design practice note - metro trains melbourne · design practice note earthing of tca surge...

DESIGN PRACTICE NOTE

EARTHING OF TCA SURGE ARRESTORS

L1-CHE-INS-016 Version: 2 Effective from: 21st August 2015

Approving Manager: Chief Engineer Approval Date: 21/08/2015 Next Review Date: 21/08/2017

PRINTOUT MAY NOT BE UP-TO-DATE; REFER TO METRO INTRANET FOR THE LATEST VERSION of 5

Table of Contents

1. Purpose ........................................................................................................................ 3

2. Scope ........................................................................................................................... 3

3. Abbreviations and Acronyms ..................................................................................... 3

4. Background ................................................................................................................. 3

5. The Issue ..................................................................................................................... 3

6. The Requirement ......................................................................................................... 4

7. Responsibilities ........................................................................................................... 4

8. References and Legislation ........................................................................................ 4

9. Appendix 1 - Transient Controls Australia Report .................................................... 5






1. Purpose

The purpose of this document is to clarify MTM requirements for the earthing of TCA surge arrestor arrestors.

2. Scope

The contents of this document are to be incorporated into all future design or renewal work.

3. Abbreviations and Acronyms

MTM Metro Trains Melbourne

PCER Prefabricated Communication Equipment Room

RRL Regional Rail Link

SER Signal Equipment Room

TCA Transient Controls Australia

VRIOGS Victorian Rail Industry Operators Group Standard

4. Background

VRIOGS 012.2, section 18.11 lists the following Transient Controls Australia type arrestors for power protection:

Model No: TC-440/80 3 phase 440VAC incoming power.

Model No: TC-150/80EI 2 wire 120VAC incoming power.

Model No: TC-150/802WE 2 wire track circuits.

Model L No: TC-150/803WE 3 wire track circuits (i.e. Point machines).

Model L No: TC-XXS/S4 xx Nominates signal voltages.

Model No: TC-RCC Communications lines.

The method of earthing for the TCA surge arrestors is not described.

5. The Issue

There has been confusion with designers and constructors with regards to the earthing of TCA surge arrestors.

Currently there are two (2) identified methods to earth TCA surge arrestors:

1. direct earth wire from the surge arrestor to the earth bar, or

2. by using an EK10 earth connector via the ‘G’ rail to the earth bar.

Earthing by the direct earth wire method results in multiple earth wires that are run in parallel on the same row. MTM do not consider this as good practice.

Earthing through an EK10 earth connector via the ‘G’ rail, is rail industry practice and there

have been no identified problems with this type of earthing of the TCA surge arrestors. This is the preferred method of earthing of the TCA surge arrestors by MTM Signal Maintenance.






6. The Requirement

The requirement is that TCA surge arrestors shall be installed on a ‘G’ mounting rail with an EK10 earthing terminal fitted directly below the TCA surge arrestor.

The TCA surge arrestor earth terminal shall be linked from its earthing terminal to the EK10 terminal. Refer to figure 1.

Where there is greater than one (1) TCA surge arrestor, one (1) EK10 terminal shall not be connected to more than three (3) TCA surge arrestors. Refer to figure 1.

To ensure separation of clean and dirty wiring, the earth wire shall be run on the dirty cable ducting side of the row.

To ensure a good earth connection is provided as per the surge protection earthing guidelines, any terminals (EK10) that require an earth connection are to be mounted as close as practicable to the earth bar.

Figure 1: Assembly arrangement

7. Responsibilities

All planned works to cater for this design requirement.

8. References and Legislation

Ref. No.

Document Title Document No.

Version

1. Specification for Signalling Supply, Construction and Installation

VRIOGS 012.2

B

2. Lightning and Surge Protection – General Requirements VRIOGS 012.7.9

A

3.

Transport for NSW – Signalling Surge Protection Installation Guidelines http://www.asa.transport.nsw.gov.au/sites/default/files/asa/reference-material/ref-04.pdf

N/A 15/06/2005

http://www.asa.transport.nsw.gov.au/sites/default/files/asa/reference-material/ref-04.pdf

http://www.asa.transport.nsw.gov.au/sites/default/files/asa/reference-material/ref-04.pdf






9. Appendix 1 - Transient Controls Australia Report

Please find the following document after this page.

Document Name Document Number Version No. of Sheets

Performance Based Specification for Lightning Protection and General Local Earthing

Sunshine SER/PCER Building.

