Keep voltage differentials between different parts of a system at a minimum which reduces shock hazard.
Keep impedance of the ground path to a minimum. The lower the impedance the greater the current is in the event of a fault. The greater the current the faster an overcurrent device will open.
Grounding the Service Entrance Panel
In the service entrance panel shown in Figure 7, the neutral from the transformer is connected to the neutral bar in the loadcenter. The neutral bar is then grounded to the earth.
Figure 7. Grounding a Service Entrance Panel
This is accomplished by running a conductor from the loadcenter’s neutral bar to a cold water pipe or copper rod buried in the ground (or other suitable grounding electrode.) Then, a Bonding screw or strap is used to physically “bond” the neutral bar to the metal enclosure.
In this way, the neutral bar, the neutral of the transformer, and the enclosure are grounded. When the neutral is grounded in this manner, the equipment ground from the branch circuits is sometimes allowed to connect to the neutral.
Equipment Grounding Bus
The Equipment Grounding Bus is connected directly to the loadcenter enclosure. For safety, all equipment (on both the feeder and branch circuits) is connected to the equipment grounding bus. This requirement keeps the connected equipment at the same potential as the enclosure itself.
Figure 8. Equipment Grounding Bus
One additional point for service entrance applications: this bonded and grounded neutral can also be used as the point of connection for equipment grounding.
Grounding Downstream Loadcenters
It is important to remember that the service entrance panel is the only ground connection point for the neutral
. The neutral is both insulated and isolated at any downstream panels.
As shown in Figure 9, the downstream panel’s enclosure is grounded with a grounding conductor running back to the service entrance panel.
Figure 9. Grounding the Downstream Loadcenter
In Figure 10, the table lamp has a short circuit. If you trace the thick line back, you will see how fault current is returned to the source. This is why the downstream loadcenter contains a branch circuit breaker. It trips, disconnecting power from the load.
Figure 10. A Short in a Downstream Load
Ground Fault
In most cases, NEC also requires equipment to have an additional grounding conductor.
This conductor connects the metal housing of an appliance to the system ground to reduce the possibility of a shock due to a Ground Fault.
A ground fault is simply current leakage from an ungrounded conductor to the Grounding Path in an electrical system. Consider Figure 11. A piece of insulation covering a conductor at an electrical outlet has flaked off. This could allow current to pass from the conductor to the metal outlet box. The box is part of the system ground. If properly grounded, the current should follow the grounding path to ground, instead of an unfortunate person’s finger that touched the box.
Figure 11. Ground Fault
Because a distribution panel takes its power from the service entrance panel (Figure 9), the neutral needs to be isolated from the ground. It is considered a branch circuit of the main service panel, and needs to have its own ground to prevent a ground fault.
If all the loads in a panel are connected evenly between A-N and B-N, no current will flow in the neutral. However, loads are rarely balanced. As a result, some current usually flows through the neutral. The amount of current flowing in the neutral conductor at any one time is the difference between the current flowing through leg A and leg B. This is why a path to ground is needed.
