Earthing system

Earthing system

Presented By:

* Md. Noman Saber Khan

The objective of a grounding system are:1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.2. To assure correct operation of electrical/electronic devices.3. To prevent damage to electrical/electronic apparatus.4. To dissipate lightning strokes.5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients.6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.

A safe grounding design has two objectives:1. To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service.

2. To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.

Introduction to GroundingThe primary goal of the grounding system throughout any facility is SAFETY. Grounding is implemented to ensure rapid clearing of faults and to prevent hazardous voltage, which in turn reduce the risks of fires and personnel injuries. Grounding serves the primary functions of referencing the AC systems and providing a means to ensure fault clearing.99.5% survival threshold 116 mA for one (1) second.367 mA for zero point one (0.1) second.

six (6) grounding systems in use1. Equipment grounds, 2. Static grounds, 3. Systems grounds, 4. Maintenance grounds, 5. Electronic grounds, 6. Lightning grounds.

Equipment grounds:Equipment grounds: An equipment ground is the physical connection to earth of non-current carrying metal parts. This type grounding is done so that all metal part of equipment that personnel can come into contact with are always at or near zero (0) volts with respect to ground. All metal parts must be interconnected and grounded by a conductor in such away as to ensure a path of lowest impedance for flow of ground fault current. Typical items (equipment) to be grounded are; electrical motor frames, outlet boxes, breaker panels, metal conduit, support structures, cable tray, to name a few.

Static grounds:A static ground is a connection made between a piece of equipment and the earth for the purpose of draining off static electricity charges before a flash over potential is reached. This type grounding system is utilized in dry materials handling, flammable liquid pumps and delivery equipment, plastic piping, and explosive storage facilities.

System grounds:A system ground refers to the point in an electrical circuit that is connected to earth. This connection point is typically at the electrical neutral. The sole purpose of the system ground is to protect equipment. This type ground also provides a low impedance path for fault currents improving ground fault coordination. This ensures longer insulation life of motors, transformers and other system components

Maintenance grounds:This type ground is utilized for safe work practices, and is a temporary ground.

Electronic and computer grounds:Grounding for electronic equipment is a special case in which the equipment ground and the system ground are combined and applied in unity. Electronic equipment grounding systems must not only provide a means of stabilizing input voltage levels, but also act as the zero (0) voltage reference point. Grounding systems for the modern electronics installation must be able to provide effective grounding and bonding functions well into the high frequency megahertz range.

Lightning protection:Lightning protection grounding requirements are dependent upon the structure, equipment to be protected, and the level of lightning protection required of desired.

Factors to be consideredThe area available for installation of the grounding system. This could lead to the requirement and utilization of chemical rods, or wells.Water table and seasonal changes to it.Soil condition and resistivity, Also elevation above sea level and hard rocky soil are concerns that would need to be addressed.Available fault currents (i.e., three (3) phase, line-to-ground, and line-to-line-to ground, etc.).NEC and ANSI/IEEE requirements. Also include here the requirements of the process equipment to be installed.Consideration to the number of lightning strikes and thunder storm days per year.Utility ties and/or service entrance voltage levels.Utilization of area were ground system is to be installed, (i.e., do not install under paved parking lot).

Defination of Protective Earth(A protective earth (PE connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It also ensures that in the case of an insulation fault, a high fault current flows, which will trigger an overcurrent protection device (fuse, MCB) that disconnects the power supply.

IEC nomenclature The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):T:direct connection of a point with earth I:no point is connected with earth (isolation), except perhaps via a high impedance.The second letter indicates the connection between earth and the electrical device being supplied:T:direct connection with earth, independent of any other earth connection in the supply systemN:connection to earth via the supply network

TN network In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer

TN

The conductor that connects the exposed metallic parts of the consumer is called protective earth PE. The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system is called neutral N. Three variants of TN systems are distinguished:

TNS:PE and N are separate conductors that are only connected near the power source

.TNC:A combined PEN conductor fulfills the functions of both a PE and an N conductor

TNCS:Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used.

TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected at any point after the building distribution point.

TN-C: combined PE and N conductor all the way from the transformer to the consuming device. .

TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.

TT network In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator..

IT network In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection.

Properties TN networks save the cost of a low-impedance earth connection at the site of each consumer. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems. TN-C networks save the cost of an additional conductor needed for separate N and PE connections. However to mitigate the risk of broken neutrals, special cable types and lots of connections to earth are needed. TT networks require RCD protection and often an expensive time delay type is needed to provide discrimination with an RCD downstream

Safety

In TN an insulation fault is very likely to lead to a high short-circuit current that will trigger an over current circuit-breaker or fuse and disconnect the L conductors.

