Ground-Fault Detection, Charging Current and
Neutral-Grounding Resistor Selection
Don Selkirk, P.Eng
3714 Kinnear Place
Canada S7P 0A6
Abstract—Choosing appropriate ground-fault circuit interrupting devices. Alarm-only systems usually
protection and neutral-grounding resistor (NGR) let- limit NGR current to 10 A or less.
through current requires careful consideration of
several factors including system voltage, ground-fault II. SYSTEM VOLTAGE CONSIDERATIONS
relay co-ordination, tripping action, harmonics, and
system charging current. In the application of resistance grounding the system
In order to facilitate system design and voltage and NGR let-through current are the primary
troubleshooting, this paper reviews charging current considerations that influence whether to deploy an alarm-
and sympathetic tripping, protective relay co- only or tripping system.
ordination, and the factors that affect NGR selection for The NGR let-through-current rating in an alarm-only
alarming and tripping applications. A systematic system should be no more than 10 A. This value of fault
method is proposed for selecting an NGR, and setting current is usually small enough to prevent burning of the
ground-fault protection. insulation and the subsequent escalation to a phase-to-
phase fault, and burning of core material on motors,
I. INTRODUCTION generators and transformers . This level is the maximum
let-through current for an alarm-only system as defined by
Resistance grounding has been used in three-phase section 10-1102 of the Canadian Electrical Code.
industrial applications for many years. Properly designed Alarm-only systems are seldom used at system
resistance grounding resolves many of the problems voltages above 5 kV because higher system capacitance
associated with solidly grounded and ungrounded systems necessitates an NGR let-though current greater than 10 A,
while retaining many of their benefits. Resistance and because higher voltage increases the probability of a
grounding can limit point-of-fault damage, eliminate ground-fault escalation to a phase-to-phase fault.  On a
transient overvoltages, reduce the risk of an arc flash, limit tripping system, an NGR with a higher let-through-current
personnel exposure to voltage, allow continuity of service rating can be used and NGR’s of 15 A and 25 A are
with a ground fault, and provide adequate current for common. Higher-voltage systems, which must trip on a
ground-fault detection and selective coordination. ground fault, can have let-through currents up to hundreds
However, proper sizing of the NGR is critical in order to of amps. Designers comfortable with solidly grounded
provide system stability. In this paper resistance grounding systems often choose NGR’s with let-through currents
refers to a resistive element connected between the much larger than necessary for either system stability or
transformer or generator neutral and the system ground on selective coordination . This practice increases potential
the secondary side. damage to equipment and danger to personnel during a
There are two levels of resistance grounding, high- ground fault.
resistance grounding and low-resistance grounding. The
difference between these levels is a matter of perception III. UNDERSTANDING CHARGING CURRENT
and, therefore, is not well defined. Generally speaking
high-resistance grounding refers to a system in which the Each phase of a distribution system has capacitance to
NGR let-through current is less than 50 to 100 A. Low- ground. Although a system may be ungrounded in that
resistance grounding indicates that NGR current would be none of its current-carrying conductors are intentionally
above 100 A. As an example a 480 V system with a 100 A connected to ground, an ungrounded system has its neutral
NGR would be considered low-resistance grounding point established by distributed system capacitance as
because 5 A is a common let-through current for a 480 V shown in Fig. 1. For simplicity, the distributed capacitance
system. A 35 kV system with a 100 A NGR could be of the system is shown as a single capacitor per phase.
considered high-resistance grounding. When the system is energized current flows between the
A better distinction between the two levels might be phase conductors and the capacitive neutral; this current is
alarm only and tripping. An alarm-only system continues referred to as charging current. Extensive damage can
to operate with a single ground fault on the system for an occur when an ungrounded power-distribution system
unspecified amount of time. In a tripping system a ground experiences a transient overvoltage condition caused by an
fault is automatically removed by protective relaying and intermittent ground fault .
Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor Selection Page 2
Fig 1: Ungrounded System
On an ungrounded system a transient overvoltage can IV. SYMPATHETIC TRIPPING
result from a re-striking ground fault. When the strike
occurs, charging current only flows for a short time and is Fig. 2 shows a system similar to Fig. 1 with a 5.0-A
then extinguished. This results in a capacitive charge that NGR added. Voltages and currents in the unfaulted case
elevates system voltage-to-ground for all phases and are the same as in the ungrounded system. If phase A is
neutral. Phase-to-phase and phase-to-neutral voltages are faulted to ground, voltages and currents in phases B and C
not affected. The repetition of the re-strike process over are also the same as in the ungrounded system; however,
several cycles can result in a voltage built up of 5 or 6 fault current is the vector sum of NGR current and system
times the nominal voltage . To prevent this transient charging current. If the system in Fig. 2 has three equal
overvoltage condition in a resistance-grounded system, the feeders, currents will be as shown in Fig. 2. Meters A1 and
grounding resistor must have a let-through-current rating A3 on the unfaulted feeders each read 1.0 A—the charging
large enough to drain the charge between successive re- current of their respective feeders. NGR current and fault
strikes. This is especially important in an alarm-only current remain unchanged at 5.0 A and 5.8 A respectively.
system that continues to operate with a ground fault. The Meter A2 on the faulted feeder will read 5.4 A—the vector
minimum let-through-current value is generally accepted to sum of NGR current and the charging currents of the
be equal to system charging current. unfaulted feeders.
Fig. 2: Resistance-Grounded System
Sympathetic tripping is when unfaulted feeders trip in the operating value of the feeder’s ground-fault relay is less
response to a ground fault elsewhere in the system, and is than the feeder’s charging current. Sympathetic tripping
an undesirable situation. Sympathetic tripping can occur if cannot occur, regardless of the
Don Selkirk, P.Eng Littelfuse Startco May 2008
Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor Selection Page 3
relative feeder sizes, if an operating value above the occurs, the nearest upstream relay is the first to operate.
charging current of the largest feeder is used for all ground- Coordination is achieved without the need for zone-
fault relays in the system. Personnel-level ground-fault selective interlocking. On an alarming system a ground
protection is difficult to achieve in a three phase system fault can be easily located by the indication given by an
because of the magnitude of charging currents. upstream relay.
Reliable ground-fault protection requires the addition
V. TRIPPING RATIO AND NGR SELECTION of continuous NGR monitoring. When a NGR fails, the
failure mode is usually open circuit, leaving the ground-
Tripping ratio is defined as the ratio of prospective return path incomplete. Current-sensing ground-fault
ground-fault current to the operating value of the ground- protection, which is the type most commonly employed in a
fault protection. An adequate tripping ratio ensures that resistance-grounded system, will not operate with an open
sufficient ground-fault current is available for detection NGR. Unless additional protection is provided, the
when a ground fault occurs. Reference  shows that a advantages of resistance grounding are unknowingly lost.
tripping ratio of at least 7 is necessary to detect a two A continuous NGR monitor provides protection against
phase-to-ground fault. It can be argued that this type of failures that previously rendered ground-fault protection,
fault should be cleared by overcurrent devices and that coordination, and annunciation systems inoperative, as well
ground-fault detection does not require a tripping ratio of 7. as leaving the system exposed to damaging transient
On the other hand, a higher tripping ratio is required to overvoltages.
provide machine-winding ground-fault protection.
Reference  states that the generally accepted protection VII. HARMONIC CURRENTS
philosophy is based on protecting 90 percent of a wye
connected winding, and that the probability of a ground The presence of harmonic-frequency voltages (integer
fault on the last 10 percent nearest the neutral is small. A multiples of the fundamental frequency) in an electrical
tripping ratio of 10 is required to meet this protection system cause harmonic-frequency currents that can affect
philosophy; however, tripping ratios of 5 are common. ground-fault detection and minimum trip set points.
If the operating value of the ground-fault relays is Harmonics can be the result of the use of adjustable-speed
greater than the charging current of the largest feeder, and drives and solid-state starters. Static switching of line
if a tripping ratio of 5 is selected to ensure adequate currents cause harmonic voltages that drive harmonic-
tripping levels and machine-winding ground-fault capacitive current from the phases to ground. Capacitive
protection, the let-through current of the NGR must be impedance is inversely proportional to frequency (Xc =
greater than 5 times the charging current of the largest 1/(2πfC) where f = frequency in Hertz). The higher the
feeder. Charging current is a function of system voltage frequency, the lower the capacitive impedance, and the
and can be measured on an existing system or estimated greater the current per volt. Except for the triplens,
from tables. Typically, charging current will be 0.5 A per harmonic currents to ground add in the same manner as the
1000 kVA on low voltage systems and 1.0 A per 1000 kVA fundamental components of capacitive current to ground.
on medium voltage systems. Consequently, 5-A, 15-A, and Only the unbalance portion contributes to neutral current.
