ear thing and lightning protection project

ELEN7018: Earthing and Lightning Protection Class Project 2011: High Level Design of a Earthing and Lightning Protection System

Abstract

A high level design of an earthing and lightning protection system for a Methanol processing plant has been developed. A risk analysis of the plants major structures and areas gives an in-dication of the lightning protection level required for a particular structure as well as the type of protection measures which would reduce the risk to loss of human life. External lightning protection including air-termination, down-conductor and earth-termination systems are spe-cified for each major structure within the plant. The design of the meshed earth termination system connecting all the individual earth termination systems is illustrated. The importance of lightning protection zones, bonding, screening, cabling, equipotentialisation and surge pro-tection devices is highlighted in the design of the internal lightning protection systems.

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Contents

1 Introduction.......................................................................................................................1

2 Background.......................................................................................................................1

3 Plant Layout......................................................................................................................2

4 Necessity of a lightning protection system......................................................................3

4.1 Structure/Area Classification.......................................................................................3

4.1.1 Maintenance, Admin, Waste control buildings........................................5

4.1.2 Processing plant: Gasifier, Methanol storage 1 and 2..............................6

4.1.3 Feedstock storage, Waste water pond and Waste ash/slag storage..........7

5 Lightning Protection System Design...............................................................................7

5.1 ELPS.............................................................................................................................8

5.1.1 Maintenance and Administration buildings.............................................8

5.1.2 Waste control building.............................................................................9

5.1.3 Processing plant: Gasifier, Methanol storage 1 and 2............................10

5.1.4 Feedstock Storage, Waste water pond and ash/slag storage areas.........11

5.1.5 Meshed earth termination system (site-wide earthing)..........................13

5.1.6 88/11kV Substation................................................................................14

5.2 ILPS............................................................................................................................14

5.2.1 LP Zones................................................................................................14

5.2.2 Bonding/screening..................................................................................16

5.2.3 Proximity, cabling and routing...............................................................18

5.2.4 SPD’s......................................................................................................18

6 Conclusion.......................................................................................................................20

7 References........................................................................................................................21

Appendix A..............................................................................................................................22

Appendix B..............................................................................................................................23

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1 Introduction

This report presents the high level design of a earthing and lightning protection system for a processing plant located in Piet Retief in Mpumalanga, South Africa. In the process, the com-plete LPS design flow as specified in the SANS62305 suite of documents is followed. The risk assessment of the plant and its structures is performed to determine their respective light-ning protection levels and the protection measures required in order to satisfactorily reduce the risk to loss of human life to below tolerable levels. The plant is then divided into the ma-jor areas/buildings for which the external lightning protection systems are determined i.e. air-termination, down-conductor and earth termination systems. This is followed by the plants internal lightning system requirements.

Section 2 gives a background to the area and the environmental conditions posed. Section 3 provides a brief summary of the plant layout and the dimensions of the structures. Section 4 serves to describe the results of the risk assessment of the processing plant. The lightning pro-tection design is covered in Section 5, where the external lightning protection and internal lightning protection requirements are discussed.

2 Background

The plant to be protected is a Wood Gasification-Based Methanol Processing Plant. The Plant is to be located in a town called Piet Retief in the Mpumalanga province, which is known to be within the timber growing region of South Africa. This is a green field’s project located close to the Pulp and Paper Processing Plants in the area. Large volumes of waste from these plants in the form of wood chips and bark can be used as feedstock in the production of Meth-anol. Methanol derived from this waste can then be seen a greener alternative to conventional petrol and diesel fuels for the large timber transportation vehicles in the region. The Methanol can be derived from a syngas produced by the gasification (thermal conversion) of the feed-stock with a limited amount of air, this step in the key component of the processing plant [1].

The following assumptions and conditions are noted for the area:

Piet Retief Ground flash density: 11.7 (SANS 10313) Soil resistivity can range from 950Ωm - 1500Ωm. Soil condition: Loose and sandy and drains easily. Rainfall: Heavy in summer.

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3 Plant Layout

The Plant layout is shown in Figure 3.1 indicating the main sections of the plant including the Admininstration building, Maintenance building, Waste treatment and storage, Feedstock Drying and Handling, Processing :Gasification and Methanol synthesis and Storage :Feed-stock and Methanol. There are five control rooms with the plant located within the buildings and areas as shown in the diagram (green blocks).

Figure 3-1: Methanol Processing plant layout and dimensions (Adapted from [1])

The substation shown to the right of the diagram provides the HV/MV transformation where a single 88kV line is used to supply power to the plant. Two 10MVA 88/11kV transformers bays are installed within the substation and the secondary plant equipment including trans-former and line protection panels are located within the Main control building. The 11kV cable ring network on cable trays (indicated by the dashed green lines) is used to reticulate the plant and the various control room’s house 11/0.4kV transformers from which LV reticulation of the building and surrounding areas is provided. The Waste treatment and storage section does not form part of the ring instead gets its supply via two underground cables within a tun -nel from the Processing plant main control building (as indicated by the solid brown line). The dimensions of the structures are shown in Table 3.1.

Table 3-1: Structure dimensions.

