EDS CUSTOMER EHV AND HV CONNECTIONS (INCLUDING GENERATION) EARTHING DESIGN AND CONSTRUCTION GUIDELINES

Transcription

1 ENGINEERING DESIGN STANDARD EDS CUSTOMER EHV AND HV CONNECTIONS (INCLUDING GENERATION) EARTHING DESIGN AND CONSTRUCTION GUIDELINES Network(s): EPN, LPN, SPN Summary: This standard provides guidance on the design and construction of earthing systems for customer demand and generation connections and the associated substations. This document is not a substitute for the specific design and construction standards but has been produced to assist UK Power Networks' staff and customers with their application. Author: Stephen Tucker Approved By: Paul Williams Approved Date: 09/09/2016 This document forms part of the Company s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue. Applicable To UK Power Networks All UK Power Networks Asset Management Capital Programme Connections External G81 Website Contractors ICPs/IDNOs Meter Operators HSS&TT Network Operations UK Power Networks Services Error! Unknown document property name.

2 Revision Record Version 2.1 Review Date 07/09/2021 Date 19/12/2017 Author Lee Strachan Why has the document been updated: Minor version update What has changed: reference of EDS changed to EDS Version 2.0 Review Date 07/09/2021 Date 07/09/2016 Author Stephen Tucker Why has the document been updated: Revised to incorporate business feedback What has changed: Scope expanded to include customer demand connections (Section 2). Earth conductor and electrode sizing revised to align with latest ENA TS (Section 5.4.2). Guidance on standalone and integral/basement substations included (Section 5.7). Guidance on combining UK Power Networks/customer earthing systems expanded (Section 5.8). Specific guidance on solar and wind farms added (Section and Appendix A). Specific guidance on high rise developments added (Section and Appendix B). ECS incorporated. Version 1.0 Review Date 05/06/2016 Date 22/05/2015 Author Stephen Tucker Why has the document been issued: New document to provide guidance on earthing associated with generator connections. UK Power Networks 2017 All rights reserved 2 of 37

3 Contents 1 Introduction Scope Glossary and Abbreviations Overview General Basic Principles Touch and Step Voltages Transfer Voltage Design Overview Applicable Design Standards Information Fault Levels Design Process Design Criteria Combining UK Power Networks and Customer Earthing Systems Earthing Arrangement HOT Sites Auxiliary Supplies Lightning Protection Electrode Systems Metallic Fences Lighting and Security Equipment Specific Earthing and Bonding Requirements Design Approval Construction General Materials Conductor Sizes Electrode Special Considerations Earth Bar and Labelling Equipment Bonding Ancillary Metalwork Bonding Customer Earth Test Link Commissioning Testing Assessment UK Power Networks 2017 All rights reserved 3 of 37

4 8.3 Records References UK Power Networks Standards National and International Standards Dependent Documents Appendix A Solar and Wind Farm Specific Requirements A.1 Solar Farm A.2 Wind Farms and Wind Turbines Appendix B High Rise Development Requirements B.1 Overview B.2 General Requirements for Customer Connections B.3 Shock Risk B.4 UK Power Networks and Customer Substations B.5 Building, UK Power Networks and Customer Earthing Systems B.6 Original London 33kV Earthing Design Parameters Appendix C Electrode Surface Area Current Density Appendix D Earthing Checklist Appendix E Drawings UK Power Networks 2017 All rights reserved 4 of 37

5 Figures Figure 7-1 Proprietary Earth Bar Figure 7-2 Earth Marshalling Bar Figure 7-3 Earthing Interconnection via a Removable Link Figure 7-4 Connection to Customer Earthing System via Removable Link Figure B-1 High Rise Building Overview Showing Common Mesh Arrangement Figure E-1 High Rise Building Typical Connections to Structural Piles and Reinforcement Figure E-2 High Rise Building Typical Earthing Arrangement Tables Table 5-1 Design Criteria Table 5-2 Maximum Touch and Step Voltages (based on ENA TS Figure 2) Table 5-3 Label Table 7-1 Tape and Stranded Conductor Specifications Table 7-2 Minimum Earth Conductor and Electrode Sizes Table 7-3 Earth Link Warning Label Table B-1 London 33kV Distribution Network Earthing Design Parameters Table C-1 Surface Area Formulae UK Power Networks 2017 All rights reserved 5 of 37

6 1 Introduction This standard provides guidance on the design and construction of earthing systems for customer demand and generation connections where the customer carries out the majority of the design and construction work. This standard includes specific guidance on solar and wind farms and high rise buildings. This document is not a substitute for the specific design and construction standards but has been produced to assist UK Power Networks staff and customers with their application and produce an earthing system that will satisfy UK Power Networks requirements. The customer s own installation will generally be designed and built by the developer with reference to appropriate standards. It is not UK Power Networks role to carry out design work for a developer. However, UK Power Networks does have a duty of care to ensure that the earthing system of any customer connected to its distribution network is adequate in terms of safety and conforms to current UK earthing standards. An audit of the earthing design shall be carried out to ensure that the design meets relevant UK Power Networks and UK standards as described in this document. All earthing designs shall be approved before construction and tested before energisation. Connection will be refused, as outlined in Paragraph 26 of the Electricity Safety Quality and Continuity Regulations (ESQC Regulations) 2002, if UK Power Networks considers a design to be unsafe. 2 Scope The guidelines in this document apply to the earthing of any substation associated with customer demand and generation connections from 6.6kV to 132kV inclusive. Refer to EDS for customer LV connection earthing. For guidance on other aspects of customer connections refer to the relevant design standard: 132kV: EDS and EDS kV: EDS , EDS and EDS kV: EDS , EDS and EDS UK Power Networks 2017 All rights reserved 6 of 37

