Hubbell offers a broad range SPD receptacles with technology that is unsurpassed. Inside Hubbell SPD receptacles are two 22mm square MOVs providing 240 joules of surge protection for each mode. The nylon component shield protects the printed circuit board from contaminants. An all-glass, conformally coated PC board provides superior moisture immunity for longer life in humid environments. In February 1968, Hubbell patented the first isolated ground receptacle. Today - when a clean, noise-free ground is more important than everHubbell is still setting the standard. Hubbell uses insulation barrier construction on many models to isolate the ground contacts from the mounting strap. The green grounding screw is connected directly to the grounding contacts. In this way, ground contacts are separated from the mounting strap and also from the conventional grounding system. The isolated ground circuit is completed by running a dedicated insulated ground wire from the system ground buss to the green grounding screw.