My previous post raised some questions on how does a three phase RCD work. If you've a three phase load then you can't just use three single phase RCD's attached to each of the phases. You'll need a specialist 3 Phase RCD such as this one from HSS.
Basically it works in the same way as a single phase one in that it's looking for a mismatch in current in and current out. It uses a "Core balance Current transformer" to sense this mismatch.
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