Schweitzer Relay- 51a & 51b

[Humble2000]

Under relay settings for 50 , I see two settings 50a,50b
50Na and 50Nb, what are these parameters represent?

 

[stevenal]

C37.2 Annex A gives several examples with schematics.

Broken delta VT secondary connection - N

Current in grounded wye neutral of transformer - G*
* G is preferred, past practice make N acceptable

Generator with high-impedance ground - G

Residually connected CT secondaries (or internally derived by device) - N

Tank ground - G

CT in delta tertiary of auto transformer - N

Donut CT enclosing three phases - G

You can see that a relay manufacturer can only guess how the fourth current input might generally be used. The same pair of terminals might be used for a donut CT, residual connection, tank ground, tertiary, or grounded neutral CT in different applications.