In the majority of TT systems the earth fault loop impedance will be too high to do this and so an RCD must be employed

In TN-S and TT systems (and in TN-C-S beyond the point of the split), a residual-current device can be used as an additional protection. In the absence of any insulation fault in the consumer device, the equation IL1+IL2+IL3+IN = 0 holds, and an RCD can disconnect the supply as soon as this sum reaches a threshold (typically 10-500 mA). An insulation fault between either L or N and PE will trigger an RCD with high probability

In IT and TN-C networks, residual current devices are far less likely to detect an insulation fault. In a TN-C system they would also be very vulnerable to unwanted triggering from contact between earths of circuits on different RCDs or with real ground thus making their use impractical. Also RCDs usually isolate the neutral core which is dangerous in a TN-C system.

In TN-C and TN-C-S systems any connection between the combined neutral and earth core and the body of the earth could end up carrying significant current under normal conditions and could carry even more under a broken neutral situation.In TN-C and TN-C-S systems any break in the combined neutral and earth core which didn't also affect the live conductor could theoretically result in exposed metalwork rising to near "live" potential

Substation Earthing Calculation MethodologyCalculations for earth impedances and touch and step potentials are based on site measurements of ground resistivity and system fault levels. A grid layout with particular conductors is then analysed to determine the effective substation earthing resistance, from which the earthing voltage is calculated.

In practice, it is normal to take the highest fault level for substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for. To determine the earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained

Earthing Materials1. Conductors: Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves usually have a cross-sectional area of 95 square millimetres, and they are laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground earthing ring is usually provided, to which all metallic substation plant is bonded.

2. Connections: Connections to the grid and other earthing joints should not be soldered because the heat generated during fault conditions could cause a soldered joint to fail. Joints are usually bolted, and in this case, the face of the joints should be tinned. 3. Earthing Rods: The earthing grid must be supplemented by earthing rods to assist in the dissipation of earth fault currents and further reduce the overall substation earthing resistance. These rods are usually made of solid copper, or copper clad steel.

4. Switchyard Fence Earthing: The switchyard fence earthing practices are possible and are used by different utilities. These are: (i) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then bonded to the grid at regular intervals. (ii) Place the fence beyond the perimeter of the switchyard earthing grid and bond the fence to its own earthing rod system. This earthing rod system is not coupled to the main substation earthing grid.

ApparatusParts to be EarthedMethod Of ConnectionPower TransformerTransformer tankConnect the earthing bolt on transformer tank to station earth. Connect the neutral to earthing systemHigh Voltage Circuit BreakersOperating mechanism, frameConnect the earthing bolt on the frame and the operating mechanism of Circuit Breaker to earthing systemSurge ArresterLower Earth PointTo be directly connected to the earth matSupport of bushing insulators, lightning arresters, fuse, etc..Device Flange or Base PlateConnect the earthing bolt of the device to the station earthing systemPotential TransformerPotential transformer tank, LV neutral.Connect the transformer earthing bolt to earthing system Connect LV neutral of phase lead to case with flexible copper conductorIsolatorIsolator frame, operating mechanism, bedplateWeld the isolator base frame, connects it to the bolt on operating mechanism base plate and station earth.Current TransformerSecondary winding and metal caseConnect secondary winding to earthing bolt on transformer case with a flexible copper conductor.

Different Equipments and Ground Connections

How to Determine Correct Number of Earthing Electrodes

Number of Earthing Electrode and Earthing Resistance depends on the resistivity of soil and time for fault current to pass through (1 sec or 3 sec). If we divide the area for earthing required by the area of one earth plate gives the number of earth pits required.There is no general rule to calculate the exact number of earth pits and size of earthing strip, but discharging of leakage current is certainly dependent on the cross section area of the material so for any equipment the earth strip size is calculated on the current to be carried by that strip.

How to Determine Correct Number of Earthing ElectrodesFirst the leakage current to be carried is calculated and then size of the strip is determined.For most of the electrical equipment like transformer, diesel generator set etc., the general concept is to have 4 number of earth pits. 2 nos for body earthing with 2 separate strips with the pits shorted and 2 nos for Neutral with 2 separate strips with the pits shorted.The Size of Neutral Earthing Strip should be capable to carry neutral current of that equipment.The Size of Body Earthing should be capable to carry half of neutral Current

How to Determine Correct Number of Earthing ElectrodesFor example for 100kVA transformer, the full load current is around 140A.The strip connected should be capable to carry at least 70A (neutral current) which means a strip of GI 25x3mm should be enough to carry the current and for body a strip of 253 will do the needful. Normally we consider the strip size that is generally used as standards.However a strip with lesser size which can carry a current of 35A can be used for body earthing. The reason for using 2 earth pits for each body and neutral and then shorting them is to serve as back up. If one strip gets corroded and cuts the continuity is broken and the other leakage current flows through the other run thery by completing the circuit.

