Electrical

Why is ground fault protection needed?

Electrical system ground fault protection is vital to ensure the safety of personnel and equipment and to reliably maintain systems based on the loads they serve

By Will McGugan and Lilly Vang September 24, 2020
Courtesy: CDM Smith

 

Learning Objectives

  • Learn about the common methods of electrical system grounding.
  • Determine the methods of ground fault protection that may be implemented.
  • Understand the importance of ground fault protection for personnel and equipment.

While the more powerful three-phase or arcing faults receives a large amount of attention when discussing electrical system protection, ground faults actually present a far more common and — in aggregate — potentially more dangerous occurrence for personnel and equipment. Grounding, bonding and ground fault protection are vital to decrease shock hazards to personnel during a ground fault such as when a current-carrying conductor’s insulation fails or inadvertently faults to ground.

Ground fault protection allows an electrical system to be maintained in a reliable manner matching the needs/loads it serves and to isolate undesirable fault conditions in a quick and ideally selective manner.

Per the 2017 edition of NFPA 70: National Electrical Code 250.4(A)(1) Electrical System Grounding, “Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.”

Ground fault protection for specific equipment such as generators, transformers or motors is worthy of several articles and will only be discussed tangentially here. Unless noted otherwise, all discussion of ground fault protection applies primarily to the coordination of distribution protective devices.

Ground, as defined in the NEC, is the earth. A ground-fault occurs when a conductor at a voltage other than the earth reference comes into contact with another conductive material connected to ground. Depending on the type of grounding system installed, this can lead to transient or sustained overvoltages, undervoltages and undesirable single-line-to-ground fault currents. If not isolated, these faults can harm personnel and damage to systems or even propagate into faults of higher magnitude.

It should be noted that an effective ground-fault current path‘s primary purpose is to keep noncurrent-carrying conductive materials as close to the system ground potential as possible and to provide a safe path for ground-fault current in the event of a fault.

However, the absence of an equipment grounding conductor, which is part of the effective ground-fault current path, does not mean an electrical system is ungrounded or that ground fault protection cannot be used. The zero-sequence impedance of a set of conductors is impacted by several factors including conductor arrangement, presence of grounding paths (equipment grounding conductor, cable shields, parallel paths, etc.) and the surrounding environment.

Figure 1: This is a switchgear installation with circuit breakers having phase- and ground fault protection capabilities. Courtesy: CDM Smith

Figure 1: This is a switchgear installation with circuit breakers having phase- and ground fault protection capabilities. Courtesy: CDM Smith

Grounding standards and codes

All electrical designs or construction must adhere to applicable standards and codes. The following are some of the items that relate to grounding and bonding of electrical systems and ground fault protection requirements:

Grounding system types

The term grounded or grounding is often loosely used in the industry. Grounding, per the 2017 NEC, is the connection to ground or to a conductive body that extends the ground connection. Grounding includes both equipment and systems. Equipment grounding refers to the connection together of the noncurrent carrying conductive parts of equipment and to the system grounded conductor or to the grounding electrode or both. System grounding refers to the connection of the power system grounded conductor or neutral to earth ground through a grounding electrode conductor.

There are different types of system grounding practiced in the industry, such as solidly grounded systems, (low or high) impedance grounded systems as well as ungrounded systems.

Ungrounded systems operate with no connection to earth ground. Per NEC 250.4 (B), noncurrent-carrying conductive materials enclosing electrical conductors or equipment or forming part of such equipment shall be connected to earth in a manner that will limit the voltage imposed by lightning or unintentional contact with higher-voltage lines and limit the voltage to ground on these materials. Two common ungrounded type systems are shown in Figure 2: an ungrounded delta type system or a wye with no grounded neutral system.

