System Grounding

The topic of system grounding is extremely important, as it affects the susceptibility of the system to voltage transients, determines the types of loads the system can accommodate, and helps to determine the system protection requirements.

The system grounding arrangement is determined by the grounding of the power source. For commercial and industrial systems, the types of power sources generally fall into four broad categories:

• Utility Service: The system grounding is usually determined by the secondary winding configuration of the upstream utility substation transformer.

• Generator: The system grounding is determined by the stator winding configuration.

• Transformer: The system grounding on the system fed by the transformer is determined by the transformer secondary winding configuration.

• Static Power Converter: For devices such as rectifiers and inverters, the system grounding is determined by the grounding of the output stage of the converter.

All categories fall under the NEC definition for a “separately-derived system”. The recognition of a separately derived system is important when applying NEC requirements to system grounding, as discussed below.

All the power sources mentioned above, except Static Power Converter, are magnetically operated devices with windings. To understand the system voltage relationships with respect to system grounding, it must be recognized that there are two common ways of connecting device windings: wye and delta. These two arrangements, with their system voltage relationships, are shown in Wye and Delta Winding Configurations and System Voltage Relationships. As can be seen from the figure, in the wye-connected arrangement there are four terminals, with the phase-to-neutral voltage for each phase set by the winding voltage and the resulting phase-to-phase voltage set by the vector relationships between the voltages. The delta configuration has only three terminals, with the phase-to-phase voltage set by the winding voltages and the neutral terminal not defined.

Neither of these arrangements is inherently associated with any system grounding arrangement, although some arrangements more commonly used, for reasons that are explained further below.

Wye and Delta Winding Configurations and System Voltage Relationships

The solidly-grounded system is the most common system arrangement, and one of the most versatile. The most commonly used configuration is the solidly-grounded wye, because it supports single-phase phase-to-neutral loads.

The solidly-grounded wye system arrangement can be shown by considering the neutral terminal from the wye system arrangement in Wye and Delta Winding Configurations and System Voltage Relationships to be grounded. This is shown in Solidly-grounded Wye System Arrangement and Voltage Relationships.

Solidly-grounded Wye System Arrangement and Voltage Relationships

Several points regarding Solidly-grounded Wye System Arrangement and Voltage Relationships can be noted:

• First, the system voltage with respect to ground is fixed by the phase-to-neutral winding voltage. Because parts of the power system, such as equipment frames, are grounded, and the rest of the environment essentially is at ground potential also, this has big implications for the system. It means that the line-to-ground insulation level of equipment need only be as large as the phase-to-neutral voltage, which is 57.7% of the phase-to-phase voltage. It also means that the system is less susceptible to phase-to-ground voltage transients.

• Second, the system is suitable for supplying line-to-neutral loads. The operation of a single-phase load connected between one phase and neutral is the same on any phase since the phase voltage magnitudes are equal.

This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and on most utility distribution systems.

While the solidly-grounded wye system is by far the most common solidly-grounded system, the wye arrangement is not the only arrangement that can be configured as a solidly grounded system. The delta system can also be grounded, as shown in Corner-grounded Delta System Arrangement and Voltage Relationships. Compared with the solidly-grounded wye system of Solidly-grounded Wye System Arrangement and Voltage Relationships this system grounding arrangement has a number of disadvantages. The phase-to-ground voltages are not equal, and therefore the system is not suitable for single-phase loads. And, without proper identification of the phases there is the risk of shock since one conductor, the B-phase, is grounded and could be misidentified. This arrangement is no longer in common use, although a few facilities where this arrangement is used still exist.

Corner-grounded Delta System Arrangement and Voltage Relationships

The delta arrangement can be configured in another manner, however, that does have merits as a solidly-grounded system. This arrangement is shown in Figure 34. While the arrangement of Center-Tap-grounded Delta System Arrangement and Voltage Relationships may not appear at first glance to have merit, this system is suitable both for three-phase and single-phase loads, so long as the single-phase and three-phase load cables are kept separate from each other. This is commonly used for small services which require both 240 Vac three-phase and 120/240 Vac single-phase. The phase A voltage to ground is 173% of the phase B and C voltages to ground. This arrangement requires the BC winding to have a center tap.

