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This is a UK install for reference.

Long story short is I essentially have a 13A plug wired direct to an RCBO. Output of the RCBO is connected to a 13A trailing socket.

There's a multimeter inline between the earth of the 13A socket and plug. Functional earth is connected to the earth pin of the 13A plug.

With the RCBO off, as expected, the current on earth is 0A. Turning the RCBO on causes a current of 6mA to appear on the earth.

Why is that?

  • How is the RCBO connected to earth? In the States we don't normally connect GFCI breakers to earth so it would surprise me that it would even have a route to leak. – Harper - Reinstate Monica Jun 11 '17 at 23:12
  • @Harper The functional earth isn't actually required in the UK to meet the current standards and some manufacturers do not provide a functional earth on some RCBOs. However, they add a second layer of protection because if for some reason the neutral is lost in the circuit the RCBO call still detect the fault and trip. The argument against is that this would be two different faults happening at the same time and as such is very unlikely but it does serve a purpose. – Ryan Walkowski Mar 11 '18 at 1:06
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Most devices have small amounts of earth leakage, electronics, computers are good examples. The casings of the devices are metallic they are bonded to the earth. EM radiation from components will work its way to the case and pass as earth leakage. They could be designed not too but the cost is prohibitive when it serves no real purpose as they are low enough to not cause a real problem.

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