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Guideline for Earthing of Buildings & Industrial plants

This guide covers the earthing of Domestic, Commercial buildings and Industrial Plants. This guide is prepared after referring to BS 7430, IEEE 142, EN 50522 and IS 3043. Readers should note that this guide is supplementary to these standards. Readers have to refer the standards before designing a system.

Before starting the design the designer should have Earth resistivity of the site, Fault current calculation results, Details about the regulation, Select the material of earthing conductor, Earthing conductor Sizing, Layout of the industry/Site/Plant, Type of source earthing to be used, Resistance of earthing system to be achieved

The Objective of earthing design is to carry out Sizing of earthing conductors, Earthing system resistance calculation and determine number of vertical driven earth rods required, Preparation of earthing layout, Estimation of total quantity of earthing materials and Prepare Bill of material BOM or Bill of Quantity BOQ. Earth Fault current is calculated using standard IEC 909 either manually or using a computer program

Earthing Conductor material can be copper, aluminium or steel . The choice of material depends on the owner’s specification, type of site etc. Earthing conductor sizing S = I tk     (BS 7430 - CL 9.7), I is fault current in A rms, and t fault duration in sec, S is in mm2

Different types of LV earthing system (BS 7430) are TN-S, TN-C, TN-CS, TT, IT. Selection of the type depends on local regulations. Source side earthing type can be T or I, T : effectively earthed ,  I : un earthed. Load side earthing can be T or N. T : load side has own earthing terminal or earth electrode, N: Load side earthing system connected to source side earthing

Earth Loop Impedance is the impedance seen by the fault current from the fault location upto the neutral point of the source where the current returns back. This impedance does not influence the fault current if solidly grounded system is used. It affects the potential rise of non conducting metal parts during a fault.

Resistance of earthing system, the earthing system consisting of several earth electrodes and interconnecting horizontal conductors gives a earthing resistance which is much less than the resistance of single electrode. Different standards specify different values. Some standards don’t specify any value. However it is a practise to achieve the resistance of the earthing system to be 1Ω.

Different types of electrodes are Plate, Pipe/Rod, Strip, Mesh. A given earthing system may consist of all of these or few of them. Normally strip electrode (Horizontal) and Rod electrode (vertical) will be used more often. Plate electrode is used to earth the neutral of LV transformers due to large area of the plate.

R plate  = 4A, = resistivity of soil Ωm, A area of plate m2 (BS 7430 CL 9.5.2, IS 3043 CL 9.2.1). R rod = 1n2πLln 8Ld -1+LSln 1.78n2.718   (BS 7430 Cl 9.5.4), n number of rods, s spacing between rods.

Typical Earthing Layout of LV MV Substations

Detailed guidelines for Earthing of Buildings and Industrial plants consist of 35 sections with 34 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