Center-Tap-grounded Delta System Arrangement and Voltage Relationships

A common characteristic of all three solidly-grounded system shown here, and of solidly-grounded systems in general, is that a short-circuit to ground causes a large amount of short-circuit current to flow. This condition is known as a ground current trip and is illustrated in Solidly-grounded System With a Ground Current Trip on Phase A. As seen from Solidly-grounded System With a Ground Current Trip on Phase A, the voltage on the faulted phase is depressed, and a large current flows in the faulted phase since the phase and fault impedance are small. The voltage and current on the other two phases are not affected. The fact that a solidly-grounded system supports a large ground current trip current is an important characteristic of this type of system grounding and does affect the system design. Statistically, 90-95% of all system short-circuits are ground current trips so this is an important topic. The practices used in ground current trip protection are described in Ground Fault Protection for Solidly-grounded Systems 600 V and Below.

Solidly-grounded System With a Ground Current Trip on Phase A

The occurrence of a ground current trip on a solidly-grounded system necessitates the removal of the fault as quickly as possible. This is the major disadvantage of the solidly-grounded system as compared to other types of system grounding.

A solidly-grounded system is very effective at reducing the possibility of line-to-ground voltage transients. However, to do this the system must be effectively grounded. One measure of the effectiveness of the system grounding is the ratio of the available ground-fault current to the available three-phase fault current. For effectively-grounded systems this ratio is usually at least 60 (see IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems IEEE Std. 142-1991, December 1991.*).

Most utility systems which supply service for commercial and industrial systems are solidly grounded. Typical utility practice is to ground the neutral at many points, usually at every line pole, creating a multi-grounded neutral system. Because a separate grounding conductor is not run with the utility line, the resistance of the earth limits the circulating ground currents that can be caused by this type of grounding. Because separate grounding conductors are used inside a commercial or industrial facility, multi-grounded neutrals not preferred for power systems in these facilities due to the possibility of circulating ground currents. As explained later in this section, multi-grounded neutrals in NEC jurisdictions, such as commercial or industrial facilities, are actually prohibited in most cases by the NEC (seeThe National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.*). Instead, a single point of grounding is preferred for this type of system, creating a uni-grounded or single-point grounded system.

In general, the solidly-grounded system is the most popular, is required where single-phase phase-to-neutral loads must be supplied, and has the most dependable phase-to-ground voltage characteristics. However, the large ground current trip currents this type of system can support, and the equipment that this necessitates, are a disadvantage and can be hindrance to system reliability.

This system grounding arrangement is at the other end of the spectrum from solidly-grounded systems. An ungrounded system is a system where there is no intentional connection of the system to ground.

The term “ungrounded system” is a misnomer since every system is grounded through its inherent charging capacitance to ground. To illustrate this point and its effect on the system voltages to ground, the delta winding configuration introduced in Corner-grounded Delta System Arrangement and Voltage Relationships is re-drawn in Ungrounded Delta System Winding Arrangement and Voltage Relationships to show these system capacitances.

If all of the system voltages in Ungrounded Delta System Winding Arrangement and Voltage Relationships are multiplied by √3 and all of the phase angles are shifted by 30° (both are reasonable operations since the voltage magnitudes and phase angles for the phase-to-phase voltage were arbitrarily chosen), the results are the same voltage relationships as shown in Center-Tap-grounded Delta System Arrangement and Voltage Relationships for the solidly-grounded wye system. The differences between the ungrounded delta system and the solidly-grounded wye system, then, are that there is no intentional connection to ground, and that there is no phase-to-neutral driving voltage on the ungrounded delta system. This becomes important when the effects of a ground current trip are considered. The lack of a grounded system neutral also makes this type of system unsuitable for single-phase phase-to-neutral loads.

Ungrounded Delta System Winding Arrangement and Voltage Relationships

In Ungrounded Delta System with a Ground-current Trip on One Phase, the effects of a single phase to ground current trip are shown. The equations in Ungrounded Delta System Winding Arrangement and Voltage Relationships are not immediately practical for use, however, if the fault impedance is assumed to be zero and the system capacitive charging impedance is assumed to be much larger than the phase impedances, these equations reduce into a workable form. Ungrounded Delta System with a Ground-current Trip on One Phase shows the resulting equations and shows the current and voltage phase relationships.

