High resistance grounding of industrial power
systems reduces outages and interruptions due to faults. It offers
many of the benefits of both the solidly grounded and ungrounded
systems, including lower levels of transient overvoltages and
reduced probability of equipment damage.
This document will attempt to clarify the advantages
of high resistance grounding and present practical ways to apply
it on low and medium voltage industrial systems.
Industrial power systems can be solidly grounded
(no intentional impedance to ground), impedance grounded (through
a resistance or reactance), or ungrounded (with no intentional
connection to ground). Table 1 compares these different grounding
schemes.
| Solidly-grounded three-phase systems are usually wye-connected, with the neutral point grounded. Less common are the:
"Red-leg" or high-leg delta (with one winding center-tapped and grounded). Corner-grounded three-phase delta system where one phase grounded. |
| Ungrounded systems can be either wye or delta, although the ungrounded delta system is far more common. |
| Resistance Grounded Systems
Resistance-grounded systems can be either wye or delta connected. In a wye connected system, grounding is accomplished by connecting the neutral point directly through the resistor to ground. Delta systems can be grounded by means of a zig-zag or other grounding transformer with a resistor in the ground connection. The size of the resistance determines the current under a line-to-ground fault. Low resistance grounding limits the fault to relatively low levels (400-3000A) compared to fault currents for solidly grounded systems which may be in tens of thousands of amperes.
High resistance grounding limits the fault current few amperes (typically 5-10A). Reactor Grounding Systems Generator neutrals are sometimes grounded through a reactor, to limit ground-fault (zero sequence) currents to values the generator can withstand. |
There is no one "best" distribution
system for all applications. In choosing a grounding method, many
attributes should be compared. The characteristics of each grounding
method system must be weighed against the requirements of power
loads, lighting loads, continuity of service, safety, and cost.
Moreover, the 1993 National Electrical Code, NEC, restricts the
use of grounding schemes according to the system configuration,
the voltage, and the types of loads used. NEC Article 250-5 calls
for solid grounding for all AC circuits and systems of 50 Volts
to 1000 volts under any of the following conditions:
BENEFITS OF HIGH-RESISTANCE GROUNDING
High-resistance provides a low fault current to minimize damage,
yet limits the potential of overvoltages to less than 250% of
the normal crest value to ground limiting the hazard to personnel
and the potential of equipment damage. This will allow the system
to continue in operation with a ground fault one phase conductor.
On ungrounded systems, arcing ground faults can cause excessive
transient overvoltages exceeding many times the normal phase-to-ground
voltage.
Exception No. 5 of NEC Article 250-5 allows the use of high resistance
grounding on ac system of 480 volts to 1000 volts where all
of the following conditions are met:
a. The conditions of maintenance and supervision assure that only
qualified persons will service the installation.
b. Continuity of power is required.
c. Ground detectors are installed on the system.
d. Line-to-neutral loads are not served.
Source: 1993 National Electrical Code®
NEC requires that the grounding impedance be inserted in the
grounding conductor between the grounding electrode of the supply
system and the neutral point of the supply transformer or generator.
The neutral conductor should also be identified, as well as fully
insulated, the same as the phase conductors. Figure 1 illustrates
a low voltage high-resistance scheme.
For low voltage systems, Article 210-6 of NEC limits the voltage
to ground of lighting fixtures to 300 volts. Thus 480Y/277-volt
systems, serving line-to-neutral lighting loads, must be operated
with solidly-grounded neutrals. Small amounts of 277V lighting
can be accommodated on high-resistance grounded systems by the
use of 480V delta-480V wye-connected transformers.
A high resistance grounding system consists of the following components:
1. A grounding resistor
2. A fault detector and an alarm system
3. A fault locating scheme
The grounding resistor determines the value of the ground fault
current. This resistor is sized such that its current, IR, during
a line-to-ground fault is slightly higher than the total system
charging current, ICO. The charging current is that current associated
with total distributed capacitance of the system to ground, made
up of
the capacitance to ground of the cables, motor windings, and other
insulation systems. In low voltage systems, the two greatest sources
of capacitance are conductors in conduit and motor windings. Another
significant source of the charging current are surge capacitors
(power factor correction capacitors are connected phase-to-phase
and do not add appreciably to the capacitance to ground). The
charging currents of ungrounded systems are typically low and
can be calculated from data on the different system components.
