Earthing

What is the maximum ground fault current in the grounded system?

The ground fault current for a solidly grounded system can range from 25% to 125% of the bolted three-phase fault current values, but in the must systems does not exceed the calculated three-phase fault current value. For low and high resistance grounded systems, the ground fault current is limited by the impedance of the grounding device and is substantially less than three-phase fault current. The maximum and minimum generation cases need to be determined, just as for three-phase faults, to determine whether the circuit protection is sensitive enough to protect against damage that could result from low level faults. Separate ground fault relays are usually applied for the ground fault protection system.

What recommendations have been made for improving earthing system provided for lightning protection system?

Lightning current is considered to be high frequency phenomenon with current rise time in the order of 10 kA/Ωs. Lightning protection standards recommend 10 Ω or less for lightning protection earth. A building earth at a source communications should have an earth resistance of 5 Ω or less. It is often believed that the lowest earth resistance will provide the most effective lightning protection. In fact this requirement is really secondary to the need to bond all metal structures and services together to provide an equipotential plant so that everything rises in potential together. As long as this is carried out equipment at a site can still be successfully protected with earth resistance of 50 or even 100 Ω.

Direct bonding is permitted by many lightning standards and is the only preferred method. Otherwise bond all earthing system via gas filed arresters with rating 150 kA for an 8/20 µs impulse. Earth potential rise can never be eliminated. That is why bonding is essential.

Impedance of the earth termination network can be lowered by the following.

  • The use of flat strip rather than circular conductor. This increases surface area, reduced high frequency resistance due to skin effect, increases both capacitive coupling and the earth contact area for a given cross-section of conductor.
  • The use of ground enhancing compound around the conductor can reduce the impedance of the earth electrode.
  • The use of centre point feed to create the effect of two parallel connected transmission lines is also effective.

Where trench (horizontal) earthing electrodes are installed, the initial surge impedance of two or more electrically parallel wires or strips, radiating symmetrically from a central connection point, will be less than the equivalent length laid as single unit. The optimum surge performance for a single horizontal earthing electrode will usually be achieved when the down-conductors attaches to its midpoint.

  • For practical purposes, the separation between vertical earthing electrodes can be taken as twice the length of the earthing electrode.
  • Where side-flashing or step and touch potentials are a design problem on a driven earthing electrode, it is good practice to sleeve the upper part of the earthing electrode with a non-conducting pipe of heat shrink tubing of adequate weather resistance and electrical insulation properties. For example, this could take the form of 2 mm thickness of polyethylene or 4 mm thickness of PVC covering the upper 2m to 3m of the earthing electrode.

What do you recommend for cable armour earthing?

Armouring and metal sheathing is generally earthed at both ends of the cable. However, when either of these protections is used on cables without the screen, earthing should be at one end only (AS 2381.7).

In some plants, where appropriate, the cable armouring may need to be connected to earth at only one end to protect the integrity of the corrosion protection system. Where this is the case, it is preferred that the armouring be earthed in the hazardous area. Armour continuity must be maintained throughout the length of the complete cable run.

For single point bonded (earthed) installations, the standing voltages on the armouring at the insulated glands shall not exceed 60V under condition of rated full load current, nor 430 V under conditions of maximum short circuit current. Where these standing voltage limits would be exceeded, the sheath or screen shall be cross-bonded. This alternative should only be considered for circuit lengths exceeding 500m.

What are the guidelines on fieldbus cable shield earthing?

When using shielded cable, connect each spur shield to the trunk shield and connect the overall shield to ground at one point. For most networks the ground point can be anywhere. For intrinsically safe installations, a specific location may be required for the ground.

Fieldbus devise shall not connect either conductor of the twist pair to ground at any point in the network. The fieldbus signals are applied and presented differentially throughout the network, and grounding either conductor would be expected to cause all devise on that bus to loss communication for the period the conductor is grounded.

The shield of the fieldbus cables, by standard practice, typically grounded at only one point along the length of the cable, and shall not be used as a power conductor. Never use a shield as a conductor or a conductor as a shield. Never connect the shield to the enclosure of device.

