OTC Tools OTC 3639 Extra Long Heavy Duty Circuit Tester. OTC Tools. Long 7" probe shank allows access to difficult to reach areas for testing electrical circuits and locating shorts, grounds, and open circuits. Features and Benefits 12-foot, heavy-duty, red coil cord lead. Checks 6- and 12-volt systems. Ergonomic, slim designed clear handle. Bright red, slim style replaceable bulb. Positive contact molded strain relief. Heavy-duty, stainless steel probe with insulated shaft. Heavy-duty clip with positive soldered connection. Email for product availability.