The ornamental design for a fencepostinsulator for plastic coated electrical wire, as shown and described.
FIG. 1 is a front, top, left side perspective view of a fence post insulator for plastic coated electrical wire, showing my new design in a state of use;
FIG. 2 is a rear, bottom, right side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a left side view thereof, shown in a state of use;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a top view thereof.
The broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.