An Overview of Earthing

Transcription

1 2018 Earthing An Overview of Earthing Gary Blackshaw, Global Business Development Manager

2 Founded by William Joseph Furse Originally as a Steeplejack company Relocated to Traffic Street More emphasis on Engineering Incorporated as W J Furse & Co Ltd William Joseph Furse passed away Premises built at Wilford Road Acquired by EV Holdings Acquired by Crown House Two buy-outs took place Acquired by East Midlands Electricity PLC Celebrated 100 years in business Acquired by Cinven Ltd Acquired by Thomas & Betts ABB acquired Thomas & Betts Celebrating 125 years in business Furse Overview History years of history & experience! 125 years of reliability & trust! May 24, 2018 Slide 2

3 ABB Organization Divisions, business units & product groups Group Divisions Discrete Automation & Motion Electrification Products Process Automation Power Grids Business Units Solar Distribution Solutions Installation Products Protection & Connection Building Products Product Groups Cable Ties, Metal Framing, Duct, Cable Tray Connectivity & Grounding Cable Protection Systems Explosion Protection Emergency Lighting Cable Apparatus & Accessories May 24, 2018 Slide 3

4 Furse Overview Where we make a difference Oil & Gas / Petrochemical Utilities / Energy Cultural & Heritage Data Centers Rail & Infrastructure High Tech & Industrial The Furse Total Solution for all project types and industry sectors worldwide May 24, 2018 Slide 4

5 Earthing Systems Earthing for Lightning Protection Systems Applicable to Lightning Protection systems IEC/BS EN Lightning Protection Standard Generally simple Power Earthing Systems Applicable to Substations, Power Stations, Transformers, Transmission Lines, Telecommunication Lines, Wind Farms, Solar Farms, Data Centres etc. Various Standards Generally very complex May 24, 2018 Slide 5

6 Earthing Systems Lightning protection earthing systems are designed for high frequency applications. For example, a lightning current will typically reach peak value between 10 and 20 microseconds whereas power earthing systems are generally designed for applications operating at relatively low frequency and time spans from 0.2 milliseconds to 5 second duration Lightning protection standards recommend a resistance to earth of 10 Ω or less in most cases Power earthing systems will typically require far lower values, calculated for each separate project To achieve the low resistance values, designing a power earthing system requires much more thought, information, and application than just simply installing an array of rods into the ground as is fairly common practice May 24, 2018 Slide 6

7 Earthing for Lightning Protection Systems

8 for Lightning Protection Systems Functions of the Earthing System Safely & effectively dissipate the lightning current into the ground / earth Earthing products for use in lightning protection systems are designed to safely & effectively dissipate lightning current to earth, whilst withstanding the stresses placed on them Equipotential bonding is equally vital to prevent dangerous sparking between the LPS and other components such as: metal installations, internal systems, external conductive parts and lines connected to the structure. The products are designed to achieve equipotential bonding of metal parts within and around the structure May 24, 2018 Slide 8

9 Lightning Protection Basic Principles of Lightning Protection 1. Capture/intercept the lightning strike (air termination network) 2. Safely conduct the lightning current to earth (down conductor System) 3. Safely & effectively dissipate the lightning current into the ground (earth termination system) 4. Provide equipotential bonding & electrical insulation (separation distance) to prevent dangerous secondary sparking 5. Protect against the secondary effects of lightning caused by surges & transients (i.e. SPDs) May 24, 2018 Slide 9

10 Lightning Protection Standard IEC/BS EN Earth Termination Systems Recommended resistance of 10 Ohms or less in most situations The standard recommends a single integrated earth termination system for a structure, combining lightning protection, power systems and telecommunication systems The main principle behind such a system is to ensure that all systems are at the same electrical potential in the event of a fault or lightning strike, thus minimising and hopefully avoiding any risk of secondary flashing or arcing between the various electrically connected parts of the structure and the equipment contained within Note - Local electrical requirements and regulations may not permit the LP and power earthing systems to be interconnected May 24, 2018 Slide 10

