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Earthing___UK_Power_Networks_Construction_Guide.pdf | Electrical Substation | Cable
Description: earthing
SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION
EARTHING CONSTRUCTION MANUAL SECTION 3 - SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION
Version 1 1.1 2.0 Date 20/02/2009 09/04/2009 20/10/2010 Description Original Legal Notice removed Document rebranded Originator Stephen Tucker John Lowe Mariann Mulligan
© UK Power Networks 2010 All rights reserved
........................................................9 3.............. 15 LV EARTHING........5 3.......................................................................7 3............................................................................2 3.............................................6 3. 16 EARTH ELECTRODE OPTIONS ....................................... 17 MATERIALS LIST ........................................................8 3.........................................................................................................1 3....................4 3.......... 21 DEFINITIONS ......................................................................................................................................................3 3...................................... 19 MEASUREMENTS......................... 4 GROUND-MOUNTED SUBSTATION EARTHING ....................10 SCOPE........ 22 25/10/2010 © UK Power Networks 2010 All rights reserved ................................................................SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION CONTENTS 3.................................... 5 POLE-MOUNTED EARTHING ................................................................................................... 18 CONNECTION TECHNIQUES ............................................................................20 EARTHING BACKGROUND INFORMATION ..........EARTHING CONSTRUCTION MANUAL SECTION 3 ................
6. Purpose of earthing . Construction techniques. Agricultural and horticultural premises. Ground-mounted substation earthing .a list of definitions and terms used in this document. However. Existing substations where a material alteration is to take place. Refer to Section 3. eg an increase in fault level etc. The earthing construction manual is not a substitute for the earthing design manual which covers the earthing design in detail. The use of polymeric cables means this contribution can no longer be relied upon and as a consequence higher voltages may appear on the site during an earth fault. 25/10/2010 © UK Power Networks 2010 All rights reserved . potential grading and a reduction in the earth resistance were convenient by-products of metallic sheathed cable systems. In the past.9 for further information. Definitions .1 SCOPE The earthing construction manual provides guidance for field staff on the practical application of earthing and applies to:     New secondary substations. Earth electrode options . Fencing replacement at secondary substations. In general the earthing design should be provided by the project planner/designer and the earthing construction manual can then be used to apply the design. Caravan/camping parks.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. 7. mean that earthing requires greater consideration than before. 8.a brief description of what happens during an earth fault and why earthing is required. Scope (this section). 10. 4. Materials list.vertical earth rod and horizontal conductor options to achieve desired resistance values in various soil types. Modern materials. 5. 9. This document includes the following sections: 1. 2. including:    Construction sites. Measurements (reserved for future use).standard earthing requirements and designs. Switchgear replacement at secondary substations. if detailed design information is not available this manual can also be used to install an appropriate earthing system. Pole-mounted equipment earthing . This document does not cover the requirements of special situations.reserved for future use. LV earthing (reserved for future use). 3. in particular polymeric cables.EARTHING CONSTRUCTION MANUAL SECTION 3 .
2. Using the available information: 1.1.5) in a cable trench at least 200mm underneath an HV cable (or if not possible alongside the HV cable).2 GENERAL EARTHING REQUIREMENTS The sections below detail the common earthing requirements that should be applied with the earthing layouts in Section 3. However.2.4 and apply the requirements of Section 3.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.8 to confirm that the required earth resistance has been achieved. As an interim solution the following criteria can be applied: 1. 2.2.1 GROUND-MOUNTED SUBSTATION EARTHING PROCESS The design of earthing for new secondary ground-mounted substations should be carried out by the designer/planner to ensure that the design criteria outlined in Section 4 of the new earthing design manual are fulfilled. 25/10/2010 © UK Power Networks 2010 All rights reserved .    For earth fault levels up to 8kA use 70mm2 bare stranded copper cable.2. HV rod electrode should be made up from 1m or 1.2. select a suitable arrangement from Section 3. The designer/planner should provide the following for each project:   An earthing design form specifying the required HV earth resistance.4. 4.1 INTERIM EARTHING DESIGN Until the earthing design manual is updated designers/planners will not be able to provide the required value of HV earth resistance and whether the site is COLD or HOT.2.EARTHING CONSTRUCTION MANUAL SECTION 3 .2.1. 3.6 and 3. The substation must be assumed to be HOT unless it is fed from the source substation by a continuous underground cable. The HV earth must have a maximum resistance of 1 unless it is a compact or micro substation fed by overhead line when a maximum HV earth resistance of 10 should be used.7.1. For earth fault levels greater than 8kA use 120mm2 (or 2 x 70mm2) bare stranded copper cable. if this is not available.2m copper clad or galvanised rods. 3. Install the earthing as detailed on the drawing provided by the project designer/planner or. Note . In these instances it is advisable to install horizontal bare copper electrode and earth rods (from Section 3. 2. 3. Select the materials and connections from Sections 3. 3.1 HV ELECTRODE Bare copper cable and copper clad or galvanised rods are used as earth electrodes. if detailed design information is not available or for existing sites this manual can be used to install an appropriate earthing system.2 to the earthing arrangement.2 3. Apply the earthing requirements shown in Section 3.2.in ground types expected to have high resistivity (eg sand/gravel/slate/shale/rock) additional HV electrode may be needed to achieve the required resistance to earth. and whether the HV and LV earths are to be combined (for COLD sites) or segregated (for HOT sites). Measure the values of the HV and (if segregated) LV earths in accordance with Section 3.2. An earthing drawing based on one of the standard layouts.
