Beyond the basic safety features for CAT II and CAT III measurement categories, the 3255-50 is distinguished from common testers by special features to protect against the unlikely event of a catastrophic short circuit resulting from failure of the tester circuit itself: internal current limiting suppresses short-circuit current and a fast-blow fuse (rated at 10 kA handling capacity) quickly and reliably breaks the circuit. High-current power lines are particularly conducive to instantaneous over-voltage conditions resulting from rapid load changes. The tester circuitry in the CAT III version of the 3255-50 is designed to enable all measurement modes to sustain over-voltages up to 6 kV. In addition, the input stage of the tester is equipped with a current-limiting resistor and fast-blow fuse to protect against short-circuit conditions that could result from an unlikely fault within the tester itself, thereby providing enhanced safety in the event of such a catastrophic failure. Above the special protective components, the multi-level safety design includes polarized terminals to prevent incorrect insertion of test leads, and dust- and water-proofing (IP54) to enable usage even in harsh environments

Functional differences from the predecessor Model 3255-50 include support for safe and simple AC load current measurements on active lines using an optional clamp-on probe. Simply match the range of the clamp-on probe with that of the 3255-50, and clamp over a conductor to read load current directly. Furthermore, the maximum display count has been increased from 3249 to 4199, extending the dynamic range of all measurement modes.

High-current power lines are particularly conducive to instantaneous overvoltage conditions resulting from rapid load changes. Comprehensive safety measures are desirable in testers to be used in such environments. Multi-level protective features in the 3255-50 include protective components in the tester circuitry and a current-limiting resistor and fast-blow fuse in the input stage to provide enhanced safety in the event of catastrophic failure.