Contents of the detailed guideline are as given below

1.0 Scope
2.0 Exclusions
3.0 Reference Standard
4.0 What is the function of earthing
5.0 What is the difference between Earthing of Substation and Building
6.0 Basic requirement before starting the earthing design
8.0 Objective of earthing design
9.0 Fault current calculation results
10.0 Material of earthing conductor
11.0 Earthing conductor sizing
12.0 Current density limitation at electrode
13.0 Minimum dimensions of conductors
15.0 Source earthing
15.6 Transformer neutral earthing
15.7 Earthing of UPS neutral
16.0 Types of earthing based on resistance
16.1 Solidly grounded system
16.2 Unearthed or ungrounded system
16.3 Reactance earthed system and resonant earthed system
16.4 Resistance earthed system
16.5 Types of Resistance earthed system
17.0 Different types of LV earthing system (BS 7430)17.1 Earth Loop Impedance
18.0 Potential gradient around the earth electrode
19.0 Earth Resistivity measurement
20.0 Resistance of earthing system
21.0 Measurement of electrode resistance
22.0 What all needs to be earthed
22.1 Source Neutral earthing
22.2 Cable Armour earthing
22.3 Cable Tray earthing
22.4 Electrical Panel and Distribution boards earthing
22.5 Junction Box earthing
22.6 Motor and Push button station earthing
22.7 Lighting poles and fixtures earthing
22.8 Tanks, Vessels, Piping earthing
22.9 Package equipments earthing
22.10 Lightning Protection system earthing
22.11 Electronic equipments earthing system
22.12 Earthing of utility pipes
22.13 Earthing of steel reinforced bars of structures and buildings
23.0 Earthing Schematic
24.0 Types of Electrodes and their resistances
24.1 Resistance of Plate electrode
24.2 Resistance of Rod or Pipe electrode
24.3 Resistance of Rod electrodes in parallel
24.4 Variation of resistance of electrode due to length and diameter of the rod (Table 5 and Table 6)
24.5 Resistance of straight Strip
24.6 Resistance of Mesh
24.7 Resistance of electrodes encased in Low resistivity materials
24.8 Earthing of steel reinforced concrete foundations
25.0 Treated earth electrodes
26.0 Auxiliary earth grid
27.0 Typical Earthing Layout of LV MV Substation
28.0 Layout Requirement
28.1 Spacing between electrodes
28.2 Distance between electrode and building wall
28.3 Depth of horizontal conductor or connecting conductors
29.0 Stray Currents
30.0 Common mode noise
31.0 Typical Calculation of Earth electrode resistance of substation
31.1 Resistance of Rod electrodes in parallel (BS 7430)
31.2 Resistance of straight Strip (BS 7430)
31.3 Resistance of Mesh (BS 7430)
32.0 Type of joints
33.0 Recommended dimensions of earthing Conductor (Table 7)
34.0 Typical Earthing design of Oil and Gas installation
34.1 Calculation and steps for typical Oil and gas installation earthing design
35 Short Circuit Current calculation for fault at Motor terminal and Earthing conductor sizing.
36 Fault Current paths
37.0 Reference

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Solar PV system Grounding, Faults and Protection

Proper earthing/Grounding of the solar PV system is very important for safety and reliability. Earthing of the PV system, fault current calculation and protections are related. Solar PV power generation is different from rest of the power generation. Solar PV System is spread over large areas, Solar PV has a mix of  AC DC system and PV cell does not store energy like a battery, or a AC generator. Grounding is required for Lightning protection, Fault current Path and detection, Equipotential bonding, Prevent of Corrosion and to carry leakage current

Different Types of Grounding are Protective earthing, System Grounding/Earthing, DC unearthed system, DC earthed system, Positive pole grounded, Negative pole grounded, Mid point grounded (Very rare), Solid earthed, Resistance earthed and Active grounding in PV system. Earthing in PV system depends of the type of cells.

Combiner box receives both positive and negative cables from each string. These box will also have fuse. Earthing conductor of recommended size has to be connected between this earthing bolt and the plant earth grid.

SOLARLOK 5-String Combiner Box

Picture courtesy: TE Connectivity Ltd 

SolKlip Grounding Clips are used for grounding of Solar Panel using 10 AWG/6 Sqmm or 12 AWG / 4 Sqmm bare copper wire. Ground Bolts made of Stainless steel is used to connect sold copper wire 6AWG (16 Sqmm) to 12 AWG (4 Sqmm). The bolts earth the aluminium frame of the module.

 

Picture courtesy: TE Connectivity Ltd 

EATON Crouse Hinds series lugs

Picture Courtecy : Eaton.com

Deterioration of the module back cover, failure of insulation in cable, Rat bites, Plastic material exposure to UV rays, pollution deposit, rain, bad ingress protection are some of the causes of the fault.

The Magnitude of short circuit current in a PV system is very close to the load current.

Fault in an Ungrounded PV system will also produce a fault current of small magnitudes which can also cause fire and safety hazard for people if left undetected. In a grounded PV system, most of the string current from healthy module will get diverted to the fault location.

PV System protection is different from the AC power system because the PV system consist of DC power from PV modules, Inverter and Step up transformer with Grid.

Eaton, BRL215CAF, 2 pole breakers with inbuilt AFCI (AC or DC)

(Picture courtesy: EATON)

Insulation monitoring is continuous process done by certain devises. These devises measure the insulation resistance continuously.

