If there is nothing connected and this specific plug trips the ELU, then there could be some carbon floating around in the switch housing where the wires are connected which could affect the plug. Had a problem years ago on an old style thermal overload on a contactor, don't know how but there was a high impedance between the live and the plate causing the ELU to trip. Could only attribute it to dust/carbon/humidity what ever, once replaced it the tripping went away. Could not measure it with a high impedance ohm meter.
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