Figure 2: The illustrations show simplified typical wiring diagrams for different types of system grounding. To the left, a grounded wye-system; in the middle, an ungrounded delta system; and to the right, an ungrounded wye system. It is important to note that the equipment grounding conductor exists in all three examples. Courtesy: CDM Smith

Figure 2: The illustrations show simplified typical wiring diagrams for different types of system grounding. To the left, a grounded wye-system; in the middle, an ungrounded delta system; and to the right, an ungrounded wye system. It is important to note that the equipment grounding conductor exists in all three examples. Courtesy: CDM Smith

Ungrounded systems, while less common in the present, offer the following advantages:

  • Low line-to-line ground fault current because ground fault current is dependent on the capacitive current returning through the network phase to ground capacitance.
  • No flash hazard for personnel for accidental first occurrence line-to-ground fault.
  • Continued operation of process on the first occurrence of a line-to-ground fault.

However, even with these advantages, having an ungrounded system can make it difficult for personnel to locate line-to-ground faults. It also places personnel at risk during maintenance and offers little control over transient overvoltage.

There are two main types of alternating current system grounding: solidly grounded and impedance grounded. The benefits of grounded systems include:

  • Provides a common reference point.
  • Improved safety.
  • Better protection for equipment.
  • Ground fault locations can be detected easily.
  • Electrostatic accidents can be better avoided.
  • Allows better control of transient overvoltages.

Solidly grounded electrical systems have a grounded conductor connected to earth ground through a grounding electrode conductor with no intentionally added impedance in the circuit. The general requirements for solidly grounded systems can be found in the NEC Article 250.4(A); with some exceptions, NEC articles 215.10 and 240.13 require ground fault protection where low-voltage protective equipment is rated 1,000 amperes or greater.

Figure 3: This shows simplified current transformer wiring schemes for residual ground, core-balance and ground-return protection systems. In modern systems, the CT secondary wires are generally installed directly to the relay. In older systems, the residual ground CTs might be connected before the relay. Courtesy: CDM Smith

Figure 3: This shows simplified current transformer wiring schemes for residual ground, core-balance and ground-return protection systems. In modern systems, the CT secondary wires are generally installed directly to the relay. In older systems, the residual ground CTs might be connected before the relay. Courtesy: CDM Smith

Solidly grounded systems are the most common type of system and offer the following advantages:

  • Provides supply to line-neutral loads.
  • Ensures that ground fault, in one phase, does not cause the voltage of the other two phases to increase appreciably.
  • No possibility of transient overvoltage.

Along with these advantages, there are also a few disadvantages, including:

  • High severity for arc flash hazards.
  • High fault current values.
  • High possibility of single-phase fault becoming three-phase fault.

Impedance grounded systems are like solidly grounded system, but with an intentionally added impedance — typically a resistor — that limits the ground-fault current. The advantages of an impedance grounded system include:

  • Limits the ground fault current to a lower level; this is controllable depending on the type of impedance equipment installed (i.e., Peterson Coil).
  • Controls transient overvoltage.
  • Reduces line voltage drop caused by ground fault.
  • For high impedance systems, continued operation of the process/facility on the first occurrence of a line-to-ground fault while the fault is being located.

Disadvantages:

  • If line-neutral connected loads are required, a separate transformer must be installed to serve those loads.
  • Equipment such as breakers must be rated for line-line voltages, e.g.,“slash” rated breakers, 480/277 volts alternating current rated breakers are not allowed to be used on an impedance grounded system.

How is ground fault protection performed?

The type of ground fault protection employed and how it is measured are mostly determined by the type of system grounding employed, but are generally limited to voltage- and current-based detection systems. Knowledge of sequence networks — especially the zero-sequence network — is vital to understanding why a certain detection system is or is not ideal for a given type of system grounding, however, that is beyond the scope of this article. NEC Article 250 provides specific requirements for the type of protection that must be implemented for a given type of grounding system.

Figure 4: In this simplified 3V0 ground fault protection scheme, wiring the three potential transformer secondaries in series yields a vector sum of the phase voltages and therefore only the zero-sequence voltage is impressed on the resistor and relay. Courtesy: CDM Smith

Figure 4: In this simplified 3V0 ground fault protection scheme, wiring the three potential transformer secondaries in series yields a vector sum of the phase voltages and therefore only the zero-sequence voltage is impressed on the resistor and relay. Courtesy: CDM Smith

Ground fault protection on solidly grounded and impedance-grounded systems generally employ current-based detection systems, though the preferred form of measuring ground-current in each system varies with the expected magnitude of ground-fault current. Each system may use residual ground-fault detection, ground-return detection or core-balance detection; however, the latter may be overly sensitive for solidly grounded systems.