Picture Courtecy : Bender GmbH & Co. KG
(www.bender.de)

Insulation measurement is done intermittently for a grounded system.  Insulation resistance measurement can only be done when the system is shutdown unlike insulation monitoring which is down when system is running.

Picture Courtesy: KYORITSU Electrical Instruments Works Ltd
www.kew-ltd.co.jp/en

RCD (Residual current device) and RCMU (Residual current monitoring unit) are used to trip an inverter in case of excessive leakage currents in transformer less system and for protection of the distribution system fed from solar PV system.

Picture Courtesy: Littelfuse, Inc
www.littelfuse.com

Sensing and measuring DC current is done with help of hall effect current transducers.

Picture Courtecy: Electrohms Pvt Ltd
www.electrohms.com

Strings, sub arrays and arrays can also be protected by DC circuit breakers instead of fuse. Combiner box will house a circuit breaker or isolator instead of the fuse.

OTDC
800...1600 A
HPMCB
S800
MCCB Tmax
T PV

Picture Courtecy: ABB

Combiner box are just like a distribution board or a small panel. Circuit breakers or fuse are located inside the combiner box.

Picture Courtesy: ABB

In ungrounded DC system the fault current magnitude will be very less. It is not possible to detect and clear fault with fuse. Special sensitive relays are required.

Picture Courtecy: Littelfuse
www.littelfuse.com

Fire in Solar PV systems like any other electrical fire has chances of electrocution to fire fighters. There is additional risk because the plant cannot be shut down like any other power plant. Solutions such as PVSTOP are available to mitigate these risks.   PVSTOP is a non-conductive, non-flammable polymer coating that is sprayed on to the PV panels to make the panel face opaque which can later be removed without damaging the PV panel. This stops the generation of power and thus cuts power at the source.

Pictures Courtesy: PVSTOP
www.pvstop.com.au

When there is a fault, the current flows into the ground and raises the potential of the non-conducting metal parts.  When a person comes in contact with these metal parts, current will enter the body. If the Metal part is grounded, the magnitude of current entering person will be lesser.

Detailed guidelines on Solar PV system Grounding, Faults and Protection consist of 28 chapters with 73 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

1 Introduction
2 Scope
3 Exclusion
4 How is solar PV power generation different from rest
5 Why Grounding is required
6 Types of Grounding
6.1 Protective grounding
6.2 System grounding
6.3 DC unearthed system
6.4 DC earthed system
6.5 Negative pole grounding
6.6 Positive pole grounding
6.7 Potential induced degradation
6.8 TCO Degradation
6.9 Surface polarisation
6.10 PV offset box
6.11 Detection of PID
6.12 Resistance earthed
6.13 Midpoint Grounding
6.14 Transformer Less Inverter and Grounding
6.15 Grounding Kit
6.16 Earthing at inverter side or PV side
7 What is Grounded in a PV system
7.1 Junction box
7.2 Connector
7.3 Module frame and support Grounding
7.4 Cable armour grounding
7.5 Bonding
7.6 Combiner box
7.7 Cable routing
7.8 Interconnection of Solar PV system and AC substation earthing
8 Grounding Accessories
8.1 Ground clips
8.2 Ground Bolt
8.3 Grounding lugs (Lay in type)
8.4 Grounding lugs (Crimping type)
9 Faults
9.1 Cause of faults in Solar PV system
9.2 Different places where fault occurs
9.3 Different types of fault
9.4 Short circuit current in PV system
9.5 DC and AC Arc
9.6 Series fault and parallel arc fault
9.7 PV module equivalent circuit
9.8 Fault current in Ungrounded PV system
9.9 Current Back feed or Reverse current in PV system
9.10 Fault current in Grounded PV system
9.11 Blindspot Fault
9.12 AC Side fault and PV Contribution
10 Fault current in PV System
10.1 Fault current in single string
10.2 Fault current in solar power plant
10.3 Fault current Calculation
11 Earthing of PV system with inverter built in module
12 Protection
12.1 Different devises to detect faults
12.2 GFPD
12.3 AFCI
12.4 Insulation monitoring
12.5 Insulation Measurement
12.6 RCMU and RCD
12.7 IDMT Earth fault relay (51G) for AC system fed from PV System
12.8 DC differential relay
12.9 Fuse
12.10 Fuse sizing and selection
12.11 Fuse coordination in PV system.
12.12 SSTDR
12.13 DC fault detection and Hall effect current sensor
12.14 Circuit breakers and Switch Disconnectors
12.15 Combiner box
12.16 DC ground fault monitoring in ungrounded solar PV system
12.17 Blocking diode
12.18 Reverse current overload
12.19 Does protection stop the fault current completely
12.20 Fault current carrying capability of PV system components
12.21 Line to Line Fault
12.22 Protection system application in PV system versus AC system
12.23 PV cell over voltage protection
13 Sizing of earthing conductor
14 PV cell characterises with respect to temperature
15 Sizing of Power cable
16 Fault current Calculation in Solar PV system
17 Fire Safety
18 Electrical Safety
19 Earthing/Grounding of Racks with piles
20 Protection by Extra Low voltage
21 Corrosion
22 Leakage current
23 Parasitic capacitance
24 Typical PV Ratings
25 Lightning Arrestor and Surge Protection devise in Solar PV System
26 Measurement of resistivity
27 Installation of PV Panels in Petro chemical installations and Industries
28 Reference