2.the outer sheath of the cable must be connected to the neutral bar in the LV pillar/cabinet in accordance with Section 4 of the LV Cable Jointing Manual.2. For surface mounting in indoor substations an alternative is 25mm x 6mm (or larger) aluminium tape. If practicable the grading electrode should be installed outside the fence at a distance of 300-500mm away from the fence. RTUs etc) must be bonded to the main transformer or switchgear earth terminal.4   LV CABLES CNE cables .5 If a metallic fence is installed within 2m of equipment:   It must be connected to the HV earth and A grading electrode of 70mm2 bare copper cable must as a minimum be installed under the fence line.2 BONDING All main items of plant (LV boards.2. ideally at a depth of 500mm (300mm minimum) and connected to the fence in order to protect staff and the public from dangerous touch potentials. If practicable it should be outside the door. and to the HV earth using flexible 16mm2 PVC covered stranded copper cable.2. Alternatively. 3. ACBs.    For earth fault levels up to 8kA use 70mm2 PVC covered stranded copper cable. FENCING AND DOORS 3.EARTHING CONSTRUCTION MANUAL SECTION 3 . Each end of the loop must be connected to the existing HV electrode and be covered with a 100mm thickness of concrete to provide protection against damage or theft. covering the same area as above. a steel or copper mesh may be installed in concrete to depth of 200mm to 300mm. or just inside. HV CABLES 3.2. For earth fault levels greater than 8kA use 120mm2 PVC covered stranded copper cable. 1m from the door front and 500mm beyond each door frame as shown below. If metallic doors are installed within 2m of equipment:  A loop of 70mm2 bare copper cable must as a minimum be installed directly under the door at a depth of 300mm to 500mm.2. The neutral conductor must be connected to the neutral bar in the LV pillar/cabinet. Each metal door must be bonded to the framework (if metallic).SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. SNE cables .2.the outer sheath and armouring must be bonded together and connected to the neutral bar in the LV pillar/cabinet.2. This must be completed in accordance with Section 4 of the LV Cable Jointing Manual.3 All cable earth screens must be bonded to the transformer or switchgear earth terminal in accordance with Section 4 of the 11kV Cable Jointing Manual.   25/10/2010 © UK Power Networks 2010 All rights reserved .
Connected to the LV neutral bar in the LV pillar/cabinet using 70mm2 PVC covered copper conductor (also laid under an LV main cable) in accordance with the LV Cable Jointing Manual. Also see fence and door replacement in Section 3. 3.6 COMBINED HV/LV EARTHS When a combined HV/LV earth is installed an LV electrode is NOT required. 3.5.1 LV ELECTRODE A segregated LV earth electrode must be:     Selected from Section 3.2.2.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION If metallic fencing or doors is installed more than 2m away from the equipment it must not be connected to HV earth but a 1m driven earth rod must be installed at each corner of the fence or door hinge post and connected to the post. Segregated from any HV electrode by at least 8m.4.EARTHING CONSTRUCTION MANUAL SECTION 3 .2. 25/10/2010 © UK Power Networks 2010 All rights reserved .3 HOT SITE EARTHING REQUIREMENTS The requirements outlined in this section must be applied when combined HV/LV earthing cannot be installed and in such cases the HV and LV earths must be segregated. 3. The LV neutral/earth link in the LV pillar/cabinet must be in place.5 to provide a resistance of 20 or less.3.2. This bonds the LV neutral/earth to the substation HV earth.2. Installed under an LV main cable in the cable trench wherever practicable to enhance its security.
