The utility model provides a special test instrument that is used for large -scale transformer and large -scale switch direct current resistance. Transformer bushing tests problem dangerous when working a telephone switchboard, that work efficiency hangs down in mainly solving current large -scale transformer maintenance and handover process. Its characterized in that: lead screw (5) is connected through universal joint (6) in insulator spindle (7) upper end, the top of lead screw (5) with clamp down mouth (2) continuous, lead screw (5) external connection has nut (4), insulator spindle (7) outside cover has sliding sleeve (9), and sliding sleeve (9) external connection has telescopic guide rail (3), and guide rail (3) and sliding sleeve (9) junction be equipped with angle stopper (8), guide rail (3) upper end connect last clamp mouth (1) on, and guide rail (3) link to each other with nut (4), on clamp mouth (1) and clamp under with and all be connected with terminal (11) on mouth (2). This testing tool can be convenient carry out test wiring of large -scale transformer bushing and resistance test, the operation is safe convenient, improves work efficiency.