Neutral wire grounding

An ungrounded system is one in which there is no intentional connection between the system conductors and earth. When the neutral of the system is not grounded, it is possible for high voltages to appear from line to ground during normal switching of a circuit having a line to ground fault. These voltages may cause failure of insulation at other locations on the system and result to damage to equipment.

Line to ground fault on ungrounded neutral systems causes a small amount of ground fault current to flow which may not be enough to actuate protective relays or other protective equipment.

Neutral grounding has been in practice in many systems all over the world. Generally, the neutrals of source transformers or generators with star connected windings are grounded. Grounding the neutral reduces the magnitude of transient voltages, improves protection against lightning, protection for line to ground fault becomes reliable, and improves reliability & safety. Also the potential of the neutral gets fixed.

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Line to ground fault on grounded neutral systems causes a large ground fault current that will very quickly burn the power feed fuse or trip other protective equipment. This means that the faults are detected quickly and the place of fault is quickly isolated from electrical distribution network (will not disturb operation of rest of electrical distribution network, and the potentially dangerous voltage at fault location are quickly cut of so reduced electrocution danger).

The typical disadvantages of grounded systems are related to high fault currents. In a typical solidly grounded three phase system, the neutral is tied directly to earth ground. This can cause high ground fault current (typically 200 to 20,000 amps) and excessive damage to transformers, generators, motors, wiring, and associated equipment. Some industrial electrical distribution networks use Neutral Grounding Resistor between neutral and ground limits fault current to a safer levels (typically 25 to 400 amps) while still allowing sufficient current flow to operate fault clearing the protective relays.

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  5. Grounding issues and minimizing EMI « Tomi Engdahl’s ePanorama blog says:

    [...] Neutral grounding has been in practice in many systems all over the world. Generally, the neutrals o… The typical disadvantages of grounded systems are related to high fault currents. The operational safety is the primary function of grounding. Grounding systems are designed so that they do provide the necessary safety functions. Grounding also have other functions in some applications (for example work as signal ground reference) but the safety should not be compromised in any case. [...]

    Reply
  6. Tomi Engdahl says:

    Choosing between grounded and ungrounded electrical system designs
    http://www.controleng.com/single-article/choosing-between-grounded-and-ungrounded-electrical-system-designs/ef08b7eecd8a3df4d717836b4776b5da.html

    Understanding both grounded and ungrounded electrical systems enables engineers to apply the appropriate grounding topology for the electrical system requirements.

    Grounding and shielding electrical systems are of key importance to electrical engineers. Understanding the basic operations between grounded and ungrounded electrical systems is necessary for matching the appropriate grounding topology to the desired electrical system performance.

    Selecting the proper grounding topology for an electrical distribution system is important to ensure facility occupant safety and health as well as reliable and safe electrical equipment operation. According to NFPA 70: National Electrical Code (NEC), Article 250.4(A)(1), the purpose of electrical system grounding is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.” The focus of Article 250 is to describe the grounding topologies available among grounded and ungrounded systems and how they operate.

    The purpose of grounding the electrical system as stated in NFPA 70: National Electrical Code (NEC) is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.” To achieve these goals, the NEC provides the framework for the selection of grounding methodologies in Article 250. The focus of this article is to describe the grounding topologies available among grounded and ungrounded systems and how they operate.

    The importance of providing a solidly grounded circuit for safety was recognized in the early editions of the NEC. According to “IAEA Soares Book on Grounding,” 100 years ago, the 1913 NEC committee required that “transformer secondaries of distributing systems must be grounded, provided the maximum difference of potential between the grounded point and any other point in the circuit does not exceed 150 V and may be grounded when the maximum difference of potential between the grounded point and any other point in the circuit exceeds 150 V.”

    Today, because grounded systems offer greater voltage stability, most of the systems described in Article 250.20 of the NEC require a grounded system, whether it is a solidly grounded system or an impedance grounded system. Historically, the most commonly used system is the solidly grounded system

    The NEC allows up to 25 ohms of ground resistance, recognizing different soil resistivities found across the U.S. However, the lower the ground resistance (or higher the ground conductivity), the better the ground fault detection system will operate. Typically, 5 ohms is a good design basis for commercial buildings.

    Fault types

    There are several types of faults that an electrical system must be designed to withstand. The worst case but less common fault is a 3-phase bolted fault with little or no circuit impedance in the fault path. Equipment is typically sized and noted with a fault current rating based on fault calculations for these situations. With little impedance in a grounded circuit, high fault current levels are possible and arc flash hazards may be present in a solidly grounded system. The high fault current levels are considered one of the main downsides of a solidly grounded system.

    Because of the importance of this current flow being high enough to trip overcurrent devices, the NEC requires that the neutral-to-ground bond be made within the service entrance equipment. This is essential for the ground fault detection scheme to operate correctly.

