Plug in , it’s that simple … I’m no electrician , but this was my first port of call before I called one out to assess my socket which had been occasionally tripping the main board … I plugged this in and it diagnosed the defect straight away , Someone had put the Live and neutral in the wrong way .. So it was easy enough to fix without spending money on a call out charge … It’s also really good just to help you with turning off the correct breaker , because it sets off a tone when plugged in , so if your working yourself once the tone goes off you know you have turned off correct switch … For the price it’s definitely worth having as an extra safety measure if doing minor repairs ..