What is the difference between "Sum 3I" and "Input Io" when setting the ANSI 50/51N protection in a Sepam relay?

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When setting the ANSI 50/51N protection, the Sepam relay provides 2 different options for residual current type: "Sum 3I" and "Input Io".
ANSI 50/51N protection Sepam
The residual current type depends on the wiring of the current sensors to the relay. If, for example the relay is wired with a standard 3-phase current connection without wiring the Ir input of the CCA634 or using a CSH zero-sequence CT, the "Sum 31" option needs to be selected. The measurement of the 3-phase currents allows the "calculation" of residual current.
3 CT connection Sepam

Note: A 2-phase current connection will not calculate residual current.

If, for example, the CCA634 Ir input is wired or a CSH zero-sequence ct is used, the Sepam will "measure" the residual current and "Input Io" needs to be selected when configuring 50/51N.