In effect these type of machines have a"Dead Mans Handle" which is a sort of misnomer. However in effect you usually can't start the machine until this handle is depressed and conversely when the handle is released the drive to the 'engine' cuts off.
So I guess there is some small comfort that when one falls out of the tree at least one shouldn't be in danger of cutting ones arm off in the process!!!!
i would use the plug in type of RCD ( bout �7-8) from argos or B&Q make sure that it has a 30 milliamp trip setting (ask them if your not sure, most are by default anyhow) that way should you cut the cable by accident it will have tripped out before coming back to shock you.