Why does an error message appear when an RCD is tested with DC?

If a type A or AC is set, you cannot perform a test with DC, because these switches are not suitable for this.

If you want to perform the measurement with the 2-pole adapter, an error message also appears.

For testing with DC, the N conductor is always required as well, since the DC fault current is generated between N and PE. So use the Schuko insert or contact the third line at the socket of the 2-pole adapter and contact the N conductor with it.

In networks without N conductor, this test cannot be performed. Lead an N conductor to the RCD and lead an additional line from the N outlet of the RCD to the profit test, then you can perform the test.