All commercial two-way radio power leads have fuses in the Pos and Neg leads. This is to protect against reversal of power leads when the aerial is connected - there is then a path from the Pos Battery, through the negative lead, the radio chassis, antenna coax shield to vehicle earth !!!

Adding a Zener diode provides no protection here ! The zener is useful for overvoltage - but is not needed in the FT817 for reverse protection. There are internal series diodes in the power (D1085)and charging (D1083) circuits to isolate any external reverse voltage. 

The battery pack is also connected via a series diode (D1084). On current production there is now a PC-board mounted 3A fuse, as well as a plastic shield attached to the battery case to prevent batteries shorting to the case while being inserted.

The FAQ should be updated with this warning to fuse the Neg Power lead

Mike VK2BMM B.Eng.