Safety Science 49 (2011) 1355–1364

Contents lists available at ScienceDirect

Safety Science

journal homepage: www.elsevier .com/locate /ssc i

Lightning protection scenarios of communication tower sites; human hazardsand equipment damage

Chandima Gomes a,⇑, Arturo Galvan Diego b

a Centre of Excellence on Lightning Protection, Universiti Putra Malaysia, Malaysiab Instituto de Investigaciones Eléctricas, Cuernavaca Morelos, Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 February 2011Accepted 14 May 2011Available online 11 June 2011

Keywords:LightningProtectionSafetyCommunicationTowerGuidelinesGrounding

0925-7535/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.ssci.2011.05.006

⇑ Corresponding author. Tel.: +60 102777895.E-mail address: chandima.gomes@gmail.com (C. G

This paper provides comprehensive analysis on the lightning protection scenarios in 48 communicationand broadcasting towers situated in similar isokeraunic contours in Sri Lanka at 79�–81� East and 5�–10�North. The investigation has been conducted to study the hazardous environment created on the towerand in the neighbourhood in the event of a lightning strike to the tower. The results show that a directstrike to an antenna structure in a metallic tower is rare irrespective of the presence of an air-terminationor a down conductor. However, side flashing or arcing to antenna structures is highly possible once theair-termination and/or down conductor is installed and attempts are made to insulate the system fromthe tower. The outcome also shows that equipotential bonding of the grounding system, a distributedgrounding network including a ring conductor and a suitable system of surge protective devices play amuch vital role in lightning protection of equipment and safety of people compared to the effects of sim-ply achieving a low grounding resistance. However, in the absence of such integrated, distributed andequipotentialized grounding system, a high value of ground resistance will sharply increase the possibil-ity of accidents and damage. Considering the observations of the investigations into account we havedesigned a concrete embedded grounding system for tower sites at problematic locations. Finally, thescenarios for safety management at telecommunication tower sites have been discussed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In most parts of the world, communication towers are all-metalstructures, which make them prime targets of lightning that maycome within their vicinity. During the last few decades, a largenumber of lightning related accidents and damages have been re-ported in many countries in connection with communication andbroadcasting tower sites (Kithil, 2006; Eriksson and Meal, 1984;Pierce, 1971). In a tower environment, lightning related hazardsmay occur at various stages of a lightning strike.

a. The attachment process: A lightning step leader may attachwith an antenna structure, aviation warning light or signal/power cable in the tower, in which case the object whichis subjected to the lightning attachment may be severelydamaged. There can also be secondary effects, as the itemstruck by lightning may be detached from the tower or frag-mented, giving rise to falling parts that will cause damage tothe objects underneath or injuries to the staff at groundlevel. The lightning current will most probably enter the

ll rights reserved.

omes).

cables connected to the object struck, and flow into the sig-nal feeding devices or power panels in the base transmissionstation (BTS) causing many other hazards to both equipmentand staff.

b. Passage of lightning current to ground level: As the lightningcurrent flow to the ground level through any possible path,melting or burning of materials and side flashing to nearbyobjects or antenna structures in the tower itself, may occurdepending on the resistance and impedance of the pathtaken by the current. The lightning current, which usuallyshows a rapidly varying double exponential waveform withsub-microsecond to microsecond scale rise time, gives riseto a large electromagnetic field in the proximity whichmay induce large voltage impulses in the nearby electricalsystems. Such voltage pulses may also damage theequipment.

c. Once the lightning current reach the ground level, a lowimpedance path should be provided to that to be dissipatedinto mother earth within a very short period. In the absenceof such path the current may take surface routes in the formof arcs and/or enter into electrical networks through theelectrical grounding system (or even by insulation break-down between the path of the lightning current and the

1356 C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364

electrical system). Such cases may lead to severe injuries oreven death of the staff in the site and also cause heavyequipment damage and triggering of fire/explosions, etc.

In the literature we find several studies done with regard tosuch hazards (Zhou et al., 2009a,b; Rizk, 1994; Pavanello et al.,2004; Rachidi et al., 2001; Rakov, 2001; Melander, 1984). Most ofthese researches have theoretical approaches to the issues thatthey address. Even in very practical issues such as the increasedlightning damage in the buildings neighbouring towers have notyet studied with a sizable database of in situ observations. Further-more, the analysis of grounding systems at locations with spacerestrictions has not been done in detail either theoretically orexperimentally. The general concepts of protection of structuresand equipment against lightning have been covered in IEC62305:2006 (2006), NFPA 780:2008 (2008), IEEE C62.41:1991(1999) and AS/NZS 1768:2007 (2007), etc. The telecommunicationstandards ITU-T REC K.20:2003 (2003), ITU-T REC K.21:2005(2005), ITU-T REC K.56:2003 (2003), ITU-T REC K.27:1996 (1996)and ITU-T REC K.31:1993 (1993), provide the guidelines for pro-tecting telecommunication related equipment and stations.

In this study, we analyze the lightning related environment intall communication and broadcasting towers, giving special atten-tion to the grounding systems of such sites, which are in mostcases situated in space restricted locations. The results can be ex-tended to radar masts, power transmission towers and many othermetal structures with comparatively small horizontal span. The pa-per first discusses the lightning protection systems and lightningaccidents pertinent to 48 tower related sites in Sri Lanka; a tropicaloceanic country (79�–81� East and 5�–10� North). Based on ourobservations we make a comprehensive list of recommendationsfor the installation of lightning protection scheme at a tower siteand also discuss a concrete encased steel reinforcement systemthat can be used as a suitable grounding system for sites with veryhigh soil resistivity.