7 3 Glossary and Abbreviations Term COLD Site DigSILENT PowerFactory Earth Conductor Earth Electrode EHV EPR Grid Substation HOT Site HV ITU LV POC Primary Substation Secondary Substation Source Substation Step Voltage Touch Voltage TN-C-S TT Transfer Voltage Definition A COLD site is a grid, primary or secondary substation where the earth potential rise is less than 430V or 650V (for high reliability protection with a fault clearance time less than 200ms) The power system analysis software used by UK Power Networks A protective conductor connecting a main earth terminal of an installation to an earth electrode or to other means of earthing A conductor or group of conductors in direct contact with the soil and providing an electrical connection to earth Extra High Voltage. Refers to voltages at 132 kv, 66kV and 33kV Earth potential rise. EPR is the potential (voltage) rise that occurs on any metalwork due to the current that flows through the ground when an earth fault occurs. Historically this has also been known as rise of earth potential (ROEP) A substation with an operating voltage of 132kV and may include transformation to 33kV, 11kV or 6.6kV A HOT site is a grid, primary or secondary substation where the earth potential rise is greater than 430V or 650V (for high reliability protection with a fault clearance time less than 200ms) High Voltage. Refers to voltages at 20kV, 11kV and 6.6kV International Telecommunication Union. ITU directives prescribe the limits for induced or impressed voltages derived from HV supply networks on telecommunication equipment and are used to define the criteria for COLD and HOT sites see below Low Voltage. Refers to voltages up to 1000V, typically 400V and 230V Point of connection A substation with an operating voltage of 33kV and may include transformation to 11kV,6.6kV or 400V A substation with an operating voltage of 11kV or 6.6kV and may include transformation to 400V The grid or primary substation supplying the new substation for the customer connection The step voltage is the potential difference between a person s feet assumed to be 1m apart The touch voltage is the potential difference between a person s hands and feet when standing up to 1m away from any earthed metalwork they are touching Terre Neutral Combined Separated. Refer to EDS for further details Terre Terre. Refer to EDS for further details The transfer voltage is the potential transferred by means of a conductor between an area with a significant earth potential rise and an area with little or no earth potential rise, and results in a potential difference between the conductor and earth in both locations UK Power Networks 2017 All rights reserved 7 of 37

8 4 Overview 4.1 General Earthing is necessary to ensure safety in the event of a fault. In general terms, the installation should be connected to the general mass of earth via an electrode system that provides a suitably low earth resistance value. In the event of an earth fault, the earth resistance needs to be low enough to limit the earth potential rise (EPR) to safe values and to operate the earth fault protection. It also needs to be capable of carrying, without damage, the fault current that will flow until the system protection can operate. In addition, bonding (low impedance connections) is required between equipment and metalwork to ensure they remain at the same voltage and to safely convey fault current without damage or danger. 4.2 Basic Principles During an earth fault the voltage of the earthing system and everything connected to it rises briefly until the protection can operate to clear the fault. Effective earthing and bonding minimises the risk to staff and public during this time. The magnitude of the voltage rise (EPR) is determined by the resistance of the local electrode system (R b ) and the fault current (I ef ) that flows into it. Typically, for overhead systems, almost all of the earth fault current will return to the source via the ground. For cable systems, the ground return current will be much smaller if the cable sheath provides a continuous metallic path back to the source substation. The ground return current percentage (I gr %) can approach 100% for overhead systems and is typically 5% to 30% for cable systems. The application of Ohm s Law gives the EPR: EPR = I ef (A) I gr (%) R b (Ω) In designing an electrode system, the value R b should be low enough to limit the EPR (and touch/step) voltages to safe values. The overall value of R b will reduce when the customer s system is connected to the UK Power Networks system, but the standalone value should be sufficient to ensure safety. In short, the customer s system shall be able to operate safely should UK Power Networks local earthing system become disconnected or otherwise compromised, and vice-versa. Faults at all relevant voltage levels need to be considered. 4.3 Touch and Step Voltages Touch and step voltages can be calculated using software or standard equations and for any given design they are a fixed percentage of the EPR. They should be calculated inside and outside of the substation/switchroom(s), as well as around any metalwork (including fences, building structures, cladding etc.) which is connected to the earthing system. Acceptable touch and step voltages are given in Table 5-2 and are dependent on the fault clearance time and surface covering. Therefore the fault clearance time should be calculated (or requested from UK Power Networks) to allow calculated touch and step voltages to be compared with the permissible limits. UK Power Networks 2017 All rights reserved 8 of 37

9 4.4 Transfer Voltage Additionally, for cable connected systems, a fault at the source substation (i.e. UK Power Networks primary or grid substation) can produce an EPR at the new installation due to voltage rise being carried along the cable sheath. The transfer voltage should be calculated and resultant touch/step voltages compared to acceptable limits based on the EPR(s) and fault clearance time(s) at the source substation. Transfer voltage can also be an issue if remote references (e.g. telephone cables, cable TV systems, water pipes etc.) are brought into an installation, and should be considered where any un-bonded services exist within the site boundary or close to it. UK Power Networks 2017 All rights reserved 9 of 37

