Single-Phase Power:

The source of single-phase (1f) power in all facilities is through generation from a single-phase generator or through utilization of one phase of a three-phase (3f) power source. Commonly, every phase of the 3f distribution system is a single-phase generator electrically spaced 120 degrees from the other two; thus, a 3f power source is convenient and practical to use as a source of single-phase power.

Single-phase loads can be linked to three-phase systems utilizing two methods. The diagram display in Figure described these connections.

Figure: Three-Phase to Single-Phase Connections

The first scheme gives for the connection of the load from a phase leg to any ground point and is referred to as a phase-to-ground scheme. The remaining scheme connects the single-phase load among any two legs of the three-phase source and is referred to as a phase-to-phase connection. A choice of schemes, phase-to phase or phase-toground, permits various voltage options depending on whether the source three-phase system is a delta or wye configuration. That will be elaborate in the three-phase segment of this theory.

An only approved method of wiring single-phase power is the scheme generally referred to as the 3-wire, single-phase Edison system. The described in Figure depicts the use of a centertapped transformer, along with the center tap grounded, providing half voltage (120 V) connections on either side or full voltage (240 V) across the both sides.

Figure: 3-Wire Edison Scheme

The physical connections to the transformer secondary include two insulated conductors and one bare conductor. The conductor will be if the conductor is a current-carrying leg or neutral leg insulated. A remaining uninsulated conductor will serve as a safety ground and will be bonded to the ground point of the system.

Within all cases, 3 wires will be presented to the load terminals and the safety ground would be bonded to every junction box, or device, inside the distribution system.

In case of half voltage (120 V) use, the intended path of the current is from the supply leg by the load and back to the source on the neutral leg. No current would be carried on the ground unless a fault occurred in the system, in that case the current would flow safely to ground.

Within the full voltage system (240 V), the insulated conductors are linked across the full winding of the transformer and the uninsulated conductor is again bonded to the grounded center tap.

Inside a balanced system, all currents will flow on the insulated conductors, and the grounded neutral will carry no current, acting only in a ground capacity. Within case of either an unbalanced load or a fault in the system, a bare conductor will carry current, other than the potential will remain at zero volts since it is tied to the ground point. As per the case of the half voltage system and the uninsulated conductor will be bonded to every device in the system for safety.