11 Lightning Protection Standard IEC/BS EN Earth Termination Arrangements Type A arrangement Vertical rods or horizontal radial electrodes Connected to each down conductor Type B arrangement Unbroken ring conductor around perimeter of structure depth >0.5m & 1m from building edge Foundation reinforcement piles or raft May 24, 2018 Slide 11

12 Power Earthing Systems

13 What Do We Mean By Earthing? By Earthing we generally mean an electrical connection to the general mass of earth. The mass of earth generally being a volume of soil/rock whose dimensions are very large in comparison to the electrical system being considered. May 24, 2018 Slide 13

14 Functions of an Earthing System Earthing is generally provided for reasons of safety To provide a definite path for fault currents from a fault point back to the associated system neutral To provide a low impedance/resistance to ensure satisfactory protection system operation under fault conditions To limit as far as it is practicable, the rise of earth potential under fault conditions to a value that can safely be transferred outside the site boundary to a third party To eliminate persistent arcing ground faults To provide an alternative path for induced currents thereby minimising the electrical noise in cables To ensure that a fault which develops between high and low voltage windings of a transformer can be detected by primary protection systems May 24, 2018 Slide 14

15 Standards

16 Standards - In Great Britain, earthing of an electricity supply system is governed by the: Electricity Safety, Quality and Continuity Regulations 2002 Electricity at Work Regulations 1989 Construction Design and Management (CDM) Regulations 1994 Breaches of the above constitute a criminal offence BS EN 50522: 2011 Earthing of power installations exceeding 1 kv a.c. BS7430: Code of practice for protective earthing of electrical installations BS7354: Design of high-voltage open terminal substations BS7671: Requirements for electrical installations BS EN IEC : Power installations exceeding 1 kv a.c. Part 1: Common rules May 24, 2018 Slide 16

17 Standards US IEEE Std IEEE Guide for Safety in AC Substation Grounding IEEE Std Guide for measuring Earth Resistivity, Ground Impedance.. IEEE Std Grounding of industrial and commercial power systems IEEE Std IEEE Recommended practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault IEEE Std IEEE Guide for Generating Station Grounding May 24, 2018 Slide 17

18 Soil Resistivity

19 Soil Resistivity One of the most important factors influencing the performance of an earthing system The resistance to earth of a given electrode depends upon the electrical resistivity of the earth i.e. the actual soil where the earth electrodes will be positioned The resistivity of soil can vary not only geographically but across the same site, and quite dramatically at different depths Different layers of strata will affect the distribution of current passing through the electrode May 24, 2018 Slide 19

20 What Factors Influence Soil Resistivity? Type of soil Moisture content Temperature Chemical composition Compactness/Density Seasonal variation Artificial treatment May 24, 2018 Slide 20

21 Soil Resistivity Measurement

22 Soil Resistivity Measurement The resistivity of soil can vary not only geographically but across the same site, and quite dramatically at different depths Different layers of strata will affect the distribution of current passing through the electrode Generally the soil is made up of varying layers of material, different thickness therefore differing resistivity values Soil resistivity measurements will determine the soil resistivity for different depths May 24, 2018 Slide 22

23 Earthing Materials and Connections

24 Dedicated Earthing Earthing System May 24, 2018 Slide 24

25 Earth Electrode Types

26 Earth Electrode - Conductor Earthing conductors form an integral part of the single earthing arrangement, whether they provide the means of connection to the final earth electrode (earth rod or plate), or whether they comprise the earth electrode itself (through an earth grid or ring earth arrangement) An earth conductor must be capable of carrying the maximum expected earth fault current and leakage current likely to occur at a structure. The size or minimum crosssectional area of the conductor must therefore be calculated through the specification of fault current, duration, and jointing type. A good earth conductor must also: Be able to withstand mechanical damage Be compatible with the material of the earth electrode Resist the corrosive effect of local soil conditions May 24, 2018 Slide 26

