A double-pole socket is used for 240-volt circuits, has four terminal screws to connect two 120-volt hot wires, and always has a ground terminal screw. Wiring a double-pole socket is similar to wiring two single-pole sockets separately.
Double- and single-pole sockets look similar except for their wiring terminals. The grounding terminals of double-pole sockets are connected to the building's main grounding system to absorb electrical surges and short circuits harmlessly, which is typically a solid copper rod driven into the soil under the main electrical panel.
Grounding wires are also required to complete the electrical circuit loop of double-pole sockets.