This mower ran well till the owner mowed a tennis ball. Now it starts right up,sounds good and within a second blows the house circuit breaker. When it's not running, the blade turns freely. There is no visible tennis ball under there, and no accumulation of grass. The plastic cowell over the engine is loose, sitting on crookedly (I haven't investigated further yet). I use a heavy extension cord; my other two B&D electric mowers run well on the same cord, same circuit, and same level lawn. Thanks for the help! What could be shorting or over drawing the current?