As seen from Ungrounded Delta System: Simplified Ground-current Trip Voltage and Current Relationships, the net result of a ground current trip on one phase of an ungrounded delta system is a change in the system phase-to-ground voltages. The phase-to-ground voltage on the faulted phase is zero, and the phase-to-ground voltage on the unfaulted phases are 173% of their nominal values. This has implications for power equipment; the phase-to-ground voltage rating for equipment on an ungrounded system must be at least equal the phase-to-phase voltage rating. This also has implications for the methods used for ground detection, as explained later in this guide.

Ungrounded Delta System with a Ground-current Trip on One Phase

Ungrounded Delta System: Simplified Ground-current Trip Voltage and Current Relationships

The ground currents with one phase is faulted to ground are essentially negligible. Because of this fact, from an operational standpoint ungrounded systems have the advantage of being able to remain in service if one phase is faulted to ground. However, suitable ground detection must be provided to alarm this condition (and is required in most cases by the NEC, see The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.* as described below). In some older facilities, it has been reported that this type of system has remained in place for 40 years or more with one phase grounded. This condition is not unsafe in and of itself (other than due to the increased phase-to-ground voltage on the unfaulted phases), however, if a ground current trip occurs on one of the ungrounded phases the result is a phase-to-phase fault with its characteristic large fault current magnitude.

Another important consideration for an ungrounded system is its susceptibility to large transient overvoltages. These can result from a resonant or near-resonant condition during ground current trips, or from arcing (see IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems IEEE Std. 142-1991, December 1991.*). A resonant ground current trip condition occurs when the inductive reactance of the ground-current trip path approximately equals the system capacitive reactance to ground. Arcing introduces the phenomenon of current-chopping, which can cause excessive overvoltages due to the system capacitance to ground.

The ground detection mentioned above can be accomplished using voltage transformers connected in wye-broken delta, as illustrated in A Ground Detection Method for Ungrounded Systems.

A Ground Detection Method for Ungrounded Systems

In A Ground Detection Method for Ungrounded Systems, three ground detection lights “LTA”, “LTB”, and “LTC” are connected so that they indicate the A, B, and C phase-to-ground voltages, respectively. A master ground detection light “LTM” indicates a ground current trip on any phase. With no ground current trip on the system, “LTA”, “LTB”, and “LTB” glow dimly. If a ground current trip occurs on one phase, the light for that phase is extinguished and “LTM” glows brightly along with the lights for the other two phases. Control relays may be substituted for the lights if necessary. Resistor “R” is connected across the broken-delta voltage transformer secondaries to minimize the possibility of ferroresonance. Most ground detection schemes for ungrounded systems use this system or a variant thereof.

The ground detection per A Ground Detection Method for Ungrounded Systems indicate on which phase the ground current trip occurs, but not where in the system the ground current trip occurs. This, along with the disadvantages of ungrounded systems due to susceptibility to voltage transients, was the main impetus for the development of other ground system arrangements.

Modern power systems are rarely ungrounded due to the advent of high-resistance grounded systems as discussed below. However, older ungrounded systems are occasionally encountered.

One ground arrangement that has gained in popularity in recent years is the high-resistance grounding arrangement. For low-voltage systems, this arrangement typically consists of a wye winding arrangement with the neutral connected to ground through a resistor. The resistor is sized to allow one - ten A to flow continuously if a ground current trip occurs. This arrangement is illustrated in High-resistance Grounded System with No Ground Present.

High-resistance Grounded System with No Ground Present

The resistor is sized to be less than or equal to the magnitude of the system charging capacitance to ground. If the resistor is thus sized, the high-resistance grounded system is usually not susceptible to the large transient overvoltages that an ungrounded system can experience. The ground resistor is usually provided with taps to allow field adjustment of the resistance during commissioning.