10 5 Design 5.1 Overview The design and installation of an appropriate earthing system will ensure that a suitably low impedance path is in place for earth fault and lightning currents and minimise touch and step voltage hazards. The main objectives are to: a) design and install an earthing system that provides sufficient safety with regard to touch and step voltage limits; b) conform with the requirements of UK Power Networks earthing standards, BS EN and BS 7430; and c) satisfy UK Power Networks that the site is safe to energise. Ideally the UK Power Networks and customer earthing systems should be designed separately and not be reliant on each other for safety. However an integrated earthing design (where the customer substation/switchroom earthing system is connected to the UK Power Networks substation earthing system) may be considered to achieve an optimal design (refer to Section 5.6 for detail). 5.2 Applicable Design Standards UK Power Networks Earthing System The earthing system for the UK Power Networks substation shall be designed in accordance with the appropriate earthing design standard: EDS for 132kV and 33kV substations. EDS for 20kV, 11kV and 6.6kV substations Customer Earthing System The earthing system for the customer installation shall as a minimum be designed in accordance with BS EN 50522,BS 7430 or the relevant the UK Power Networks standard. The standard(s) used shall be clearly stated in the design submission. Where the UK Power Networks earthing sytem is reliant on interconnection to the customer earthing system for safety the customer earthing system shall be designed to the relevant UK Power Networks standard in Section Further guidance can be found in ENA TS which provides for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations. Its scope includes all equipment within HV and EHV substations. It is adopted by all Distribution Network Operators (DNOs) in the UK, is referred to in the UK Distribution Code and is written into DNO s own standards. Note: UK Power Networks earthing design standards derive from the industry standard documents including ENA TS and ENA ER S34 and ENA ER S36. UK Power Networks 2017 All rights reserved 10 of 37

11 5.3 Information UK Power Networks will usually provide the following information to assist with the earthing design: Maximum earth fault level, earth resistance value, earth potential rise and HOT/COLD classification of the source grid or primary substation. Details of the cable or overhead line network between the source and the proposed point of connection. Maximum earth fault level at the proposed point of connection. Fault clearance time for an earth fault at the proposed point of connection. Additionally for 11kV secondary substations an estimated target earth resistance to achieve a COLD site. 5.4 Fault Levels EPR and Safety Calculations The EPR and safety voltage calculations shall be based on the foreseeable worst case earth fault level. This shall be (at least) the maximum earth fault level at the point of connection including any contribution from the generator plus 10%. An example of the PowerFactory fault level format is shown below. The RMS break value (Ib) should be used for the EPR calculations. PowerFactory Studies Name Ik" A (ka) Ik' A (ka) Ib A (ka) ip A (ka) ib (ka) Sub-transient Transient RMS Break Peak Make Peak Break Busbar Poc Earth Conductor and Electrode Sizing Earth conductor (i.e. connections to the earth grid) size shall be based on the worst-case symmetrical RMS three-phase fault level or the switchgear three-second rating. Earth electrode (i.e. conductors in direct contact with the soil) size shall be based on 60% of the worst case symmetrical RMS three-phase fault level or worst-case earth fault current. However for substations connected at 33kV or 11kV the worst-case earth fault level at the source (grid or primary) substation may be used to size the earth conductor and electrode. The selected values should allow for reasonable growth or future network reconfiguration. Backup protection clearance times, i.e. 3 seconds, shall be used for conductor and electrode sizing purposes. The minimum sizes are given in Table The proposed fault levels are based on the working draft of ENA TS which is based the requirements in BS EN UK Power Networks 2017 All rights reserved 11 of 37

12 5.5 Design Process A summary of the design process outlined below. Obtain soil resistivity data. Design earthing system to optimise resistance in relation to soil structure. Calculate earth grid resistance. Determine ground return current using site specific earth fault levels and supply arrangements. Calculate EPR using ground return current. Calculate/model touch and step voltages and compare to limits. Modify design to achieve compliance. Check electrode surface area current density (Appendix C). Modify design to achieve compliance. Determine site ITU classification (i.e. HOT/COLD). Determine external voltage contours and effect on third parties. If required redesign the earthing to eliminate/reduce the impact of external voltage contours on third party equipment If EPR exceeds threshold values determine necessary mitigation requirements. Produce HOT zone plot for third parties (if necessary). Select earth conductor and electrode sizes to match thermal requirements. Produce earthing report and construction drawing. Submit design for approval. 5.6 Design Criteria The UK Power Networks earthing system and the customer earthing system shall each satisfy the following requirements: Maximum HV electrode earth resistance as specified in Table 5-1 to ensure correct source protection operation. Note: The touch and step voltage requirements detailed below may require a much lower value. Touch and step voltages within the limits specified in Table 5-2 based on the installed substation earth electrode system only (for an agreed integrated earthing system this may include the combined customer/uk Power Networks earthing system). Dedicated electrode systems may be included in this calculation but any contribution from the wider distribution network or auxiliary electrodes such as building foundations or solar/wind farm structures shall not be included in these safety calculations. See Section 5.7 An earth potential rise less than 430V (or 650V for high speed protection) to classify the site as COLD (the earthing contribution from the wider distribution network, building foundations and the customer earthing system may be included in this calculation) or below 2kV if the site is to be classified as HOT. Sufficient earth electrode surface area in contact with the ground to ensure the ground around the electrode does not dry out and increase in resistance during an earth fault (refer to Appendix C and ENA TS for further details). UK Power Networks 2017 All rights reserved 12 of 37

13 Table 5-1 Design Criteria Voltage Max Earth Resistance Typical Fault Clearance Time Earth Fault Level for EPR and Safety Voltage Calculations 132kV n/a 0.2s POC Maximum Earth Fault Level + 10% 33kV n/a 0.5s POC Maximum Earth Fault Level + 10% 11kV or 6.6kV 10Ω 1.5s POC Earth Fault Level from the Substation Earthing Design Tool Table 5-2 Maximum Touch and Step Voltages (based on ENA TS Figure 2) Fault Clearance Time (s) Touch Voltage (V) Soil 2 Step Voltage (V) Concrete/Chippings (150mm) 3 Touch Voltage (V) Step Voltage (V) Asphalt/Tarmac 4 Touch Voltage (V) Step Voltage (V) Soil limits based on ENA TS Figure 2. 3 Concrete/chipping limits based on ENA TS Figure 2. 4 Asphalt/Tarmac limits extrapolated from ENA TS for a minimum resistivity of ohm-metres for Tarmacadam 50 to 100mm thick. UK Power Networks 2017 All rights reserved 13 of 37