01051301/PE01 A

01/05/2013 17

Ref : 01051301/PE01 rev A 01th May 2013 Sunshine SER/PCER Building. Performance Based Specification for Lightning Protection and General local Earthing. Attention: Mr. Mathew Taylor (RRL Footscray-Deer Park). Copy: Mr. Mike Coonan (Transient Controls Australia).

Transient Controls Australia PO Box 290 Hillarys W.A. 6923 Tel : 08 9408 0931 Email : [email protected]

tca Transient Controls Australia

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Scope of work. Transient Controls Australia to conduct Earthing and Lightning Protection

Survey of Sunshine SER/PCER Compound located within the Inner Melbourne railway line and provide an Engineering Report of findings and recommendations.

1. Review existing earthing drawings provided by TBBJV. 2. Conduct a site survey of the Sunshine SER/PCER compound. 3. Rationalize the existing earthing design in accordance with AS3000,

VRIOGS 12.2, VRIOGS 12.8 and Victracks standard. 4. Provide a design report with recommended earthing design along with any

supporting calculations and assumptions.

Specifications/References. 1. VRIOGS 012.8- Signaling Equipment Housing- Revision 0.4 2. VRIOGS 012.2- Specification for signaling supply, Construction and

Installation- Revision B. 3. AS 1768- Lightning Protection. 4. AS 3000- Electrical Installations. 5. AS/ACIF S009 6. TS-AST 026- Victrack- Telecommunication Equipment enclosures

(communication huts) specifications. 7. Victrack- Sunshine Prefabricated communication Equipment Room (PCER)

specifications.

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Building Grouping. Buildings can be categorized into one main group for general engineering comments. The general engineering comments regarding earthing and lightning protection cover all three buildings along with recommendations. Risk Assessment calculations on all three buildings had the same result of 14 need for protection on all incoming and outgoing cabling, Note : For risk assessment purposes the equipment has been classified as essential public supply equipment, All sites are within the average annual 20-40 thunder day map zone and have lightning flash rate of 8 flashes per square km per year. These figures would rate the sites as “Category C risk external Services”. Due to the length and exposure of incoming feeder power supply this area has been classified as “ Category C High risk external services “. The need for structural lightning protection to the buildings is a combination of the following factors : 1. The geographical location of the structure. 2. The effective collection area of the structure. 3. The intended use of the structure. 4. The type of construction. 5. What is housed within the structure. 6. The location of the structure (Is it located in a built up area or isolated area). 7. The topography of the location/countryside.

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Earth Grid Specification. The final earth grid specification is a result of Soil Resistivity tests and Depth site survey tests. These tests were performed at two locations on site and an optimum design has been achieved as a result of these tests. Measurement of soil resistivity and depth site surveys were taken at two locations and it was not possible to vary spike direction in all four directions due to site access restrictions. It is to be noted that a 90deg grid test variation of the Soil resistivity was possible. Soil Resistivity Tests indicated homogeneous soil. Due to the lack of area to perform additional tests Werner depth surveys were incorporated with standard Line of traverse testing to obtain final resistivity results. A resistivity map was not an option due to limited area for testing. The site had Line Traverse Soil Resistivity tests taken using 200mm/4M and 400mm/8M (Four pole a/20 method). From these tests and using the Wenner Resistivity calculation Nomogram, earth grids for each building were mapped out and plotted. (See drawing A4600). The earth grids plotted have an optimal rod spacing resulting in 100% efficiency in rod separation to depth ratio. The calculated earth impedance to discharge high transient currents in this area is under 5 ohms, this value would guarantee a secondary transient in the order of 100kA (8/20uS) current pulse would discharge safely. A target value of 2 ohms has been specified by the customer and to achieve this over the entire year taking into account extreme weather conditions the earth building grids have been designed. Site conditions and soil resistivity tests would indicate parallel rod electrodes in a matrix arrangement with inter rod coupling would be best suited at all buildings. A target figure of 2 ohms should be achieved via this method which will ensure that the local ground dissipates lightning energy and other dangerous electrical fault currents to a safe level at high speed. The earth is (relatively) infinite in its size compared to the grounding system and can absorb a virtually unlimited supply of current. In practice this unlimited path is limited by the metal electrode-earth soil interface. With a rod electrode the resistance of the soil is the sum of the series of resistances of virtual shells of earth, located progressively outward from the electrode. The shell nearest the rod has the smallest circumferential area of cross section so it has the highest resistance.