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Guideline for Neutral Earthing

This guide covers the Neutral earthing of Generator, Transformer which is also called system earthing. Neutral earthing is done to protect people and equipment. Earthing will have impact on fault current and system overvoltage. Certain types of earthing reduce the earth fault current and protects the equipment like generators, motors from damage during fault. Neutral earthing also prevents over voltages in a system.

Low resistance grounding, In this method the fault current is restricted to a value which is much below the actual fault current but still sensible by the normal relays.

Picture courtesy: M S Resistances France

NGR (Neutral grounding resistors) are used in LRNG system between the neutral and earth to restrict fault current

Picture courtesy: Advanced Power Technologies (APT) (LRNG and HRNG)

High resistance neutral grounding (HRNG) is mostly used for Unit connected generators. It is also used of LV distribution and MV distribution system. Resistive current from NGR to fault, should be equal are greater than capacitive charging current from stray capacitance to fault. Normally HRNG consist of NGR and neutral grounding transformer (NGT).

 

Picture Courtesy: Pyrotech India (NGR with NGT , HRNG System)

Picture Courtesy: HEINE Resistors GmbH ( Metallic resistor and NGR with NGT )

Picture Courtesy: Meister international, LLC (NGR with NGT)

Monitoring of NGR is very important. In case of power plant and industrial application It is not mandatory to monitor NGR, but standards for mines requires monitoring of NGR. Monitoring of the resistor is done by injecting a current through the resistor and by measuring the injected current, neutral to ground voltage and the continuity.

Picture courtesy: Littelfuse, Inc

Zigzag transformer are used to derive neutral in an ungrounded system. Neutral earthing resistor along with the transformer are used to reduce the fault current level.

Picture Courtesy: Trench Austria GmbH (combination of zigzag transformer with arc suppression coils)

Resonant grounding, In this method tuned reactor in neutral in parallel with system capacitance provide a very high impedance due to resonance. The fault current is limited to a value much lower than high resistance grounding. There will be no transient over voltage as there will be no arc and re-strikes. Cost of arc suppression coil is more than resistor.

Picture Courtesy: Trench Austria GmbH

Peterson Coil or another name for resonant grounding, construction will be similar to dry type or oil type transformer or reactor. The coil consists of Iron core, winding, body with or without oil. If coil reactance is adjustable it will consist of either tapping or some other mechanism either a motor or capacitors as per the vendor design.

Picture Courtesy: Trench Austria GmbH

Tuning of arc suppression coil is done by regulators which measure the capacitive reactance of the network and adjust the inductive reactance to match the capacitive reactance. If exact resonance is achieved fault current will be very less.

