A shared neutral is a connection in which a plurality of circuits use the same neutral connection. This is also known as a common neutral, and the circuits and neutral together are sometimes referred to as an Edison circuit.
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In a three-phase circuit, a Neutral is usually shared between all three Phases. Commonly the system Neutral is derived from the star point on feeding transformer. This is the reason that the secondary side of most three phase distribution transformers is Wye or Star wound. Three Phase transformers and their associated neutrals are usually found in an Industrial distribution environment.
It is most common to refer to the Neutral as a "Neutral phase". Commonly the Neutral is "taken to ground" or "Earthed" through the Neutral-Earth link, a bond between the Neutral bar and the Earth Bar. It is common on larger systems to monitor any current flowing through the Neutral/Earth link and use this as the basis for Neutral Fault protection.
This Neutral-Earth link provides a zero voltage reference for the system, so that a three phases can be measured against Earth/Neutral and give a voltage reading. In a three phase and neutral system of say 480Volts phase to phase, the phase to Neutral/Earth voltage will be 277Volts (480÷√3). Without this Neutral-Earth link, the Phase to Earth voltage can read zero on an electricians multimeter, but will still be dangerous. The Neutral/Earth link allows any Phase to Earth fault to develop enough current flow to "trip" or operate the Circuit breaker protection. Some jurisdictions enforce calculating the magnitude of a potential downstream fault to ground and ensuring that the fault loop impedance back to the Neutral-Earthling is sufficiently low to generate enough current to trip the protection (specifically this is required in AS3000:2007 Fault loop impedance calculation).
In the case of two phases sharing one neutral, the worst-case current draw is one side has zero load and the other has full load, or when both sides have full load. The latter case results in 1 + 1@120deg = 1@60deg, i.e. the magnitude of the current in the neutral equals that of the other two wires.
In a three-phase circuit with three balanced loads, the Neutral has zero current in it when the loads are equal, and at most one unit of current in it, where the maximum load on any one branch is one unit. It is best to think of the outgoing current on say the Red (L1) phase going out and coming back on say the Yellow (L2) and Blue (L3) phases, rather than the Neutral Phase. It is important to remember that the three Phases are out of phase by 120 degrees. In some jurisdictions it is common to allow 1/2 sized or 1/4 sized Neutrals conductors for balanced loads. This can be dangerous (the Neutral current can exceed the rating of the 1/4 sized cable) if the Phases become unbalanced due to the addition of load.
The current drawn by non-linear loads, such as fluorescent & HID lighting and electronic equipment containing switching power supplies, often contains harmonics. Triplen harmonic currents (odd multiples of the third harmonic) are additive, resulting in more current in the shared neutral conductor than in any of the phase conductors. In the absolute worst case, the current in the shared neutral conductor can be triple that in each phase conductor. Some jurisdictions prohibit the use of shared neutral conductors when feeding single-phase loads from a three-phase source; others require that the neutral conductor be substantially larger than the phase conductors. It is good practice to use four pole Circuit breakers (as opposed to the standard three pole) where the fourth pole is the neutral phase, and is hence protected against over current on the neutral conductor.
In split phase house wiring, for example, a duplex receptacle in a kitchen is typically connected with a cable that has three conductors, in addition to ground. The three conductors are usually colored red, black, and white. The white serves as a common neutral, while the red and black each feed, separately, the top and bottom hot sides of the receptacle. Typically such receptacles are supplied from a ganged breaker, i.e. a breaker in which the handles are tied together for a common trip, so that if one kitchen appliance malfunctions and pops the breaker, the other side of the duplex receptacle will be shut off as well. This is called a multiwire circuit.
Common trip is always required if there are loads that use more than one phase simultaneously. It is also required by the NEC 2008 for any loads using only one phase.
In situations where the electrical code doesn't require it, common trip is still generally advisable for the following reasons:
An inexperienced user might only check one of the sockets and think the circuit is off when turning off only one breaker, i.e. a homeowner might go into the kitchen, plug in a lamp, and turn off breakers until the lamp goes off, not realizing that the two outlets of the duplex receptacle are fed from different breakers. Subsequent electrocution could result; Loop currents could be present, when someone is working on the equipment, or otherwise.