On December 30, 1997, at 1400 hours Pacific standard time, a Cessna 172RG, N5404V, was substantially damaged during landing with the gear partially extended at the Van Nuys, California, airport. There were no injuries to the flight instructor or student pilot. The flight departed Van Nuys on a local instructional flight and no flight plan was filed. Visual meteorological conditions prevailed for the personal flight. The flight instructor stated in his report of accident that the student pilot had extended the landing gear on the downwind leg of the traffic pattern, but then retracted it after being told to go around by the control tower. On the second downwind leg, the student again placed the gear switch in the down position; however, neither the student pilot nor the instructor visually verified that the gear was down and locked. During the final flare for landing, the instructor noted that the gear was not down and that the 5 amp gear warning circuit breaker had opened. He then closed the circuit breaker and noted the gear starting to extend but it had achieved only an intermediate position at the time of touchdown. The airplane subsequently settled on to the runway and incurred damage to the underside, the right wing, and the right horizontal stabilizer. A review of the Cessna 172RG Service Manual, Drawing No. 2471000, "Wiring Diagram-Landing Gear System" indicated that subject circuit breaker, in addition to providing power to the gear indicator lights and warning horn, also powers the main gear downlock switches and the stall warning horn. This was verified by a telephone conversation with the Cessna Aircraft Engineering Department.