the sum of the 4 (signed) currents at any time in a 3 phase system should be zero (or for those who have trouble visualising negative currents total out must equal total in)
any devitation from this causes a magnetic field
if this field get big enough the thing trips in roughly the same way as the mag trip on a mcb
What ever goes out on the RED (sorry BROWN) will come back in the neutral. (or if the YELLOW and BLUE phases are loaded, the RED current will flow back the yellow, blue and neutral.)
What ever goes out the rcd, will come back through it on one phase or the neutral or a mixture of all. Any current missing coming back (ie gone to earth) will cause a tiny magnetic field and induce a current into the trip winding, and operate the RCD.
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