For new systems, see example below. For existing systems, it should
measured. For low voltage systems a good-rule-of-thumb is to use
the approximation of 1 ampere per 2000 kVA of transformer or system
capacity.
| 0.1 A per 1000 ft | |
| 0.05 A per 1000 ft | |
| 0.01A per 1000 hp |
The resistor for a High-resistance scheme is
sized according to the following steps:
1. Measure the charging current or estimate the capacitive reactance, XC to ground, of the system.
2. Select the size of the grounding resistor such that it is equal to or slightly less than one third the system* :
or 
This provides a low fault current to minimize damage, yet it will
limit transient Overvoltages to less than 2.5 times the normal
crest value to ground. (Under a no fault condition, the grounding
resistor will bleed off the system capacitance thus limiting the
overvoltages.)
The occurrence of a fault is typically detected
by measuring the voltage across the neutral grounding resistor.
A voltage-sensitive device, such as overvoltage relay, or a voltmeter-relay
(contact making voltmeter) can be connected across the grounding
resistor to detect the occurrence of the fault. When a ground
faults occurs, the relay or the voltmeter-relay will pickup a
timer set for 4-6 seconds. The purpose of the timer is to inhibit
alarming on faults with transitory nature. Upon timing out, an
auxiliary relay is energized to alarm, or trip.
A very popular fault locating scheme in use
today is one which utilizes current pulses and permanently installed,
or clamp-on, ammeters to trace the current pulses. The principal
of operation of this scheme is simple. The ground current is pulsed
to a higher value (typically from 5A to 10A intermittent) by shorting
out part of the tapped grounding resistor. At the point where
the High-resistance grounding is connected, the different feeders
are checked with the ammeter to determine which feeder has the
fault on it (this is confirmed by the presence of the current
pulses). The faulty feeder can be traced with the ammeters by
eliminating branch circuits where the pulsing disappears and thereby
locating the faulted circuit. Shorting out part of the grounding
resistor is made with a pulsing contractor. A cycle timer is also
needed to generate the pulse train.
Like everything else, high-resistance comes
with certain caveats :
A High-resistance grounding scheme should not
be used as a substitute for proper system maintenance.
High-resistance grounding scheme should not
be used on a four-wire systems.
For economic purposes, high-resistance grounding
scheme should not be applied when a large portion (greater than
30%) of the load is of the line-to-ground nature. Such loads must
be isolated by separate transformers.
When converting from a solidly grounded system
to a high resistance system, attention should be placed on the
checking the insulation of the different components, especially
for medium voltage systems.
For low voltage system, and in a nonessential
circuit, some users alarm only if the fault is located, removing
the fault in two to ten hours. In an essential circuit which cannot
be de-energized, the fault duration extends to possibly several
days, with the accompanying higher risk of fault escalation. Alarm-only
high-resistance grounding has been most successful in low-voltage
systems provided the following limitations are observed:
It is applicable only to three-phase, three-wire
systems. The exclusion of three-phase, four-wire (such as 480
wye/277 volt) systems is disallowed by NEC.
There is a remote possibility that a phase-to-ground
fault may result in a phase-to-ground-to-phase fault that may
trip as many as two feeder breakers. This phenomenon can be a
result of the appearance of full phase-phase voltage on the unfaulted
phases on a High-resistance system following a ground fault. This
higher than normal voltage can lead to a second ground fault may
develop on weakened and damaged insulation.
Hence prompt location and removal of the first
ground fault must be stressed to insure successful operation of
the high-resistance grounding concept
For medium voltage, some users trip
on the first indication of a ground fault requiring the need for
selective ground-fault relaying in the presence of small (up to
8A) ground faults.
Figures 2 and 3 illustrate three high-resistance
schemes. For 480 volts systems, practical grounding resistors
can be made from strip heaters rated about 750-1250 W, as in Figure
2. The grounding resistor is usually tapped to facilitate the
use of the fault locating equipment, as described below.
For medium voltage applications, the use of
a distribution transformer and a secondary resistor is an economic
consideration. With high-resistance grounding it is generally
less expensive to use low voltage resistors. Such a system is
illustrated in Figure 3.
Back to Power Engineering Notes Page