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4 Necessity of a lightning protection system

The Wood Gasification-Based Methanol Processing Plant is a chemical plant thus according to SANS62305-02, lightning could cause fires and malfunctioning of the plant with extremely harmful consequences to the environment. Thus the lightning protection level 1 is specified for a structure at risk of exploding and being dangerous to the environment where the effi-ciency of the lightning protection system is required to be above 98%.

4.1 Structure/Area Classification

Within the plant not all the areas/structures could be classified as being dangerous to the en-vironment thus a risk assessment for the major structures and areas is conducted to determine the efficiency of the lightning protection system required. The major structures can be determ-ined by the relative collective areas of the structures as shown in Figure 4.1.

These areas include the following:

i. Maintenance building

ii. Admin building

iii. Waste treatment and storage area

iv. Feedstock storage area

v. Methanol storage area

vi. Processing plant (Determined by the effective collection area of the Gasifier unit)

From SANS10313 (2005) the efficiency required and the relevant protection level for a struc-ture is given by (1):

EC=1−NC / ND (1)

From the analysis the Maintenance building, Administration building, Waste control building, Processing plant: Gasifier, Methanol storage 1 and Methanol storage 2 can be classified as lightning protection level (LPL) I and the Feedstock storage, Waste water pond and Waste ash/slag storage are classified as LPL III.

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Figure 4-2: Structures and their respective effective collection areas.

The lines dividing the overlapping collection areas are shown in Figure 4.1, thus these new areas are taken into account when calculating the required risk component values. As the ef-fective collection areas of certain structures completely overlap other close smaller structures, only the main structures are taken into account in the risk analysis.

The main risks which should be analysed is Risk type 1 (R1 : Loss of human life or permanent

injury) and Risk type 4 (R4 : Loss of economic value) for the major structures identified. The risk analysis performed is presented in Appendix A and a summary of the results is shown in Tables 4.1 and 4.2.

Table 4-2: Risk analysis for Risk type 1 for the structures within the plant

It can be observed that the tolerable risk (RT ) for loss of human life is exceeded for the fol-lowing structures: Processing plant, Feedstock storage, Methanol Storage and Maintenance, Administration buildings. Thus LPS’s with a suitable lightning protection level are required to be installed in order to reduce the risk to below the tolerable risk value.

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Table 4-3: Risk analysis for Risk type 2 for the structures within the plant

The tolerable risk for loss of economic value is not specified in SANS62305-02 and it de-pendant on how much loss the owner is willing to accept. The risk of loss of economic value is calculated and shown in Table 4.2. The cost analysis is presented in Appendix A although the costing of the protection devices and measures are fictitious and is only shown to illustrate the complete analysis process. The following protections measures are required in order to decrease the risk to human life within or near any of the structures analysed.

4.1.1 Maintenance, Admin, Waste control buildings

Reduction of PA to 10−5

Electrical insulation of down conductors Effective soil equipotentialization Warning notices

Reduction of PB to 10−3

“Structure with a metal roof or an air-termination system, possibly including natural components, with complete protection of any roof installations against direct lightning strikes and a continuous metal or reinforced concrete framework acting as a natural down-conductor system.” (SANS62305-03)

Reduction of PC to 10−2

Install surge protection devices with Lightning protection level 1 ratings.

Reduction of PM to 10−4

The probability PM that a lightning flash near a structure will cause failure of internal systems depends on the adopted lightning protection measures (LPM), according to a factor KMS. The KMS value was obtained using the following requirements:

Minimum 10m space between parallel down conductors or the reinforced concrete framework acting as a natural LPS.

Shielded cable with shield resistance 5<RS ≤20 Ω/km Equipment/apparatus to have 11kV impulse withstand levels.

Reduction of PU to 10−7

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Lower value between PSPD andPLD multiplied by PA .

Reduction of PV =PW=PZ to 10−2

Lower value between PSPD andPLD or PLI

4.1.2 Processing plant: Gasifier, Methanol storage 1 and 2


Electrical insulation of down conductors Effective soil equipotentialization Warning notices



Reduction of PC to 10−3

Smaller values of PSPD are possible in the case of SPDs having better protection char-acteristics (higher current withstand capability, lower protective level, etc.) compared with the requirements defined for LPL I at the relevant installation locations.

Reduction of PM to 10−4

The probability PM that a lightning flash near a structure will cause failure of internal systems depends on the adopted lightning protection measures (LPM), according to a factor KMS. The KMS value was obtained using the following requirements:

Minimum 10m space between parallel down conductors or the reinforced concrete framework acting as a natural LPS.

Shielded cable with shield resistance 5<RS ≤20 Ω/km Equipment/apparatus to have 6kV impulse withstand levels.



Reduction of PV =PW=PZ to 10−3

Lower value between PSPD andPLD or PLI

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4.1.3 Feedstock storage, Waste water pond and Waste ash/slag storage


1. Electrical insulation of down conductors 2. Effective soil equipotentialization 3. Warning notices



PC=PM=PV =PW=PZ=1

No Incoming service therefore no coordinated SPD protection. LPL level III to be as-sumed.