2. Methodology

Forty-eight communication tower sites in Sri Lanka have beeninvestigated during the 5-year period from 2003 to 2008. In thisinvestigation we have;

� Checked the nature and installation features of1. Air-termination system.2. Current path to ground level.3. Grounding system.

Fig. 1. A foot of the base of a tower made of X cross-sectioned painted re-bars madeof galvanized steel and the copper grounding tapes. The base plates are installed onburied concrete platforms. In some cases the base plates have been installed onconcrete platforms of 30–100 cm height.

� Taken quantitative measurements of1. Ground resistance of the system.2. Average soil resistivity of the site.

� Collected confirmed lightning related damage records for theprevious 1–3 years.

The ground resistance was measured with a three-pole digitalearth resistance meter (KYORITSU MODEL4105A), under dry earthconditions. The information to calculate soil resistivity of theground was obtained by taking measurements from a four-poleground resistivity meter (MEGER DET5/4R), under the same dryconditions. The continuity check of metallic parts (especially thatof the grounding system) has been done with digital multi-meter(Fluke-115 or Fluke-85).

The ground resistance was taken twice (in V directions, wher-ever it is possible) at each location, where a conductor intendedfor grounding, enters the earth. Information for the calculation ofsoil resistivity was taken by taking measurements at three randomplaces of the site within about 10 m from the tower.

The materials used for various parts of the lightning protectionsystem, and layout of the grounding system have been determinedby the site-engineer-provided data and visual observations.

The installation of air-terminations and upper parts of downconductors have been observed from the ground level or from loca-tions at few meters above ground level by binoculars (Nikon 8X42WP Trailblazer ATB).

3. Observations

3.1. Types of towers

The 48 towers inspected in this study are all-metal (made ofmetal re-bars making a steel lattice that stands on concrete plat-forms), self supported structures (no guy wires except in four tow-ers) with heights range from 40 m to 100 m. All the towers areeither square or triangular cross-sectioned (having four legs orthree legs) and except for the four which are guy wired, they aretapered over the entire height (i.e. legs are inclines to the vertical).The tubular or X/I/L cross sectioned re-bars are typically made ofpainted galvanized steel and have cross sectional area over150 mm2. In general, these towers are certified by a civil engineerfor its mechanical stability. The towers are either used for signaltransmission in telecommunication or for broadcasting. A foot ofthe base of a tower is shown in Fig. 1.

The sites have been selected so that they are situated in areas ofsimilar contours of isokeraunic level (annual thunder days). Thecontour map issued by the Sri Lanka Meteorology Department isbased on 30 year data (from 1970 to 1999) has assigned the level120–140 days per year to the contours, however, it is said thatthe data has been collected during the peak periods (March/Apriland October/November) and normalized to 1 year. Despite the factthat this data interpretation is erroneous, the region can be treatedas a high lightning density zone.

3.2. Lightning protection components

3.2.1. Air terminationIn this paper we use the term ‘‘air termination’’ to refer any

metallic object (typically a rod) that specifically installed at thetop of the tower to intercept with lightning stepped leader. A sum-mary of the observations on the air termination is given below.

� Metal rod that covers all antenna structures in the tower withina cone of vortex angle 45�: 18.

C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364 1357

� Metal rod that does not cover all antenna structures in thetower within a cone of vortex angle 45�: 06.� ESE rod of which the physical height covers all antenna struc-

tures in the tower within a cone of vortex angle 45�: 13.� ESE rod of which the physical height does not cover all antenna

structures in the tower within a cone of vortex angle 45�: 08.� Other types of air-termination of which the physical height

cover all antenna structures in the tower within a cone of vortexangle 45� (lightning prevention type): 01.� No air termination: 02.

The vortex angle 45� has been taken as a general reference.

3.2.2. Down conductorsThe term ‘‘down conductor’’ is used to refer any metallic part (in

the form of wire or tape) that is specifically installed to drive light-ning current from top of the tower to ground level.

� Single copper tape strapped to one of the tower legs: 33.� Single copper tape taken from the middle of the tower (typically

strapped to the ladder): 3.� Two copper tapes strapped to tower legs: 02.� Insulted down conductor (and insulated air-termination) that is

isolated from the tower material: 04.� No down conductor:0 6.

The term ‘‘insulated down conductor’’ is referred to a metallicpipe or tape which is covered by an insulating material for thedeliberate purpose of electrically isolating the conductor fromthe metals of the tower. The conducting tapes which may be elec-trically isolated (continuously or at some locations) due to thepainting of the tower (an in the absence of metal screws that fixingthe tape to the tower) are not considered as ‘‘insulated down con-ductors’’. In these cases the air-termination was fixed directly tothe tower by metal screws.

3.2.3. Grounding systemThe term ‘‘grounding system’’ is used to refer any metallic part

that is specifically installed to connect the down conductors ormetallic parts of the tower to mother earth.

� Only the down conductors are grounded: 04.� The down conductors and the four legs are grounded and inte-

grated: 39.� The down conductors and the four legs are grounded but not

integrated: 05.

The ground resistance of the grounding system was in the fol-lowing ranges (note that in the cases of non-integrated groundingof down conductors and tower footing, the following values referto the grounding system of the down conductors). In all the infor-mation given in this section (values) the marginal cases have beenincluded in the lower range.

� 0–2 O: 04.� 2–10 O: 22.

� 10–20 O: 09.� 20–100 O: 06.� 100–1000 O: 02.� Resistance could not be measured due to inaccessibility: 03.� Resistance could not be measured due to lack of soil: 02.

