I would also add that to have any general lighting circuit in a dwelling unit loaded to 15 amps would be an extremely rare situation and this is one of the problems with this type of tester.
If it could apply say 4 to 8 amps it would be more realistic but I would say in my house most general lighting circuits (excluding the kitchen and bathroom) never see more than 2 to 3 amps in a normal day.
Roger
