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The Basics of Electrical Systems

Jan 1, 2002 12:00 PM, By Mike Holt, Mike Holt Enterprises, Inc.

Do you understand neutrals and fault current paths?

Neutral connections, fault current paths, and grounding considerations can be confusing, but if you understand the basic principles of electricity, you can clear up the confusion every time.

Consider current flow. Electrons leaving a power supply are always trying to return to it — they are not trying to go into the earth. Alternating current applied to a transformer primary induces a voltage in the secondary. This induced voltage causes electrons to leave one end of the transformer secondary, travel over the conductors, through the load, and return over the remaining conductors to the other end of the transformer secondary. Comparing what utilities do on their side of the meter to what we do on ours illustrates key concepts.

Multipoint grounded neutral (utility).

Utilities use a multipoint grounded neutral system, in which the neutral conductor is electrically connected to earth approximately every quarter mile of distribution lines and at distribution transformers and service equipment (see the Figure).

Neutral current path.

Utilities ground the neutral conductor to the earth at multiple locations (parallel path for neutral current) to reduce the impedance of the return neutral current path. This reduces neutral voltage drop and line-to-ground voltages; it also helps clear line-to-neutral faults.

Fault current path.

Utilities connect metal parts of equipment to earth and to the neutral conductor (parallel path for fault current) to provide a low-impedance path for clearing line-to-case faults. If the neutral conductor opens, the earth should still have sufficient low impedance to clear a high-voltage line-to-case fault. For example, consider a typical 7.2kV line protected by a 100A fuse. With an impedance of 25 ohms, the earth should have no problem carrying sufficient fault current to open a 100A fuse (I5E4Z, I57,200V425 ohms, I5288A).

Single-point grounded neutral (our side of the meter).

Premises wiring not over 600V uses a single-point grounded neutral system, in which the neutral conductor connects to the earth and metal parts of the electrical system only at service equipment and at separately derived systems.

Neutral current path.

Neutral current should flow only on the grounded (neutral) conductor, not the effective fault current path or the earth.

Fault current path.

Metal parts of premise wiring should be bonded to an effective fault current path that provides a low-impedance path necessary to clear line-to-case faults. Fault current should flow only on the effective fault-current path, not the grounded conductor (neutral) or earth.

For systems operating at 600V or less, the earth will not carry sufficient fault current to clear a line-to-case fault. For example, a 120V fault to the earth (assuming a 25-ohm resistance) will draw only 4.8A (I = E/Z, I = 120/425 ohms, I = 4.8A), which won't open a 15A protection device.

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