Forums » Electrical Engineering

grounding system

    • 149 posts
    February 15, 2019 11:21 PM PST

    Dear all,

    for mission-critical facilities which type of grounding system do you prefer on low voltage transformer side?

    • solidly grounding system
    • low resistance grounding system
    • high resistance grounding system

    Thanks in advanced.


    This post was edited by Hameedullah Ekhlas at February 15, 2019 11:22 PM PST
    • 70 posts
    February 16, 2019 3:48 AM PST

    Mostly used direct grounding system but for Low Voltage System low resistance and high resistance are also applicable and more safety.

    • 25 posts
    February 16, 2019 4:03 AM PST

     

    Solidly grounded. This type of grounding system is most commonly used in industrial and commercial power systems, where grounding conductors are connected to earth ground with no intentional added impedance in the circuit.

    Advantages

    • Good control of transient overvoltage from neutral to ground.

    • Allows user to easily locate faults.

    • Can supply line-neutral loads

    Disadvantages

    • Poses severe arc flash hazards.

    • Requires the purchase and installation of an expensive main breaker.

    • Unplanned interruption of production process.

    • Potential for severe equipment damage during a fault.

    • High values of fault current.

    • Likely escalation of single-phase fault to 3-phase fault.

    • Creates problems on the primary system.

     

    High-resistance grounding

    High-resistance grounding (HRG) systems are commonly used in plants and mills where continued operation of processes is paramount in the event of a fault. High-resistance grounding is normally accomplished by connecting the high side of a single-phase distribution transformer between the system neutral and ground, and connecting a resistor across the low-voltage secondary to provide the desired lower value of high side ground current.

    Should I install an ungrounded, solid, or high-resistance grounding system? That is the question asked by many designers and installers. The answer to this question depends on many factors. To make the correct decision, you must completely understand the pros and cons of each type of system. But first, you must also understand the different types of faults that can occur on your system and in what frequency they may appear.

    Faults and Failures. Faults can damage equipment and facilities, drive up costs due to lost production time, and lead to employee injuries, and even fatalities. The four types of faults include:

    • Line-to-ground faults, which represent about 98% of all failures.

    • Phase-to-phase faults, which account for about 1.5% of all failures.

    • 3-phase faults, which make up less than 0.5% of all faults and are often caused by human error. Failure to remove a grounding breaker, leaving ground clusters on systems, and lifting a truck bed into an open wire system can cause this type of fault.

    • Arcing faults are intermittent failures between phases or phase-to-ground. They’re discontinuous currents that alternately strike, extinguish, and strike again.

    Now that we’ve addressed the different types of faults that can appear on an electrical system, it’s time to provide an overview on the three main types of grounding systems you may encounter in the field.

    Grounding systems.

    1. Ungrounded. Electrical power systems that are operated with no intentional connection to earth ground are described as ungrounded. Although these systems were standard in the ’40s and ’50s, they’re still in use today. The main advantage of this type of grounding system is that it offers a low value of current flow and reliability during a fault. Unfortunately, this type of system also offers some big disadvantages. One major disadvantage to an ungrounded system is in the difficulty in locating a line-to-ground fault. Finding the fault is a time consuming process. For that reason, it’s often done on the weekends so a company doesn’t have to shut down its normal production processes. In addition, the fault must be located and repaired quickly because if a second fault occurs, the fault acts like a phase-to-phase fault extending the repair process.

    Advantages

    • Offers a low value of current flow for line-to-line ground fault (5A or less).

    • Presents no flash hazard to personnel for accidental line-to-ground fault.

    • Assures continued operation of processes on the first occurrence of a line-to-ground fault.

    • Low probability of line-to-ground arcing fault escalating to phase-to-phase or 3-phase fault.

    Disadvantages

    • Difficult to locate line-to-ground fault.
    • Doesn’t control transient overvoltages.

    • Cost of system maintenance is higher due to labor involved in locating ground faults.

    • A second ground fault on another phase will result in a phase-to-phase short circuit.

    2. Solidly grounded. This type of grounding system is most commonly used in industrial and commercial power systems, where grounding conductors are connected to earth ground with no intentional added impedance in the circuit. A main secondary circuit breaker is a vital component required in this system, although it has no bearing in other grounding systems. This component is large in size because it has to carry the full load current of the transformer. Back-up generators are frequently used in this type of grounding system in case a fault shuts down a production process. When this happens, the generators become solidly grounded. However, it’s important to note that the generators aren’t designed for the larger short circuit current associated with solidly grounded systems.

    A solidly grounded system has high values of current ranging between 10kA and 20kA. This current flows through grounding wires, building steel, conduit, and water pipes, which can cause major damage to equipment and shut down production processes. When a line-to-ground fault occurs, arcing can create flashes–generally in the terminating box. In this enclosed area, water is turned to steam, causing the terminating box. To locate the fault, all you need to do is follow the smoke.

    Advantages

    • Good control of transient overvoltage from neutral to ground.

    • Allows user to easily locate faults.

    • Can supply line-neutral loads.

    Disadvantages

    • Poses severe arc flash hazards.

    • Requires the purchase and installation of an expensive main breaker.

