Ungrounded and high-impedance systems

Every system Dana Whitfield had installed in eleven years at Brightwater Electric was solidly grounded. So when her practice exam asked what happens on an ungrounded 480 volt system when one phase faults to a motor frame, she picked the answer she would expect from a grounded system: the breaker trips. It does not. On an ungrounded system, the first ground fault draws only a tiny current, nothing trips, the motor keeps running, and the other two phases rise to 480 volts above ground. Ray Ochoa had worked in a paper mill early in his career, where 480 volt ungrounded and high-impedance grounded systems were normal, and he walked her through why plants choose them, what the code demands in return, and what the exam tests. This lesson is his explanation, organized around 250.20, 250.21, 250.26 and 250.36.

As always, the rules are summarized in our own words with illustrative values stated as this course's examples. Confirm every condition and value in your own adopted edition.

Why anyone would choose an ungrounded system

A solidly grounded system clears a ground fault immediately by tripping the circuit. That is ideal for safety in most buildings, but in some industrial processes an unexpected shutdown is itself dangerous or very expensive. A glass furnace that loses power solidifies. A chemical reactor that loses its cooling pumps can run away. A continuous paper machine that stops can take a day to restart. For processes like these, an ungrounded system offers continuity: the first ground fault does not trip anything, so the process keeps running while maintenance staff find and repair the fault on their schedule.

The price of that continuity is a hidden hazard. While the first fault is in place, the system is effectively corner grounded through the fault, and the two unfaulted phases sit at full line-to-line voltage above ground. If a second ground fault occurs on a different phase, the two faults together form a phase-to-phase short circuit through the equipment frames and the bonding conductors, with large currents and possibly arcing at two separate locations. The code therefore permits ungrounded systems only in limited cases and requires ground detectors so the first fault is found and fixed before a second one happens.

When a system may be ungrounded

Lesson 3 covered 250.20(B), which requires grounding for systems where the voltage to ground can be held at 150 volts or less, for four-wire wye systems using the neutral, and for four-wire delta systems using a winding midpoint. Systems that fall outside those conditions, such as a 480 volt three-phase, three-wire delta, are not required to be grounded. Section 250.21(A) lists systems that are permitted to be grounded but are not required to be: systems used exclusively to supply industrial electric furnaces for melting, refining, tempering and similar purposes; separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives; separately derived systems supplied by transformers with primaries under 1,000 volts, used for control circuits, where only qualified persons service the installation, ground detectors are installed, and continuity of control power is required; and other systems that 250.20(B) does not require to be grounded.

Ground detectors and marking

Section 250.21(B) requires ground detectors on ungrounded alternating current systems permitted by 250.21(A)(1) through (A)(4) that operate at not less than 120 volts and not more than 1,000 volts. The detectors are installed as close as practicable to where the system receives its supply. A simple ground detector is a set of three lamps or voltmeters, one connected from each phase to ground. In normal operation all three show the same reading. When one phase faults to ground, its lamp goes dark and the other two burn brighter, because their voltage to ground rises from line-to-neutral to line-to-line. Modern installations use insulation monitoring relays that alarm and indicate which phase has faulted.

Section 250.21(C) requires ungrounded systems to be legibly marked at the source or first disconnecting means of the system to identify the system as ungrounded and state the operating voltage between conductors. The marking must be durable for the environment. The purpose is simple: a worker who assumes a system is grounded may assume a phase reads 277 volts to ground when it can read 480.

Ground detectors only help if someone responds to them. Ray's old mill had a written procedure: when the detector alarmed, an electrician was dispatched within the shift, feeders were opened one at a time during a planned window until the alarm cleared, and the faulted circuit was repaired before the next production run. The code does not prescribe that procedure, but it is why 250.21(A)(3) and 250.36 tie these systems to qualified persons. On an exam, a scenario with an ungrounded system and no one assigned to watch the detector is describing exactly the hidden hazard the code is trying to prevent.