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Successive shells outside the inner one have progressively larger areas and thus progressively lower resistances. As the radius from the earth rod increases the incremental resistance per unit of radius decreases to nearly zero. The path of ground current outward from the earth rod surface consists of successive cylindrical and hemispherical shells. As the distance from the rod increases, so do the cross-sectional areas of the individual shells. As their areas increase their individual series resistances decrease inversely proportionally with the area. The first 1.5 meters from the earth electrode is the most important as this has the highest resistance. It is recommended from site results that a minimum rod depth of 4 meters be used with a grid layout on the building outer limits, the earth grids should be located a minimum of one meter from the buildings foundations. It is to be noted that the depth/spacing ratio is ideal and no additional spur electrodes were required. It is imperative that good earths be installed locally at each building to effectively dissipate transient faults and maintain a good safety earth for personnel and equipment. The return path from any new local earth grid should be physically routed away from all other clean earthing or clean input/output cabling. Each local earth grid requires a separate earth connection (two connections per grid is recommended) to the local earth buss bar, these connections should be 50mm copper cable routed in an electrically dirty designated path avoiding all electrically clean wiring. This physical barrier is critical to the effective elimination of reentrant transients and cross coupling of multiple low level transients. The Earth grid deep vertical rods are at four meter depth and have been specified at 19mm diameter, the 19mm rods are only required if the rods are to be driven into the ground by Kanga hammer or similar mechanical method. Should the sites be drilled then a 16mm diameter rod is acceptable and the drilled holes should be backfilled with Bentonite. The advantages of the Bentonite backfill are to Increases the surface area of the Earth-Electrode, thus lowering its earth resistance and to reduce the soil resistivity by retaining moisture. The interconnection of the deep driven earth rods is a critical part of the final earth grid design. These connections are in effect used as a radial electrode and form part of the final calculated design. It is essential that the trench depth be at minimum 1 meter depth for final earth resistance value and to reduce the voltage gradient to a safe personnel level.

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Local Earth Grid Hardware Components. 1. Deep driven earth rods. Earth rods 19mm diam * 3meter length, 99.9% pure electrolytic copper, molecularly bonded onto steel core minimum thickness 0.25mm. Low carbon steel core with high tensile strength. Rod threads should be rolled for strength and to ensure uniform layer of copper. 2. Rod coupling. Long length silicon and aluminum bronze counter bored threads. Couplings must ensure rod to rod coupling. (19mm coupling). 3. Rod to cable clamp. Cast gun metal body and phosphor bronze bolt. (19mm rod to 50-120mm cable). 4. Inter grid connecting cable. Interconnecting grid cables should be bare copper 70-120mm. The interconnecting grid should be buried to a depth of 1-1.5 meters. 5. Earth inspection pits. Polymer shatterproof earth inspection pit, chemical resistant, load level 5000kg, resistant to oil, petrol, and diesel. Note : No underground jointing of grid cables, all clamps above ground within the inspection pit. Earth inspection pit must be a minimum of 250mm of clear rod to surface. 6. Air Terminal. Solid Copper Earth Terminal (1m ). 7. Air Terminal Base. Cast gun metal air terminal base. 8. Copper Tape. High conductivity annealed copper tape. Note : The 19mm diameter rods and couplings can be replaced by 16mm rods and couplings if sites are to be drilled for the vertical earth rods.

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Transient Protection. The buildings lend themselves to a physical/Electrical barrier approach where all outgoing and incoming cabling can be physically routed via a “dirty” path and all clean protected cabling can be physically isolated from any “dirty” areas. Main earth transient return paths are classified as a “dirty” earth and should be included as part of the physical barrier. This alone will reduce a large amount of damage from cross-coupling of transient faults. All incoming cables should have transient clamps fitted and these clamps should be incorporated in the following areas. 1. Input Power and Backup Power. 2. All signaling cabling. This should include lamps, field contacts, point’s

machines and track circuits. 3. Communications cabling. This should include the data transfer multidrop

system and any modem links. Installing earthing grids/Low impedance high current earth discharge

paths and bonding at all sites. Equipment location. All equipment is to be located within the buildings. Use of high speed three mode transient clamps of correct clamp levels

for all equipment to be protected. All external mains power sources protected on incoming and outgoing

lines with correct fault level protection for area classification. As per AS-1768:2009. (Mains power input 70kA, signal/control inputs/outputs 20kA).