    • Unplanned interruption of production process.

    • Potential for severe equipment damage during a fault.

    • High values of fault current.

    • Likely escalation of single-phase fault to 3-phase fault.

    • Creates problems on the primary system.

    3. High-resistance grounding. High-resistance grounding (HRG) systems are commonly used in plants and mills where continued operation of processes is paramount in the event of a fault. High-resistance grounding is normally accomplished by connecting the high side of a single-phase distribution transformer between the system neutral and ground, and connecting a resistor across the low-voltage secondary to provide the desired lower value of high side ground current. With an HRG system, service is maintained even during a ground fault condition. If a fault does occur, alarm indications and lights help the user quickly locate and correct the problem or allow for an orderly shutdown of the process. An HRG system limits ground fault current to between 1A and 10A.

    Advantages

    • Limits the ground fault current to a low level.

    • Reduces electric shock hazards.

    • Controls transient overvoltages.

    • Reduces the mechanical stresses in circuits and equipment.

    • Maintains continuity of service.

    • Reduces the line voltage drop caused by the occurrence and clearing of a ground fault.

    Disadvantages

    • High frequencies can appear as nuisance alarms.

    • Ground fault may be left on system for an extended period of time.


    This post was edited by David Welson at February 16, 2019 11:49 PM PST
    • 149 posts
    February 16, 2019 1:00 PM PST

    Thanks

    • 200 posts
    February 18, 2019 11:44 AM PST

    Thanks for the suggestion for this topic, Hameed. The reply is in 3 parts.

    Knowledge sharing for all Z4E Members first:

    Mission Critical: it is any factor of a system (component, equipment, personnel, process, procedure, software, etc.) that is essential to business operation, to a community or to an organization. Failure or disruption of Mission Critical factors will result in serious impact on business operations or upon a community or an organization, and even can cause social turmoil and catastrophes.

    Mission Critical Facility: it is a facility that provides services and functions essential to a company, a community or an organization, especially during and after a failure, disruption, critical event or a disaster. Definition of Mission Critical Facilities (and systems within) is not only concern with life and death operations, or critical business operations, but also with damage to a business’ reputation, that has a much wider scope. In today’s technologically-advanced world, end users expect services and systems to be readily available whenever they want or need them. Should downtime occur, even for a few minutes, it could lead to significant damage to reputation;

    -- The companies and some private organizations consider a Mission Critical Facility as a facility that is essential to the overall operations of a business or process within a business. Essentially, something that is critical to the primary mission of a company or private organization. While there are computer data centers that are mission critical, places like laboratories, public safety centers, hospitals, military facilities and other locations are considered mission critical as well. The facilities (and processes and operations running within) that directly support an organization’s End Users and Customers are Mission-Critical because they are core to the company’s mission and, if they fail, they can cause significant financial or reputation damage to the organization. Examples of mission critical facilities (and systems within) are: online banking system centers, railway/aircraft operating and control centers, electric power stations, and computer systems centers that will adversely affect business and society when they fail;


    -- Certain critical infrastructure, government and military facilities, if they go down, they may also have an impact on national security. The U.S. government Federal Insurance and Mitigation Administration (FEMA) consider such facilities as (Mission) Critical Facilities: Police stations, Fire stations, critical vehicle and equipment storage facilities, and Emergency Operations Centers. Medical facilities such as: hospitals, nursing homes, blood banks, and health care facilities, including vital medical records storage centers. Schools and day care centers, power generating stations and other public and private utility facilities, drinking water and wastewater treatment plants, structures or facilities that produce, use, or store highly volatile, flammable, explosive, toxic, and/or water-reactive materials.

     


    This post was edited by Alex de Moura at February 18, 2019 11:45 AM PST
    • 200 posts
    February 18, 2019 11:57 AM PST

    Saying that, let's try to reply Hameed's question:

    -- Hameedullah Ekhlas said: "...which type of grounding system do you prefer on low voltage transformer side?", not sure if "preference" (of an Engineer or Technician) can be correctly applied to this topic. At first sight we could suppose it was a poll among Z4E Members;

    -- For this specific question the reading of EATON's "Grounding methods in mission critical facilities" is advised - here we can find few Rules of Thumb (RoT) that sometimes, depending on the Risk Analysis of Facility installation and the installed Transformer, are mandatory - and not a preference;

    -- UNGROUNDED: NOT RECOMMENDED for most modern electrical systems, specially on Mission-Critical ones;

    -- CORNER GROUNDED DELTA (CGD): if the installed Transformer is a Delta then a simple method of grounding is CGD although mandatory. It can establishes a ground reference for each current carry conductor, low cost, eliminates the problems (over-voltage, transient) of ungrounded systems. However, need to mark grounded phase throughout distribution system, cannot use lower-cost “slash-rated” circuit breakers (e.g., 480/277 or 240/120) and Ground Fault (GF) sensing is undefined for the grounded phase, so CANNOT BE USED at locations where GF sensing is required, such as healthcare facilities;
    [insert image: Figure 2. Corner Grounded Delta=imgsnap100.png]