3.EARTHING CONSTRUCTION MANUAL SECTION 3 .2. warning notices as detailed below are required. 13A sockets must be removed from transformer mounted fuse cabinets.2. LV pillars and LV boards unless the earth terminal is connected to the HV earth and an RCD is provided.3. 3.3 WARNING NOTICES FOR SEGREGATED EARTHS Where the HV and LV earths are segregated. 1 These labels will be added as stores items in the future. Label Type Plate Plate Label Text Separate HV & LV Earths Do Not Close Earth Link between Separate HV & LV Earths CAUTION: Separate HV & LV Earths Treat LV Neutral as Live Wear HV Rubber Gloves to Operate Location Install in prominent position inside fence/enclosure Install in LV equipment adjacent to LV Neutral .2 NEUTRAL-EARTH LINK The HV/LV neutral-earth link must be removed for a segregated HV/LV earth. Therefore at HOT sites:   Metallic light switches.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.3. 13A sockets and conduits must not be installed within 2m of any metalwork bonded to the HV earth.HV earth link Install in LV cabinet or next to fuse board in close proximity to LV neutral On HV switchgear near operating position Stores Code 19208X 19210L Plate Not currently available procure locally1 19209G Adhesive Label 3.2.4 LIGHTING AND SOCKET SUPPLIES Care must also be taken with lighting and socket supplies to avoid operator contact between different earthing systems. 25/10/2010 © UK Power Networks 2010 All rights reserved .
           Standard GRP enclosure unit/package substation Standard GRP compact substation Standard GRP micro/pad-mount unit substation Standard GRP HV metering substation Fully-bunded GRP enclosure unit/package substation Fully-bunded GRP compact substation Brick-built unit/package substation Brick-built unit/package substation with ACB Double brick-built unit substation Compact and micro/pad-mount outdoor substations Connection techniques EI 07-0102-0002 EI 07-0102-0012 EI 07-0102-0008 EI 07-0102-0006 EI 07-0102-0004 EI 07-0102-0014 EI 07-0102-0017 EI 07-0102-0020 EI 07-0102-0023 EI 07-0102-0024 EI 07-0102-0025 3. Fence and door replacement. for the following types of ground-mounted secondary substation:       Standard GRP substations.refer to the drawings above for specific designs.4 SECONDARY SUBSTATION EARTHING DESIGNS The section contains general earthing arrangements designs. showing the installation of the HV and LV (if required) earthing electrode. Existing outdoor substations.4.5 STREET LIGHTING COLUMNS Metal street lighting columns within 2m of a HOT site must have their columns earthed via a separate earth rod installed adjacent to the column and not use the neutral/earth of a PME supply.2.2. 3.1 GRP AND BRICK-BUILT SUBSTATIONS (COLD SITE) The general earthing arrangement for GRP and brick-built substations with a combined HV/LV earth (COLD site) is shown below .SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. 25/10/2010 © UK Power Networks 2010 All rights reserved . These are based on the following standard substation design drawings contained in EI 07-0102. Compact and micro pad-mount outdoor substations.EARTHING CONSTRUCTION MANUAL SECTION 3 . Brick-built substations.2.3. Fully-bunded GRP substations.
2.refer to the drawings above for specific designs.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.4. 25/10/2010 © UK Power Networks 2010 All rights reserved .2 GRP AND BRICK-BUILT SUBSTATIONS (HOT SITE) The general earthing arrangement for GRP and brick-built substations with a segregated HV/LV earth (HOT site) is shown below .EARTHING CONSTRUCTION MANUAL SECTION 3 .
SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 25/10/2010 © UK Power Networks 2010 All rights reserved .EARTHING CONSTRUCTION MANUAL SECTION 3 .
25/10/2010 © UK Power Networks 2010 All rights reserved .3.EARTHING CONSTRUCTION MANUAL SECTION 3 . These are generally installed in the overhead network and therefore the HV and LV earths should be segregated as shown below and detailed in section 3.3 COMPACT AND MICRO (PADMOUNT) SUBSTATIONS The earthing arrangement for a compact/micro substation without an enclosure is shown below.2.4.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.2.