Picture Courtesy: A. Eberle GmbH

In low resistance and solid earthed network, the fault current will be high enough to enable a current based relay sense the fault. Whereas in resonant earthed system it is very difficult to sense the fault with only current. Measuring the neutral to earth voltage will indicate the fault in the network but the feeder where fault has taken place cannot be determined by these voltage-based relays. Hence various advanced methods are used to determine the faulty feeder.

Picture Courtesy: Trench Austria GmbH

Ground fault neutralisers are an improvement over arc suppression coil, they consist of the variable reactors which fully compensates the capacitive current and in addition it has a residual current compensator which will completely eliminate the active part of the current thus reducing the fault current to a very low value.

Picture Courtesy: Swedish neutral

Detailed Guideline for Neutral Earthing consist of 38 sections with 70 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

Contents of the detailed guideline are as given below
1.0 Scope
2.0 Exclusions
3.0 Reference Documents
4.0 What is the function of Neutral earthing
5.0 Inputs required to carry out the design
6.0 What causes a fault
7.0 Coefficient of grounding and Earth fault factor
8.0 Type of grounding
9.0 Neutral treatment:
10.0 Selection of type of earthing
11.0 Different types of earthing practise used in
12.0 Low resistance neutral grounding
13.1 Advantages of resistance grounding
13.2 Low resistance earthing of MV system with VFD
13.3 Low resistance grounding with Single generators
13.4 Low resistance grounding with Single generators
13.5 Low resistance grounding with multiple generators
13.6 Effect of Third harmonics in parallel Generators on NGR design
14.0 High resistance grounding
14.1 HRNG Basics
14.2 Sequence network High resistance neutral grounding system
14.3 HRNG current limit
14.4 History and evolution of high resistance grounding
14.5 Overvoltage and insulation
14.6 Charging current contributions
14.7 Cause of stray capacitance
14.8 High resistance grounding for Generators with Unit transformer
14.9 Fault current circulation due to field discharge system
14.10 Open circuit time constant
14.11 NGR/NGT design methodology
14.14 Basic insulation level
14.15 Construction of NGR
14.16 Ingress protection
14.18 NGR time rating
14.20 Temperature coefficient of the NGR
14.21 Requirement of Neutral Grounding Transformer
14.23 Typical calculation of NGR NGT and Brief specification
14.24 Arc hazard eliminated by High resistance grounding
14.26 Oil mines earthing
14.27 High resistance grounding for MV distribution system
14.28 NGR failure and its associated problems
14.29 Monitoring of NGR with relays
14.30 Earth fault Protection for HRNG system
14.31 Locating fault in High resistance grounded system
14.32 Why HRNG were used in the beginning with only alarm
15.0 Arc energy and damage limits
16.0 Deriving neutral artificially
16.1 Deriving a neutral in Ungrounded power system
16.2 Zigzag transformer used to derive neutral in ungrounded system
16.3 Earthing transformer/ Zig zag specification
16.4 Calculation of fault current for network with zigzag transformer
16.5 Protection methods when using zig zag transformer
17.0 Resonant grounding
17.1 Different terminology for a group of similar technology
17.2 Resonant grounding system for Unit connected Generator
17.3 Calculation for resonant grounding of Unit connected generators
17.4 Resonant grounding for distribution system
17.5 Typical Arc suppression coil application SLD
17.6 Fault current of 11kV isolated network earthed with Peterson coil
17.7 Peterson Coil construction
17.8 Type of arc suppression coil
17.9 Regulator for Peterson coil
17.10 Detection of fault feeder in resonant earthed system
17.11 Voltage at the point of fault
17.12 Converting Isolated network to one with Peterson coils
17.13 System modification requirement- ASC added in solid earth system
17.14 Changing Peterson coil to Low resistance grounding
19.0 Rating of neutral grounding reactors
20.0 Advantage of ground fault current limiters over phase fault current limiters
21.0 Active earthing system or Ground fault neutralisers (GFN)
22.0 Neutral grounding resistor versus Neutral grounding reactor
23.0 Type of generator connection and related earthing system
24.0 What is hybrid system
25.0 Different types of Earthing of generator and utility system combination
26.0 LV unearthed system
27.0 Grounding of DC system
28.0 Common grounding resistor for generator earthing
29.0 Time rating of neutral grounding equipment
30.0 Voltage versus Current
31.0 Clearing ground fault within a definite time
32.0 Earthing, tripping philosophy, fault duration, time rating of neutral equipment
33.0 Types of earthing system used in generation, transmission and distribution
34.0 Conversion of one system of earthing/grounding to other system
35.0 Effect of earthing system on damage, overvoltage and relay operation
36.0 Vendor supported in preparing this guidelines
37.0 Allowable touch voltage in 11kV system
38.0 Reference