14 5.7 Combining UK Power Networks and Customer Earthing Systems Ideally the UK Power Networks and customer earthing systems should be designed as separate systems that are capable of operating safely in the absence of the other. In this way, safety is ensured should they become disconnected from each other or if the customer network is decommissioned. In most situations it is then possible (and preferable) to connect the customer and UK Power Networks earthing systems together (the exception being certain HOT sites detailed in EDS see Section 5.9). However in some circumstances an integrated earthing design (where safety of the UK Power Networks and/or customer earthing system relies on the interconnection with the other) may be required to achieve an optimal design and is acceptable if the reasons can be justified. As a general rule, only the parts of the customer earthing system that are immediately adjacent to the UK Power Networks earthing system should be included in the safety calculations. Where an integrated earthing system is specified the customer system shall be constructed to UK Power Networks standards (in terms of electrode/conductor sizing, method of installation and touch/step considerations). However care is needed if the customer system should become decommissioned or compromised; clear labelling and test facilities shall be provided to enable UK Power Networks to assess whether any customer contribution has been lost. 5.8 Earthing Arrangement The earth electrode system provides the basic functional earthing for the site and is used to satisfy the safety criteria Standalone Substation The earth electrode system for a standalone substation (e.g. switchroom for a solar farm or wind turbine) shall consist of the following: A ring of earth electrode buried in soil around the perimeter of the substation at a depth of 0.6 metres. An earth rod at each corner of the substation (unless a standard earthing arrangement with only two rods as specified in EDS is being used). Duplicate connections between the perimeter electrode and the substation earth bar. Multiple connections to the rebar or reinforcement mesh in the substation foundation. Additional electrode and earth rods to achieve the required earth resistance value. Duplicate connections between the UK Power Networks and the customer earthing systems (refer to Section 7.8). Earth rods, earth conductor and earth electrode using the minimum sizes specified in Table 7-2. UK Power Networks 2017 All rights reserved 14 of 37

15 5.8.2 Integral Substation Where the substation/switchroom is within a building (e.g. high rise developments) it is not usually possible to install an earthing arrangement as outlined in Section and the earth electrode system shall be based on the following: Multiple earth rods in the substation floor or the basement directly into natural soil. A mesh embedded in the substation concrete floor. Use of specifically designed vertical piles and horizontal rebar. Duplicate connections between the mesh and the substation earth bar. Multiple connections to the rebar or reinforcement mesh in the building foundation. Duplicate connections between the UK Power Networks and the customer earthing systems (refer to Section 7.8). Earth rods, earth conductor and earth electrode using the minimum sizes specified in Table 7-2. Refer to Appendix B for further information. 5.9 HOT Sites Overview A COLD site is preferred where possible. HOT sites introduce difficulties particularly if the HOT zone extends beyond the site boundary (and is unlikely to be acceptable in urban areas due to the close proximity to other structures). These main issues are described in the following sections External LV Systems in the HOT Zone It is necessary to consider touch voltages at nearby third-party buildings where they are either inside the HOT zone or where the LV system (e.g. pole-mounted transformers) that supplies them is within the HOT zone or provides supplies into the HOT zone. If touch voltages are not within acceptable limits the earthing system shall be redesigned or specific mitigation measures shall be implemented. The appropriate touch voltage limit to consider is the relevant transferred voltage limit (430V or 650V, as appropriate). Refer to EDS for further information Telecommunications ENA ER S36 defines the criteria for classification of substations and power stations as HOT sites. Special precautions are required with telecommunications plant and strict working procedures adopted in the immediate vicinity of substations where the earth potential rise could under fault conditions exceed 430V/650V. Where this limit is exceeded the site is classified as HOT. For safety reasons it may be necessary for mitigation to be applied at and in the vicinity of the HOT site. If the UK Power Networks substation/customer installation is classified as a HOT site the customer shall consult with the telecommunications operator (e.g. BT Openreach) to establish if any mitigation is required and provide written confirmation that the telecommunications operator agree to energisation before it is energised. It is recommended that assessments at the design phase are conservative and that consultation occurs at an early stage to avoid prolonged delays to the energisation date. UK Power Networks 2017 All rights reserved 15 of 37

16 5.10 Auxiliary Supplies General Requirements LVAC supplies shall be provided in accordance with EDS The following sections provide further detail on the specific earthing arrangements where the auxiliary supply is derived from a secondary 11kV (or 6.6kV) substation or customer switchroom DNO 11kV and 6.6kV Substations Where a secondary 11kV (or 6.6kV) substation is installed to provide auxiliary LV site supplies it shall have an earthing system designed in accordance with EDS and installed in accordance with ECS The earthing system may be interconnected with other earthing systems if required. Where the EPR of the overall site is greater than 430V and it has been classified as a HOT site the segregation of HV and LV earthing systems is not usually required provided the secondary substation: Is within the 430V contour. Is only supplying the site (and not capable of providing supply outside site boundary e.g. backfeeds, etc) and warning label EDS (Table 5-3) is fitted. Has no metallic connection (cable screen or otherwise) outside the site boundary. Note: If the site is HOT (EPR > 430V) due to other HV or EHV supplies, care shall be taken to ensure that dangerous or damaging EPRs are not exported to the 11kV (or 6.6kV/20kV) networks. Refer to EDS for further information. Table 5-3 Label Situation/Location Reference 5 Specification Label Next to the source of LV EDS mm x 37.5mm adhesive label Customer Switchroom Where an auxiliary LV supply is provided from the customer switchroom into the DNO substation the LV cable sheath, armour or earth conductor shall not be connected to the substation earthing system (i.e. it shall only be earthed in the customer switchroom). The armour shall be covered or otherwise protected to prevent shock hazard in the DNO substation LV Earthing System The LV earthing arrangement will usually be TN-C-S unless one of the EDS options requiring a TT earth is used. 5 Refer to EAS for label availability. UK Power Networks 2017 All rights reserved 16 of 37