5 Lightning Protection System Design

In this section, the design aspects of the lightning protection system are discussed and presen-ted. The design would follow the requirements specified within the risk assessment of the plant thus both the external and internal lightning protection system (ELPS and ILPS) require-ments are provided for each structure ensuring that the risk of physical damage due to light -ning flashes and the effects of lightning electromagnetic pulses on internal systems are re-duced to within tolerable levels. The following buildings require Class I LPSs to be installed: Maintenance, Administration, Waste control buildings, Processing plant: Gasifier, Methanol storage 1 and Methanol storage 2. The Feedstock storage, Waste water pond and Waste ash/slag storage areas would be protected by Class III LPSs. The design methodology follows the LPS design flow as shown in [2] and described in the SANS62305 suites of documents. A conscious effort has been made to integrate the naturally conductive aspects of the structures into the design of the LPS such that the effectiveness of the LPS is maximised at minimal cost. As these requirements are to be provided during civil construction, the architects and builders should be made aware such they are included in their plans.

5.1 ELPS

The external lightning protection system per structure including air-termination, down-con-ductor and earth termination systems are determined based on the SANS62305-03 standard.

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5.1.1 Maintenance and Administration buildings

It is assumed that the buildings would be constructed from a reinforced concrete framework which would be designed taking into account the LPS requirements stipulated. The roofs will also be made of a flat, concrete slab. The Maintenance building roof top houses a metal water-tank of dimensions are 2x2m. The Administration building roof top houses a metal-encased air-conditioning unit of dimensions are 1x2m.

5.1.1.1 Air-termination systemFor both buildings, the roof is made of a non-combustible material (concrete), the air-termina-tion conductors may be positioned on the surface of the roof. The preferred option would be

to install a grid/mesh of stranded 35mm2 copper conductors elevated above the surface of the

roof although it is acceptable to install the conductors directly onto the roof [SANS62305-3]. The use of the steel reinforcement in concrete roof as part of a natural air-termination system is not preferred as damage to the waterproof layer may occur due to a lightning strike.

As the buildings are classified as LPL I the mesh size required is 5x5m. Also for a LPL I structure, a rolling sphere of radius 20m would be used to determine the area to be protected in complex structures. In this case the mesh is elevated thus the rolling sphere method is used to determine the height above the roof surface [2]. By using the calculation shown in Ap-pendix B the height of the meshed conductors obtained is 0.16m. The use of the metal para-pet cladding as a “Natural” air-termination conductor is specified for providing an equipoten-tial ring conductor around the mesh. Each piece needs to be bonded to each other to ensure continuity using a flexible conductor and a corrosion-resistant joint. The water tank on the

roof needs to be bonded to the conductor mesh at two points using 16mm2 copper bonding

conductor. Side strikes are neglected as the building is less then 60m in height. Figure 5.1 il-lustrates the air-termination on top of the Maintenance building. Note that even though the preferred bonding method is welding, copper cross and t-shaped corrosion resistant joints are to be used at the relevant conductor intersections.

Figure 5-3: Air-termination system for the Maintenance (left) and Administration (right) building.

5.1.1.2 Down-conductor system As the structures have a reinforced concrete framework this would form an ideal down-con-ductor system if the framework is electrically continuous. Thus this can be ensured by weld-ing or clamping the majority (over 50%) of the interconnections of vertical and horizontal re-bars and these need to be welded over a length not less than 30 mm. Vertical bars should be overlapped over 20 times their diameter and are to be secured tightly. Furthermore a termina-tion joint should be provided on the roof (preferably within the parapet) at specified points such that the air-termination system could be connected easily. Where the air-termination sys-tem is to be connected directly to the reinforcing steel, multiple connections are preferred. As

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the bonding would be between dissimilar metals sufficient care should be taken such that the bond/joint is sealed from moisture ingress through the use of epoxy or other moisture-proof coatings [2]. The resistance of the down-conductor system should be less than 0.2Ω. The ter-mination joints should be provided at each corner of the building and at a minimum of 10m apart along the walls.

5.1.1.3 Earth-termination system As the buildings are quite large they would typically have a foundation which is reinforced. Thus the reinforcing rods within the foundation, foundation slab and outer walls below the surface of the soil form an excellent, minimal cost foundation earth electrode. SANS62305-03 stipulates that an additional meshed network of horizontal conductors lashed to the reinfor-cing rods should be installed in order to ensure good joints. The resistance of the foundation earth electrode needs to be less than the 10Ω resistance specified in the standard. Furthermore termination conductors, required to connect the foundation earth electrode and any external earth-termination system, should be provided at the specified intervals. In this case the ter-mination conductors would be required at 10m intervals from each corner. As the reinforcing steelwork of the walls and pillars are used as the down-conductor system, the reinforcing rods needs to be connected to the reinforcing rods of the foundation.

5.1.2 Waste control building

It is assumed that this building would also be constructed from bricks. The roof will be made of 0.5mm galvanised steel metal sheeting covering the building.

5.1.2.1 Air-termination systemAs the roof is made of metal sheeting and the thickness is greater than 0.5mm, the sheeting could be used as a “natural” air-termination system provided that the electrical continuity of between the components is made robust (welding, brazing, seaming, crimping screwing or bolting) and the sheeting is not clad in insulating material. Furthermore as the control building does not house combustible material, the risk of puncture is acceptable. The air-termination

system (metal sheeting) needs to be bonded to the down conductor system using 16mm2 stran-

ded copper conductors.