In three of the cases where resistance could not be measured,the down conductor has been extended for about 20–60 m belowthe tower foot level (tower is on a solid rock) and dumped in aninaccessible location. In the other two cases the down conductorswere inserted into a bore drilled in the rock and the cavity has been

filled with a cement-like material. Investigators could not acquireany information regarding the depth of the bore, electrodearrangement or the materials used. The ground soil resistivityhad the following variation,

� 0–15 O m: 07.� 15–100 O m: 20.

� 100–1000 O m: 12.� 1000–10,000 O m: 04.� Resistivity could not be measured due to inaccessibility or lack

of soil: 05.

3.2.4. Grounding system configurationIn the 04 cases of only the down conductors are grounded (all

four cases had one down conductor); a deep driven copper tubehas been integrated with three radials that runs for 3–4 m fromthe deep driven rod (at a depth of about 0.5 m). The four towers,belong to the same company, are situated in areas where soil resis-tivity was in the ranges 0–15 O m (two sites) and 15–100 O m(three sites). The ground resistance in the four cases was in therange 2–10 O.

In the 39 cases where four legs are grounded and integrated; in35 cases the four legs have been interconnected by a ring conduc-tor. The ring conductor was connected to either radials or deep dri-ven rods/plates. The 06 cases of no down conductors and the 04cases of resistivity in the range 1000–10,000 O m come under thiscategory. The ground resistance in 32 cases was below 20 O whileonly in three cases (where soil resistivity is in the range 1000–10,000 O m) the value was in 20–100 O range. At the other foursites (out of 39), the tower footings and down conductor were sep-arately connected into a deep driven rods (at an inspection pit).The resistance in the four cases was below 10 O.

In the five cases that the tower footing and the down conductorrelated grounding systems are not integrated, the grounding resis-tance of down conductor and that of tower footing are given in Ta-ble 1 (note that in 01 case the four legs have been separatelygrounded so that the average value of the four systems is given.In the other four cases the four legs have been interconnectedabove surface level and grounded at one point).

4. Information and discussion

4.1. Damage to equipment installed on the tower

There were eight instances (on six towers), when damage hasbeen detected and recorded in antenna structures (melted orripped-off material) during a period of 3–5 years prior to the timeof inspection; once in five towers and three times in one tower.Summary of observations is given in Table 2.

It is interesting to note that in all cases the damaged structurewas within the cone of protection with a vortex angle of 45�. How-ever close analysis revealed that it is only in one case (the last caseindicated in the table) the possibility of direct strike to the dam-aged structure is justified. In the other cases there are strong evi-dences to conclude that the damaged objects have beensubjected to arcing from the down conductor.

One of the authors of this paper is a consultant to the towerowners, hence had the authority to demand the removal of theinsulated down conductors at the four towers including the siteswhere the damage has been reported. The inspection of thesedown conductors revealed that the insulation has been ripped offin all four cases, in the sections of the cable mostly towards thetop end of the tower. The marks of burning and insulation rippingoff reveal that the arcing has been taken place at many points alongthe conductor. Many of these arcing incidents have gone unnoticedeither due to the sparking into the tower (no damage to equip-

Table 1The ground resistance of the down conductor termination and tower footing of thecases where the two systems are non-integrated. Three cases out of the five arerelated to the towers with insulated down conductors (Cases 1, 2 and 3).

Ground resistance of the groundtermination of down conductor (O)

Ground resistance of the groundtermination of tower footing (O)

1 2 172 9 243 7 54 11 325 8 13

1358 C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364

ment) or the limitations of visibility at ground level to figure outdamages to antenna structures. In one of the cases of investigation,we could obtain the photographs of the damaged parts of the insu-lated cables before it has been removed. Hence, we could clearlyfigure out the arcing marks. Fig. 2 depicts several photographs ofthis case. The tower is 95 m tall and the majority of insulationdamage of the down conductor could be seen from a height ofabout 40 m onwards. Note that there are number of picturing ofthe insulation material (Fig. 2c) could be seen in abundance in alldown conductors, which could be detected only at close range. Inline with the insulation breakdown of cables, there are burningmarks on the tower which could not be properly photographedat higher levels due to practical constraints. Fig. 3 shows such anincident at about 5 m, close to a point at which cable sheaths of sig-nal wires are grounded. Note that the down conductor is placedalong with the cable bunch. Similar arcing related to insulateddown conductors have been reported elsewhere (by personalcommunication).

Let us consider a lightning strike with peak current 50 kA andmaximum current derivative 50 kA/ls striking the air terminationof the tower. If we assume the peak current and peak current deriv-ative occur at the same time (an assumption that has no impact onthe result as we show latter), the potential at a given height withrespect to ground is given by

V ¼ IRþ Ldidt

where R is the resistance of the conductor and L is the inductance.

Table 2The details of cases where tower-installed equipment were damaged due to lightning.

No. oftowers

No. of timesdamaged

Air termination Down c

01 03 ESE rod isolated from the tower Insulatethe bod

Physical height covers the damaged structureat 45� vortex angle

02 01 each ESE rod isolated from the tower Insulatethe bod

Physical height covers the damaged structureat 45� vortex angle

01 01 Copper rod Copper

Height covers the damaged structure at 45�vortex angle

01 01 Copper rod Copper

Height covers the damaged structure at 45�vortex angle

01 01 ESE rod Copper

Physical height covers the damaged structureat 45� vortex angle

The typical values of R and L per unit length of a copper conduc-tor having cross sectional dimensions as per the IEC 62305-3(2006), assuming them to be flat, are 3 � 10�4 O/m and 1.5 lH/mrespectively. Hence at a height of about 50 m;

IR ¼ 0:75 kV and Ldidt¼ 3:75 MV

Therefore, irrespective of the resistance, the inductance alone willcontribute to a very high potential difference between the downconductor and the tower or antenna structures which are essen-tially at ground potential.