In some plants it is standard practice to ground the some shield at more than one point over the length of the able run. This practice may be acceptable in 4-20 mA DC control loops, but can cause interference in a fieldbus system. If a multi-point grounding scheme is used as standard practice, the requirements for and location of grounds for cable shields should be re-visited. If multiple grounded are required for an excessively environment, it should be used capacitive grounding. It means that the connection to earth is made through a capacitor rather than a wire, thus only high frequencies are grounded.

What are the guidelines on instrument cable shield earthing?

All instrument screens shall be connected to the instrument earth at the non-filed end only. Screen continuity and insulation from earth and other screens must be maintained throughout the length of the circuit, through filed and marshalling cables. All screens shall be insulated from earth at field end and in junction boxes where screens are connected in contiguous cabling.

Which value is recommended for the resistance of earthing system?

Earthing system can be applied for different application as mentioned below.

AS/NZS 1768, Lightning Protection System (LPS)
Low earth resistance is desirable and all practical measures should be taken to achieve 10Ω or less for the whole interconnected LPS (Lightning Protection System) earth termination network.

For any system incorporating two or more down conductors, it should not be necessary to install a total length of more than 50 m of widely separated horizontal or vertical earthing electrodes per down conductor, regardless of the earthing resistance. Depending upon site and operating conditions, it may be possible to obtain overall protection by using one earthing system. Where it is considered that one common earthing system may be adapted to comply with all the requirements, it is necessary to ensure that the value of earth resistance does not exceed 4Ω.

AS/NZS 3000, section K11.4.1, HV stations
Combined earthing system
A combined earthing system is one in which the high voltage and low voltage electrical equipment is earthed to a common terminal bar. The combined earthing system shall have a resistance to earth not greater than 1Ω. The resistance of 1Ω may be achieved by connections to electrode systems, metallic cable sheaths or low voltage neutrals, provided that when any such connection is temporary provided for test or maintenance purposes the resistance of the remaining earthing connections does not exceed 30Ω.  A resistance to earth greater than 1Ω may be appropriate where step and touch potentials are satisfactory.

Separate earthing system
The resistance to earth shall not exceed the following

–       High voltage system 30Ω
–       Low voltage system

  • Aggregate transformer rating up to 50 kVA 30Ω
  • Aggregate transformer rating over 50 kVA but not more than 500 kVA 15Ω
  • Aggregate transformer rating over 500 kVA 10Ω.

The resistances specified for separate earthing system shall be achieved independently of any connection between the neutral conductor and earth at other points within the electrical installation.

AS/NZS 4853, section 8.4, Lightning mitigation on metallic pipelines
An electrode resistance of 5Ω to earth at each end of a pipeline section (typically 10 to 100 km) is recommended.

AS/NZS 2832.1, section G3.7, Lightning mitigation on metallic pipelines
Field experiences on pipelines suggest that on earth resistance of 5Ω, installed at each end of a 100 km pipeline section would discharge any lightning charge to a safe value in around 0.01s. Such discharge systems need to be capable of carrying high currents for a very short time and the conductors should have a cross-sectional area of at least 25 mm2.

AS/NZS 1020, section 4.2.2, Earthing for static charge dissipation
The total resistance between object and earth not exceeding 1MΩ is sufficient to prevent charge accumulation. If there is any possibility of this resistance being exceeded under condition of normal operation, the installation of a separate earthing conductor is recommended.

IEEE 80, section 14.1, IEEE Guide for Safety in AC Substation Grounding
For most transmission and other large substations, the ground resistance is usually about 1Ω or less. In smaller distribution substations, the usually acceptable range is from 1Ω to 5Ω.

What is recommended for earthing system separation in a HV station?

For major substations such as zone and transmission substations a single combined earthing system shall be used. It is only under special circumstances such as those involving underground mining installation that a design departing from this principle may be necessary. Furthermore, the HV and LV system shall be interconnected if the LV system is totally confined within the area covered by the HV earthing system. For distribution substations (e.g. 11 kV/415 kV or 22 kV /415 kV) a common earthing system is the preferred configuration. If reasonable attempts to meet voltage limits will not succeed, the segregation of HHV and LV earthing systems may be considered.