    -- SOLIDLY (effectively) GROUNDED (SG): if the installed Transformer is a Wye (Y or Star), a SG connects the system Neutral directly to Earth Ground, it ensures that the Neutral and Ground voltages are equal, and each individual Phase (or Line) to Neutral voltage is fixed with a reference to Ground, ensuring that Phase-to-Ground over-voltages do not occur. PROPER INSTALLATION will bond the Neutral Ground at only one point, failing to pay attention to the location of bond points can disrupt Ground Fault (GF) sensing systems. Also, the large amounts of current that can flow during a GF require that the source be disconnected immediately upon the GF detection, since GFs are approximately 100 times more common than other types of Fault, SG can be a vulnerable design and it may NOT BE THE BEST CHOICE for Mission Critical applications;
    [insert image: Figure 3. Solidly Grounded System=imgsnap101.png]

    -- LOW RESISTANCE GROUNDED (LRG): also for installed Wye (Y or Star) Transformer, LRG is used to reduce damage caused by high currents flowing during Ground Faults (GFs). Very common on Medium Voltage systems, they can be used on Low Voltage systems. Even if the GF current can be reduced (from kA to x100A), the GF current is still large enough that requires the source be disconnected immediately. Unless there are overriding reasons as with the Solidly Grounded, a LRG system may NOT BE THE FIRST CHOICE for Mission Critical applications;

     

    -- HIGH RESISTANCE GROUNDED (HRG): HRG systems are similar to LRG systems, except that a Neutral Grounding Resistor (NGR) has a higher resistance value. This reduces the Ground Fault (GF) current to lower values, typically less than 10A on MV systems, and usually less than 5A on LV systems. It eliminates the need to trip immediately, allowing time to locate and clear the GF. HRG systems can include features that assist to locate the GF, in certain applications, HRG systems are RECOMMENDED for Mission Critical applications. However, while a Ground remains in the system, the Neutral voltage can be higher than Ground potential, and some equipment such as Uninterrupted Power Supplies (UPSs) can view a Neutral-to-Ground voltage as a Fault and inhibit transferring to and from bypass, so HRG systems may NOT BE RECOMMENDED for powering UPSs installed Facilities such as Oil & Gas, Information & Communication Technology (ICT) centers. The Designer need to weigh the benefits of increased power availability into the UPS (due to GFs not causing source trip) with potential problems resulting when UPSs switch to bypass while a GF is present. In all cases HRG can be configured to trip the upstream Breaker, "mimicking" a SG system, providing arc flash reduction in a single GF event because the NRG limits the GF current to low levels. HRG system is DESIRABLE for Mission Critical applications. However, the U.S. NEC and Canadian CEC place additional RESTRICTIONS on situations when HRG systems can be used, but tend to not apply to motor loads. In Mission Critical installations, larger motors are rarely connected Line-to-Neutral (one of PROHIBITIONS when using HRG), Mission Critical facilities could use HRG on the mechanical loads while using a solidly grounded system to power the ICT loads.
    [insert image: Figure 4. Resistance Grounded System=imgsnap102.png]


    This post was edited by Alex de Moura at February 18, 2019 11:57 AM PST
    • 200 posts
    February 18, 2019 12:04 PM PST

    Now, let's see some uncommon grounding methods also used in Mission Critical Facilities:

    -- RESONANT GROUNDED (RSG): RSG systems, also for installed Wye (Y or Star) Transformer, are grounded through a variable inductance called a "Petersen coil" instead of a NRG, the coil is in series with the Line-to-Ground capacitance of each Phase conductor and is tuned to produce an impedance based on the Series LC circuit. Since the coil can be adjusted based on what is connected at the time, Petersen coil would potentially need to be re-adjusted as different loads are switched on and off, so it is NOT BE THE BEST CHOICE for Mission Critical Facilities with frequent load variations;
    [insert image: Figure 5. Resonant Grounded System=imgsnap103.png]

    -- HYBRID HIGH-RESISTANCE GROUNDED (HHRG): are systems were developed for the case of PROTECTING GENERATORS from internal Ground Faults (GFs) - in the Wye (Y or Star) Transformer Primary connected to the Generator. They operate as a LRG system with a bypass device closed (2 parallel resistors) under NORMAL conditions. Note: the value of 2 resistors in parallel is less than the lowest resistance. When a Fault is detected, the bypass device opens and Ground current is forced through the higher NRG resistor as a HRG system. This reduces the current and internal generator lamination damage;
    [insert image: Figure 6. Hybrid High-Resistance Grounded System=imgsnap104.png]

    -- The Risk Analysis of the context where the Transformer is being installed SHALL BE DONE. The Table 1 below is intended to assist the Engineering/Technician to choose the best ground options to that specific context in Mission Critical Facility. However it is necessary to check the restrictions of all options in the text above:
    [insert image: Table 1. Overvoltage and Continuity Benefits=imgsnap105.png]

    I hope this helps. Thanks and regards to you all.
    Hameed, keep your good work - very good question by the way.


    This post was edited by Alex de Moura at February 18, 2019 12:05 PM PST
    • 149 posts
    February 18, 2019 12:55 PM PST

    Thanks Alex very good information.

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