1 TRANSFORMER TRANSFORMER 3 3 1 HV SWITCH GATE HV SWITCH LV PILLAR 2 Wooden Fence 1 4 LV PILLAR 1 Metallic Fence HV earth electrode . and consequently a standard design is rarely appropriate.2. Often the internal loop of electrode around the switchgear will not exist and the new electrode loop for the fence can provide this. Buried bare copper earth electrode ring around and the switchgear and any other equipment and connected to the earth rods.5 apply: the fence must be connected to the HV earth and a grading electrode must be installed directly underneath or just inside the fence at a depth of 300 to 500mm.70mm2 bare stranded copper cable Gate/gate post bonding . Outdoor secondary substations (surrounded by fencing or walls) must consist of:    A buried ring cable electrode around and between the switchgear. Typical outdoor secondary substation layouts with wooden and metallic fences are shown below. At least two main earth rod electrodes connected to the internal buried earth electrode.EARTHING CONSTRUCTION MANUAL SECTION 3 .2. The electrode should ideally be at a depth of 500mm but if this is difficult to achieve.4. a minimum depth of 300mm is needed and two additional earth rods should be installed near the front of the substation in a location free of buried cables. For metallic fencing the requirements of Section 3. 25/10/2010 © UK Power Networks 2010 All rights reserved .SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.2. Buried bare copper earth electrode in front of the switchgear where an operator stands when using the switchgear.16mm2 PVC covered stranded copper cable Earth rods Connection to equipment earth terminal 1 2 3 4 3m earth rods. If metallic fencing is present install the grading electrode directly underneath (or just inside the fence) and connected to the earth rods.4 OUTDOOR SUBSTATIONS (EXISTING) Outdoor secondary substations are not used for new build. instead earthing is an issue during asset replacement work at existing sites.
metallic fences and doors will nearly always be within 2m of the equipment and the approach below should be adopted. Due to the small size of most secondary substations.2.5 apply to the replacement of fences and doors. and it is unlikely that a fence earthing system will exist. Bond between gate and gate posts. Connection to the HV earth in four places.EARTHING CONSTRUCTION MANUAL SECTION 3 .2.16mm2 PVC covered stranded copper cable 4 1 1 2 3 4 5 Earth grading electrode either directly underneath the fence line (or just inside fence). Metallic fences even if painted or powder coated must be considered as bare metal unless covered in an approved insulated coating that will not degrade over time. Expamet.2. Ensure that no other metallic fencing or conducting material is within 2m of the substation fence. ideally at a depth of 500mm (300mm minimum) below ground level (not the level of any shingle).5 FENCE AND DOOR REPLACEMENT The requirements detailed in Section 3. 4 3 2 GATE 3 4 5 4 Fence grading buried electrode 70mm2 bare stranded copper cable Gate/gate post bonding . nor is it sufficient merely to bond metallic fence panels together above ground without a buried electrode system. Bond between gate posts. 25/10/2010 © UK Power Networks 2010 All rights reserved . It is not sufficient simply to replace wooden panelling with metallic. Care should be exercised when replacing wooden fencing with a metallic type (eg Pallisade.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.4. 358 etc) since its bonding requirements are more onerous.
refer to the pole-mounted section of the earthing design manual.3 POLE-MOUNTED EARTHING Reserved for future use .SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. 25/10/2010 © UK Power Networks 2010 All rights reserved .EARTHING CONSTRUCTION MANUAL SECTION 3 .
refer to the LV section of the earthing design manual.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.EARTHING CONSTRUCTION MANUAL SECTION 3 . 25/10/2010 © UK Power Networks 2010 All rights reserved .4 LV EARTHING Reserved for future use .