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Guideline for Earthing of Substation

Connecting above ground non conducting metal part to below ground buried metal is earthing or grounding. It is done to protect humans against electric shock, protect equipment from over voltage, provide safe path for lightning currents and prevent accumulation of static charges. When there is a fault large amount of energy is released. It should be diverted to some safe place i.e. earth. While this energy flows, it should not harm anyone nearby. Hence we earth the metallic parts.

During a fault the fault current will flow from live conductor to non conducting metal parts. With help of earthing, the fault current flows down to the earth grid. When current flows from non conducting metallic part to earth grid, the potential of earth grid and the non conducting metallic part rises. When someone touches that, they receive a shock.

Fault current does not flow directly through the human body, instead fault current develops some potential at the non conducting metallic surface. When a person touches this surface, some amount of current flow through their body. When the current passes through the body it has different effect at different magnitude.

The main purpose of earthing grid design is to limit the current through the human body below the value which leads to fibrillation or death. We can allow 116mA for 1sec, 164mA for 0.5sec, 211mA for 0.3sec currents through 50kg human body. The duration of current flowing through the body depends on operation of the main protection and backup protection. 

The paths in which current can flow into human body

  1. Hand to foot 
  2. Foot to foot (rare)
  3. Hand to Hand ( very rare)

Permissible touch and step voltage are the voltages which will keep the current through the body below the danger level.   Etouch =  IB (RB+1.5   )  Estep   =  IB (RB+6   ) 

Earthing Grid Consist of mesh of horizontal conductors and earth roads penetrating below the ground. The grid material is either copper or Galvanised iron rods or flats. The size depends on the fault current.  

Mesh Voltage Em=ρ Km Ki IG Lm    Step Voltage Es=ρ Ks Ki IG Ls , these are actual voltage developed during a fault. The Objective of earthing design is to keep the mesh and step voltage developed below the Permissible values.  Developed voltages depends on the geometry of the earth grid. By proper design the voltage developed can be brought down below tolerable values. 

Detailed guidelines on  Guideline for Earthing of AC substation consist of 31 pages with few sample calculations. This detailed guideline gives many practical examples, calculations, illustrations which will help an application engineer to actually design a plant. This full guidelines can be accessed in below link. 

Contents in the detailed guidelines are 

1. What is earthing
2. Why do we do earthing
3. What is the Scope of this guideline
4. How to protect a person in a substation against electrical shock
5. What happens during a fault
6. Why fault take place
7. What happens when the current flows to the ground or earth
8. How is earthing design of domestic and industrial system different from substation
9. What happens during a shock10. At what current does a person feel an electric shock and when does a person die
11. What is the current allowed through human body
12. What should be the value of ts
13. What if weight of person is more or less than 50kg
14. What is the value of Resistance for the current through the body
15. What are the paths in which current can flow into human body
16. Permissible touch and step voltage
17. Correction factor for foot resistance
18. Permissible touch and step voltage with Cs
19. Earthing Grid
20. What needs to be connected to the earthing Grid
21. Sizing of earthing grid
22. Fault current distribution (Sf)
23. Decrement Factor
24. Grid Geometry
25. Mesh Voltage calculation
26. Step Voltage calculation
27. GPR- Ground potential rise
28. Transferred potential
29. What Causes GPR
30. Grid Resistance
31. Sample Calculations
32. Case 1: Fault inside substation with design fault current and duration 1sec
33. Case 2: Fault inside a substation with fault current 20.5kA and duration 100ms
34. Fault current distribution

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