27 Earth Electrode - Rods Copperbond Rod Molecularly bonding 99.99% pure electrolytic copper on to a low carbon steel core (not sheathed type) No interface or gap between the two metals due to the bond at molecular level which means a dissimilar metal reaction cannot occur and the copper cannot be separated from the steel Highly resistant to corrosion High tensile strength steel core means they can be driven to great depths Copperbonded / Solid Copper / Stainless Steel May 24, 2018 Slide 27

28 Earth Electrode - Rods Solid Copper Rod 99.99% pure copper Offers greater resistant to corrosion Ideally used in applications where soil conditions are very aggressive, such as soils with high salt content Lower strength Copperbonded / Solid Copper / Stainless Steel May 24, 2018 Slide 28

29 Earth Electrode - Rods Stainless Steel Rod Stainless Steel Highly resistant to corrosion Used to overcome many of the problems caused by galvanic corrosion which can take place between dissimilar metals buried in close proximity High strength Copperbonded / Solid Copper / Stainless Steel May 24, 2018 Slide 29

30 Earth Electrode - Comparing Copperbonded & Galvanised Steel Rods Copper is resistant to corrosion in most soils Zinc is sacrificial in most soils and with respect to most metals Corrosion protection mechanisms are different; The copper coating is designed to prevent corrosion of the steel core The zinc coating will delay corrosion of the steel core by providing a sacrificial barrier May 24, 2018 Slide 31

31 Earth Electrode - Comparing Copperbonded & Galvanised Steel Rods Earth Electrode Rods excavated after 12 years ¾ Galvanised Steel Earth Rod The loss of zinc on the galvanized steel earth rod resulted in excessive corrosion of the steel The copperbonded steel earth rod showed minimal corrosion 5/8 Copperbonded Steel Earth Rod May 24, 2018 Slide 32

32 Earth Electrode - Comparing Copperbonded & Galvanised Steel Rods Galvanised Earth Electrode Rod excavated after 11 years Galvanised Steel Earth Rod The loss of zinc resulted in excessive corrosion of the steel. One area is reduced from a ¾ diameter to approximately a ¼ diameter due to the corrosion The eventual failure could result in a potential, critical earthing system collapse! May 24, 2018 Slide 33

33 Corrosion Copper is one of the better and commonly used materials for earth electrodes. Solid copper is particularly suitable and recommended where high fault currents are expected May 24, 2018 Slide 34

34 Corrosion Earth electrodes, being directly in contact with the soil, shall be made of materials capable of withstanding corrosion. The factors associated with the corrosion of metals in contact with soil that should be considered are; The chemical nature of the soil ph value (acidity/alkalinity) Salt content Differential aeration / drainage Presence of bacteria The material has to resist the mechanical influences during their installation as well as those occurring during normal service May 24, 2018 Slide 35

35 Earth Electrode - Plates & Mats 100V 50V Difference 1000V 400V Difference 50V Difference in voltage potential minimized through use of earth mat 600V Voltage potential curve Image is for illustration purposes only May 24, 2018 Slide 36

36 Earth Electrode - Plates & Mats Copper Earth Plates 99.99% pure copper Highly resistant to corrosion Alternative style of electrode where there is high resistivity soil or where rock conditions prohibit the driving of rods Copper Earth Lattice Mat 99.99% pure copper Highly resistant to corrosion Designed to minimize the danger of exposure to high step and touch voltages to operators in situations such as high voltage switching May 24, 2018 Slide 37

37 Earth Electrode Connections / Joints It is critical that the earth electrodes connections / joints are conductively and mechanically stable and reliable Mechanical (compression, bolted etc.) connections / joints rely on surface contact and physical pressure to maintain connection Exothermic welded connections / joints form permanent, high quality electrical connections Compression Connection Mechanical Connection Exothermic Connection May 24, 2018 Slide 38