All of these measures should be viewed as cumulative and not alternatives.

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Input Power. Input power to SCR building is transformer isolated normally 2.2kV/110V transformer. The power reticulation supply path is underground and is a common source of transient entry to control rooms. Transformer isolation does not provide transient over voltage protection. Distribution transformers will certainly provide a degree of protection but unfortunately they also have the characteristic faults of “sneak paths” which allow transients from the HV primary directly through to the LV power distribution supply. High voltage to Low voltage transformer action theoretically eliminates transients with respect to earth. That is Line to earth and Neutral to earth. The problem is stray capacitance between primary and secondary windings within the transformer will provide the transient fault with a high frequency path from primary to secondary. In the case of Line to Line transient or differential transients then the transformer is of little or no protective value. These transient faults will pass easily through transformer windings. Also Resistive coupling of transients within transformers is common with differential earth reference points. Live to earth transient laboratory testing on transformers show surprisingly large transient over voltages let-through. The recorded transient let-through level was in the order of half the input due to capacitive coupling. The transformer must not be viewed as an effective transient protection device. A practical solution would be to use a common and differential mode transient clamp at the secondary of the transformer. This would give effective safe transient clamp protection and also in the rare case of unit failure to earth isolation from earth reference. Recommended transient clamp for this purpose would be the TC-150/802EI. The TC-150/802EI would give the required protection in both the common and differential modes with a low let-through voltage that would eliminate transient damage and erroneous system tripping and shutdown. All back-up 240V supplies should be protected using either a TC-440/80 or a TC-150/80, this will ensure integrity of the safe zone and stop transient entry via this path.

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Signal Circuits. Signaling circuits are broken down into three main groups. 1. Direct signaling circuits such as lamp drivers and interface contacts. 2. Points machines, main dive circuits. 3. Track circuits. The direct signaling circuits should be protected by series clamps. These clamps are to be installed as close as possible to the incoming safe area designated zone. The recommended family of clamps is the TC-**S/S4, this family has a working voltage range of 5V to 200V and has been designed specifically for the rail industry with fail to safe functionality. See sample specification below. TC-18S/S4 1. Maximum working Voltage 18V D.C. 2. Peak clamp let thru Voltage 28V peak. 3. Response time less than 10nS. 4. D.C. resistance less than 0.1R end to end. 5. Working current 4amps. 6. Discharge current 20ka (8/20uS) per line, 40kA per unit minimum. 7. Healthy indication. 8. Snap on Din or G rail mounting.

9. Multi strike capability (not one shot fuse type unit).

10. Earth connection isolated from all other wiring and mounting. To avoid dangerous voltage levels occurring within the panel.

11. Earth connection is dirty earth and should be separated from all clean safe wiring, 4mm isolated earth stud mounted on top of unit.

12. No DIN rail earthing of unit should be used as transient reentrant loops will occur. Also the danger of arcing from DIN rail to unit has a high failure due to high discharge currents causing high potential differences.

13. Dimensions are critical low density racking. 12.5W*45H*75L.

14. Plug and base units a not acceptable due to the unsuitability of the pressure wipe contact. Under transient fault conditions the earth and or line contacts have to conduct 20kA, this high energy pulse will either totally destroy the contact or seriously degrade it. The degradation may not be evident initially but will upon next transient fault it will cause a high differential across the contact resulting in very high let thru voltage.

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Points Machines. Point’s machines are protected at point of return from field, the transient is effectively clamped at safe area and type of unit used will depend upon power out to point’s machine. One such clamp is the TC-150/802WE.

Note : Point of entry is the actual physical entry point into the control room or cabinet, of a cable or group of cables. Local earth grid take off points are classified as an entry cable and must be treated as dirty or field wiring. The required segregation distances must be observed when installing such cables.

Communications circuits. All communication input and output circuits should have transient clamps fitted. The multidrop communication circuits should have transient protection clamps fitted on every loop input located at point of entry to SER. It is also recommend that this unit have a healthy local indication for maintenance. One such clamp is the TC-RCC.

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Track Based Primary Protection.