5.1.2.2 Down-conductor system The Waste control building is a bricked building therefore a suitable “Natural” down-con-

ductor system is not available. Thus a down-conductor system made up of 16mm2 stranded

copper conductors are used to connect the air-termination system to the earth-termination sys-tem at each corner (Dimensions of the building are 10x10m therefore the distance between these down-conductors are adequate for the LPL I). A test joint is installed on each down-con-ductor at the base of the building and a horizontal ring conductor should be installed around the building at this level to interconnect all the down-conductors.

5.1.2.3 Earth-termination system As the building is specified as a LPL I building with an unsuitable foundation, the earth-ter-mination system selected is the type B arrangement. This arrangement is suited for buildings with a large amount of electronic equipment therefore it is ideal for the control building. The

type B arrangement consists of an 50 mm2 copper ring earth electrode be buried at a depth of

0.5m and the radius of the ring is dependent on the LPL defined for this building. For a Class 1 LPS and with a soil resistivity of 1500Ωm the specified radius is roughly 37m

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(SANS62305-03). The area of the building is 10x10m and if one were to stipulate that the ring earth electrode should be 1m away from the building it would mean the mean radius of the ring earth electrode is 6m, it would leave a shortfall of roughly 31m. Thus additional four

50 mm2 copper vertical earth electrodes of 15.5m lengths should be installed at the corners of

the ring earth electrode (at the point at which the down-conductors connect to the earth-ter-mination system). The connections between the systems need to be made secure by any one of the following methods: Brazing, welding, clamping, crimping, seaming, screwing or bolting. It should be noted that the down-conductors need to be securely clamped to the building every 1m. Provision needs to be also made for additional termination points such that the ring earth electrode could be connected to the plants meshed earth termination system.

Figure 5-4: External LPS for the Waste control building.

5.1.3 Processing plant: Gasifier, Methanol storage 1 and 2

It is assumed that the Processing plant: Gasifier would be constructed from mild steel with thickness 10mm and would resemble a continuous, enclosed, metallic container having the following dimensions: Radius-3.75m and height: 18m. Furthermore the entire Processing plant, Feedstock Handling and Drying area, Methanol storage area is positioned on the top of a reinforced foundation. The Methanol storage 1 and 2 structures are to be continuous, en-closed, metallic containers having the following dimensions: Radius-2.5m and height-5m. The containers would be made of steel with the thickness of 5mm.

5.1.3.1 Air-termination systemFor the three structures, an air-termination system is not necessarily required according to SANS62305-03 as the structures themselves would act as “natural” air-termination systems. As the thickness of the steel structure walls are 0.5mm or greater the risk of puncturing or melting the metal in the structure walls is effectively reduced to below tolerable levels.

5.1.3.2 Down-conductor system As the structures have a continuous metallic structure, the need for additional down-conduct-ors is not required as the containers themselves would also act as “natural” down-conductor systems. Furthermore, termination joints should be provided at the base of these structures at specified points such that the “natural” air-termination and down-conductor systems could be easily connected to the earth-termination systems. As the bonding would be between dissim-ilar metals (copper and steel) sufficient care should be taken such that the bond/joint is sealed

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from moisture ingress through the use of epoxy or other moisture-proof coatings [2]. The ter-mination joints should be provided at a minimum of 10m apart along the walls of the struc-

tures. A 16 mm2 copper bonding conductor is specified for the connections to the earth ter-

mination system.

5.1.3.3 Earth-termination system As the Processing plant, Feedstock Handling and Drying area and methanol storage area ddi-mensions are quite large it would typically have a foundation which is reinforced. Thus the reinforcing rods within the foundation below the surface of the soil form an excellent, min-imal cost foundation earth electrode. SANS62305-03 stipulates that an additional meshed net-work of horizontal conductors lashed to the reinforcing rods should be installed in order to ensure good joints. The resistance of the foundation earth electrode needs to be less than the 10Ω resistance specified in the standard. Furthermore termination conductors should be provided at the specified intervals to connect the foundation earth electrode to objects above the foundation (i.e. Gasifier, Methane storage tanks, Dry woodchip silos, conveyer belt sys-tems, Main control building, etc) and any external earth-termination system. SANS62305-03 states that the storage tanks and the Gasifier need only be connected to the earthing founda-tion at one point as long as the greatest horizontal dimension is less than 20m. Although even though this is the case all objects on the plant foundation should be connected to the founda-

tion earth-termination system atleast twice using 16 mm2 copper bonding conductors.

5.1.4 Feedstock Storage, Waste water pond and ash/slag storage areas

It is assumed that the feedstock storage building, waste water pond and waste ash/slag storage buildings would be constructed from bricks. The roof of the buildings will be made of 1mm galvanised steel metal sheeting and the pond would be an open tank.

5.1.4.1 Air-termination systemFeedstock and Waste ash/slag storage buildings: As the roof is made of metal sheeting the use of it as a “natural” air-termination system could be limited as the Feedstock building houses combustible material (wood chips) thus the metal thickness is increased to double the required amount (0.5mm). Furthermore the following needs to be ensured: the electrical con-tinuity between the components is made robust (welding, brazing, seaming, crimping screw-ing or bolting) and the sheeting is not clad in insulating material. The air-termination system

(metal sheeting) needs to be bonded to the down conductor system using 16mm2 stranded

copper conductors. Figure 5.2 does illustrate the layout of the “natural” air-termination system based on an inclined roof with metal sheeting.

