RCD не должен срабатывать?

"Multimeter Testing: Measure voltage potential between the fluid/pipe and a true earth ground; readings above 0.5V typically indicate active stray current."

вот ещё от gemini:
Inspect the Immersion Heater: If your Monsoon pump handles a hot water cylinder, a split element in your immersion heater is the #1 culprit for leaking electrical current directly into the hot water supply channel of the pump.