Ground fault protection on ungrounded systems are normally voltage-based, as no ground-fault current will flow if only one conductor is faulted to ground. If a second conductor were to fault to ground, the fault would be phase-to-phase and would not introduce significant ground-fault current. As the line-to-ground voltage on the unfaulted phases of an ungrounded system experiencing a fault will increase to match the line-to-line voltage, an overvoltage relay must be used to protect these systems per NEC 250.21(B).

High-impedance grounded systems can operate very similarly to ungrounded systems, so a protection system based upon voltage detection may also be advisable for these systems.

While current transformers may be installed in multiple arrangements, thanks to advancements in relay and protection technologies, it is rarely necessary to install them in anything other than a wye-arrangement. For existing systems that have CTs installed in another manner, such as delta, the methods of ground fault detection discussed may not apply.

Residual ground-fault detection systems use CTs installed for each set of phase conductors (and neutral, if applicable) to determine the ground-fault current. Traditionally, this was accomplished by combining one set of CT secondaries before the relay or trip unit such that the phasors would sum/cancel to yield the unbalanced current equal to the ground-fault current. In most modern relay protection systems, the protective device calculates the current unbalance internally from the direct input of the phase CTs, eliminating the need for additional CTs.

Core-balance ground-fault detection systems are referred to by a wide-variety of names: zero-sequence, window-CT or donut-CT, but all do the same thing: install a single large CT around all phase conductors (and neutral, if applicable). The CT measures the vector sum of the current-carrying conductors in a similar, but more direct manner than the residual ground-fault detection method, but does not need to be sized for the full fault current as is needed for individual phase CTs.

Figure 5: This shows a switchgear installation with relays having phase and ground fault protection capabilities. Courtesy: CDM Smith

Figure 5: This shows a switchgear installation with relays having phase and ground fault protection capabilities. Courtesy: CDM Smith

Care must be given when installing cables that provide a ground- or neutral-current return path (i.e., cable shields, integral ground conductors, etc.) where core-balance detection systems are employed to ensure the return-path conductors either do not pass through the window-CT or where they do, pass back through the same CT in the opposite direction. If the shielding is not installed correctly, the relay could operate prematurely.

Ground-return ground-fault detection systems use a CT installed on the ground-fault return path to the source — generally at the ground bus. While this system provides good sensitivity and control, it is not popular due to the potential for alternate ground-fault return paths (such as through a conductive conduit bonded to structural steel that is itself grounded); medium-voltage impedance grounded systems often use this method by measuring the current at the connection from the impedance to ground.

Ground differential protection systems can be considered a subset of ground-return detection systems when examining three-phase systems: CTs are installed in the ground fault return path and each feeder; if the current measured by the CT installed in the ground-fault return path differs from the sum of the currents measured by the other CTs, the system would operate.

Figure 6: Shown is a switchgear installation with neutral grounding resistor mounted on top of the transformer radiation fins. Courtesy: CDM Smith

Figure 6: Shown is a switchgear installation with neutral grounding resistor mounted on top of the transformer radiation fins. Courtesy: CDM Smith

Ground/neutral voltage detection: When a single phase in an ungrounded system faults to ground, the wye point of the source increases from effectively zero to the normal operation line-to-ground voltage. An overvoltage relay installed to either directly measure the voltage at the wye point or to measure the secondary voltage of a transformer installed between the wye point and ground can therefore be used to detect the fault.

In many ungrounded systems, a special type of ground fault relay, a broken delta voltage (3V0) overvoltage relay is used to measure zero-sequence components to provide sensitive protection. The ground/neutral voltage detection method may also be used in high-impedance low-voltage systems.

