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Put simply a coupler is a large industrial version of the electrical plug and socket system you use in your home. While the plugs in your house use single phase power and have a power, neutral and ground contact; most industrial systems use a three phase system and include additional safety features due to the higher voltage and power draw. The result is that couplers have three power contacts, one or two ground contacts depending on the application, and up to four ground-check (pilot) contacts. The pilot circuit(s) is used to monitor the continuity of the ground wire from the power source to the end of the cable where the load is located. The pilot circuit is strictly a safety device,  the pilot contacts within the coupler are designed to physically open before any others causing the breaker at a remote location to open. However the circuit is not designed to be used as a switch and the circuit breaker should be visibly open before operation of the coupler. Opening an energized coupler may result in injury to person or equipment.