The primary track protection should be high energy low let thru voltage parallel clamps. These clamps should be high energy 500kA parallel high speed low let thru voltage clamps due to the application and the vulnerability of the equipment to be protected. There is no doubt that a large amount of transient coupling occurs on the long length of the tracks, the tracks are insulated and direct and indirect lightning will be induced on to these tracks. This will result in high elevated voltage levels onto the tracks with respect to ground. The result of this is flashover at various points is large common mode and differential mode transients entering the control cabinets. It is strongly recommended that HIGH energy HIGH speed transient clamps be installed at the track take off points in the field. These clamps should have local earth deep driven rods to dissipate the transient before it has chance to flashover and enter the cabinets via the field wiring. The high energy 500kA clamp should have the following minimum specifications. 1. Response time less than 25nS. 2. Working Voltage 120V A.C. 3. Peak clamp voltage of 327V. 4. Discharge current of 250kA (8/20uS) per line. 5. Provide repeated protection in high lightning intense areas. 6. Local indication of system healthy. Clamps should be connected from rail to a local dedicated earth. All cables lengths to be kept as short as possible. All cables including earth cable should be segregated from any other wiring by a minimum of 100mm. Note : If using high energy spark gaps care should be taken during evaluation and selection. The standard spark gap has a slow response time and a very high let thru voltage. This let thru voltage in the order of 4-5kV is one of the main reasons for the careful selection of HIGH energy HIGH speed primary transient clamps. The very low level let through voltage is critical in this application. (See data sheet TC-150/250)

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Structural Lightning Protection. The rail system is classified as an essential public service and the buildings critical to this operation. The buildings house items of sensitive electronic equipment and from risk management calculations the result is need for structural lightning protection at all buildings. Due to the classification of all the buildings the required minimum earthing resistance should not exceed 5ohms. The structural lightning protection systems can be broken down into five main groups. 1. Air Termination Network. 2. Down conductors. 3. Test links. 4. Earth terminations. 5. Earthing electrodes. Air Termination Network. All buildings having metal clad roofing (coulour bond) fixed to metal roof beams. The roofs have no large variations in height so the roof beams can be used as an Air Termination Network with some additions to make it an electrically continuous network. The roof beams if not continuous in length should be electrically bonded at joints with a 50mm earth strap. All roof beams at edges of roof should be cross bonded with earth strap to adjacent beam. This will ensure a continuous Air Termination Network that covers the entire roof area with a High discharge network with low impedance. The parts of flat or semi flat roofs most likely to be struck by lightning are the corners and edges. Down Conductors. The down conductors should be installed at each external corner of the SCRs and also if the SCRs are greater than 20m in length then a central down conductor is necessary. As the SCRs walls are constructed of precast reinforced concrete it is recommended that the steel mesh reinforcing be used as the down conductors. Any overlapping non continuous steel mesh within the casting should be welded together to form an electrically continuous path. The roof and foundation points of the

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wall casting at the corners (and centre if over 20m) should have take off points exiting the casting for electrical bonding. It is common practice to use a steel strap welded to the mesh exiting the casting with 10mm studs welded for electrical connections. Note : It is possible to use copper tape on the outside of the building clipped to the precast walls at earth electrode points if the internal steel mesh is inaccessible. Test Links. The test links of each down conductor should be provided in the earth pit of each earth electrode and this is for testing purposes only. The test link also serve as the earth termination points. Earthing Electrodes. The Earthing Electrodes are the four (six on buildings over 20m) general earthing points around the building. This in effect is the common bonding network. The concrete reinforced floor should have two exit points from the steel mesh connected directly to the Lightning Protection Network to complete a common bonding network. These exit points should be at opposite corners of the buildings if possible. The electrical network should have as few joints as possible and be mechanically and electrically sound. Where overlapping joints are used the overlap must not be less than 20mm. All joints must be clean and inhibited from oxidation with suitable corrosion-inhibiting compound.

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High Voltage supply. The following High Voltage Installation requirements are assumed to have been addressed in the preliminary considerations with the Regulatory Supply Authority before construction. 1. Access. 2. Correct circuit control and protection. 3. Labeling 4. High Voltage control incoming. 5. Busbar /Switchgear design 6. Transformer general. 7. Emergency procedures. 8. Electrical clearances. 9. Fire protection. 10. Safety signs. All of the above items are to comply with Australian Standards and are assumed to have been completed.

Earthing. The earthing networks or Local Earth Electrode Grids at each building should be linked at the common earth bar within the building. Two 50mm copper cables should be used for this connection (see drawing A4601).

Combined Earthing System.