Ground fault coordination

Ground-fault overcurrent protection in circuit breakers generally takes the form of a single pickup with a definite time delay band, making selective coordination of ground-fault circuit breakers in some ways much easier than the selective coordination of phase-fault protection. It should be noted that many relays allow for the use of more advanced protection features such as inverse-time curves and multiple pickup and delay settings. The following standards and guides provide recommendations on the setup and selection of ground-fault protective device settings:

  • IEEE 242-2001: Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book). Chapter 8 Ground fault protection.
  • IEEE C37.101-2006: Guide for Generator Ground Protection.
  • IEEE C37.102-2006: Guide for AC Generator Protection.
  • IEEE 3003.1-2019: Recommended Practice for System Grounding.
  • IEEE 3003.2-2014: Recommended Practice for Equipment Grounding and Bonding in Industrial and Commercial Power Systems.
  • IEEE 3004.1-2013: Recommended Practice for the Application of Instrument Transformers in Industrial and Commercial Power Systems.
  • IEEE 3004.5-2014: Recommended Practice for the Application of Low-Voltage Circuit Breakers in Industrial and Commercial Power Systems.
  • IEEE 3004.8-2016: Recommended Practice for Motor Protection in Industrial and Commercial Power Systems.
  • IEEE 3004.11-2019: Recommended Practice for Bus and Switchgear Protection.

Coordination of ground-fault protective devices follows a similar process to that of phase overcurrent protective devices: the closer to the power source a device is, the higher pickup setting and time delay it will have.

However, there are two challenges unique to the coordination of ground-fault overcurrent protective devices that may arise, which can limit the ability of a system to be selectively coordinated. NEC 230.95 requires that the ground-fault pickup at a service be no more than 1,200 amperes and that the protection must operate in one second or less for ground-faults greater than or equal to 3,000 amperes, thus setting an upper bound for all ground-fault settings.

In systems where ground fault protection is installed throughout multiple levels of distribution (at the same voltage level), an engineer may reach a point where the ground-fault settings of a downstream protective device cannot be adjusted lower than the preceding protective device creating an area of miscoordination.

The second challenge may arise when ground-fault protective devices feed equipment that does not have ground fault protection. In such situations, the ground fault protection should be coordinated with the instantaneous setting of the downstream protective devices. As ground fault protection is generally low in magnitude and instantaneous phase protection is generally high in magnitude, it may not be possible to selectively coordinate these devices without negatively impacting the coordination of upstream protective devices. In such a situation, the engineer performing the coordination study should select the protective device settings such that any miscoordination is as far from the source and/or important equipment as possible.

Zone selective interlocking is often employed for both ground- and phase-fault protection schemes to ensure selective coordination and to improve coordination where the above limitations exist. When a fault occurs in a system with ZSI enabled, the protective device closest to the fault sends a signal to the next upstream device to block it from tripping for a given amount of time if the fault is not cleared during that time, the next upstream device then may trip and hopefully clear the fault; the ZSI trip/block signals propagate up to the highest level of the system containing ZSI. The major drawback of ZSI is that it requires control cables to be installed between each level of protection which can be costly depending on the devices’ proximity to each other.

Figure 7: This time-current curve shows selective coordination of the main and tie ground-fault devices with feeders, two also having ground fault protection and one having only phase-protection. Courtesy: CDM Smith

Figure 7: This time-current curve shows selective coordination of the main and tie ground-fault devices with feeders, two also having ground fault protection and one having only phase-protection. Courtesy: CDM Smith

Although ground faults do not receive the same infamy as three-phase or arcing faults, it is critical to have ground fault protection because ground faults are the most commonly seen fault type. Ground fault occurs when there is an unwanted connection between the ungrounded system conductors and ground. This type of fault can go unnoticed and potentially propagate into a higher fault current or causes equipment to be damaged over time. Undetected ground faults can pose potential safety risks to personnel that range from equipment malfunctions, to fire and electric shock.


Will McGugan and Lilly Vang
Author Bio: Will McGugan is an electrical engineer at CDM Smith, focusing in the design and analysis of electrical power systems. Lilly Vang is a junior electrical engineer at CDM Smith, where she focuses on electrical power system design and analysis.