Cool Shared Neutral On Lighting Circuit

Sparkwright 7 oct 2012 3.

Shared neutral on lighting circuit. A shared neutral in electrical circuits is a common connection for the neutral lines that usually carry a near zero net current. Re energise the upstairs lighting mcb and if the landing light comes on voila borrowed neutral. In the case of a three phase circuit the neutral line current for a balanced three phase load is zero or very minimal.

The use of shared neutral circuits produces significant copper savings when two branch circuits are close to each other but far from the circuit breaker panel. More important even if the return conductor is not part of a multiwire circuit removing a conductor from the grounded terminal bar when the circuit is energized could result in injury due to shock or arcing. If this happens voltage will dramatically increase on the circuits.

Now to check for a borrowed neutral disconnect the neutral of the downstairs lighting circuit with both mcbs turned off. Never remove the grounded neutral conductor from the grounded terminal bar in the panel board if the phase conductors are energized. Split phase single phase systems this means going from 120 to 240 volts.

In essence your appliances become a series circuit between the supply lines in u s. Shared neutrals are a problem on two or more different circuits if not covered by the same rcd. If two separate circuits fed from separate supply lines share a neutral the main danger is when the neutral becomes disconnected.

Turn off downstairs lighting mcb and landing light goes off. For split phase electric power when there are balanced loads on each of the two phases there is almost zero net current on the common neutral. A shared neutral circuit uses 3 conductor nm b wire to join two adjacent circuit breakers in the panel to a junction box near the branch circuit loads outlets see figure 2.

Fires fried appliances or worse. The grounded neutral conductor you remove could be part of a multiwire branch circuit so this could result in destruction of electrical equipment.