Note that a minimum separation distance of 4m is suggested between the HV and LV earthing systems.

How can an earthing system separation be achieved in a HV station?

Based on AS/NZS 3000, section K11.4.1, separation on high voltage and low voltage systems can be achieved as below.

Electrodes of the high voltage earthing system shall be installed not less than 2 m from those of the low voltage system. Conductors or metallic parts that are connected to the high voltage earthing system shall be provided with a clearance of not less than 35 mm from any conductors or metallic parts that are connected to the low voltage system.

What is recommended for earthing system separation in a normal plant?

Some installations may necessitate up to three distinct earthing systems.

  • Earthing for electrostatic protection (AS 1020)
  • Earthing fir lightning protection (AS 1768)
  • Earthing of metallic enclosures for electrical wiring and equipment (AS 3000).

Since it is not always practicable to electrically separate these systems, bonding of the systems is recommended.

How can interaction of cathodic protection systems with pipeline protective earthing systems be avoided?

LFI (Low Frequency Induction by power lines) should normally be designed first. This is not only because it protects personnel from likely and repeated electrical hazard, but also because it imposes significant problems on the subsequent CP (cathodic protection) concept. LFI electrodes may be isolated for low DC potentials by fitting polarization cells. Such cells, both metal/alkali and semi-conductor units, must be capable of carrying full LFI current at 50 Hz. This may be around 500A, and typically for around 0.25 s. refer to AS/NZS 4853 for more details.

How can cathodic protection of metals be achieved?

In most situations, coating provide the primary corrosion protection system, which cathodic protection providing back-up corrosion protection to the structure at points where failure of the coating or damage to the coating has occurred.

Based on AS 2832.1 two types of cathodic protection systems are available as follows.

Galvanic anode systems
Galvanic anode systems employ metallic anodes that are consumed to provide the source of protection of the structure. The driving voltage for the prospective currents comes from the natural potential difference that exists between the structure and the second metal (the galvanic anode).

Impressed current systems
In impressed current systems the driving voltage for the protective current between the structure and the anode is supplied by an external direct current power source.

In both systems the anodes will be consumed, and will require replacement at intervals. Metals may be protected from corrosion by application of direct current to lower and maintain the potential of the metal sufficiently negative with respect to the environment. The criteria for the protection of a buried ferrous structure is to maintain a potential on all parts of the structure equal to, or more negative than, -850 mV with respect to a saturated copper/copper sulphate reference electrode. This voltage limit is different for different metal types as below.

  • Ferrous structure: -850 mV
  • Copper/copper alloy structure: -300 mV
  • Lead structured (aerated condition): -650 mV
  • Lead structure (anaerobic condition): -800 mV

For protection to be achieved, a structure should

  • be immersed or buried in an electrolyte;
  • be electrically continuous; and
  • be electrically isolated from structures not requiring protection.

Insolating joints should not be positioned in enclosed or defined hazardous areas where combustible gases may be present unless electrical surge protection is provided. Surge protection devices in such areas should be of a type that cannot cause an exposed arc.

As a matter of personnel safety, the open circuit voltage of a cathodic protection power supply shall not exceed 50 VDC, unless appropriate safety precautions have been taken.

Where a colour code is not specified, the following insulation colours are recommended for cathodic protection system.

  • Primary structure: Black
  • Anode feeder cable: Red
  • Reference electrodes: Yellow
  • Earthing electrodes: Green or Green/Yellow

Which earthing places are recommended for pipelines?

The earthing of pipelines can be complex matter. In general, lightning earthing will be positioned at section ends, and low frequency induction (LFI) earthing positioned at each end of parallel power transmission exposure.

What does LFI stands for?

Under normal operating conditions a three phase power line may be expected to be operating as a balanced system such that the surrounding electromagnetic field is small. However, some induction will result due to the slightly different distances of each phase conductor from a nearby pipeline, or due to the current imbalance between phases. The voltage inducted on pipes by power lines is called Low Frequency Induction (LFI).

