A Coast Guard inspector testified here today that a loose steering motor switch could have caused a sulfur tanker to strike the Benjamin Harrison Bridge near Hopewell, Va., last month.

Cdr. K. L. Yancy said the switch was improperly seated and could have caused the motor to kick out.

Yancy, of the Coast Guard's marine inspection office in New Orleans, testified as the Coast Guard resumed its hearing to determine why the tanker Marine Floridian rammed the bridge instead of staying in the James River channel and passing beneath its raised draw span.

The hearing had been adjourned for three weeke while Yancy, at the request of the hearing officer, made an independent investigation of the ship.

He said there was play in the switch that is used to transfer power from one steering motor to a backup motor.

Yancy said "the only logical conclusion you can come to" to explain how the switch could have moved in such a way as to increase the load on the motor and cause the circuit breaker to trip is "through vibration."

During the first phase of the hearing the ship's master had testified that the rudder would not turn to steer the vessel beneath the draw-span.

A port engineer from New York had, like Yancy, blamed the problem on the steering motor switch. But a General Electric Co., field engineer had testified that while the switch was "sloppy" it is unlikely it could have vibrated loose so that steering power would be shut off.

The bridge was severed when the tanker struck it Feb. 24, cutting off Hopewell's only passage across the James River.