17 5.11 Lightning Protection Electrode Systems Lightning protection systems, where installed, will usually consist of air terminations, downleads and earth electrodes in accordance with BS EN These may be connected to the customer earth electrode system and will provide a contribution to overall earth resistance. However, this contribution should not be relied on for safety unless the conductors and electrodes are shown to be corrosion resistant, sufficiently rated to carry fault current, and measures (e.g. duplicate connections and labelling) are in place to prevent inadvertent disconnection. Lightning protection electrodes that are intended to be permanently and securely combined with power system earths should be included in analysis of step and touch voltages Metallic Fences General All metallic fences shall be earthed, either independently (preferred) or bonded to the substation earthing system. Occasionally it may be appropriate to use both methods at the same site for different fence sections, providing there is adequate insulation between them, e.g. an insulated fence panel mounted on stand-off insulators. Where the fence panels are supported by steel posts that are at least 1 metre deep in the ground, the posts can be considered as earth electrodes. Additional care and precautions are required for powder coated fences and galvanised mesh fences. Refer to ECS for further information on these and all other aspects of fence earthing Independently Earthed Substation Fence Where an independently earthed fence is used earth rods shall be installed and connected to the fence as follows: At all fence corners. 1 metre either side of where overhead live HV conductors cross the fence. Additional locations such that the distance between rods does not exceed 50 metres Bonded Earthed Substation Fence The fence is usually bonded to the substation earthing system where a 2 metre separation between equipment and fence cannot be maintained. The fences shall be connected to the substation earthing system using discrete but visible connections as follows: At all fence corners. 1 metre either side of where overhead live HV conductors cross the fence. Additional locations such that the distance between connections does not exceed 50 metres. UK Power Networks 2017 All rights reserved 17 of 37

18 Additionally where the perimeter fence is connected to the substation earthing system and the touch voltage on it could exceed the safety limits the precautions below are required: A bare electrode conductor shall be buried in the ground external to the perimeter fence at a distance approximately of 1 metre away and at a depth of 0.5 metres. The conductor shall be connected to the fence and to the earthing system at intervals of approximately 50 metres such that it becomes an integral part of the substation earthing system Site Perimeter Fence The site perimeter fence shall also be earthed however the requirements specified in the previous sections may be relaxed following an assessment of the risks at the site e.g. token earth rods at each corner and every 100 metres Gates An earth bond shall be installed between the gateposts of each gate. A flexible earth strap shall be fitted between each gatepost and gate Lighting and Security Equipment Care is required to ensure that metal supports or columns for lighting and security equipment are correctly earthed, maintain the required separation from other earthing systems and do not inadvertently connect different earthing systems together. For specific requirements refer to ECS Specific Earthing and Bonding Requirements Solar Farms and Wind Turbines Further guidance on the earthing and bonding requirements for solar farms and wind turbines is included in Appendix A High Rise Developments Further guidance on the earthing and bonding requirements for high rise developments, particularly those supplied at 33kV in London, is included in Appendix B. UK Power Networks 2017 All rights reserved 18 of 37

19 6 Design Approval The submitted design should include sufficient information (drawings, reports, calculations, justification etc.) to enable UK Power Networks to understand and assess the design. The requirements are summarised below: Substation layout with earthing arrangement. Main earth electrode(s) and depth, earth rods, rebar/reinforcement connections etc. Bonding to equipment, metalwork etc. Type and sizes of earth electrode, earth rods, bonding conductors etc. Site boundary and the position of any metallic fencing, street furniture or other metallic buildings or structures. Earth resistance. Ground return current. Earth potential rise (EPR) calculations. Touch and step voltage calculations. Transfer voltage calculations. ITU limit calculations and plot of HOT zone (if applicable). The information should be clearly presented in such a way that UK Power Networks can quickly understand and assess the proposed design without having to repeat the design process or calculations. If software plots are included they should be clearly explained and any key values summarised in a table. A basic earthing report template is included in EDS F. UK Power Networks 2017 All rights reserved 19 of 37

20 7 Construction 7.1 General The earthing system shall be constructed in accordance with: ECS for 132kV and 33kV substations. ECS for 11kV substations. This section provides further guidance on specific items. 7.2 Materials The specification for rods, tape and stranded conductor is given in Table 7-1. For all other approved earthing materials refer to EAS Note: The use of galvanised steel is not permitted in any UK Power Networks earthing system or any earthing system that UK Power Networks relies on. Table 7-1 Tape and Stranded Conductor Specifications Material Specification Earth Rods Copper bond steel earth rods to ENA TS Copper Tape* High conductivity copper tape to BS EN Copper Conductor Hard drawn stranded copper to BS 7884 with a minimum strand radius of 3mm Copper Braid High conductivity copper wire to BS 4109-C101 Aluminium Tape* Hard drawn to BS or aluminium alloy to BS 3242 (above ground only) *Tape should be embossed with Property of UK Power Networks 7.3 Conductor Sizes Minimum earth rod, earth electrode and equipment connections are given in Table 7-2. Table 7-2 Minimum Earth Conductor and Electrode Sizes Voltage Minimum Earth Rod Size Max Fault Level Earth Electrode Minimum Copper Size Equipment Connections Minimum Copper Size 132kV 3.6m 26kA 40mm x 4mm 40mm x 4mm (duplicate) 33kV 3.6m 12kA 25mm x 4mm 25mm x 4mm (duplicate) 20,11 or 6.6kV 2.4m 16kA 25mm x 6mm or 2 x 70mm 2 25mm x 6mm or 2 x 70mm 2 12kA 25mm x 4mm or 120mm 2 25mm x 4mm or 120mm 2 8kA 25mm x 3mm or 70mm 2 25mm x 3mm or 70mm Electrode Special Considerations When routing earth electrodes (including those of a combined lightning/power system), extra care is required if the electrode will pass close to an area frequented by people with bare feet, UK Power Networks 2017 All rights reserved 20 of 37