38 Earth Electrode Connections / Joints FurseWELD Exothermic Welding offers the following advantages; Connections are designed to have a larger cross-sectional area than the conductors being joined Equivalent or greater current carrying capacity Joints can therefore handle higher fault currents than using mechanical clamps or brazing Better corrosion properties Permanent connections that will not loosen May 24, 2018 Slide 40

39 FurseWELD Exothermic Welding Where to use it? Infrastructure projects Utility projects Power plants Substations Rail Windfarms Solar farms OHL Telecoms May 24, 2018 Slide 41

40 Earth Electrode Backfill Materials

41 Earth Electrode Backfill Materials Typical Application An earth electrode backfill material may be used to reduce the contact resistance and increase the effective size of earth electrodes, e.g. as a backfill for earth rods installed in drilled holes or as a layer encapsulating horizontal earth conductors buried in a trench. May 24, 2018 Slide 43

42 Earth Electrode Backfill Materials Bentonite Bentonite is a moisture retaining clay consisting largely of sodium montmorillonite, which when mixed with water swells to many times its dry volume. Its main advantage as far as earthing is concerned, is that it has the ability to hold its moisture content for a considerable period of time and to absorb moisture from the surrounding soil. May 24, 2018 Slide 44

43 Earth Electrode Backfill Materials Bentonite Bentonite will absorb up to five times its weight in water and swell up to thirteen times its dry volume. At six times its dry volume it is a very dense, pasty clay that can hold its own shape and will adhere to any surface it touches. These two characteristics solve the compaction and soil to rod contact problems Bentonite hydrates chemically, holding water in its structure. The material is a natural clay formed years ago by volcanic action. It is noncorrosive, stable and will not change characteristics as time elapses The resistivity of Bentonite varies from about 3 Wm upwards depending on its moisture content (BS7430 clause 8.5) Generally not used in very dry or free draining locations May 24, 2018 Slide 45

44 Earth Electrode Backfill Materials FurseCEM FurseCEM is a granulated electrically conductive aggregate that replaces normal concrete fine aggregates such as sand, permitting electrically conductive concretes to be designed by applying conventional concrete technology May 24, 2018 Slide 46

45 Step and Touch Potential

46 Step and Touch Potential When the human body is accidentally introduced into the circuit between live (faulted) metalwork and earth a current may flow that could be lethal Current flow is dependant on many factors such as duration, body impedance, footwear impedance, surface resistivity etc. The evaluation of step and touch potentials are required by most international earthing standards Most earthing standards set tolerable limits for step and touch potentials which are determined by the product of allowable body current and the impedance of the electrocution circuit model Definitions of voltage limits varies between standards May 24, 2018 Slide 48

47 Step Potential Step Potential is the difference in surface potential experienced by a person s feet bridging a distance of 1m without contacting any other grounded surface Step Potential can be controlled by the use of a properly designed ground electrode system (grid) or the use of insulating ground coverings such as rock chips 50% Voltage drop between feet Same potential between feet May 24, 2018 Slide 49

48 Touch Potential Touch Potential is the potential difference between EPR and the surface potential at the point where a person is standing, while at the same time having hands in contact with a grounded structure Touch Potential is controlled by proper bonding and protective systems, such as personnel safety mats and insulating ground coverings (rock chippings) No Protection Same potential as tower May 24, 2018 Slide 50

49 Earthing Design Overview

50 Design Overview A vital first part of the earthing design is the accurate measurement and interpretation of Soil Resistivity Accurate soil resistivity data together with other system design information are of vital importance as the inputs to complex computer modelling processes This data is used to determine Rise of Earth Potential values under system fault conditions The data is also used to calculate values of potentially hazardous touch, step and transfer voltages and determine the Hot or Cold nature of the site Hot Site A site where the rise of earth potential, under the maximum earth fault current condition, can exceed the value either 430 V or 650 V depending upon the fault clearance time Cold Site A site that has a earth potential rise below the telecommunication authorities limits (430 and Hzs) May 24, 2018 Slide 52