It is recommended that the Combined Earthing System (CES) be adopted. The CES is a system where the high voltage earth and low voltage electrical earths are taken to a common earth bar. The earthing system overview is shown in drawing A4600 revA. The CES is common to all buildings. There are a number of strict rules that must be adhered to for CES installations. 1. There must be at least two earth connections to the main earth grid. These connections should be at diagonally opposite corners of the earth grid network. 2. All accessible exposed conductive parts containing or supporting high voltage equipment must be earthed to the CES. 3. All metal enclosures within the H.V. room are connected to the CES. All exposed metal work in and around the structure must be bonded to the CES. 4. Low voltage Earths and Neutrals to be connected to the CES.

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5. All High and Low voltage cable sheaths are connected to the CES. The H.V. incoming cable metal sheath must have a direct and dedicated earth complete with disconnect link to the CES and marked . 6. All underground services such as water pipes must be bonded to the CES. 7. Low and High Voltage surge diverting devices must have their return paths bonded to the CES. 8. The reinforcing network of the building must be bonded to the CES. 9. Minimum H.V. earthing connection 25mm/sq (Main earth grid earth connections minimum of 50mm/sq copper cable). 10. H.V. insulating matting should be installed in and around all H.V. incoming cable entry, transformers/switchgear. The matting should have a dielectric strength of 30,000V for 1min and a Voltage test of 15,000 for 1min with a continuous rating of 3000V minimum. The CES should have a resistance to earth not greater than 1ohm. Note: This is a requirement of AS3000 and all sites should achieve this with the standard design at present. All sites also have low level step and touch potentials and meet all Australia and International safety standards.

H.V. Earth Faults/Step and Touch Potentials. It is assumed that all supplies have been tested for make and break currents at full working load and at fault level load with respect to source impedance. The 2.2kV H.V. supply is a fully floating supply and has no reference to earth in normal operation or in fault conditions. The use of fully floating systems in low level H.V. supplies needs special considerations regarding earthing. The mechanical steel wire armoring and shield screen if any should be earthed at both ends and this earth clearly identified at termination point CES earth bar (H.V. Earth). All standard H.V. precautions should be applied and only authorized personnel have access to H.V. rooms. The H.V. standard earth fault testing does not apply to isolated fully floating supplies other than any return fault path should be capable of carrying fully fault level currents and protection equipment should be able to operate with no damage to any return path. The structural lightning protection earth network has the capacity to discharge a direct lightning strike and precautions have been taken to ensure that the Australian and International standards are meet. The step and touch potential has been addressed with respect to lightning and this also covers a supply fault to earth. The cross bonding of all exposed metalwork with

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insulation of down conductors and rod entry level to ground for the lightning protection system all covers the step and touch potentials. The deep driven earth rods with interconnection copper links act not only as a structural lightning protection earth network but also as a discharge path below ground level (250mm minimum earth rod). The earth resistivity and depth testing results gave final design of deep driven rods the only effective method of creating a local earth. The maximum current discharge over the average rod would occur at a depth of 4/6 meters, the top 300/500mm of soil at all sites would be greatly affected by a number of factors which include temperature, moisture and soil composition. The step potential is greatly affected by these variables and if fault current from source was to be discharged via the grid (which is not possible with fully isolated/floating systems) then under the most severe conditions e.g. 40degC and 30% moisture by weight then step potential would still be extremely low at surface level. Remember that the fault current has a distributive effect over the entire earth network and a step potential is measured at 1metre spacing. Direct lightning strike to the building has to be treated differently and risk factors have to be weighted for practical and effective discharge with safety from step and touch potentials to taken into account.

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General Comments.

The need for structural protection to the buildings based upon site viewing would indicate a high level of zone protection exists. However the classification of the rail network and the critical operating nature of the buildings to that network results in the need for Structural Lightning Protection. The protection of all incoming and outgoing cables is essential and the physical separation barrier is critical to ensure an effective transient barrier for the SERs. The earthing grid has been standardized where possible for ease of construction. It is essential that all dirty earths are segregated from all clean wiring. The earth grid should be connected to the main SER isolated earth and all other earths should be common bonded. The communications earth can be isolated from all other earths but it is highly recommended that an electrically isolated earth clamp be used. (ref TC-EC). If communications earth is isolated the perimeter fence must have isolation sections with equalization clamps fitted for personnel safety requirements. A number of data sheets and drawings have been included in this package of standard power/signal protectors and construction sheets as a general guide. Should any further information or clarification be required Transient Controls Australia would be available to provide.

Yours sincerely, Peter Evans Engineering Director Transient Controls Australia.

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