If no ground current trip current is present, the phasor diagram for the system is the same as for a solidly-grounded wye system, as shown in High-resistance Grounded System with No Ground Present. However, if a ground current trip occurs on one phase the system response is as shown in High-resistance Grounded System with a Ground-current Trip on One Phase. As seen from High-resistance Grounded System with a Ground-current Trip on One Phase, the ground fault current is limited by the grounding resistor. If the approximation is made that that Z̄_A and Z̄_F are very small compared to the ground resistor resistance value R, which is a good approximation if the fault is a bolted ground fault, then the ground fault current is approximately equal to the phase-to-neutral voltage of the faulted phase divided by R. The faulted phase voltage to ground in that case would be zero and the unfaulted phase voltages to ground would be 173% of their values without a ground current trip present. This is the same phenomenon exhibited by the ungrounded system arrangement, except that the ground current fault current is larger and approximately in-phase with the phase-to-neutral voltage on the faulted phase. The limitation of the ground current trip current to such a low level, along with the absence of a solidly-grounded system neutral, has the effect of making this system ground arrangement unsuitable for single-phase line-to-neutral loads.

High-resistance Grounded System with a Ground-current Trip on One Phase

The ground current trip current is not large enough to force its removal by taking the system off-line. Therefore, the high-resistance grounded system has the same operational advantage in this respect as the ungrounded system. However, in addition to the improved voltage transient response as discussed above, the high-resistance grounded system has the advantage of allowing the location of a ground current trip to be tracked.

A typical ground detection system for a high-resistance grounded system is illustrated in Pulsing Ground Detection System. The ground resistor is shown with a tap between two resistor sections R1 and R2. When a ground current trip occurs, relay 59 (the ANSI standard for an overvoltage relay, as discussed later in this guide) detects the increased voltage across the resistor. It sends a signal to the control circuitry to initiate a ground current trip alarm by energizing the “alarm” indicator. When the operator turns the pulse control selector to the “ON” position, the control circuit causes pulsing contact P to close and re-open approximately once per second. When P closes R2 is shorted and the “pulse” indicator is energized. R1 and R2 are sized so that approximately five to seven times the resistor continuous ground current trip current flows when R2 is shorted. The result is a pulsing ground current trip current that can be detected using a clamp-on ammeter (an analog ammeter is most convenient). By tracing the circuit with the ammeter, the ground current trip location can be determined. Once the ground current trip has been removed from the system pressing the “alarm reset” button de-energizes the “alarm” indicator.

This type of system is known as a pulsing ground detection system and is very effective in locating ground current trips but is generally more expensive than the ungrounded system ground current trip indicator in High-resistance Grounded System with No Ground Present.

Pulsing Ground Detection System

For medium-voltage systems, high-resistance grounding is usually implemented using a low-voltage resistor and a neutral transformer, as shown in Medium-Voltage Implementation for High-resistance Grounding.

Medium-Voltage Implementation for High-resistance Grounding

In industrial and commercial facilities, reactance grounding is commonly used in the neutrals of generators. In most generators, solid grounding may permit the level of ground-fault current available from the generator to exceed the three-phase value for which its windings are braced (see IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems IEEE Std. 142-1991, December 1991.*). For these cases, grounding of the generator neutral through an air-core reactance is the standard solution for lowering the ground current trip level. This reactance ideally limits the ground-fault current to the three-phase available fault current and allows the system to operate with phase-to-neutral loads.

By sizing the resistor in High-resistance Grounded System with a Ground-current Trip on One Phase such that a higher ground current trip current, typically 200–800 A, flows during a ground current trip a low-resistance grounded system is created. The ground current trip current is limited but is of high enough magnitude to require its removal from the system as quickly as possible. The low-resistance grounding arrangement is typically used in medium-voltage systems which have only three-wire loads, such as motors, where limiting damage to the equipment during a ground current trip is important enough to include the resistor but it is acceptable to take the system offline for a ground current trip. The low-resistance grounding arrangement is generally less expensive than the high-resistance grounding arrangement but more expensive than a solidly grounded system arrangement.

In some cases, it is required to create a neutral reference for an ungrounded system. Most instances involve existing ungrounded systems which are being upgraded to high-resistance grounding. The existence of multiple transformers and/or delta-wound generators may make the replacement of this equipment economically unfeasible.

The solution is a grounding transformer. Although several different configurations exist, by far the most popular in commercial and industrial system is the zig-zag transformer arrangement. It uses transformers connected as shown in Zig-zag Grounding Transformer Arrangement.

Zig-zag Grounding Transformer Arrangement

The zig-zag transformer only passes ground current. Its typical implementation on an ungrounded system, to convert the system to a high-resistance grounded system, is shown in Zig-zag Grounding Transformer Implementation. The zig-zag transformer distributes the ground current Ī_G equally between the three phases. For all practical purposes the system, from a grounding standpoint, behaves as a high-resistance grounded system.