The Waste water pond is constructed from brick therefore no “natural” air-termination com-

ponents would exist therefore 35mm2 stranded copper air termination wires elevated above the

pond are used, these wires would protect the pond and its surrounding from direct lightning strikes based on the height at which they are elevated. The rolling sphere method with a radius of 45m for the class III protection is used to determine the height above which the wires are to be elevated. Using the method shown in Appendix B the wire height equates to 0.07m above the pond. The distance between vertical conductors has been equally spaced around the struc-ture as shown in Figure 5.3 which also illustrates the remaining components of the ELPS for the waste water pond.

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Figure 5-5: External LPS for the Waste water pond (Adapted from [3]).

5.1.4.2 Down-conductor system The Feedstock and Waste ash/slag storage buildings and the waste water pond are bricked structures therefore suitable “Natural” down-conductor systems are not available. Thus a

down-conductor system made up of 16mm2 stranded copper conductors are used to connect

the air-termination system to the earth-termination system at 20m intervals (corresponding to LPL III) in the case of the Feedstock and Waste ash/slag storage buildings starting at each structure corner.

As shown in Figure 5.3, the 16mm2 stranded copper down-conductors are positioned around

the pond such that they are equally spaced around the structure.

Test joints are to be installed on each down-conductor at the base of the structures and a hori-zontal ring conductor should be installed around the building at this level to interconnect all the down-conductors.

5.1.4.3 Earth-termination system As the structures have foundations which are unsuited for use as a “natural” earth-termination system, the earth-termination system selected is the type B arrangement.

The type B arrangement consists of an 50 mm2 copper ring earth electrode be buried at a

depth of 0.5m around the structures. The mean radius of the earth ring is dependent on the LPL defined for this building. For a Class III LPS and with a soil resistivity of 1500Ωm the specified mean radius is roughly 5m (SANS62305-03).

The mean radius of the ring earth electrode around each of the structures is as follows: Feedstock storage – 56mWaste ash/slag storage buildings – 21mWaste water pond – 6m

As these mean radius values are greater than the stipulated 5m requirement, additional hori-zontal or vertical earth electrodes are not required. The connections between the down-con-ductor and earth-termination systems need to be made secure by any one of the following methods: Brazing, welding, clamping, crimping, seaming, screwing or bolting. It should be

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noted that the down-conductors need to be securely clamped to the structure every 1m. Provi-sion needs to be also made for additional termination points such that the ring earth electrode could be connected to the plant’s site-wide earth mat.

5.1.5 Meshed earth termination system (site-wide earthing)

Installing a site-wide earthing mat provides the following benefits i) electrical protective earthing for protection of people and objects, ii) lightning protective earthing for the disper-sion of lightning currents to the ground iii) equipotential earthing for uninterrupted and safe operation of electrical and electronic installations.

Figure 5-6: Meshed earth termination system for the Methanol Processing Plant

Furthermore having separate earth termination systems for protective earthing is very risky as this could lead to voltage potentials which could lead to discharges. These discharges in the plant environment could lead to the ignition of fuel vapours and a subsequent fire and explo-sion could result.

As per SANS62305-03, the earth-termination systems of the buildings in the plant should be interconnected to form the plant’s meshed earth termination system. It is recommended that the size of the mesh be 20x20m around the buildings and other objects and beyond a 30m dis-tance the mesh size can be increased to 40x40m. It should be noted that all the earth-termina-tion systems, objects, cable trenches, cable trays need to be connected to the meshed earth ter-

mination system. 50 mm2 copper stranded conductors are specified for the meshed networks.

Figure 5.4 illustrates the meshed earth termination system for the Methanol Processing plant. The cables from the Main control building in the processing plant to the Waste control build-ing will be run in a underground tunnel between the two locations. Care must be taken such that termination joints from the meshed earth termination system can be accessed within the tunnel at the intersection points such that the conduits and or cable trays enclosed can be con-nected with ease to the meshed earth termination system. The tunnel itself should be made from concrete with reinforced steel thus also provides an effective shield against interference.

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5.1.6 88/11kV Substation

Shielding lines are installed above the incoming overhead conductors thus these provide an effective air-termination system until the conductors are terminated on the structures within the MV yard. These shielding lines are to be terminated and bonded onto the earth mat (earth foundation system). Further air-termination system is not required to protect the transformers and equipment within the MV yard as the yard is within the effective collection area of the Processing plant: Gasifier.

The MV transformers are specified to be resistively earthed and connected to the earth mat below (earth foundation system) as recommended for industrial plants (SANS10200). It should be noted that all surge protection arrester and cables need to be rated at full line voltage (11kV).

5.2 ILPS

The internal lightning protection system is used to provide protection from the effects of light-ning current flowing in the ELPS or conductive components of the structure and to reduce the risk of dangerous sparking between these components and also lines entering the structure. Furthermore the cables and equipment within the structures in the plant are susceptible to in-terference and noise injection caused by lightning strikes, switching events and even earth fault events on the power supply. This noise injection could degrade critical control and meas-urement signals and possibly even destroy the equipment. As the plant contains a significant amount of electronic equipment for process control and measurement purposes, some tech-niques are employed to reduce the noise and disturbance into the electronic and cabling sys-tems.

