Plug-in transient, surge, lightning, amp spike protection for 24V systems. To use, plug the system's 24VAC transformer output leads onto the PRO-24's line side wires (the side with the inline fuse). Plug the PRO-24's load side wires where the transformer's leads used to go. Connect green wire terminal to a secure ground point. Insert proper fuse / place sticker nearby. Enjoy continuous protection.