The ornamental design for an extensioncableseparator, as shown and described.
FIG. 1 is a perspective view of an extension cable separator showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof; and,
FIG. 5 is a side view thereof.
The broken line showing of the front and rear bore holes is included for the purpose of illustrating a portion of the extension cable separator that forms no part of the claimed design.