Right. the better sniffers can be adjusted for sensitivity. The one I have used an oddball wafer type battery. NFG when the battery is dead, in fact, dangerous. If conditions are right, you can check polarity on an ungrounded receptacle. If they've installed a polarized receptacle, like a 3 slot with no ground, the sniffer will tell if the narrow or wide slot is energized. Back to the OP, yes, I try to test 240 outlets if they are accessible, sorry.