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A recent job

Three-phase main switch fault in Double Bay

Peter's own video from the job, filmed on his phone in the main switch room: how a fault upstairs turned out to be the main switch downstairs.

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In short

In October 2026, Ambient Electrical's Peter Stafford was called to a property in Double Bay where the three-phase isolator on a sub-board upstairs, which controls the oven and power circuits, was not working properly. He traced the fault to the 32 A three-phase main switch downstairs: one phase was not getting through it. The fix is to replace it with a new 32 A three-phase main switch from a reputable brand.

Watch Peter's video from the job

Filmed on Peter's phone (4:06). It plays from YouTube when you tap it.

What was wrong?

The three-phase isolator on a sub-board upstairs, which controls the oven and power circuits, was not working properly. Peter believes it also controls the cooktop.

How did Peter find the fault?

He went down to the main switch room and tested the three-phase main switch on both sides. On a three-phase supply, the voltage between any two phases should read about 415 volts.

  • Tested the 32 A three-phase main switch in the main switch room, on its supply side and its load side
  • Supply side: 415 volts between every pair of phases, as it should be
  • Load side: 415 volts between two of the phases, but 240 volts across the other two pairs, where 415 should be
  • Found no continuity through the main switch on the third phase (the blue cable)

Why did a fault upstairs come from downstairs?

The sub-board upstairs is fed through the main switch. With one phase not getting through the main switch, the circuits on that phase upstairs could not work properly, even though nothing was wrong with the sub-board itself. In Peter's words, it is an unusual situation, but it does happen from time to time.

What is the fix?

The fault was in the main switch downstairs, not the sub-board upstairs. The fix is to replace it with a new 32 A three-phase main switch from a reputable brand. In NSW, replacing a main switch is licensed electrical work, certified with a Certificate of Compliance for Electrical Work (CCEW).

Peter's advice on main switches

The faulty switch was a cheaper one that had been installed before. Peter's advice is to use a recognisable brand, such as Schneider, Hager or Clipsal (or NHP, to a lesser extent), and to steer clear of cheaper versions for this reason.

My recommendation, folks, is to stay with the reputable brands.

Peter Stafford

Peter does electrical fault finding across the Eastern Suburbs, and main switches are part of every switchboard upgrade. If you need an electrician in Double Bay, call 0415 684 120. More jobs are on the projects page.