To prevent spark-over through insulation breakdown, such con-dition requires, for an example, either

a. 11.25 cm thick insulation covering of cross-linked polyethyl-ene (assuming 1.2/50 ls voltage impulse) or

b. 2 m of air-separation even at level IV protection according tothe following equation given in IEC 62305-3 (2006).

s ¼ kikc

kml

where s is the minimum separation, ki the factor depends on le-vel of protection (0.04 in this case which is corresponding to le-vel iii/iv), kc the factor depends on the number of downconductors (one in this case), km the unity for air and l is the dis-tance from the possible flash over point to the nearest equipo-tential plane (ground plane in this case).The isolation (or insulation) provided for the down conductor or

for the air-termination in any of the commercially available prod-ucts (or installations) was much lower than such values obtainedabove. It is well understood that implementation of such insula-tion/isolation has many practical constraints, thus, it is not com-mercially viable. Therefore, the five cases of damage to theantenna structures of the three towers with insulated down con-ductor is justified as due to the flash over from the down conduc-tor. The splitting of insulation in the insulated down conductors ofother two towers also shows that there may be sparking to thetower or antenna structures. However, the damage in such cases

onductor Grounding system

d conductor electrically isolated fromy of the tower

Integrated to the grounding ofthefootingGround resistance: 7 O

d conductor electrically isolated fromy of the tower

Integrated to the grounding of thefootingGround resistance: 8 O

Ground resistance: 5 O

tape strapped to one leg of the tower Integrated to the grounding of thefootingGround resistance: 68 O

tape strapped to one leg of the tower Integrated to the grounding of thefootingResistance could not be measured dueto lack of soil

tape strapped to one leg of the tower Integrated to the grounding of thefootingResistance could not be measured dueto inaccessibility

Fig. 2. Insulation breakdown of the insulated down conductor installed in a 95 m tower. (a) A large part of the insulation of the cable has been ripped off at a height of about45 m. (b) Melting of the insulation material of the cable. Such marks are observable in the entire upper part of the cable. (c) Puctures that cannot be observed at ground leveleven with the aid of binoculars. (d) The down conductor is grounded separately from the tower foot ground.

C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364 1359

may be either negligible or beyond the detection from groundlevel.

In the two cases of copper-rod/copper tape; the damaged struc-tures were within 1 m of the down conductor and about 15–20 mbelow the top. There are several antenna structures on the othersides of the tower and also above the damaged object. The downconductors were supported to the tower leg by PVC straps in onecase and by PVC holders in the other case. The copper tape wasloosely in contact with the tower at many places; however, thethick painting of the tower may have prevented the tape from get-

ting into an electrical contact with the tower. As per the informa-tion from the site engineers the damage to the antenna structures,which were in the form of punctures, were in the sides facing thedown conductor (it is very unlikely to have a direct strikes to thesides of an antenna). These were the evidence to justify that thedamages were due to arcing from the down conductors.

The last case, in the Table 2, depicts no evidence for arcing fromthe down conductor. The antenna structure was on a side oppositeto that of the down conductor and the damage is on the outwardsurface of the antenna. In such geometry, the damage due to arcing

Arcing marks on the tower

Damage to the cable insulation

Fig. 3. Arcing from insulated down conductor to the tower.

1360 C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364

from the down conductor is highly unlikely. However, the possibil-ity of a side flash from the air-termination or from another part ofthe tower cannot be discarded.

The observations show that the ground resistance, the integra-tion of down conductor to the grounding system of tower legs orthe coverage of the air-termination has no significance to the pos-sibility of arcing from down conductor to the objects in the tower.

Interestingly, there was no damage to tower-installed objects inthe following cases

a. Towers with no air-termination.b. Towers of which air-termination does not cover all the

equipment installed on the tower, within a cone of protec-tion with 45� vortex angle.

c. Towers with no down conductors.

Most often the antenna structures are attached to the tower re-bars about 5 m below the towers of height above 40 m. Hence thechances of the antenna structures covered by a suitable protectiveangle either by the air termination or the top of the tower itself arehigh. However our investigation does not provide indication tomake definite conclusions on the significance of air-terminationfor lightning protection of metal towers. In the only case in whichwe have justifiable evidence for a lightning strike to an antenna,the damaged object was within a protected cone subtended byan angle of 45�.

4.2. Damage and injuries at ground level

In the 48 sites that we have investigated, there were 102 occa-sions of damage to the equipment in the base transceiver station(BTS) at 27 tower site and 13 cases of personal injuries at 08 sites.

The exact nature of damages is highly complicated to analyze.Basically, the effects are in the form of damage to the electroniccards of the transceivers (TRXs) in the BTS at the tower site. In mostcases of visible damage the burn marks are close to the power in-put ports of the card which indicates that the transient has enteredfrom the power side. In several cases, there were multiple damagesin a single occasion.

The personal injuries were in the form of temporary paralysisdue to step potential or electric shock. In 09 cases, the victims werestanding close to the tower and the descriptions indicate that theyhave been subjected to step potential. In the other four cases, thevictims were inside the station (brick buildings in all four cases)and are in contact with the equipment cabinets or wire shields.