Fault and steady state on HV distribution power lines (e.g. 22 kV) may be more than sufficient to result in potentials requiring mitigation on adjacent structures. It should not be assumed that only HV power transmission lines require consideration.

Is it necessary to mitigate the pipeline capacitive coupling?

In general, mitigation of capacitive coupling is required mainly during the construction phase of structures such as pipelines, when they are strung above ground during operations such as welding.

What is the voltage limitation on LFI mitigation?

Acceptable voltage limits caused by LFI are specified in AS/NZS 4853. In addition, the breakdown voltage of the structure coating should not be exceeded. It should be also noted that continuous application of relatively low levels of AC voltage may cause reduction in the effectiveness of cathodic protection and even corrosion. It is generally accepted that no more than 15 V AC should be continuously present, although further research is in progress that may result in reduction in this value in situations.

Based on AS2832.1, at locations with exposure to voltages greater than 1000 V due to LFI or EPR (earth potential rise), earthing grids should be installed at accessible locations such as at CP (Cathodic Protection) test points and within facilities compounds to reduce touch and step potentials. Long-term exposure of the structure to alternating current induced from electric power lines should be designed to limit value of not greater than 15 V AC.

What measures should be taken when earth grid passes the pipelines?

As specified by some companies, where conductors of the earth grid pass over or under the buried pipelines, the conductors should generally be installed in a heavy duty, rigid PVC conduit for a distance of 1m either side of the centre line of the pipeline. This distance is 2m where power, instrument cables or earth conductors pass over or under the centre line of the pipeline.

How shall an IS earthing system be implemented?

Installation utilizing intrinsically safe components that require an earth connection to maintain their integrity under fault condition (such as Zenner or shunt diode barriers) shall have a separate earth bar for IS circuits. The earth bar shall be located as close as practicable to the safety barriers. The IS system earth bar shall be connected to the main earth using a minimum of two separate earth conductors. Each conductor shall be a minimum of 6 mm2 cross sectional area copper conductor. The impedance between the intrinsically safe system earth bar and the site supply transmitter or alternator neutral star point or, where it is not available, the main power system earth bar shall not exceed 1Ω.

What is ground mat?

Ground mat is a solid metallic plate or a system of closely spaced bare conductors that are connected to earth system and often placed in shallow depth above the ground grid or elsewhere at the earth surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people. Grounded metal gratings placed on or above the soil surface, or wire mesh placed directly under the surface material, are common forms of ground mat.

How can I design a grid (earth system) for a plant?

As per IEEE 80, the basic aspects of grid design are as below.

  • A continuous conductor loop should surround the perimeter to enclose as much area as practical. This measure helps to avoid high current concentration and, hence, high gradient both in the grid area and near the projecting cable ends. Enclosing more area also reduce the resistance of the grounding grid.
  • Within the loop, conductors are typically laid in parallel lines and, where practical, along the structures or rows of equipment to provide for short ground connection.
  • A typical grid system for substation may include 4/0 (120 mm2) bare copper conductors buried 0.3-0.5 m below grade, spaced 3-7 m apart, in a grid pattern. At cross-sections, the conductors would be securely bonded together. The conductor diameter has negligible effect on the mesh voltage. The area of the grounding system is the single most important geometrical factor in determining the resistance of the grid. The larger the area grounded, the lower the grid resistance and thus the lower GPR.
  • Ground rods may be at the grid corners and at junction points along the perimeter. Ground rods may also be installed at major equipment, especially near surge arrestors. In multilayer or high resistivity soils, it might be useful to use longer rods installed at additional junction points.
  • This grid system would be extended over the entire substation switchyard often beyond the fence lines. Multiple grounding leads or larger sized conductors would be used where high concentration of current may occur, such as at the neutral to ground connection of generators, capacitor banks, or transformers.
  • The ratio of the sides of the grid meshes usually is 1:1 to 1:3, unless a precise (computer aided) analysis warrants extreme values. Frequent cross-connections have a relatively small effect on lowering the resistance of the grid. Their primary role is to assure adequate control of the surface potentials. The cross-sections are also useful in securing multiple paths for the fault current, minimizing the voltage drop in the grid itself, and providing a certain measure of redundancy in the case of conductor failure.

