One of the most difficult topics to discuss in-depth about a home is the electrical systems. In particular why neutrals and grounds should be bonded on the main service panel but is prohibited on a sub-panel. To dissolve some of the mystery we must dispel common misconceptions, learn a fact or two, and talk about a couple of cases.
Misconceptions
A common myth is electrons leaving a grounded power supply are trying to get back to Earth ground.
Electricity only takes the path of least resistance.
Fact
Electricity wants to return to the source. In the case of man-made electricity, this is the transformer on the service pole, and it returns to the transformer via a neutral conductor in the main panel. Electricity will take ANY conductive path back to its source, and does so proportionally to the conductor’s resistance it travels on.
Grounding
Grounding electrodes are used to divert over-voltages on lines from static or lightning.
Grounding conductors are utilized in the case of a ground fault, providing a path back to the transformer via a neutral conductor in the main panel, and then from the transformer to the overcurrent device (breaker), clearing the fault.
Now that we have cleared all that up, it’s time to discuss why grounds and neutrals should not be bonded in sub-panels.
Case 1. – Neutral and ground bonded with ground fault
If a ground fault occurs, it needs ONE low impedance path (ground wire) to the main panel which transfers current to the transformer and back to the main panel’s breakers to clear the fault. With ground and neutral bonded, current can travel on both ground and neutral back to the main panel.
Case 2. -Neutral and ground bonded with an unbalanced load
If the load becomes unbalanced and ground and neutral are bonded, the current will flow through anything bonded to the sub-panel (enclosure, ground wire, piping, etc.) and back to the main panel. Obvious shock hazard! Remember, electricity will take any path back to the source.
In more layman terms, all neutral wires should be attached to a floating bar, and all grounds should be attached to a bar directly attached to the panel.
At IM Home Inspections, we check sub-panels to make sure the neutrals and grounds are properly separated.
To book your home inspection, call 818-298-3405 or book online here.