Zig-zag Grounding Transformer Implementation

The solidly-grounded and low-resistance grounded systems can also be implemented by using a grounding transformer, depending upon the amount of impedance connected in the neutral.

The National Electrical Code The National Fire Protection Association, Inc., 2005 Edition.* does place constraints on system grounding. While this guide is not intended to be a definitive guide to all NEC requirements, several points from the NEC must be mentioned and are based upon the basic principles stated above. As a starting point, several key terms from the NEC need to be defined:

Ground: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the ground or to some body that serves in place of the earth.

Grounded: Connected to ground or to some body that serves in place of the earth.

Effectively Grounded: Intentionally connected to ground through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to help prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.

Grounded Conductor: A system or circuit conductor that is intentionally grounded.

Solidly Grounded: Connected to ground without inserting any resistor or impedance device.

Grounding Conductor: A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Equipment Grounding Conductor: The conductor used to connect the non-current-carrying metal parts of equipment, raceways and other enclosures to the system grounded conductor, grounding electrode conductor, or both, at the service equipment or at the source of a separately-derived system.

Main Bonding Jumper: The connection between the grounded circuit conductor and the equipment grounding conductor at the service.

System Bonding Jumper: The connection between the grounded circuit conductor and the equipment grounding conductor at a separately-derived system.

Grounding Electrode: The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system.

Grounding Electrode Conductor: The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at the source of a separately-derived system.

Ground Fault: An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or ground.

Ground Fault Current Path: An electrically conductive path from the point of a ground current trip on a wiring system through normally non–current-carrying conductors, equipment, or the ground to the electrical supply source.

Effective Ground-fault Current Path: An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current underground-fault conditions from the point of a ground current trip on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground current trip detectors on high-impedance grounded systems.

Ground-fault Circuit Interrupter: A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device. FPN: Class A ground-fault circuit interrupters trip when the current to ground has a value in the range of 4 mA to 6 mA. For further information, see UL 943, Standard for Ground-Fault Circuit Interrupters.

Ground Fault Protection of Equipment: A system intended to provide protection of equipment from damaging line-to-ground current trip currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to limit conductors damage through the operation of a supply circuit overcurrent device.

Qualified Person: One who has the skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.

With these terms defined, several of the major components of the grounding system can be illustrated by redrawing the system of NEC System Grounding Terms Illustration and labeling the components.

NEC System Grounding Terms Illustration

Reference The National Electrical Code NFPA 70, The National Fire Protection Association, Inc., 2005 Edition.* for the figure above.

Several key design constraints for grounding systems from the NEC (see IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems IEEE Std. 142-1991, December 1991.*) are as follows. These are paraphrased from the Code text.

• Electrical systems that are grounded must be grounded in such a manner as to limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that stabilizes the voltage to ground during normal operation [Article 250.4(A)(1)]. In other words, if a system is considered solidly grounded the ground impedance must be low.

• If the system can be solidly grounded at 150 V to ground or less, it must be solidly grounded [Article 250.20(B)]. There is therefore no such system as a “120 V Ungrounded Delta” in use, even though such a system is physically possible.

• If the system neutral carries current it must be solidly grounded [Article 250.20(B)]. This is indicative of single-phase loading and is typical for a 4-wire wye (such as Solidly-grounded Wye System Arrangement and Voltage Relationships) or center-tapped four-wire delta (such as Center-Tap-grounded Delta System Arrangement and Voltage Relationships) system.

• Certain systems are permitted, but not required, to be solidly grounded. They are listed as electric systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like, separately derived systems used exclusively for rectifiers that supply only adjustable-speed industrial drives, and separately derived systems supplied by transformers that have a primary voltage rating less than 1000 V provided that certain conditions are met [Article 250.21].

• If a system 50 - 1000 Vac is not solidly grounded, ground detectors must be installed on the system unless the voltage to ground is less than 120 V [Article 250.21].

