Neutrals, system grounding and ground faults in AC systems

The code rules for grounding and bonding covered in earlier courses make sense only when the behavior of neutrals and fault currents is understood. Why does a lost neutral on a 120/240 V service burn out some appliances and leave others dim? Why must the neutral be bonded to ground at the service but not downstream? Why does a ground fault on an ungrounded system trip nothing, while the same fault on a solidly grounded system trips the breaker instantly? This lesson applies the AC circuit theory of Lessons 1 to 6 to neutral currents in single-phase and three-phase systems, multiwire branch circuits, the dangers of open neutrals, the different ways systems are grounded, and the path and size of ground-fault current.

The neutral in a 120/240 V single-phase system

Most homes and small buildings in North America are supplied by a single-phase three-wire system: a center-tapped transformer secondary provides two ungrounded conductors (lines) 240 V apart and a grounded neutral at the center tap, 120 V from each line. The two line voltages are 180° apart with respect to the neutral. Loads of 120 V connect from either line to neutral; 240 V loads connect line to line.

Because the two halves are 180° apart, the neutral carries only the difference between the currents on the two lines (for resistive loads):

If 120 V loads are balanced between the two lines, the neutral current is zero, and the neutral could be smaller than the lines, which is why services were historically allowed reduced neutrals. With unbalanced loads it carries the imbalance, never more than the larger line current.

Multiwire branch circuits

A multiwire branch circuit shares one neutral between two or three ungrounded conductors from different phases or lines, saving copper and reducing voltage drop. On a 120/240 V system, two circuits on opposite lines share a neutral carrying only the difference. On a 120/208 V system, three circuits on three phases share a neutral carrying the vector difference of balanced currents, ideally near zero for linear loads (but large with triplen harmonics, Lesson 3).

Multiwire circuits have two dangers:

• If both ungrounded conductors are connected to the same line by mistake, the neutral carries the sum of the currents instead of the difference, and can overheat without any breaker tripping.
• If the shared neutral is opened while the circuit is energized, for example when someone disconnects a device on one circuit and breaks the neutral splice, the loads on the two circuits are left in series across 240 V (or 208 V), as described in the next section. Opening the neutral while one circuit is still live can also expose the worker to the full voltage.

NEC 210.4(B) therefore requires a means to simultaneously disconnect all ungrounded conductors of a multiwire branch circuit at the point where it originates (a multipole breaker or identified handle ties), 210.4(D) requires the conductors of each multiwire circuit to be grouped, and 300.13(B) prohibits relying on device terminals for neutral continuity: neutrals are spliced with a pigtail so that removing a device does not open the neutral to other loads.

The open neutral

An open neutral is one of the most destructive faults in a building. When the neutral of a 120/240 V service or multiwire circuit opens, 120 V loads on the two lines are no longer connected to the center point; instead, the loads on line 1 and those on line 2 are in series across 240 V, and the voltage divides according to their impedances.

Open neutrals on services are often caused by corroded or loose connections at the meter base, weatherhead or utility drop, or by damage to underground service conductors. On three-phase four-wire systems, an open neutral shifts the neutral point toward the most heavily loaded phase, overvoltaging the lightly loaded phases in the same way.

Why the neutral is grounded, and where

Electrical systems are grounded, meaning one conductor (usually the neutral) is intentionally connected to earth, for three reasons: to limit the voltage to ground of the system conductors during normal operation, to limit overvoltages caused by lightning, line surges or accidental contact with higher-voltage lines, and to stabilize the voltage to earth.

The neutral (grounded conductor) and the equipment grounding (bonding) conductors are connected together at one point only: the service, or the source of a separately derived system such as a transformer secondary or generator. Downstream, they must be kept separate. If the neutral were also bonded to metal enclosures and grounding conductors at subpanels or equipment, part of the normal neutral current would return over the grounding conductors, conduit and building steel. This objectionable current creates voltages on metal parts, magnetic fields that disturb sensitive equipment, and confusing readings on ground-fault protection, and if the neutral opened, all the neutral current would flow on the grounding path.

Kinds of system grounding

The high-leg (center-tapped) delta, in which one transformer of a 240 V delta bank is center-tapped to provide 120/240 V single-phase and 240 V three-phase, is still found in older US commercial buildings. The phase opposite the center tap, the high leg, is about 208 V to ground; NEC 110.15 requires it to be marked orange, and 408.3(E) requires it to be the B phase in panelboards. It must never be used for 120 V loads.

Ground faults in solidly grounded systems

When an ungrounded conductor contacts a grounded metal enclosure, current flows through the fault, back through the equipment grounding conductor to the service or source bond, and through the neutral (or the transformer winding) to complete the circuit. The current is limited only by the impedance of this loop: the transformer, the ungrounded conductor, and the return path. If the return path has low impedance, the current is large enough to trip the breaker quickly, and while the fault lasts the voltage on the enclosure is limited.

Earth itself is a poor return path. A ground rod typically has a resistance of 5 to 25 Ω or more to remote earth, so a fault relying on the earth to return current would draw only a few amperes, far too little to trip a 20 A breaker, while leaving the enclosure energized at dangerous voltage. This is why an effective ground-fault current path through bonding conductors back to the source is required, and why the ground rod is not a substitute for bonding.

Ground faults in ungrounded and resistance-grounded systems

In an ungrounded system, the phases are coupled to ground only through the capacitance of cables and equipment. A first ground fault on one phase simply connects that phase to ground: little current flows (a few amperes of capacitive charging current), nothing trips, and the system keeps running, which is why ungrounded systems were favored in continuous-process industries. But the other two phases now sit at full line-to-line voltage to ground, stressing insulation, and if a second fault occurs on another phase before the first is found, the result is a phase-to-phase short circuit through the grounding system, possibly in two different pieces of equipment. Intermittent arcing ground faults can also build up transient overvoltages of several times normal through repeated charging of the system capacitance.

Ungrounded systems require ground detectors that show when a phase is faulted. The simplest uses three lamps connected from each phase to ground: normally all glow equally dimly; a ground fault on one phase darkens its lamp and brightens the other two. Modern systems use monitoring relays.

High-resistance grounding keeps the continuity advantage while avoiding the overvoltages: a resistor between the neutral and ground limits fault current to a value slightly above the system's capacitive charging current (typically 1 to 10 A), which damps arcing transients. A first fault alarms rather than trips; a pulsing feature switches part of the resistor in and out so that the pulsing current can be traced with a clamp meter to locate the fault. High-resistance grounding is widely used on 480 V and 600 V systems in industrial plants and data centers, but not for systems supplying line-to-neutral loads, because the neutral is not solidly grounded.