing) or nothing — pointing at the switch contacts or its terminations.
Safety first
The supply side of a main switch/incomer is live even with it open. Isolate upstream where possible, prove dead, and treat as licensed, high-energy 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
Main switch pole not making
Most likelyOne pole of the main switch isn't closing/conducting, dropping a phase downstream.
- 2
Loose/burnt incomer termination
#2A loose or burnt incoming/outgoing terminal on one phase opens or partially opens it.
- 3
Upstream supply loss of a phase
#3A phase is already missing on the supply side (service fuse/supply authority).
- 4
Failed main switch
Least likelyThe switch itself has failed mechanically/electrically.
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.
Measure all three phases on the supply side and load side of the main switch.
Three phases present both sides when the switch is on.
All phases pass — the loss is downstream; trace from the board.
A phase present on supply side but missing on load side → the main switch/termination is the fault.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Incomer not making all phases
→ step 2 - 2decision
Are all three phases present on the load side of the main switch?
Yes→ step 3No→ step 4 - 3result
All phases pass — the loss is downstream; trace it.
- 4decision
Are all three phases present on the supply side?
Yes→ step 5No→ step 6 - 5result
Switch pole/termination dropping a phase — rectify/replace.
- 6result
Phase missing at supply — escalate upstream (service/authority).
Common mistakes apprentices make
- Assuming a downstream fault when the main switch is dropping a phase.
- Not measuring both sides of the switch.
- Overlooking an upstream/service phase loss.
- Working on the live supply side without proper isolation.
When to stop & escalate
Supply-side/service phase loss is the supply authority's domain. Main switch/incomer rectification is high-energy licensed work — isolate properly.
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
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
No power to the whole house
The entire home has no power — nothing works. Could be a supply outage, the main switch/main safety switch, or a main fault, and the first job is to tell which.
Phase-failure / monitoring relay has tripped the circuit
A phase-failure or phase-sequence monitoring relay has dropped out and is holding the control circuit off, stopping the equipment — even though the panel looks powered.