The five sites where the grounding was achieved (or tried toachieve) either by using chemical-filled bores on the rock/small

amount of localized chunk of soil available in the site or by extend-ing a copper tape for a long distance until a mass of water or soil isreached, have the largest number of damages and personal inju-ries. Altogether in these five sites, there were 42 occasions of dam-age to equipment and five personal injuries (all were step potentialrelated injuries).

Interestingly, the damage record and the grounding resistanceat the sites, excluding the above 5, has a not-very-significant corre-lation as over 50% of the cases the resistance was below 10 O. Onthe other hand there were three sites with ground resistance be-tween 70 O and 100 O and one site with resistance 180 O whichhave not encountered any damage to the electronics during a per-iod of operation over 3 years. On the other hand, sites not havingintegrated grounding system with distributed electrodes and a ringconductor most often recorded damages; despite a comprehensiveSPD system in one case.

Among the sites that have recorded damage, in 21 cases therewere no surge protection devices (SPDs) installed at the time ofaccident. In the other 06 cases there were either limited applica-tions of SPDs or erroneous installations.

The limited applications of SPDs include;

a. SPD only at the power entrance.b. No SPDs for the data lines.

c. Inappropriate specifications of the installed SPDs.

Erroneous installation includes;

a. More than one point connected to external ground.b. Routing the grounding wire/tape in the same cable tray with

other signal lines.

In five sites, we observed the following erroneous installation ofgrounding connection, recommended by the same electrical engi-neering company, which is depicted in Fig. 4. A typical bulkheadat a tower site before the cable installation is shown in Fig. 5.

The main ground bar in the building is routed via cable traysand connected to the bulkhead which is installed at the entranceof cable bunch into the BTS (outside of the wall), The abovearrangement has been justified by the consultants to the clientby stating that it is a requirement of single point grounding. Inall five cases equipment damage has been reported and in threesites there were more than one occasion of damage.

In most of the cases, the bulkhead is installed about 2.5–3.0 mabove ground level. Hence, even if the bulkhead is connected tothe grounding grid right beneath it, potentials in the range of20 kV may build up at the bulkhead with respect to the groundinggrid. With non-distributed grounding system with sizable ground-ing resistance, voltages over 100 kV can be developed between thegrounded parts of the equipment in the BTS and the power lines ofwhich the neutral is grounded at a distant substation. In such con-dition, in the absence of SPDs for equipotentialization, arcing isvery possible from ground to line/neutral. Even when there areSPDs, they will be frequently stressed, thus probability of SPD fail-ure or late activation of SPDs is high. In the case of arcing fromground to line/neutral or in the operation of a SPD the transientcurrent will flow along the grounding wire inducing voltage pulsesin the signal lines which are running along with it. Such voltagesmay have sufficient energy to destroy sophisticated parts of theelectronics.

The personal injury record shows a recognizable pattern. In thenine cases of step potential related injuries the sites had no ringconductor and the grounding resistance was either high (above70 O) or the resistance could not be measured due to the reasonsmentioned earlier. Note that at a tower site the direction of the po-tential gradient in the surrounding soil (or rock) is modified by thearrangement of the grounding system, thus a person need not be

Cable bunch from the tower

BTS

Grounding wire that runs along the cable tray together with signal wires

Cable tray Bulkhead

Main ground bar

Fig. 4. Erroneous installation of grounding path for the main ground bar.

C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364 1361

standing along a radial direction (away from the tower) to be sub-jected to step potential. Therefore, unless there is a ring conductorand a rather low grounding resistance it will be difficult to preventa person being subjected to step potential at a tower site.

The four cases of electric shock have happened at the sites de-scribed in Fig. 4. As it was described previously in the event of alightning strike to the tower, the potential at the grounding barof such sites may increase to 20–30 kV which is sufficient to drivean electric shock in a human body. Such shock may result momen-tary paralysis causing the victim to fall unconscious. Such accidentmay lead the victim even to death depending on the fall. In one ofthe cases the victim has been thrown almost onto a bare 230 V ter-minal which was opened for repairing purpose during the time ofthe accident.

4.3. Model design

The investigators were asked to provide a solution for thegrounding at two sites where there were no soil to be found inthe proximity of the towers. The towers were on a hard rock ineach case and the access to the nearest large mass of soil requireda metal extension of nearly 400 m. There was a recommendationby another consultant to extend three parallel copper tapes fromthe tower site to this location; and the recommendation has beenput on a hold due to the high cost and more importantly the pos-sibility of theft of copper (the client has not considered the techni-cal limitations of the recommendation).

By considering our observations in the investigations describedin this study, and the experience that has been gathered during theprofessional careers, we designed the following groundingarrangement for the sites (Fig. 6). We suggest that the model de-sign is suitable for any site where the soil resistivity is very highor there is no easy access to a large mass of soil in the near vicinity.

Bulkhead

Cable tray

Grounding of the bulkhead

Fig. 5. A typical bulkhead (before the cables are installed).