Corner-grounded delta systems

A three-phase, three-wire delta system may also be grounded by connecting one phase conductor to ground, a corner-grounded delta. Section 250.26 tells you which conductor is grounded on each type of system, and for a three-phase, three-wire delta, it is one phase conductor. The grounded phase must be identified as a grounded conductor, white or gray, and the overcurrent rules in Article 240 forbid placing an overcurrent device in that grounded conductor except where it opens all conductors simultaneously. Corner-grounded systems clear a ground fault on either ungrounded phase like any grounded system, but they are uncommon today, and a worker who assumes a phase-to-ground reading of 277 volts on a 480 volt corner-grounded system will be wrong: the two ungrounded phases read 480 volts to ground.

High-impedance grounded neutral systems

High-impedance grounding offers a middle path. Instead of leaving the system ungrounded or connecting the neutral solidly to ground, the neutral point is connected to ground through a resistor that limits ground-fault current to a few amperes. A first ground fault then draws a small, controlled current that does not trip the circuit, so the process continues, but the current is large enough to detect and locate easily, and the resistor keeps the system's voltage to ground stable, which prevents the dangerous transient overvoltages that can build up on truly ungrounded systems during intermittent arcing faults.

Section 250.36 permits high-impedance grounded neutral systems for three-phase alternating current systems of 480 volts to 1,000 volts, where three conditions are met: the conditions of maintenance and supervision ensure that only qualified persons service the installation, ground detectors are installed on the system, and line-to-neutral loads are not served. The third condition is the one exam writers like. A high-impedance grounded system cannot serve 277 volt lighting, because the neutral is not solidly grounded and would not hold a stable voltage to ground under load; the lighting must be served from a separately derived, solidly grounded system, such as a transformer.

The installation rules follow from how the system works. The grounding impedance is installed between the grounding electrode conductor and the system neutral point; where no neutral exists, the neutral point is derived from a grounding transformer. The neutral conductor from the neutral point to the impedance must be fully insulated, have an ampacity at least equal to the maximum current rating of the impedance, and in this course's examples be not smaller than 8 AWG copper or 6 AWG aluminum. The system neutral may not be connected to ground except through the impedance. An unspliced equipment bonding jumper connects the equipment grounding conductors to the grounded side of the impedance, and the grounding electrode conductor connects anywhere from the grounded side of the impedance to the equipment grounding connection at the service or first disconnecting means.

Sizing the grounding resistor

The design of the resistor is an engineering matter, but master exams sometimes ask for the arithmetic. The resistor is chosen so that the ground-fault current it allows is slightly larger than the system's capacitive charging current, the small current that flows to ground through the capacitance of cables and motor windings. Allowing a resistive current at least equal to the charging current damps the transient overvoltages. On a 480 volt system, charging currents of 1 to 5 amperes are typical, and resistors are commonly sized for a few amperes more.

Note that the fault current is fixed by the resistor at 5 amperes no matter where the fault is. That is the point. Compare that with a solidly grounded system, where the same fault might draw thousands of amperes and trip a feeder, or an ungrounded system, where the fault draws only the 2.5 amperes of charging current and can let the system's voltage to ground swing dangerously during intermittent arcing.

A calculator you can edit

The listing below computes the phase-to-ground voltages on an ungrounded system before and after a fault, and sizes a grounding resistor for a chosen fault current. Change the voltage and currents to practice.

import math

def line_to_neutral(v_ll):
    return v_ll / math.sqrt(3)

def resistor(v_ll, fault_amps):
    v = line_to_neutral(v_ll)
    return v / fault_amps, v * fault_amps

v_ll = 480
print(f"Normal: each phase about {line_to_neutral(v_ll):.0f} V to ground")
print(f"After one phase faults: faulted phase 0 V, others {v_ll} V to ground")
for charging in (1.5, 2.5, 4.0):
    target = 2 * charging
    ohms, watts = resistor(v_ll, target)
    print(f"Charging {charging} A -> resistor for {target} A: {ohms:.1f} ohms, {watts:.0f} W during a fault")

Normal: each phase about 277 V to ground
After one phase faults: faulted phase 0 V, others 480 V to ground
Charging 1.5 A -> resistor for 3.0 A: 92.4 ohms, 831 W during a fault
Charging 2.5 A -> resistor for 5.0 A: 55.4 ohms, 1386 W during a fault
Charging 4.0 A -> resistor for 8.0 A: 34.6 ohms, 2217 W during a fault