51 Design Overview Earthing System Using the soil model and taking account of the power system bonding requirements, an economical earthing system layout can be developed and analysed Example of a 3D earth electrode layout consisting of vertical electrodes and horizontal interconnecting conductor tapes May 24, 2018 Slide 53

52 Furse Earthing Design C DEGS Soil resistivity measurements System design Validation of existing designs Step & touch potential calculations Hot / Cold site parameters

53 Glossary

54 Glossary Earth Potential Rise Voltage between an earthing system and reference earth Reference Earth (remote earth) Part of the earth considered as conductive, the electric potential which is conventionally taken as zero, being outside the zone of influence of the relevant earthing arrangement Hot Site A site where the rise of earth potential, under the maximum earth fault current condition, will exceed the value either 430 V or 650 V depending upon the fault clearance time Cold Site A site that has a earth potential rise below the telecommunication authorities limits (430 and Hz) Rise of Earth Potential (ROEP) - The radial ground surface potential around a earth electrode referenced with respect to remote earth Local Earth Part of the earth which is in electric contact with an earth electrode and the electric potential of which is not necessarily equal to zero Foundation Earth Electrode Conductive structural embedded in concrete which is in conductive contact with the earth via a large surface May 24, 2018 Slide 56

55 Glossary Earth Fault Fault caused by a conductor being connected to earth or by the insulation resistance to earth becoming less than a specified value Fault Level The fault level in amps that may be expected to flow through the earth grid and on which calculations will be based Earth Fault Current Current which flows from the main circuit to earth or earthed parts at the fault location Resistivity The reciprocal of conductivity. It is the inherent resistive property of a material. Dimensionally it is resistance x length for a 1 metre cube in Ω/m May 24, 2018 Slide 57

56 ABB Furse Quality Expectations IEC/BS EN Lightning Protection Component Standard

57 Quality Expectations IEC/BS EN Lightning Protection Component Standard

58 IEC/BS EN Recognised Manufacturing Product Standards IEC/BS EN Lightning Protection System Components (LPSC) Parts 1 7 Governing lightning protection components quality & performance Introduced to be the direct replacement of BS EN 50164

59 IEC/BS EN Recognised Manufacturing Product Standards IEC/BS EN Lightning Protection System Components (LPSC) IEC/BS EN :2012 Lightning protection system components (LPSC) Part 1: Requirements for connection components IEC/BS EN :2012 Lightning protection system components (LPSC) Part 2: Requirements for conductors and earth electrodes IEC/BS EN :2012 Lightning protection system components (LPSC) Part 3: Requirements for isolating spark gaps (ISG) IEC/BS EN :2011 Lightning protection system components (LPSC) Part 4: Requirements for conductor fasteners IEC/BS EN :2011 Lightning protection system components (LPSC) Part 5: Requirements for earth electrode inspection housings and earth electrode seals IEC/BS EN :2011 Lightning protection system components (LPSC) Part 6: Requirements for lightning strike counters (LSC) IEC/BS EN :2011 Lightning protection system components (LPSC) Part 7: Requirements for earth enhancing compounds

60 IEC/BS EN Product Test Standards In order to comply with IEC/BS EN standard the components & materials used shall comply with the IEC/BS EN series Governs lightning protection component quality and performance Has fully replace BS EN LPSC which conform to this standard offers assurance that their design and manufacture is suitable for use in LPS installations.

61 IEC/BS EN IEC/BS EN Lightning protection system components (LPSC) Part 1: Requirements for connection components A performance specification attempt to simulate actual installation conditions Preconditioning or environmental exposure followed by three 100kA 10/350 s electrical impulses (simulating lightning discharge)

62 IEC/BS EN Examples of components before and after testing

63 IEC/BS EN IEC/BS EN Lightning protection system components (LPSC) Part 2: Requirements for conductors and earth electrodes A performance specification attempt to simulate actual installation conditions Dimensional checks radial copper thickness & adhesion Preconditioning or environmental exposure Bend testing

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