Electronic – Benchtop circuit breaker

fusesmainspower supply

I repair electric appliances and tools. Every once in a while, the main circuit breaker trips when I plug in a device. I don't want to prevent a circuit breaker from doing its job, but I would prefer not reset the breaker at the panel every time it happens.

How could I configure this to prevent nuisance panel-breaker trips?
(I don't even know what search terms to use.)

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Bench test mains power supply.

How it works

  • The transformer XFMR1 is a 1:1 mains power transformer. You need to figure out what's the most powerful appliance you need to test and size accordingly. e.g., for 1000 W use a 1000 VA transformer.
  • The transformer will isolate the load from the mains and if there is an earth leakage fault on the appliance it will prevent your main RCD and the one on this setup from tripping. (More later.) It will also protect you while the EARTH TEST switch is open.
  • The lamp serves as a current limiting test device. Open the FULL ON switch before you plug in the device. The brightness of the lamp will give an indication of the current draw. If it goes to full brightness on a small appliance, for example, you may have a short circuit but current will be limited to 0.5 A (230 V AC lamp) or 1 A (120 V AC lamp). If all appears well then switch FULL ON.
  • If all is well then close the EARTH TEST fault switch. Now if there is an earth fault on the appliance some of the current will return on the earth-wire instead of the neutral (N). The RCD will detect a difference between the L and N current and trip out. This would usually only trip if there's an earth fault to a metal-cased appliance.
  • Size the RCD (current rating) to protect the transformer.

That should help you sort out most problems on the bench.

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