5.2.1 LP Zones

A ‘lightning protection zone’ can be defined as an area which is classified based on a certain level of threat posed by either direct or indirect effects of lightning. Lightning protection zones can be classified as either outer or inner zones with each successive zone exhibiting a greater degree of electromagnetic shielding and/or surge protection (SANS62305-4).

LPZ 0 A : The outer-most zone is exposed to direct lightning strikes and full lightning electro-magnetic impulses (LEMPs). Any internal system within this area is exposed to full lightning surge current.

LPZ 0B : This outer zone is shielded against direct lightning strikes but is exposed to full light-ning electromagnetic impulses. Any internal system within this area is exposed to partial lightning surge current.

LPZ 1 : This zone is shielded against direct lightning strikes. Any internal system within this area is exposed to significantly less lightning surge current and lightning electromagnetic im-pulses when compared to the outer zones. The current surge is limited by surge protection devices and current sharing at the boundary of the zone. The reduction in the electromagnetic impulses is due to spatial shielding.

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LPZ 2 : This zone is also shielded against direct lightning strikes. Any internal system within this area is exposed to significantly less lightning surge current and lightning electromagnetic impulses when compared toLPZ 1 . The current surge is further limited by surge protection devices and current sharing at the boundary of the zone. The reduction in the electromagnetic impulses is due to further spatial shielding being introduced [2].

By providing greater protection against lightning electromagnetic impulses and surge currents at each zone boundary, lightning protection zones assist in defining areas with a certain de-gree of threat and by doing so one could implement the zonal system in order to protect equipment with different levels of withstand capabilities within a structure. This is achieved by using the cascading approach where the clamping voltage of the surge protective devices at consecutive zone boundaries is decreased slightly; the surge currents are diverted in a con-trolled manner with most of it being dealt with at each zone boundary. As such, the surge cur-rents then entering the building are smaller therefore lower rated and cheaper surge arresters may be used inside the building at each zone boundary [2].

The following lightning protection zones (LPZs) are specified for the structures as follows:

Maintenance building: LPZ0, LPZ1 and LPZ2Administration building: LPZ0, LPZ1 and LPZ2Waste control building: LPZ0 LPZ1 and LPZ2Processing plant main control building: LPZ0 and LPZ1Methanol storage control building: LPZ0 and LPZ1

It should be noted that the 11kV cable ring network on a continuous overhead cable tray is used to reticulate the Maintenance building, Administration building, Methanol storage con-trol building and the Processing plant main control building. And the various control build-ing’s house 11/0.4kV transformers from which LV reticulation of the building and surround-ing areas are supplied. The Waste control building also houses a 11/0.4kV transformer but does not form part of the ring instead gets it’s supply from two underground cables within the tunnel from the Processing plant main control building. As seen in Figure 5.5, the transformer is housed within the buildings although it still forms part of LPZ0. This diagram would illus-trate the zones specified for the Maintenance building, Administration building and Waste control building. LPZ2 would normally specify the zone which houses the electronic equip-ment although for the Processing plant main control building and the Methanol storage con-trol building, LPZ1 would be used to house the electronic equipment thus the surge protection devices at the border of the zone needs to be specified such that it would divert a higher than normal surge current.

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Figure 5-7: Typical layout of the zones within the Administration and Maintenance buildings.

5.2.2 Bonding/screening

As discussed previously, a significant amount of signal and power cables will be run through-out the plant. As such the bonding and screening requirements for these cables are discussed below: From [2] the noise is introduced into mostly cable systems by the following electro-magnetic conditions:

1. Galvanic coupling (Resistive coupling) whereby interference is caused when there is direct electrical contact between the noise generation source and the signal channel.

2. Electrostatic coupling (Electric or capacitive coupling) whereby noise is introduced through the inherent capacitances existing between parallel conductors and cables. A voltage difference exist between the conductors causes a change in the charge on the conductors thereby introducing a capacitive current flow through the conductors. The noise is therefore caused by voltage being dropped across the inductance and resist-ance of the conductor.

3. Electromagnetic induction (Magnetic or inductive coupling) whereby a current carry-ing conductor produces a magnetic field and a portion of the flux lines link with adja-cent conductors. By Faraday’s law, a change in the current (and subsequent magnetic field) in the first conductor will cause a voltage to be induced into the second con-ductor. This voltage would represent noise in the second conductor.

As discussed in [2], effective earthing and shielding techniques can be taken to minimize the effects of the above factors on signal channels, electronic and cable systems. The objective in electromagnetic compatibility is to minimize, divert, or eliminate one of the three elements necessary for a noise problem.

In the case of galvanic coupling we can consider a simple example where two pieces of equip-ment are connected via a signal line. This voltage potential can be minimized by either min-imising the impedance path to earth or by bonding the two pieces of equipment together. The paralleling of multiple bonding conductors minimises the impedance providing a low imped-ance connection. Thus it is specified that cable trays are to be used for all the cables within the plant as it provides a suitable technique to reduce voltage potentials by offering a low induct-ance path and wide bandwidth performance assuming it is galvanically continuous along its

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length and is galvanically bonded to the equipment at both ends. The bond to the equipment could also be implemented through the building steel work, conduits armouring or bonding conductors. A good rule of thumb is to also bond the cable trays to the buildings steelwork every 30m in order to provide a low impedance path. In the case of the tunnel between the Processing plant control building and the Waste control building the cable trays should be bonded to the reinforcing steelwork of the tunnel and the termination joints from the meshed earth termination system atleast every 30m.

