two sets of cables and wires, which led to considerable annoyance in the field when equipment from different vendors could not be plugged together. By 2011, the MC4 was already in a strong leadership position, which led to the introduction of compatible products from a variety of major connector vendors. Among these are the Amphenol Helios H4 and SMK PV-03. Description The MC4 system consists of a plug and socket design. The plugs and sockets are inside plastic shells that appear to be the opposite gender - the plug is inside a cylindrical shell that looks like a female connector but is referred to as male, and the socket is inside a square probe that looks male but is electrically female. The female connector has two plastic fingers that have to be pressed toward the central probe slightly to insert into holes in the front of the male connector. When the two are pushed together, the fingers slide down the holes until they reach a notch in the side of the male connector, where they pop outward to lock the two together. For a proper seal, MC4s must be used with cable of the correct diameter. The cable is normally double-insulated (insulation plus black sheath) and both UV and higher temperature resistant (most cables deteriorate if used outdoors without protection from sunlight). Connectors are typically attached by crimping, though soldering is also possible. The MC4 connector is UL rated at 20A and 600V maximum, depending on the conductor size used. Standards efforts in Europe also allow special 1000V versions and 30A or higher when used in pairs. Application and safety MC Multilam Technology claims that constant spring pressure provides reliable low contact resistance. However, it is very important to never connect or disconnect them under load, even on low-voltage (12–48V) systems. An electric arc may form which can melt and seriously damage contact materials, resulting in high resistance and subsequent overheating. This is partly because direct current (DC) continues to arc, whereas commonly used alternating current (AC) more readily self-extinguishes at the zero-crossing voltage point. Large arrays of panels are commonly interconnected in series, made of strings of panels generating 17 to 50V each, with overall voltages up to 600V per string or 1500V in special large arrays with high voltage panels. Connectors made by other manufacturers may be mated with original Stäubli parts and are sometimes described as "MC