The main features of the above design has been listed out below

a. The site is encircled by a concrete beam of cross-sectionaldimensions 40 cm � 30 cm. A gutter is made at the topwith walls of height 6 cm and width 6 cm. In areas ofelongated dry periods and also in areas where mosquitoproblem prevails, we recommend a concrete lid of suitablethickness to cover the gutter. The perimeter of the con-crete beam is recommended to be about 80 m or more.It may have any convenient shape that does not containsharp corners (in the above case a rectangular shape withcurved corners). The concrete beam is reinforced with sixsteel bars of cross-sectional diameter 1.5–2.0 cm. Theextension of the steel bars (usually, each bar is 4–5 mlong) is done by thermo-welding. The six parallel barsare interconnected at intervals of 1–2 m by steel wires ofcross-sectional diameter of about 5 mm. The connectionis again achieved by thermo-welding.

b. The waste water of the residential worker/s (water frombody and cloth washing; without bio-wastes to preventbad odour with time) is diverted to the gutter on the con-crete beam by PVC drain pipes of suitable cross sectionaldimensions. We also recommend harvesting rainwater byinstalling rain gutters to the roof of the building and collect-ing the water in a 1000 l PVC container. Such collected watercan be released in a controlled manner to the gutter duringthe absence of residential workers or at a site that has noresidential workers. A water outflow is made at a point asfar as possible from the water feed by reducing the heightof a segment (of about 20 cm) of the outward gutter wallto 5 cm.

c. To facilitate vehicle movement in and out of the tower site,the concrete beam is made into a hump at the entrance tothe site. However, the steel reinforcement continues throughthe hump as well, so that the steel bars make a completering conductor.

d. The legs of the tower, foundation of the building, maingrounding bar of the power system, bulkhead and any othermetal part (electrically floating) should be connected to thegrounding network which in turn is connected to the steelreinforcement of concrete beam at several places. Thegrounding network is made of galvanized steel tapes of crosssectional dimensions 25 mm � 2 mm. They are covered by a10 cm layer of concrete beam tapered at the edges to avoidphysical accidents to the workers. The bulkhead should beconnected to the grounding grid right below that. The bulk-head should be a galvanized steel plate of thickness 3 mmand suitable cross sectional area. All connections should bedone by galvanized steel.

e. All the shielding layers and metal sheaths of the cable bunchand the cable tray should be terminated at the bulkhead.There should be no electrical connection between the metalparts inside the building and the bulkhead (except via theexternal grounding system).

f. A Class I arrester (typically a current handling capacity of50 kA per phase for the 10/350 ls impulse) is recommendedat the main grounding bar. Power feeders to all sophisticatedequipment are routed through a Class III arresters (typicallya current handling capacity of 10 kA per phase for the 8/20 ls impulse with voltage protection level of less than0.6 kV for the 3 kA, 6 kV combinational wave form.)

g. Data lines that feed signals into sophisticated equipment orports are recommended to be connected via suitable SPDs.The specifications of the SPDs are decided on case by case.The grounding of such SPDs is integrated with the powergrounding.

Galvanized steel tape covered in 10 cm concrete layer (tapered at edges)

1000 litre PVC container to harvest rainwater

PVC drain pipe to carry waste water/rain water to the gutter in the concrete slab and also rain water from rain gutters of the building to the PVC container

A

A

B

B

A - A

B - B

3 m

40 cm

30 cm

15 cm 6 cm

6 cm

Reinforcement steel bars

Main grounding bar

BulkheadCable bunch

in a metal tray

BTS Cabin for residential workers

Concrete beam

Tower

Hump to facilitate vehicle movement

Water outflow

Fig. 6. The model grounding arrangement designed for the a tower site with extremely high ground resistivity.

1362 C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364

h. The building is provided with only Mobile phones or CDMAconnections (no wired phones).

i. The transformer that feeds the building is asked to maintaina good grounding system (from the electricity supplier). Insome problematic sites isolation transformers are recom-mended to be installed at the site. The transferring of highpotentials into the neighbourhood that is supplied powerfrom the same transformer should be taken care of by thepower company.

j. Suitable air termination conductor is suggested for towerswhere the tower top (tip) does not provide adequate cover-age for the antenna structures. The angle of protectionshould be addressed with further investigation of the mat-ter. A separate down conductor is not recommended. Air-terminations or down conductors insulated from the towerare strongly condemned.

k. A suitable rainwater draining system is suggested to preventflooding inside the site in the event of heavy rain.

l. A warning sign has been placed at all sides of the sitedemanding the people not to be outside the beam (at closerange) in the presence of a thunder storm.

Note that the entire grounding system and connecting paths aremade of galvanized steel, instead of copper. Such selection reduces

the cost of metal by about 75% and also prevents galvanic corrosiveeffect between many iron or steel parts of the site and the ground-ing network.

At many of the sites that we have visited in South Asia (over 200sites), a concrete beam has been laid around the site, on which a pro-tective fence is built up to prevent undesired trespassing. Therefore,the concrete beam that we have recommended (although it is largerin dimensions than what has been observed at the sites) is not onlyuseful for the grounding purpose alone. A fence can be easily built upon the beam without affecting its electrical purpose.

The suggested design has been implemented at two on-the-rocksites in 2007 and found to be accident and damage free for the last3 years.

4.4. Safety management

The lightning safety of equipment and operational staff at acommunication tower site, is a special issue that has not beenaddresses, so far, in safety guidelines such as OccupationalSafety Hazard Association (OSHA) (Zwetsloot et al., 2011;Zwetsloot, 2000) or any other safety guidelines, except for avery recently published article on the field guidelines for surgeprotection that can be applied to such cases (Gomes, 2011).Requirement of strategy on lightning safety is further enhanced

C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364 1363

in the cases of tower related sites situated in regions of highisokeraunic level, locations of elevated altitudes (on rocks andmountains) and built on the grounds of high soil resistivity.Safety management of such workplaces, which are usually iso-lated from normal workplace environment, should be plannedcarefully taking into account several significant issues listedbelow.