21 e.g. swimming pools or showers and may include areas close to, but outside the immediate site boundary. Consideration should extend to livestock, which can be very susceptible to step voltages. If in doubt consult an earthing specialist. Further information can be found in BS7671, BS 7430, and ENA ER G Earth Bar and Labelling All earth connections shall be connected via separate connections to a dedicated proprietary earth bar (Figure 7-1) or earth marshalling bar (Figure 7-2). Each connection onto the earth bar shall be labelled with the destination/function (e.g. perimeter electrode, rebar, switchgear, customer etc.) using an approved label. Any earth bar shall be located in an accessible position and shall not be located in the cable pit or trench. Where required the earth bar shall employ links to allow the customer earthing system to be disconnected as detailed in Section 7.8. Figure 7-1 Proprietary Earth Bar Figure 7-2 Earth Marshalling Bar 7.6 Equipment Bonding All equipment shall be bonded to the substation earth bar using the minimum sizes specified in Table 7-2. All equipment at 132kV and 33kV shall use duplicate connections. 7.7 Ancillary Metalwork Bonding All exposed and normally un-energised metalwork, including doors, door frames, staircases, ventilation ducts, cable supports, PV panel supports etc., shall be bonded to the main earth using 16mm 2 covered copper cable or an approved equivalent to avoid any potential differences between different items of metalwork. Where ventilation ducts and cable supports are either out of reach and more than 2 metres from other earthed metalwork or there is a likelihood of a transfer voltage to other parts of the building that is outside the substation earthing system they may, with agreement from UK Power Networks, be left un-bonded. Refer to ECS and ECS for further guidance. 7.8 Customer Earth Test Link Where a customer substation or building earthing system is to be connected to a UK Power Networks earthing system duplicate connections shall be used as shown in Figure 7-3. Each connection shall include a clearly labelled removable earth link to allow the systems to be separated for testing purposes. UK Power Networks 2017 All rights reserved 21 of 37

22 UK Power Networks Customer Earth Bar Link Earth Bar UK Power Networks Earthing System Customer Earthing System Figure 7-3 Earthing Interconnection via a Removable Link The removable earth link(s) shall be incorporated into the earth bar as shown in Figure 7-3 A warning label EDS (Table 7-3) shall be fitted next to each link. Refer to EAS for the availability of labels. Earth Link Earth Bar Duplicate Connections to Customer Earthing System Figure 7-4 Connection to Customer Earthing System via Removable Link Table 7-3 Earth Link Warning Label Situation/Location Reference 6 Specification Label At the removalable link connecting two separate earthing systems EDS mm x 50mm adhesive label 6 Refer to EAS for label availability. UK Power Networks 2017 All rights reserved 22 of 37

23 8 Commissioning 8.1 Testing The earthing system shall be tested and commissioned in accordance with: ECS and ECP for 132kV and 33kV substations. ECP and ECP a for 20kV, 11kV and 6.6kV substations. Further information on earthing testing and measurements can be found in ECS Assessment A post construction report shall be provided for all earthing systems associated with all substations above 11kV and any non-standard 11kV or 6.6kV earthing systems to demonstrate that the design has been satisfied. The report should include as a minimum: Measured earth resistance, the measurement method and the measurement route. Calculation of EPR using measured value of earth resistance. Calculation of the touch/step voltages and comparison to the appropriate limits. Confirmation that all design recommendations have been implemented. 8.3 Records The original as built drawing, earthing reports and test results shall be sent to the project manager and delivery manager. A copy of the as built drawing and test results shall be left on-site. A copy of the as built drawing, test form and any earthing reports (design, survey, measurement etc.) shall sent to earthingenquiries@ukpowernetworks.co.uk for loading into the earthing database and the document management system. UK Power Networks 2017 All rights reserved 23 of 37

24 9 References 9.1 UK Power Networks Standards EDS EAS EDS EDS EDS ECS ECS ECS EDS EDS EDS EDS EDS EDS EDS EDS ECP ECP HOT Site Management Earthing Materials Grid and Primary Substation Earthing Design Secondary Substation Earthing Design Customer LV Installation Earthing Design Grid and Primary Substation Earthing Construction Secondary Substation Earthing Construction Earthing Measurements Secondary Substation Civil Design Standards Grid and Primary Substation Civil Design Standards Customer Supplies at 33kV & 11kV Notes of Guidance for New Demand & Generation Connections kV Secondary Distribution Network Design Customer HV Supplies EHV Network Design London 33kV Distribution Network Design and Customer Supplies Customer Supplies at 132kV & 66kV Notes of Guidance for New Demand & Generation Connections (new document) Earthing Test Form Secondary Substation Earthing Commissioning Procedure 9.2 National and International Standards ENA TS ENA TS ENA ER S34 7 ENA ER S36 7 BS EN 50522:2011 BS 7430:2011 BS EN IEC for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations Earth Rods and their Connectors A Guide for Assessing the Rise of Earth Potential at Substation Sites Procedure to Identify and Record HOT Substations Earthing of Power Installations Exceeding 1kV AC Code of Practice for protective earthing of electrical installations Protection against lightning. General principles Wind turbine generator systems. Lightning protection 7 Available from UK Power Networks 2017 All rights reserved 24 of 37

25 10 Dependent Documents The documents below are dependent on the content of this document and may be affected by any changes. EDS EDS EDS EDS Grid and Primary Substation Earthing Design Customer LV Installation Earthing Design Customer Supplies at 33kV & 11kV Notes of Guidance for New Demand & Generation Connections Customer Supplies at 132kV & 66kV Notes of Guidance for New Demand & Generation Connections (new document) UK Power Networks 2017 All rights reserved 25 of 37