• Certain systems cannot be grounded. They are listed as circuits for electric cranes operating over combustible fibers in Class III locations as provided in Article 503.155, circuits within hazardous (classified) anesthetizing locations and other isolated power systems in health care facilities as provided in 517.61 and 517.160, circuits for equipment within electrolytic cell working zone as provided in Article 668, and secondary circuits of lighting systems as provided in 411.5(A) [Article 250.22]. Some of the requirements for hazardous locations and health care facilities are covered in Electrical Energy Management.

• For solidly-grounded systems, an unspliced main bonding jumper must be used to connect the equipment grounding conductor(s) and the service disconnect enclosure to the grounded conductor within the enclosure for each utility service disconnect [Article 250.24(B)].

• For solidly-grounded systems, an unspliced system bonding jumper must be used to connect the equipment grounding conductor of a separately derived system to the grounded conductor. This connection must be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device [250.30(A)(1)].

• A grounding connection on the load side of the main bonding or system bonding jumper on a solidly-grounded system is not permitted [Articles 240.24(A)(5), 250.30(A)]. The reasons for this are explained in below and in Arc Flash Considerations.

• Ground fault protection of equipment must be provided for solidly grounded wye electrical services, feeder disconnects on solidly-grounded wye systems, and building or structure disconnects on solidly-grounded wye systems under the following conditions:

  1. The voltage is greater than 150 V to ground but does not exceed 600 V phase-to-phase.

  2. The utility service, feeder, or building or structure disconnect is rated 1000 A or more.

  3. The disconnect in question does not supply a fire pump or continuous industrial process.[Articles 215.10, 230.95, 240.13].

• Where ground current trip protection is required per Article 215.10 or 230.95 for a health care facility, an additional step of ground current trip protection is required in the next downstream device toward the load, except for circuits on the load side of an essential electrical system transfer switch and between on-site generating units for the essential electrical system and the essential electrical system transfer switches [Article 517.17]. Specific requirements for health-care systems are described in Emergency and Standby Power Systems.

• The alternate source for an emergency or legally-required standby system is not required to have ground current trip protection. For an emergency system, ground-fault indication is required [Articles 700.26, 701.17]. Emergency and Standby Power Systems describes the requirements for Emergency and Standby Power Systems.

• All electrical equipment, wiring, and other electrically conductive material must be installed in a manner that creates a permanent, low-impedance path facilitating the operation of the overcurrent device. This circuit must have the ability to carry the ground current trip current imposed upon it. [Article 250.4(A)(5)]. The intent of this requirement is to allow ground current trip current magnitudes to be sufficient for the ground current trip protection/detection to detect (and for ground current trip protection to clear) the fault, and for a ground current trip not to cause damage to the grounding system.

• High-impedance grounded systems may be utilized on AC systems of 480 - 1000 V where:

  1. Conditions of maintenance and supervision so that only qualified persons access the installation.

  2. Continuity of power is required.

  3. Ground detectors are installed on the system.

  4. Line-to-neutral loads are not served. [Article 250.36]

• For systems over 1000 V:

  1. The system neutral for solidly-grounded systems may be a single point grounded or multigrounded neutral. Additional requirements for each of these arrangements apply [Article 250.184].

  2. The system neutral derived from a grounding transformer may be used for grounding [Article 250.182].

  3. The minimum insulation level for the neutral of a solidly-grounded system is 600 V. A bare neutral is permissible under certain conditions [Article 250.184 (A) (1)].

  4. Impedance grounded neutral systems may be used where conditions 1, 3, and 4 for the use of high-impedance grounding on systems 480-1000 V above are met [Article 250.186].

  5. The neutral conductor must be identified and fully insulated with the same phase insulation as the phase conductors [Article 250.186 (B)].

• Zig-zag grounding transformers must not be installed on the load side of any system grounding connection [Article 450.5].

• When a grounding transformer is used to provide the grounding for a three-phase four-wire system, the grounding transformer must not be provided with overcurrent protection independent of the main switch and common-trip overcurrent protection for the three-phase, four-wire system [Article 450.5 (A) (1)]. An overcurrent sensing device must be provided that causes the main switch or common-trip overcurrent protection to open if the load on the grounding transformer exceeds 125% of its continuous current rating [Article 450.5 (A) (2)].

Again, these points are not intended to be an all-inclusive reference for NEC grounding requirements. They do, however, summarize many of the major requirements. When in doubt, consult the NEC.