Some of the reasons for using the combined system of vertical rods and horizontal conductors (grid) are as follows.

  • In substations a single electrode is, by itself, inadequate in providing a safe grounding system.
  • If the magnitude of current dissipate into the earth is high, it seldom possible to install a grid with resistance so low as to ensure that the rise of a ground potential will not generate surface gradients unsafe for human contact. Then, the hazard can be eliminated only by control of local potentials through the entire area. A system that combines a horizontal grid and a number of vertical ground rods penetrating lower soils has the following advantages.
  1. While horizontal (grid) conductors are more effective in reducing the danger of high step and touch voltages on the earth’s surface, provided that the grid is installed in the shallow depth (usually 0.3-0.5 m below grade), sufficiently long ground rods will stabilize the performance of such a combined system. for many installations this is important because freezing or drying of upper soil layers could vary the soil resistivity with seasons, while the resistivity of lower soil layers remain nearly constant.
  2. Rods penetrating the lower resistivity soil are far more effective in dissipating fault current whenever a two-layer or multilayer soil is encountered and the upper soil layer has higher resistivity than the lower layers. For many GIS and other space-limited installation, this condition becomes in fact the most desirable one to occur, or to be achieved by the appropriate design means (extra-long ground rods, grounding well, etc.).
  3. If the rods are installed predominately along the grid perimeter in high-to-low or uniform soil conditions, the rod will considerably moderate the steep increase of the surface gradient near the peripheral meshes.

To make conclusion, grounding rods will reduce the influence of the weather on grounding system. They are much more effective in dissipating of the fault currents into the lower soil layers with lower resistivity. They will reduce the earth potential steep when properly installed at the perimeter of the grid.

Note that for equally spaced ground grids, the mesh voltage will increase along meshes from the centre to the corner of the grid. The rate of this increase will depend on the size of the grid, number and location of ground rods, spacing of parallel conductors, diameter and depth of the conductors, and the resistivity profile of the soil.

Rods will transfer high currents from upper level to the lower levels. This results in lower potential gradients in upper levels. Therefore if using a computer software to calculate the GPR (ground potential rise), put the rod in the places with high GPR and repeat the calculation.

Auxiliary ground electrodes also help improving the grid system. Auxiliary ground electrodes are underground metal structures and reinforcing bars encased in concrete, if connected to the grounding grid. Auxiliary ground electrodes may have a limited current carrying capacity.

What is recommendation for connecting equipment to ground?

Those facilities that are most likely to supply or carry a high current, such as transformer and circuit breaker tank, switch frames, and arrester pads, should be connected to the grid with more than one ground lead. The loads should preferably be run in opposite directions to eliminate common mode failure. One possible exception is grounding of the secondaries of potential and current transformers. The grounding of such devices usually must be restricted to the single point to avoid any parallel path that could cause undesirable circulation of currents affecting the performance of relays and metering devices.

What is Transient Enclosure Voltage (TEV)?

TEV is very fast transient phenomena, which are found on the grounded enclosure of GIS systems. Typically, ground leads are too long (inductive) at the frequencies of interest to effectively prevent the occurrence of TEV. The phenomenon is also known as transient ground rise (TGR) or transient ground potential rise (TGPR).

Typically, The GIS installation necessitates 10-25% of the land area required for conventional equipment. Because of smaller area, it may be difficult to obtain adequate grounding solely by conventional methods, particular attention should be given to the bonding of the metallic enclosure of the GIS assembly, as the enclosures carry induced currents of significant magnitude, which must be confined to specific paths. In this respect, grounding recommendation by the manufacturer of the given GIS usually need to be strictly followed.

What is the impact of the earth grid on steel structure corrosion?

A grid of copper or copper-clad steel forms a galvanic cell with buried steel structures, pipes and any of the lead-based alloys that might be present in cable sheaths. This galvanic cell may hasten corrosion of the latter.

