I am thinking of rewiring some of my workshop with a RCD spur for my machine tools and bench and an emergency stop. I use an ESD plug to ground an antistatic mat when working on electronics on the bench.

The RCD spur will have something in the earth line to sense current - is this going to have any effect on the effectiveness of the ESD ground, or should I add another ground for ESD use? Is having a second return path going to reduce the effectiveness of the RCD for the machine tools (not that I normally use both at the same time)?


Don't quote me but.... an RCD doesn't measure earth current directly - it infers it by measuring the "difference" current between the two AC wires. This difference current has to be an earth current whether it's thru the earth pin or thru the guy's hand who's building a big spark generator (related to a different question!!!).

This means it doesn't alter the earth impedance in any way

  • \$\begingroup\$ I recently disassembled an RCD and can confirm that at least some types don't measure ground current at all, just the difference. This one was a 3-phase model and the phase and neutral wires were wound on a core as a single turn transformer. The other side of the transformer had a thinner winding with more turns which was connected to an electromechanical actuator which would trigger and cut off phases once current through it exceeds certain level. \$\endgroup\$ – AndrejaKo Mar 27 '13 at 22:42
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    \$\begingroup\$ This how all RCD's work. \$\endgroup\$ – RJR Mar 27 '13 at 23:00
  • \$\begingroup\$ @AndrejaKo sounds about right dude \$\endgroup\$ – Andy aka Mar 27 '13 at 23:01

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