26 Appendix A Solar and Wind Farm Specific Requirements A.1 Solar Farm A solar farm typically consists of a UK Power Networks intake substation, metering, customer switchgear, inverters and solar panels. The UK Power Networks substation and the associated earthing system shall be designed in accordance with the relevant part of this document to control the touch and step voltages. The UK Power Networks earthing system should be sufficient to ensure operator safety in the absence of the solar farm earthing system. The solar farm shall have an electrode system sufficient to limit touch voltage in and around the solar farm. Generally, PV arrays and support structures shall be treated as if they are (or could become) bonded to the earthing system, even if they are separated from the AC system and its earthed components. The main reason for this is that inverters (generally) cannot be relied upon to provide isolation under fault conditions (e.g. insulation failure within an inverter could cause one or both of the DC terminals to become connected to the solar farm earth). Consequently, the metallic array structures shall be bonded to the solar farm main earth terminal and measures taken to establish an equipotential zone by bonding (or barriers) in line with normal best practice. Refer to BS 7430 and BS EN for more information. Metallic fences surrounding the site shall be earthed in accordance with Section In most situations, the UK Power Networks and solar farm earthing systems will be combined. Provision should be made for testing of each system in isolation prior to commissioning the site (see Section 7.8). A fall-of-potential measurement over 400 metres or more (depending on site size and constraints) will usually be required to obtain an accurate earth resistance measurement of each earthing system in isolation. Any cable armours (e.g. LV cables or small wiring) between the UK Power Networks and the solar farm substations should remain disconnected until such time as testing is completed. Refer to ECS for further information. LV supplies to the solar farm need care, particularly if derived from a nearby network or if the solar farm is HOT (refer to Section 5.10). UK Power Networks 2017 All rights reserved 26 of 37

27 A.2 Wind Farms and Wind Turbines A wind farm typically consists of a UK Power Networks intake substation, metering, customer switchgear and one or more wind turbines. The UK Power Networks substation and the associated earthing system shall be designed in accordance with the relevant part of this document and integrated into the wind turbine earthing system as required to control the touch and step voltages. Generally each wind turbine will have its own dedicated earthing system, designed to control touch and step voltages and dissipate lightning impulses. The former is achieved by installing potential grading electrodes, whilst the latter is achieved by obtaining an earth resistance of 10 ohms or less, and providing a direct path to earth free of bends, kinks, and re-entrant loops. Refer to BS EN and IEC for further information on lightning protection. The electrode system for each wind turbine in practice normally consists of two perimeter earthing rings of copper conductor. The first (inner ring) installed 0.5 to 1m away from the turbine enclosure, at a depth of 0.5 to 1m, and the second (outer ring) just beyond the edge of the concrete base and at greater depth (typically 1-3m deep). Four earth rods (length dependent on soil structure) are installed on the outer perimeter electrode to help further reduce the resistance, together with bonding of the rebar to moderate touch voltages and provide some electrode effect. If further electrode is needed to achieve 10 ohms, then two additional independent radials up 80m long, 180 degrees apart can be installed. If the site is classified as HOT care is required to prevent dangerous potentials being exported from the site. In some situations, segregated earthing or other measures may be required. Alternatively, interconnecting wind turbines with bare electrode (laid with cables or otherwise) may offer a useful reduction in electrode resistance sufficient to make the site COLD. These design options should be explored with the aid of computer modelling software if necessary to arrive at an economic and practicable solution. Post-construction verification via measurement is also required as per Section 8. UK Power Networks 2017 All rights reserved 27 of 37

28 Appendix B High Rise Development Requirements B.1 Overview The guidance in this section has been taken from ECS (which has been withdrawn) and primarily applies to 33kV supplies in and around the City of London described in EDS ; however the general principles may be applied to any high rise development with supplies derived from an integral 33kV or 11kV substation. This guidance was originally produced to assist developers and ensure the proposed earthing systems are appropriate and adequate for a building containing 33kV equipment. In some situations, poor earthing design can lead to shock risk (to public both inside and outside the building) and fire risk or structural damage (due to overheating earth conductors and/or rebar). The earthing requires special consideration because the supply arrangements are unusual. In particular, buildings are generally supplied from a specific cable or cables and do not obtain any parallel earth benefits from adjacent buildings or substations. A set of earthing design parameters for the London 33kV distribution network is given in Table B-1. For all other aspects of network design and customer supplies associated with the London 33kV distribution network refer to EDS A checklist is provided in Appendix E to assist with the assessment of an earthing design. B.2 General Requirements for Customer Connections It is generally necessary to apply substation design techniques to any buildings housing highvoltage equipment and care is needed (particularly with metalclad buildings) to consider any shock risk which may occur in and around the building under all fault conditions. The most general and overriding requirement is that the installation needs to be designed to prevent danger, which falls into two areas: Prevention of shock. Prevention of fire/thermal damage. Shock risk can arise in terms of hand-to-foot, hand-to-hand or foot-to-foot whenever two points of differing voltage can be simultaneously accessed. This can be managed by equipotential bonding (to keep things at the same potential) and electrode placement at specific locations to control hand-to-feet voltages. Fire/thermal damage, in the context of earthing systems, can be prevented by ensuring all conductors are adequately sized for the current that they will carry in all foreseeable fault conditions. Also, it is necessary to ensure that significant stray current will not flow in parts of any building structure, or other services, that could lead to damage. This is best prevented by the installation of dedicated low impedance bonds in strategic locations to safely convey the majority of fault current. A dedicated electrode system should be sized to cope with the maximum earth fault level (that it will see locally). It is not sufficient to rely solely on lightning protection systems, piles, support structures, rebar etc. to carry high fault currents since these can overheat. Electrode sizing calculations should confirm that the surface current density (Appendix C) will not cause drying/separation at the electrode-soil interface or other damage if the electrode is encased in concrete or another agent. UK Power Networks 2017 All rights reserved 28 of 37

