The ornamental design for an outer casing for a high-voltage apparatus, as shown and described.
FIG. 1 is a front elevational view of an outer casing for high- voltage apparatus showing my new design; the rear elevational view being a mirror image thereof.
FIG. 2 is a partial right side elevational view thereof.
FIG. 3 is a left side elevational view thereof; and,
FIG. 4 is an enlarged partial top perspective view thereof.