a. The staff members are in regular contact with electricaldevices: As the BTS usually equipped with many electricaldevices which need regular operational attention, the work-ers have a high probability of being in contact with suchdevices or metallic components during a thunderstormwhich expose him/her for touch potential hazards.

b. The resident site officer may be in isolation: In many of theremote tower sites a single or dual operational staff isfound permanently housed for operational and mainte-nance purposes. Such personnel may sometimes have faceto face contact with fellow staff only in few days (or some-times few weeks). This monotonous life-style may havepsychological impact on the person in following safetyguidelines (Kouabenan and Cadet, 2005; Culyer, 2007;Baker et al., 2009).

c. Need for stepping out of the safety guidelines to attendbreakdowns: A thunderstorm environment greatly enhancesthe probability of equipment, connection or power failuresin or outside the station. Due to the demand of uninter-rupted service, the worker may be compelled (or his duty-bound mentality) attend repairing or inspection work evenduring the lightning period, which expose him into a severerisk of being subjected to lightning effects.

d. Social and cultural concepts: Depending on the region socialand cultural perceptions on safety may differs in a broadrange. Especially in a third word perspective, the adherenceto the pre-issued safety guidelines may be considered as anon-serious matter. Furthermore, even at engineer’s levelsometimes, long standing social beliefs on natural phenom-ena such as lightning persist. Some of these beliefs may leadto disastrous situations under hazardous conditions (Koua-benan, 2009; Chew, 1988; Dake, 1991; Baram, 2009; Mearnsand Yul, 2009).

Planning the management of lightning at a communicationtower site should be done together with overall safety planningand the lightning safety procedures should be integrated withthe master safety guidelines. A stepwise guidance for addressingthe lightning safety issue is given below. Note that these guidelinesshould be incorporated with a master safety plan.

a. Design and implement the total lightning protection to thesystem (both structural and surge) appropriate to the site,before the commencement of the operation.

b. Provide training on lightning safety and routinely mainte-nance to the resident site officer/worker, before he/she isposted for the job. The training should be repeated annuallyif the same personnel continue to work at the site.

c. Conduct periodic tests to ensure the ‘‘good operational con-dition’ of the lightning protection components; Measure-ment of earth resistance, checking the continuity of tapesand cables, faulty-free condition of SPDs, status of circuitbreakers (usually MCCBs) connected in series with SPDs,etc. the tests should be carried out at least twice a year, pref-erably prior to the acute lightning seasons. The inspectionand testing procedures are specified in above mentionedstandards (IEC 62305 1-4, 2006; NFPA 780:2008, 2008; IEEE

C62.41:1991 (1999); AS/NZS 1768:2007, 2007; ITU-T RECK.20:2003, 2003; ITU-T REC K.21:2005, 2005; ITU-T RECK.56:2003, 2003; ITU-T REC K.27:1996, 1996; ITU-T RECK.31:1993, 1993.)

d. In case that the grounding is achieved only through equi-potential bonding (without achieving a low earth resis-tance) adequate number of warning signs should bedisplayed for the resident staff ordering not to leave thesafe area and for the neighbouring public not to reachproximity of the site boundaries in the event of a thunder-storm. The local legislation and civil regulations should bereferred to verify the conditions under which such publicwarning can be issued.

e. A guideline should be issued on the protocol of attending torepairing operations under thunderstorm conditions. Theprotocol should be prepared by a telecommunication statu-tory body considering both the safety concerns and the ser-vice priorities (Baker et al., 2009; Aven, 2009). Hence in thisstudy we refrain from dictating such a protocol.

f. The residential site officer should maintain a log register ondamage records in which a special chapter should be allo-cated for lightning related effects. The log register shouldbe audited every year to determine the adequacy of light-ning protection and plan any rectification or augmentationneeded.

5. Conclusions

This study leads us to the following conclusions

a. We do not have strong evidence to make any conclusiveremarks regarding the necessity of an additional air-termi-nation for the protection against lightning strikes.

b. There are no evidences for the requirement of a separatedown conductor for all-metal towers. The tower re-barsseem sufficient of providing safe passage to lightningcurrent.

c. As per the clear evidence of damage to tower related equip-ment from insulated down conductors we strongly recom-mend the authorities to develop standards that rejects theusage of insulated down conductors for metal towers.

d. The tower premises should be provided with distributed andintegrated grounding system incorporated with a properlycoordinated system of surge protective devices.

e. With the analysis done and the data collected, we proposemethodologies in designing the best suited grounding sys-tem, for a given tower site. Our analysis show that at towersites on extremely high resistive grounds; rocks and sandysoil, the transient equipotentialization is more suitable forthe safety of people and protection of equipment insteadof attempting to achieve low ground resistance.

f. As per this result we propose a model grounding system,made of steel reinforced concrete, for a tower site on a solidrock with no soil in the near vicinity.

g. We also addressed the other erroneous engineering prac-tices; technical negligence or lack of technical commit-ments; that adversely affect tower site related equipmentunder lightning conditions. The most observed such mal-practices are the inappropriate cable routing, multiplegrounding references, inadequate surge protection systemsand lapses in routinely maintenance. Several case studiesare presented in this regard and remedial measures are alsoproposed.

h. The safety management of a tower site is also discussedunder the risk scenarios and safety guidelines needed to beimplemented. It has been shown that the present safety

1364 C. Gomes, A.G. Diego / Safety Science 49 (2011) 1355–1364

guidelines have not addressed the lightning safety issues oftower sites and it was emphasized for the implementationof such safety measures.