Electric coupling between adjacent conductors can be limited by screening the conductors such that capacitive current would flow along the screen instead of along parallel signal cables. Thus all signal cables used within the plant need to be shielded with a shield resistance

5<RS ≤20 Ω/km. Even though increasing the distance between cables would provide less ca-pacitive effect, this is not always possible due to the space limitations within cable trays. A further screening effect is provided by the cable tray holding the cables. Although earthing the screen at one end is effective for minimizing electric coupling, it is good practise to earth the screen at both ends in order to prohibit voltages being induced in the signal cables when surges occur during lightning.

Magnetic coupling between conductors is reduced by twisting pairs of signal cables. This ef-fectively causes the polarity of the induced voltages in each loop to be reversed thus the net induced voltage along the cable length would be zero assuming the noise voltage is induced in equal magnitudes due to the similar size of twists. As mentioned with electric field coupling, it is good practise to bond the cable trays and screens to the equipment at both ends using 16

mm2 copper bonding conductors, this is also vital for providing a low impedance path for in-

duced current flow which reduces the net induced voltage in the signal cables.

Equipotentialisation refers to an overvoltage protection technique which aims to obtain a single zero signal reference grid between all the internal systems, the LPS, structural metal parts/installations and external conductive parts. Equipotentialisation is important as it re-duces the risk of the fire and explosion hazards as well as loss of life by effectively distribut-ing the lightning current into the earth without causing dangerous potential differences. It is

achieved by bonding and interconnecting all the metallic objects mentioned to a 50mm2

cooper bonding bar using bonding conductors or surge protection devices where bonding con-ductors cannot be used [2]. Within the structures (Maintenance, Administration buildings) where the reinforced concrete framework is used as the down-conductor system and earth-ter-

mination system, a 50mm2 copper bonding bar which is bonded to the steelwork within the

concrete walls should be provided at each floor for the internal LPS. Solid 50mm2 copper

bonding bars need to be installed within the Waste control, Processing plant main control and Methanol storage control buildings close to the point of cable entry and be connected to the earth-termination system with a bonding conductor not longer than 0.5m.

As the cables enter the buildings at a single point, all the cable screening and cable trays need to be bonded to the bonding bar at the point of entry. This is the case for all 200 cables and their respective cable trays between the Processing plant control building and the Waste con-trol building as well as the 11kV cable ring and its cable tray between the remaining struc-tures specified. It should be noted that all metallic components within the structures need to

be bonded to the bonding bar within each of the structures using 5mm2 copper connecting

cable.

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A zero signal reference grid (ZSRG: conductor grid) needs to be installed within or on top of the floor of the rooms within which sensitive electronic equipment will be located. These loc-ations include the Processing plant main control, waste control, Methanol storage control and the control rooms within the Maintenance and Administration buildings. Each panel/cabinet within the room needs to be bonded securely to the ZSRG and the ZSRG needs to be bonded to the bonding bar or building steelwork close to the cable entry point. All the cabinets/equip-ment needs to be interconnected through the use of the galvanically continuous cable trays.

Where optic fibre is used, no special precautions are required as optic fibre is immune to all forms of electrical noise therefore are ideal for use as signal cables in environments where noise is inherent. Thus using twisting signal cables, optic fibre and good galvanically continu-ous bonding conductors i.e. cable trays, bonded at both ends (to equipment) and bonded at 30m intervals to the buildings steel work provides a very useful technique to limit the effects of galvanic, electric and magnetic coupling.

5.2.3 Proximity, cabling and routing

The location of the electronic equipment is important within buildings made of brick: Pro-cessing plant main control, Methanol storage control, Waste control buildings. These are clas-sified as poorly shielded buildings thus the equipment should be located within the centre of the building.

With regards to routing of the 200 signal and power cables between the Processing plant con-trol building and the Waste control building, it is recommended that large loop areas between the power and communication cables are avoided by running them in adjacent cable trays within the tunnel. Power and communication cables are not to be placed in the same cable tray. As the tunnel space specified is 1m wide by 3m high, the power cable trays need to be installed above or below the communication cable trays along either side of the walls. Based on the dimensions of the tunnel, each cable tray would be 20cm wide therefore housing a maximum of 15 communication cables each with the 2 power cables being placed at either side of a single cable tray.

A suitable separation distance needs to be ensured between the ELPS and all metallic objects connected to the equipotential bonding of the structure.

5.2.4 SPD’s

Where more than LPZ has been defined in a structure (Maintenance and Administration buildings) SPD’s are required to be installed at the boundary point between different protec-tion zones. And in structures where only LPZ1 has been identified (Processing plant main control, Methanol storage control, Waste control buildings) the SPD shall be located at the line entrance.

