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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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Training of Neutral Grounding

Training of Neutral Grounding (LRNG, HRNG, Peterson coil, Active system) Venue : Intide, above SBI RBI layout, Bengaluru Date : Feb 9-10 2019 (2 days) Type : Lecture Registration Details: Download Photo Gallery  

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