How to Determine Correct Number of Earthing ElectrodesSimilarly for panels the no of pits should be 2 nos. The size can be decided on the main incomer circuit breaker.For example if main incomer to breaker is 400A, then body earthing for panel can have a strip size of 256 mm which can easily carry 100A.Number of earth pits is decided by considering the total fault current to be dissipated to the ground in case of fault and the current that can be dissipated by each earth pit. Normally the density of current for GI strip can be roughly 200 amps per square cam. Based on the length and dia of the pipe used the number of earthing pits can be finalized.

Recommended Practices for Grounding

Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.Careful consideration should be given to installing a grounding system thatexceeds the minimum NEC requirements for improved safety and power quality.

Recommended practices for grounding

Equipment Grounding ConductorsIsolated Grounding SystemBranchCircuit GroundingGround ResistanceGround RodsGround RingGrounding Electrode SystemLightning Protection SystemSurge Protection Devices (SPD) (formerly called TVSS)

1. Equipment Grounding Conductors

The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.

1. Equipment Grounding Conductors

2. Isolated Grounding System

As permitted by NEC 250.146(D) and NEC 408.40 Exception, consider installing an isolated grounding system to provide a clean signal reference for the proper operation of sensitive electronic equipment.Isolated grounding is a technique that attempts to reduce the chances of noise entering the sensitive equipment through the equipment grounding conductor. The grounding pin is not electrically connected to the device yoke, and, so, not connected to the metallic outlet box. It is therefore isolated from the green wire ground.A separate conductor, green with a yellow stripe, is run to the panelboard with the rest of the circuit conductors, but it is usually not connected to the metallic enclosure. Instead it is insulated from the enclosure, and run all the way through to the ground bus of the service equipment or the ground connection of a separately derived system. Isolated grounding systems sometimes eliminate ground loop circulating currents.Note that the NEC prefers the term isolated ground, while the IEEE prefers the term insulated ground.

2. Isolated Grounding System

3. Branch-Circuit Grounding

Replace branch circuits that do not contain an equipment ground with branch circuits with an equipment ground. Sensitive electronic equipment, such as computers and computer-controlled equipment, require the reference to ground provided by an equipment grounding conductor for proper operation and for protection from static electricity and power surges.

Failure to utilize an equipment grounding conductor may cause current flow through low-voltage control or communication circuits, which are susceptible to malfunction and damage, or the earth.

Surge Protection Devices (SPDs) must have connection to an equipment grounding conductor.

4. Ground Resistance

Measure the resistance of the grounding electrode system to ground.Take reasonable measures to ensure that the resistance to ground is 25 ohms or less for typical loads. In many industrial cases, particularly where electronic loads are present, there are requirements which need values as low as 5 ohms or less many times as low as 1 ohm.Measuring earth resistance with fall of potential method (photo credit: eblogbd.com) For these special cases, establish a maintenance program for sensitive electronic loads to measure ground resistance semi-annually, initially, using a ground resistance meter. Ground resistance should be measured at least annually thereafter.When conducting these measurements, appropriate safety precautions should be taken to reduce the risk of electrical shock.Record the results for future reference.Investigate significant changes in ground resistance measurements compared with historical data, and correct deficiencies with the grounding system. Consult an electrical design professional for recommendations to reduce ground resistance where required.

4. Ground Resistance

5. Ground Rods

The NEC permits ground rods to be spaced as little as 6 feet apart, but spheres-of-influence of the rods verlar.

Recommended practice is to space multiple ground rods a minimum of twice the length of the rod apart. Install deep-driven or chemically-enhanced ground rods in mountainous or rocky terrain, and where soil conditions are poor. Detailed design of grounding systems are beyond the scope of this document.

5. Ground Rods

6. Ground Ring

In some cases, it may be advisable to install a copper ground ring, supplemented by driven ground rods, for new commercial and industrial construction in addition to metal water piping, structural building steel, and concrete-encased electrodes, as required by Code.Grounding rings provide a convenient place to bond multiple electrodes of a grounding system, such as multiple Ufer grounds, lightning down-conductors, multiple vertical electrodes, etc.Install ground rings completely around buildings and structures and below the frost line in a trench offset a few feet from the footprint of the building or structure. Where low, ground impedance is essential, supplement the ground ring with driven ground rods in a triplex configuration at each corner of the building or structure, and at the mid-point of each side.The emergency generator connected to the ring-ground, and additionally grounded to reinforcing rods in its concrete pad (photo credit: psihq.com) The NEC-minimum conductor size for a ground ring is 2 AWG, but sizes as large as 500 kcmil are more frequently used. The larger the conductor and the longer the conductor, the more surface area is in contact with the earth, and the lower the resistance to earth.