    Although a designer must account for the worst-case scenario, the 3-phase fault is quite rare. In fact, line-to-ground faults account for 90% to 95% of all recorded fault events in industrial settings. These faults can manifest themselves as arcing faults, which can cause current flow at a lower level than the overcurrent device rating. This is considered a serious drawback of the solidly grounded system because these faults may go undetected until equipment damage is done.

    Modern low-voltage transformers are primarily designed and constructed with delta primaries and wye secondaries. In most commercial and industrial applications, the standardized voltage is 480 Y/277 V on the secondary side. Early versions of the NEC didn’t require systems to be grounded on the secondary side for voltages higher than 150 V. Grounding the secondaries of these service transformers for safety and to minimize equipment risk didn’t gain momentum until the mid-1930s. A cost-effective solution was to ground a corner of the delta secondary. Therefore, many historic structures still have operating delta-delta service transformers where one corner of the transformer has been grounded to provide 120 V/240 V power within the facility.

    The primary goal for a solidly grounded system is to open the circuit as quickly as possible to limit damage and risk to life. For large process and industrial plants, stopping the process can be equally hazardous. Prior to the mid-1930s, the concept of an ungrounded system was still in favor because of the service continuity benefits that the ungrounded system provided. A fault on an ungrounded system doesn’t cause the source circuit breaker to trip. In fact, the system will keep operating until the operator tracks down the fault or until a second fault causes a major component in the electrical system to fault to ground, during which large magnitudes of current flow

    While theoretically this system is ungrounded, in reality the three phases are capacitively coupled to ground

    Rather than a true ground, it is the system capacitance that helps to stabilize the voltage during normal operating conditions. However, during a fault—typically from line to ground (via the system capacitance)—there is no direct ground connection, and there is no high current flow that would otherwise trip the circuit breaker to isolate the fault. Instead, it causes the phase voltage to rise 1.73 times the voltage on the other phases without tripping the breaker

    Because circuit breakers don’t trip, faults in an ungrounded system are difficult to trace and often go undetected until major equipment damage occurs during a second fault. Because of these issues, some industrial plants in the 1930s began converting their electrical infrastructures to grounded systems.

    Ungrounded, resistance grounded systems

    Although the NEC requires the majority of electrical systems to be grounded, some are actually required to be ungrounded. There are only five different electrical power systems/subsystems noted in NEC Article 250.22 where the code committee has determined the hazards of grounding to outweigh safety benefits of grounding.

    One of these system types is an isolated power system, which is a distribution power system of limited size, typically for use in hospital operating rooms. These areas are required to have an ungrounded system because it would be considered unacceptable to have a power outage during a surgical procedure.
    The 120 V equipment connected to these systems will continue to operate after the first fault, just as in an ungrounded system.
    The installation of the isolated power panel is alarmed locally, so if there is a ground fault, the team will be notified, but any ongoing procedures needn’t be interrupted.

    During the 1970s, language was added to the NEC to require ground fault trip sensors to feeders 1,000 A and above on 480 V grounded electrical systems. The need for electrical service continuity for the industrial process sector drove the need for a hybrid system to combine the stability and safety benefits of the grounded system with the continuous service benefits of the ungrounded system. During this time, resistance grounded systems began gaining traction.

    Impedance grounded systems include high resistance ground (HRG) and low resistance ground (LRG) configurations.
    In a wye connected HRG system, intermittent faults that cause so much trouble in ungrounded systems will be eliminated by the neutral system ground resistor because its insertion limits the total current flow to ground.
    System continuity is maintained because, although ground fault alarms occur, the overcurrent devices do not operate. This current flow in a low-voltage system (480 V to 600V) will be limited typically to 10 A so that the fault can be located and then repaired at a scheduled time without exposing staff to hazardous fault levels

    LRG-grounded systems are typically used for 15 kV medium-voltage applications where the charging current may be too high to match an HRG. LRG systems tend to operate more similarly to the solidly grounded system than the ungrounded system. In this case, the added resistor limits the fault currents between 200 A and 400 A, which is too high to allow continuous operation during a fault.

    Conclusion

    The NEC provides the framework for applying grounded and ungrounded systems.

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  7. Tomi Engdahl says:

    Neutral Ground Resistors for CSA Code Special Inspection
    http://www.eeweb.com/news/neutral-ground-resistors-for-csa-code-special-inspection

    Vishay Intertechnology, Inc. announced that Vishay Milwaukee (a product line of Vishay Dale Resistors) NGR series neutral ground resistors are now available for CSA code special inspection.

    NGR series resistors are designed to provide ground fault, overvoltage, and short circuit protection for generators and transformers in wye (star) configurations without exceeding the temperature limitations outlined by IEEE-32. Devices available for CSA code special inspection combine high line-neutral voltages to 8 kV and system voltages to 13.8 kV with high-temperature performance to 760 °C. Offering a tied live design to eliminate floating voltages in the assembly, the resistors feature current ratings from 100 A to 1000 A and a resistance range from 1.39 Ω to 80 Ω.

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