29 Shock and thermal damage risks can be minimised by installing a dedicated and low resistance copper earth grid underneath the footprint of any building, and bonding all items of equipment to it. B.3 Shock Risk B.3.1 Outside the Building A building that contains the 33kV or 11kV equipment can theoretically rise in potential during a fault and could cause a shock risk to public if the building is (or could become) metal clad or otherwise has accessible exposed metallic parts (e.g. handrails). For this reason it should usually have a perimeter grading electrode or similar installed in the soil around the building (i.e. a loop of conductor buried approx. 0.5m to 1m away from the building, and connected to the main earth terminal). The absence of such grading electrodes means that the full EPR could appear as a touch voltage risk to those outside the building, whereas their presence will reduce the touch voltage to a smaller percentage of the EPR. The developer will be expected to demonstrate this, or other measures to prevent danger to the public. Unless the building houses a network substation (Section B.5.4), or provides a low impedance connection to such, no LV or HV services should be taken outside the building where they could come into proximity of LV or HV services from other sources. The earths from these different sources could be at different potential and thus should be separated by at least 2 metres. One example is metallic streetlight columns supplied from the DNO s LV network. B.3.2 Shock Risk Inside the Building This is usually managed by equipotential bonding between structures and plant to ensure that all are at a similar potential. In theory, provided no remote references are introduced, a high EPR will introduce little or no risk to building occupants if the building structure acts as a Faraday Cage 8. In practice, localised voltage gradients can occur around certain items of plant. For this reason, bonding of rebar beneath switchgear/plant is required to provide additional safety to operatives. Bonding should be designed to ensure that it will not be possible for a person to bridge two items of equipment that might be at different voltages. Current flows for 33kV and 11(6.6)kV earth faults should be considered. In many cases the lower voltage levels can produce more onerous current flows (and resultant voltage gradients). Earth bonds should be a minimum size in operational areas, as described in Table 7-2. Smaller sized conductors might be justified in other areas of the building (e.g. to bond items of secondary equipment) provided that the developer can demonstrate that they will be appropriate to withstand foreseeable levels of fault current for faults on that equipment and elsewhere. 8 An enclosure formed by conducting material or by a mesh of such material. UK Power Networks 2017 All rights reserved 29 of 37

30 B.4 UK Power Networks and Customer Substations B.4.1 Common Features Buildings supplied at 33kV generally contain a UK Power Networks substation/switchroom and a customer substation. The overriding requirement in both of these operational areas is that touch voltage is adequately controlled. This generally requires a rebar mesh in the floor screed below the equipment which bonded to the main earthing system. The customer shall ensure that all operational areas are bonded to a common earthing system and that sufficient electrode (rod, tape) is provided to achieve the required overall earthing resistance. UK Power Networks substation will follow the same philosophy, and in addition will have local rod and/or electrode connections sufficient to ensure safety should the customer earthing be absent. In many cases this will involve additional rod electrodes or local pile connections within the footprint of the customer overall earthing system. B.4.2 Earth Resistance A sufficiently low earth resistance is required to ensure both the safety voltages remain within the limits and the EPR is below 430V for a COLD site; a HOT site design is not appropriate in high rise buildings. This is typically 0.5 ohms for the London 33kV distribution network (refer to Table B-1). Note: The terms COLD and HOT do not relate to safety, and it should not be implied that a COLD site is necessarily safe in terms of step/touch voltages. The earthing design shall include results from calculations or modelling to demonstrate that touch voltages are acceptable (refer to Section B.3). In rare cases if it is necessary for a customer s installation to rely on a contribution from UK Power Networks earth for safety or to achieve a COLD site, this shall be discussed with UK Power Networks at an early stage. However given typical London soil structures, a suitable earth resistance should be achievable in most situations and requests to rely on UK Power Networks contribution will not normally be agreed without good reason. B.4.3 Earth Electrode and Conductor Sizing The earth electrode and main bonding conductors shall be sized in accordance with Section and use the minimum sizes specified in Table 7-2. Electrodes may be sized to carry earth fault current since this will provide some design economy but care is required to ensure the current rating of individual rod electrodes is not exceeded in terms of surface current density; there shall be sufficient electrode surface area to convey current into soil without excessive drying/heating and consistent with normal earthing design practice. UK Power Networks 2017 All rights reserved 30 of 37

31 B.5 Building, UK Power Networks and Customer Earthing Systems B.5.1 Building The building earth electrode system provides the basic functional earthing for the site and is used to satisfy the safety criteria. The earth electrode system shall consist of some or all of the following: A ring of earth electrode buried in soil around the external perimeter of all buildings at a depth of 0.6 metres, or other measures sufficient to control touch voltages outside the building(s). Duplicate connections between the perimeter electrode and the substation earth bar or main earthing system. An interconnected mesh of copper tape laid underneath the overall building footprint. An earth rod at each corner of the UK Power Networks substation (unless a standard earthing arrangement with only two rods as specified in EDS is being used). Additional radial electrode, laid with incoming cables or otherwise, sufficient to reduce overall resistance and EPR (and to comply with surface current density requirements). Duplicate connections between the UK Power Networks and the customer earthing systems (refer to Section 7.8). Multiple connections to the rebar/piles or reinforcement mesh in the substation foundation. Duplicate connections from main earthing system to rebar mesh (or similar) laid direct in concrete screed below switchgear/plant, to control hand-to-feet touch voltage. A typical layout is shown in Figure B-1; a copper mesh is established underneath the building, and extends beyond the perimeter of the building to reduce touch voltages. The rods are established around the perimeter of the site and connections from main items of equipment to the mesh are made close to the equipment (and duplicated where necessary). Exothermic Weld at Each Connection Building Footprint Bare Copper Tape Earth Grid Earth Rods Figure B-1 High Rise Building Overview Showing Common Mesh Arrangement UK Power Networks 2017 All rights reserved 31 of 37