Acknowledgements

The authors would like to acknowledge the Department ofElectrical & Electronics Engineering, Universiti Putra Malaysiaand Department of Physics, University of Colombo for placingexcellent research facilities to complete the work and also NationalScience Foundation, Sri Lanka for the Grant No. RG/2004/E/01.

References

American Standards NFPA 780, 2008. Standard for the Installation of LightningProtection Systems.

Australia/New Zealand Standards AS/NZS 1768:2007, 2007. Lightning Protection.Aven, T., 2009. Perspectives on risk in a decision-making context – review and

discussion. Safety Science 47, 798–806.Baker, R., Chilton, S.M., Jones-Lee, M.W., Metcalf, H.R.T., 2009. Valuing lives equally

in a benefit-coast analysis of safety projects: a method to reconcile theory andpractice. Safety Science 47, 813–816.

Baram, M., 2009. Globalization and workplace hazards in developing nations. SafetyScience 47, 756–766.

Chew, D.C.E., 1988. Effective occupational safety activities: findings in three Asiandeveloping countries. International Labour Review 1, 129–145.

Culyer, A.J., 2007. The effectiveness of occupational health and safety managementsystem interventions: a systematic review. Safety Science 45 (3), 329–353.

Dake, K., 1991. Orienting dispositions in the perception of risk: an analysis ofcontemporary worldviews and cultural biases. Journal of Cross-CulturalPsychology 22, 61–82.

Eriksson, A.J., Meal, D.V., 1984. The incidence of direct lightning strikes to structuresand overhead lines. In: Lightning and Power Systems, London: IEE Conf.Publication. No. 236, pp. 67–71.

Gomes, C., 2011. On the selection and installation of surge protection devices in a TTwiring system for equipment and human safety. Safety Science.

IEC 62305 1-4, 2006. Ed-01. Protection against Lightning.IEEE C62.41-1991, 1999. IEEE Recommended Practice for Surge Voltages in Low-

Voltage AC Power Circuits.ITU-T REC K.21:2005, 2005. SERIES K: Protection against Interference; Resistibility

of Telecommunication Equipment Installed in Customer Premises toOvervoltages and Overcurrents.

ITU-T REC K.20:2003, 2003. Series K: Protection against Interference; Resistibility ofTelecommunication Equipment Installed in a Telecommunications Centre toOvervoltages and Overcurrents.

ITU-T REC K.27:1996, 1996. Bonding Configuration and Earthing Inside aTelecommunication Building.

ITU-T REC K.31:1993, 1993. Bonding Configuration and Earthing Inside aSubscriber’s Building.

ITU-T REC K.56:2003, 2003. Series K: Protection against Interference; Protection ofRadio Base Stations against Lightning Discharges.

Kithil, R., 2006. Lightning protection for telecommunications facilities. In:International Lightning Detection Conference, Tucson, AZ, April.

Kouabenan, D.R., 2009. Role of beliefs in accident and risk analysis and prevention.Safety Science 47, 767–776.

Kouabenan, D.R., Cadet, B., 2005. Risk evaluation and accident analysis. Advances inPsychology Research 36, 61–80.

Mearns, K., Yul, Steven., 2009. The role of national culture in determining safetyperformance: challenges for the global oil and gas industry. Safety Science 47,777–785.

Melander, B.G., 1984. Effects of tower characteristics on lightning arcmeasurements. In: Proc. Int. Conf. on Lightning and Static Electricity, Orlando,FL, pp. 34/1–34/12.

Pavanello, D., Rachidi, F., Rubinstein, M., Bermudez, J.L., Nucci, C.A., 2004.Electromagnetic field radiated by lightning to tall towers: treatment of thediscontinuity at the return stroke wave front. Journal of Geophysical Research109 (D6), D06114.

Pierce, E.T., 1971. Triggered Lightning and Some Unsuspected Lightning Hazards.Stanford Research Institute, Menlo Park, CA, p. 20.

Rachidi, Farhad, Janischewskyj, Wasyl, Hussein, Ali M., Alberto Nucci, Carlo,Guerrieri, Silvia, Kordi, Behzad, Chang, Jen-Shih, 2001. Current andelectromagnetic field associated with lightning – return strokes to tall towers.IEEE Transactions on Electromagnetic Compatibility 43(3).

Rakov, Vladimir A., 2001. Transient response of a tall object to lightning. IEEETransactions on Electromagnetic Compatibility 43(4).

Rizk, F., 1994. Modelling of lightning incidence to tall structures, part I & II. IEEETransactions on Power Delivery 9 (1), 162–193.

Zhou, H., Theethayi, N., Diendorfer, G., Thottappillil, R., Rakov, V.A., 2009. Effectiveheight of towers on mountain tops in lightning incidence studies: sensitivityanalysis. In: 4th International Symposium on Lightning Physics and Effects,Vienna, Austria, May 25–27.

Zhou1, Helin, Theethayi, Nelson, Diendorfer, Gerhard, Thottappillil, Rajeev, Rakov,Vladimir, 2009. A new approach to estimation of effective height of towerson mountain tops for lightning incidence studies: sensitivity analysis. In:International Symposium on Lightning Protection, Curitiba, Brazil, November.

Zwetsloot, G.I.J.M., 2000. Developments and debates on OHSM systemstandardisation and certification. In: Frick, K., Quinlan, M., Langaa Jensen, P.,Wilthagen, T. (Eds.), Systematic Occupational Safety and Health Management –Perspectives on an International Development. Pergamon–Elsevier Science, pp.391–412.

Zwetsloot, G.I.J.M. et al., 2011. Regulatory risk control through mandatoryoccupational safety and health (OSH) certification and testing regimes (CTRs).Safety Science.