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As is documented here among other places, the Line Impedance Stabilization Networks used to make measurements of conducted EMI as per CISPR 22, FCC Part 15, and suchnot contain large line-to-ground capacitances that cause significant mains frequency current to flow to the equipment grounding conductor (my computations show this current is in the vicinity of 15mA steady-state). This means that they cannot be plugged into outlets that are ground-fault protected for personnel (5mA differential trip current as per UL 943 Class A).

In a building where GFCI protection of this nature is provided everywhere (using the NFPA 70 406.4(D)(2) GFCI retrofit provisions to compensate for missing equipment grounding conductors in an older building, for instance), how can I measure the conducted EMI of my designs, given that simply plugging a LISN in guarantees gloomy sadness?

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You have to use an isolation transformer, and you should also isolate the ground of your test area from the building ground.

If you can't isolate the ground of the test area, then you need to make sure that the capacitive coupling through the isolation transformer does not allow too much ground current to flow.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The isolation transformer makes sense, at least -- I have read it mentioned before. What do you do when there's no building ground there to hook your test setup to, though? \$\endgroup\$ – ThreePhaseEel Jan 8 '15 at 1:21
  • \$\begingroup\$ Then it's isolated, which is what you want. Remember, this is just for testing conducted emissions; you won't actually be using this setup in normal use. But if your LISN isn't connected to building ground, then why is ground current an issue to begin with? \$\endgroup\$ – Dave Tweed Jan 8 '15 at 2:07
  • \$\begingroup\$ then the LISN case is live enough to bite a bit, no? (The documentation I have read on them says that they need to be connected solidly to the equipment grounding system for safety's sake...) \$\endgroup\$ – ThreePhaseEel Jan 8 '15 at 4:52
  • \$\begingroup\$ The LISN case is only a danger if you get between it and the actual building ground. \$\endgroup\$ – Dave Tweed Jan 8 '15 at 5:10

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