How can I calculate the resistance of an earth grid?

IEEE 80 introduces equations for simplified calculations of ground resistance based on grid design considering grid depth, total buried length of conductors and the area occupied by the ground grid. These include Sverak and Schwartz’s equations. Schwartz developed a set of equations to determine the total resistance of a grounding system in a homogeneous soil consisting of horizontal (grid) and vertical (rods) conductors. I also recommend below equation which is very simplified and give a very rough values for earth resistance.

ρ/L gives a rough value of the earth resistance of a rod with the length of L (m) buried vertically in the soil with resistance of ρ (Ω.m).

2ρ/L gives a rough value of the earth resistance of a grid loop with a large diameter made by 120 mm2 copper conductor buried horizontally with the total length of L (m) in the soil with resistance of ρ (Ω.m).

Different types of system earthing

TN
which includes

  • TN-S: Neutral (N) and protective conductor (PE) are separated
  • TN-C: Neutral (N) and protective conductor (PE) are combined as PEN conductor.

TT
Separate earths for power system and exposed conductive parts.
Notes:

  • Equipment shall be protected by residual current device.
  • When the earth resistivity is high such a earth system may not be accepted.

IT
Neutral isolated from earth or earthed through sufficiently high impedance.

What does TN-C or TN-S stands for?

IEC 60364 describes the MEN system as a TN-C or TN-S system with the letters signifying:

  • T: The distribution system is directly connected to earth at the neutral point of the supply transformer.
  • N: The exposed conductive parts are connected to the earthed point of the distribution system at the MEN connection.
  • C: The neutral and protective conductor functions are combined in the single conductor (the neutral conductor of the distribution system)
  • S: The protective conductor function is separated from the neutral-separate conductors within the installation.

Earthing of static electricity (AS1020)

Working surfaces can be protected from static by antistatic floors provided that the resistance between earth and the object to be discharged cannot exceed 1MΩ. Electrical charges in non-conducting objects cannot be rapidly dissipated by earthing or bonding. The surface of same material can be made conducting by metal painting, painting with a conducting paint, wetting with conducting liquids or coating with graphite or glycerine.

The resistance across pipe connections should not exceed 1 MΩ under any conditions of normal operation. Where this value is liable to be exceeded, the pipes on either side of the connection should be bonded together by suitable conductor and be earthed. In practice, the resistance across mot pipe connections would be less than 1 MΩ, but one exception is where insulating interests have been especially incorporated for reasons such as cathodic protection. In such cases, it may be necessary to shunt the insulating joint with a resistance approaching the order of 1 MΩ, thus enabling static charges on both sides of the joint to equalize while at the same time preserving the essential properties of an insulated joint.

Transfer of pure gas generates no static. However, when a gas contains entrained solids or liquid particles static generation can be expected to result from the flow of gas. The static will accumulate on any electrically isolated body in the gas or vapour stream.

What is the best place for earthing frameworks?

The earthing framework should be earth bonded near foundation but not less than 500 mm above ground level to prevent corrosion (as per the requirements of AS1768).

What is recommended for burring grid outside of the fence or inside of the fence?

The maximum step voltage is assumed to occur over a distance of 1m, beginning at and extending outside of the perimeter conductor at the angle bisecting the most extreme corner of the grid.

It is usually possible, by burring the grid ground conductor outside the fence line, to ensure that the steeper gradients immediately outside this grid perimeter do not contribute to the more dangerous touch contacts (IEEE std 80-2000, 16.6 (b)).

Is it required to connect bonding system of a plug-and-socket-outlet generator set to the general mass of earth?

The connection of the generating set bonding system to the general mass of earth through an earth electrode IS NOT required or recommended for plug-and-socket-outlet connecting generating sets.

What is recommendation for the size of bonding conductor?

For single phase and low voltage motors use a 35 sq mm stranded copper conductor.

For high voltage motors use a 70 sq mm stranded copper conductor.

For LV heaters use a 35 sq mm and for HV heaters use a 70 sq mm stranded copper conductor.

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