First of all, our HS200 have four lines for connection: one live line, one load line, one neutral line, and one ground line. https://www.tp-link.com/support/faq/1128/ There are 3 lines must be connected: one live line, one load line and one neutral line. Our HS200 work principle is to control the load line, which intends to control the appliances that connect to our HS200. But when you replace HS200 with your old switch, there is a great probability to find that your powerline socket only contains 3 lines (or less), the reason is some old switches don’t need Neutral Line to make it work. The main reason is, not only our HS200 is a switch, but also it is a powerline appliance. So please find a Neutral Line in your power socket for connection. Note1: Please don’t twine two Live Lines (Live Line and Load Line), and connect them to your socket powerline. Note2: If you really can’t find out a Neutral Line in your Powerline socket, please contact professional electrician to reform your socket powerline, or you can’t install our HS200. For a better connection, we recommend you don’t forget to twine wire nuts to make sure the copper conductor is fully concealed.