The likely induced currents at the structures are obtained as follows:

As 204 cables (excluding optic fibre cables) are routed on cable trays via the tunnel between the Waste control building and the Processing plant main control buildings. It is assumed that 2 cables will be used for electric power service, 100 pair cables for data and 1 pair cable will be used for telephone service.

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The Waste control building has been classified as a LPL III structure therefore the design cur-rent is 100kA. The number of services (n) is 2 excluding the telephone service. And the num-ber of conductors for the electric power service (m) is 4, for the telephone service (m) is 2 and for the data service (m) is 200.

The current magnitude in each service is as follows:

Electric power:I a=

502

=25 kA, Telephone: I b=5 kA , Data:

I c=502

=25kA

The current magnitude in each shield is as follows:

Electric power: I ca( shield )=0. 95 x 25 kA=23 .7 kA

, Telephone: I bc( shield )=0 .95 x 5 kA=4 .75 kA

,

Data: I cc( shield)=0 . 95 x25 kA=23. 7 kA

And the current magnitude in each conductor is as follows:

Electric power:I ca=312 . 5 A , Telephone:I bc=125 A , Data: I cc=6 . 25 A

The same process is followed for the Processing plant main control building, which is a LPL I structure, although the calculations need to take into account an additional 4 electric power cables (two from the overhead ring cable network and two from the two transformers) the cur-rents in each conductor is as follows:

Electric power:I ca=125 A , Telephone:I bc=250 A , Data: I cc=12 .5 A

The following Table 5.1 provides the current magnitude information for the remaining control rooms (All LPL I). It is assumed an additional 10 pair data cable is routed between these con-trol rooms in the ring on a separate cable tray.

Table 5-4: Current magnitudes for structures

It should be noted that additional metallic services including water piping would reduce the induced currents further thus these values could be used as a worst case scenario.

The SPD’s for Processing plant main control and Methanol storage control buildings are spe-cified to be superior to regular class I SPD’s. For all the remaining structures with incoming

services, it is recommended that SPD’s which are tested with I imp , which is the typical 10/350µs waveform, be installed at the LPZ0/LPZ1 boundary. Furthermore SPD’s tested with I n , which is the typical 8/20µs waveform, should be installed on all incoming services enter-ing LPZ2, these typically need to be connected at the cable entry points to the ZSRG. As spe-cified earlier equipment including SPD’s need to be rated at full line voltage i.e. 11kV.

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6 Conclusion

The objective of providing a high level design which is easily understood has been achieved. In doing so, the various aspects of a typical lightning protection system including the external and internal components were identified and studied in detail. The application of the different types of systems and their effectiveness in dealing with the lightning phenomena on various structures are examined and the most suitable systems are recommended. The material types and dimensions are chosen such that corrosion does not significantly affect the performance of the systems over a reasonable amount of time. The South African ‘Protection against light-ning’ suite of documents has been found to be very useful in performing the various design aspects.

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7 References

1. Nixon K.J., Van Coller J.M. and Jandrell I. (2011). ELEN7018, Lightning and Earthing Protection Notes, School of Electrical and Information Engineering, University of Wit-watersrand.

2. SERI (1987). Economic Feasibility Study of a Wood Gasification-Based Methanol Plant. National Technical Information Service. Springfield. VA.

3. Bouquegneau C. (2007). LIGHTNING PROTECTION OF OIL AND GAS INDUSTRIAL PLANTS. IX International Symposium on Lightning Protection.

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Appendix A

A risk analysis for Risk type 1 (R1 : Loss of human life or permanent injury) is presented for the Methanol processing plant and it’s structures based on the steps shown in the lightning protection standard: SANS62305-2. The assumptions are shown in Table 1.

Table 5: Parameters and assumptions used in the risk assessment.

The change of probabilities following the introduction of protection measures is shown in Table 2.

Table 6: Probabilities after introduction of protection measures.

The cost analysis of the protection measures identified and the annual savings are shown in Table 3.

Table 7: Cost analysis of protection measures.

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R

d/2

d

R-h

h

Roof surface


Appendix B

Large, flat roof areas have elevated meshes or air termination conductor systems to effectively attract lightning leaders approaching from above. In effect the upward streamer and sub-sequent leader from the air termination conductor system would attach to the downward leader first therefore efficiently conducting the current down to earth instead of being inter-cepted by a upward leader initiated from the roof surface. In order to determine the height above the roof to which the air conductor system is required to be installed, the rolling sphere method is utilised. The radius of the sphere is chosen depending on the lightning protection level required. For example if the structure to be protected, requires lightning protection level I, with a large, flat roof a sphere of radius 20m is used [2]. A mesh of grid conductors is used above this structure with 5x5m dimensions for the grid blocks. The conductors at the corners of the grid blocks are determined based on the radius of the sphere used. The height of these conductors is required to be the equal to the radius of the sphere for good protection.

As shown in Figure 1, the radius of the sphere is given by R and the distance between adja-cent air-termination (lightning rods) is given by d. And the parameter which we are required to calculate is given by h which is the height of the rods above the roof surface.

By the Pythagoras theorem the following expression (1) can be obtained:

R2=( d /2 )2+(R−h )2 (1)

And by solving (1) using the following parameters for protection level 1: R=20m and d=5m, the height (h) obtained is 0.16m.

Figure 0-8: Rolling sphere method for determining the height of lightning rods on large, flat roof surface.

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