5Ω 1 x 1. Earth rods .SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.5Ω 17m 39m 87m 555m 1200m 27m 62m 140m 870m 1890m 50m 113m 250m 1540m SITE SPECIFIC DESIGN REQUIRED Notes: 1.70mm2 bare copper.EARTHING CONSTRUCTION MANUAL SECTION 3 .5Ω 20Ω 10Ω 5Ω 1Ω 0.6m (spaced 4m apart) 7 x 3.6m 6m 2 x 6m (spaced 6m apart) 12 x 6m (spaced 6m apart) 28 x 6m (spaced 6m apart) 2 x 3.6m (spaced 4m apart ) 106 x 3.5Ω 20Ω 10Ω 5Ω 1Ω 0.6m 1 x 6m 5 x 6m (spaced 6m apart) 12 x 6m (spaced 6m apart) 3.16mm diameter rods. 2. buried at a minimum depth of 600mm. Soil Type Resistance Earth Rods (see note 1) Horizontal Conductor (see note 2) 1.2m 1 x 3. 25/10/2010 © UK Power Networks 2010 All rights reserved .6m (spaced 4m apart) 50% Clay + 50% Sand/Gravel (200Ωm) 20% Clay + 80% Sand/Gravel (300Ωm) Slate/Shale/Ro ck (500Ωm) 20Ω 10Ω 5Ω 1Ω 0.5Ω 20Ω 10Ω 5Ω 1Ω 0.6m (spaced 4m apart) 46 x 3.5Ω 20Ω 10Ω 5Ω 1Ω 0. Horizontal conductor .6m (spaced 4m apart) 3 x 3.5m 3m 7m 50m 115m 3m 7m 17m 115m 250m 7m 17m 39m 250m 555m 2 x 9m 2 x 20m 2 x 45m 2 x 285m 2 x 615m 2 x 14m 2 x 32m 2 x 72m 2 x 444m 2 x 965m 2 x 26m 2 x 28m 2 x 129m 2 x 787m Horizontal Conductors (min 90 apart) Loam (25Ωm) Chalk (50Ωm) Clay (100Ωm) 20Ω 10Ω 5Ω 1Ω 0.5 EARTH ELECTRODE OPTIONS The table below shows the earth electrode options using both vertical earth rods and horizontal conductor) to achieve desired resistance values in various soil types.
The options are to either remove the insulation from 120mm2 PVC covered stranded copper cable (05867D) or use 2 lengths of 70mm2 bare stranded copper cable (05823E) connected in parallel.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. 2.6 MATERIALS LIST Description Earth rod extensible galvanised 1m x 16mm diameter Earth rod driving point Earth rod driving head Earth rod coupling ferrule 70mm2 bare stranded copper cable (BS 6360) 120mm2 bare stranded copper cable (BS 6360) 16mm PVC covered stranded copper cable (BS 6004) 35mm2 PVC covered stranded copper cable (BS 6004) 70mm2 PVC covered stranded copper cable (BS 6004) 120mm2 PVC covered stranded copper cable (BS 6004) 40mm x 6mm bare aluminium tape (4m length) 80mm lap joint 40mm x 6mm cleat self locking Aluminium right angle fishplate Bulldog clamp NE13 Brass connector 16mm2 M10 mechanical lug (BET060-12/10) 70mm2 M10 mechanical lug (BET060-12/10) 120mm2 M10 mechanical lug (BET120-12/10) 2 Component Earth rods (see note 1) Stores Code 02463T 02465Q 01625R 02467K 05823E See note 2 05847L 05864Y 05865J 05867D 01041Y 01629F 01619W 01630B 02466A 02829A 02228U 02228U 02236J Earth electrode Bonding Bonding (above ground only) Connectors Notes: 1.EARTHING CONSTRUCTION MANUAL SECTION 3 . This manual will then be revised to include these changes. 120mm2 bare stranded copper cable is not currently available from UK Power Networks’ Stores.2m copper clad rods with appropriate fittings. The earth rod specification is currently being revised and the rods will be changed in the near future to 1. 25/10/2010 © UK Power Networks 2010 All rights reserved .
Notes: 1. 2. cleaned and have a protective coating of grease applied. all connections to metalwork must be free from paint.EARTHING CONSTRUCTION MANUAL SECTION 3 . Examples of other mechanical connections are shown below and also detailed on drawing EI 07-0102-0025.7 CONNECTION TECHNIQUES HV and LV cables must be connected in accordance with the relevant section of the 11kV or LV jointing manual. 25/10/2010 © UK Power Networks 2010 All rights reserved . All surfaces should be cleaned before jointing. All joints involving galvanised steel and copper should be completely covered in Denso tape to prevent moisture ingress and corrosion.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.
SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3.EARTHING CONSTRUCTION MANUAL SECTION 3 .8 MEASUREMENTS Reserved for future use .refer to the measurements section of the earthing construction manual and use existing practices. 25/10/2010 © UK Power Networks 2010 All rights reserved .
2.9.2 DESIGN CRITERIA To satisfy the above requirements the following design criteria are used for secondary substations: 1. 3. To prevent dangerous voltages appearing on the customers LV neutral/earth. To operate the source protection a maximum HV earth resistance of 10 is required.creating possible danger (touch and step potential) to anyone in the vicinity. Where a separate LV electrode is required the LV earth must have a maximum resistance of 20Ω (to satisfy the PME regulations). 4. A large current will flow along the cable and return to the source via the cable sheath and the general mass of earth. 2.1 EARTHING BACKGROUND INFORMATION PURPOSE OF EARTHING When an earth fault occurs on the electricity distribution network (see diagram below): 1. If the EPR is greater than 430V the site is HOT and the HV/LV earths must be segregated. 4. The current will flow until the source protection operates. 5. To prevent dangerous voltages appearing at the substation and causing danger to staff or the public.9. An EPR of less than 430V is deemed to be a safe value for the secondary distribution network.9 3. If the EPR is greater than 430V the substation is a HOT site and additional precautions are necessary.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. To prevent dangerous potentials appearing on the LV system the EPR must as far as practicable be less than 430V. Protection Device Source Transformer Load Fault Therefore the purpose of earthing is to: 1. To pass the fault current during an earth fault back to the system neutral and operate the source protection.EARTHING CONSTRUCTION MANUAL SECTION 3 . To prevent damage to sensitive equipment (eg communications). 4. This excessive rise in voltage maybe transferred onto adjacent power and communication cables creating possible danger to anyone who might be in contact with them . The EPR can be limited by reducing the fault current or the earth resistance at the substation or site. The current flowing through the earth can cause a considerable rise in voltage (known as earth potential rise or EPR) on the earth and any earthed metalwork near the fault . Provided the EPR is less than 430V the HV/LV earths can be combined. 3. 3. 2. 25/10/2010 © UK Power Networks 2010 All rights reserved . 3. To manage both the earth resistance and the dangerous voltages appearing on equipment a ring electrode is installed enclosing and bonded to all equipment.this may be some distance from the actual fault.
Installing a buried grading electrode around the site and equipment and bonding exposed metalwork together helps to reduce potential differences between a person's hands and feet such that any current flowing through the body is kept to a safe level. Cold Site A cold site is a primary or secondary substation where the EPR is less than 430V.some current will flow through the cable sheath back to the source and some will flow through the ground.EARTHING CONSTRUCTION MANUAL SECTION 3 . Combined Neutral Earth (CNE) Cable A CNE cable has a combined neutral and earth metallic outer sheath with a PVC covering and is used in a PME (protective multiple earthing) LV earthing system. and other relevant earthing information. it is only the current that flows through the ground that causes the earth potential rise. Further details can be found in Section 1 and 2 of the earthing manual. This cable is generally used in a TN-S (separate neutral and earth) LV earthing system. This has a combined neutral and earth up to the cutout and then a separate neutral and earth on the customers side. At a hot secondary site the HV and LV earths must be segregated so that the potential on the HV earth during an earth fault is not transferred to the customer's LV earth terminal. EPR Database The EPR database (Intranet > Applications > Reporting Centre) is UK Power Networks' database of grid and primary substations with details of the EPR. 25/10/2010 © UK Power Networks 2010 All rights reserved . Earth Potential Rise (EPR) EPR is the potential (or voltage) rise that occurs on any metalwork due to the current that flows through the ground when an earth fault occurs on the HV or LV network.10 DEFINITIONS This section defines some of the terms used in this guide. Touch Potential The touch potential is the potential difference between a person’s hands and feet when standing up to 1m away from any earthed metalwork they are touching. Step Potential The step potential is the potential difference between a person’s feet assumed to be 1m apart. Hot Site A hot site is a primary or secondary substation where the EPR is greater than 430V. whether the site is hot or cold. Note . Generally the neutral conductor is a fourth core and the earth conductor forms a protective sheath. in BS 7671 (IEE Wiring Regulations) this is called TN-C-S.SECONDARY DISTRIBUTION NETWORK EARTHING CONSTRUCTION 3. Separate Neutral Earth (SNE) Cable An SNE cable has separate neutral and earth conductors.
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