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I supposed there is a sense coil inside the RCCB (RCD)of this tankless water heater. But why is there a fuse inside one of the load side to the circuit? Isn't the round circle the sense coil where the 2 load wire passes? Why isn't the fuse in one of them?

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The fuse is part of the RCCB control circuitry.

I suppose this is an North American system where RCCB and GFCI require selftest functionality. (UL 943)
The control circuit performing this test has a fuse to prevent fire in case of a failure.
This selftest is a regular intentional current across the sensing coil to make sure it detects.

It is wired on the load side so it will also be off in case of a fault.

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  • \$\begingroup\$ Looking at the colors, terminology and general construction, it seems more IEC than American, but it still looks pretty unusual. \$\endgroup\$ Commented Jun 11, 2021 at 23:50

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