25-A grounding resistors are common. Triplen harmonic-frequency currents present a special
case. In a three-phase system, triplen harmonics are in
VI. RELAY COORDINATION phase and their sum is three times the individual
magnitude. In a 60-Hz system, 180-Hz, 360-Hz, 540-Hz,
Resistance-grounded facilities benefit from orderly etc. components add to the 60 Hz fundamental component
ground-fault clearing. On a solidly grounded system a and can cause false ground-fault trips.
bolted fault can result in a relay race in that several Harmonic-frequency current components can make it
protective devices, both overcurrent and ground fault, necessary to set undesirably high ground-fault current-
detect the fault and begin to trip at the same time. The fault protection pickup levels to avoid nuisance tripping. To
is then cleared by the device that operates the fastest rather eliminate this problem, use a ground-fault protection device
than the device that would remove the least amount of that ignores dc-offset and harmonic-frequency current, and
equipment from the system. This problem can be solved responds only to the fundamental-frequency component of
using zone-selective interlocking but zone-selective current. The filtering characteristic must be fast to allow a
interlocking requires a great deal of control wiring. short ground-fault trip time. Digital filtering of the zero-
By contrast a resistance-grounded system can be sequence-current waveform provides a fast and accurate
designed so that the available ground-fault current is much solution to many low-level ground-fault detection
smaller than the pickup setting of overcurrent devices— problems.
thus only ground-fault relays will respond to a ground fault.
Since ground-fault current is controlled to a level that will VIII. NGR AND GROUND-FAULT RELAY
reduce fault damage, and an arc flash will not occur, some SELECTION
time is permitted to coordinate tripping.
Time-selective ground-fault coordination on a tripping The following list summarizes the items required
system requires ground-fault relays to be set to operate on when selecting NGR let-through current and ground-fault
the same current value, with ground-fault relays furthest protection.
from the supply set to operate with the least delay. Moving Determine the system charging current.
toward the source the time delay is increased. When a fault
Don Selkirk, P.Eng Littelfuse Startco May 2008
Ground-Fault Detection, Charging Current and Neutral-Grounding Resistor Selection Page 4
o This can be done via estimation, calculation References
or measurement.  D. Shipp and F. Angelini, “Characteristics of different
o See reference 5 and 6 for more information power systems neutral grounding techniques: fact &
on determining charging current. fiction,” in Conference Record of IEEE IAS Annual
Determine whether the system will be alarm only or Meeting, 1988.
o As per reference 2 alarm only should be  J.R. Dunki-Jacobs, "The reality of high-resistance
considered only for systems where system grounding," IEEE Trans. Ind. Appl., vol. 1A-13, No.
voltage is less than 5 kV and NGR let- 5, September/October 1977.
through current is less than 10 A.
o If the system is tripping determine the  G.E. Paulson, “Monitoring neutral-grounding
amount of time that a fault will be allowed resistors,” IEEE Pulp and Paper Industry Technical
to remain on the system. Conference, Jun 1999.
Determine the desired trip level for ground-fault
protection.  D. Beeman, “Industrial Power System Handbook –
o The trip level should be above the charging First Edition,” 1955, Page 286 – 289.
current of the largest feeder.
o Ground-fault relaying with harmonic  J. Stoddard, "Sensitive earth fault protection in mines,"
filtering should be used to eliminate false IEE Conference on Electrical Safety in Hazardous
operation on harmonic current. Environments, March 16-18, 1971.
Select an appropriate tripping ratio, usually between
5 and 10.  J. Lewis Blackburn and Thomas J. Domin, “Protective
o The higher the tripping ratio the better the Relaying, Principals and Applications, Third Edition,”
ground-fault relays will be at detecting high- December 2006, Page 204 – 208.
resistance ground faults. If there is more
than 10 A of ground-fault current a tripping  David S. Baker, “Charging Current Data for
system should be used. Guesswork-Free Design of High-Resistance Grounded
Determine the NGR let-through current based on Systems,” IEEE Transactions on Industry
the ground-fault protection trip level, and the Applications. March/April 1979.
Include continuous NGR monitoring as an NGR
failure results in the loss of current sensing ground-
fault protection and the loss of many of the benefits
of resistance grounding.
Don Selkirk, P.Eng Littelfuse Startco May 2008