6. Ground Ring

7. Grounding Electrode SystemBond all grounding electrodes that are present, including metal underground water piping, structural building steel, concrete-encased electrodes, pipe and rod electrodes, plate electrodes, and the ground ring and all underground metal piping systems that cross the ground ring, to the grounding electrode system.

Bond the grounding electrodes of separate buildings in a campus environment together to create one grounding electrode system.

Bond all electrical systems, such as power, cable television, satellite television, and telephone systems, to the grounding electrode system. Bond outdoor metallic structures, such as antennas, radio towers, etc. to the grounding electrode system. Bond lightning protection down-conductors to the grounding electrode system.

7. Grounding Electrode System

8. Lightning Protection SystemCopper lightning protection systems may be superior to other metals in both corrosion and maintenance factors. NFPA 780 (Standard for the Installation of Lightning Protection Systems) should be considered as a minimum design standard.

A lightning protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.

8. Lightning Protection System

9. Surge Protection Devices (SPD) (formerly called TVSS)

The use of surge protection devices is highly recommended. Consult IEEE Standard 1100 (The Emerald Book) for design considerations. A surge protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.

Generally, a surge protection device should not be installed downstream from an uninterruptible power supply (UPS). Consult manufacturers guidelines.

9. Surge Protection Devices (SPD) (formerly called TVSS)

Earthing definitions- as per BNBC-

Earth Conductors in panel and to circuits Minimum Cross-sectional Area of Copper Earth Conductors in Relation to the Area of Associated Phase Conductors

Cross-sectional Area of PhaseConductor(s) (mm)Minimum Cross-sectional Area of the CorrespondingEarth Conductor (mm)Less than 16Same as cross-sectional area of phase conductor but notless than 14 SWG (3.243 mm)16 or greater but less then 351635 or greaterHalf the cross-sectional area of phase conductor

Earthing definitions- as per BNBC-NB- In case of copper wire being used as earth conductors, the size of the wire shall not be less than half the area of the largest current carrying conductor supplying the circuit.Restriction-Aluminium or copper clad aluminium (clad (PP of clothe) sojjito) conductors shall not be used for final connections to earth electrodes

Earthing definitions- as per BNBC-Earth LeadEarth lead is the link which provides connection between the earth conductor(s) and the earth[alvaa]electrode(s). Earthing leads shall be run in duplicate down to the earth electrode so as to increase the safety factor of the installation. Copper wire used as earthing lead must not be smaller than 8 SWG (12 mm).In normal provision the earthing lead connects the earth electrode to a bus bar called earth bus bar. earth conductor is connected to Earth bus bar

Earthing definitions- as per BNBC-Earth ElectrodesThe earth electrode shall, as far as practicable, penetrate into permanently moist soil preferably below ground water table. The resistance of earth electrodes shall not be more than one ohm. The following types earth electrodes are recognized for the purpose of Code -Copper rods, Copper plates, Galvanized iron pipes. Copper rods shall have a minimum diameter of 12.7 mm. Copper plates shall not be less than 600 mm x 600 mm in size, with 6 mm thickness.GI pipes shall have a minimum diameter of 50 mm. Earthing cable sizes should be adequate with respect to the phase.

Phase cable sizeEarthing cable sizeBelow 4 rmEarthing not less than 3.243 mm2 or 14 SWG.But for the unavailability of this size it can be considered as 4 rm. So it should not be less than 4 rm4 rm to 16 rmSame size of the phase25 rm and 35 rm16 rm35 aboveHalf of the phase cable sizeIn case of multiplication at LT panelAs it is power cable like 240/300/400/500 rm. Then earthing cable size should be half of the total size. Say,if the phase cable is 6X300 rm then earthing cable will be 6X150 rm or 3X300 rm

Earthing definitions- as per BNBC-

Earthing definitions- as per BS 7671-

Earthing definitions- as per BS 7671-

Earthing definitions- as per BS 7671-

Earthing definitions- as per BS 7671-

Earthing definitions- as per BS 7671-

Earthing definitions- as per BS 7671-Requirements of using RCDRCDs are designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 530 mA) and disconnecting quickly enough (