IX-19
Lightning and Surge Protection of Cellular
Transmit Sites
D. Kokkinos, N. Kokkinos, I. Vlaseros, ELEMKO SA, K. Dimas, M. Koulizos, P. Garoufalis, J.
Sotiriadis TIM – STET HELLAS
Abstract-- This paper aims to analyze the design,
installation and the components/devices used in a lightning
protection system of a cellular transmit site base station.
Additionally a wide number of difficulties that were faced
during the implementation of the above will be presented.
The experience over 10 years of continues installations
(more than 200) all over Hellas and statistical data recorded
during these years will also be presented emphasizing the
important and sensitive parts of a station and showing
various techniques and devices that can be used to
successfully minimize the damage and how to overcome
specific difficulties. Due to the long term experience and the
variety of difficulties that were detected various case studies
will be presented (i.e. isolated stations, high earth resistance,
high lightning activity etc).
Index Terms-- Bonding, Earthing, GSM, Surge protection
devices, TOV
I. INTRODUCTION
The erection of the first mobile telecommunication base
stations in Hellas started in 1993. STET – HELLAS
(TIM) was one of the first two companies in Hellas,
which created their own network. There are two general
types of stations those used in rural (known as GSM base
stations) and those in urban areas. The ones used in rural
areas are of the interest of this paper since they are
isolated tall antennas, which make them an easy target to
lightning flashes. Also in order to succeed efficient and
wide coverage on an area, the GSM antenna is installed
on high altitudes, although nowadays this is limited due to
environmental issues.
Contact Address:
Dr. Nicholaos Kokkinos
ELEMKO SA,
Tatoiou 90 str, 144 52 GR,Metamorphosis, Attiki,, Hellas
e-mail: nkokkinos@elemko.com
Considering that Hellas has the highest lightning activity
in Europe and the high altitude of the GSM stations many
of them are facing severe damages either due to direct or
indirect lightning flashes. The only protection measure
against lightning surges was an isolation transformer after
the electricity supply meter with T2 [1] surge protective
devices on the secondary site.
In 1996 a collaboration between STET – HELLAS and
ELEMKO SA was established by installing a complete
Lightning Protection System (LPS) on the GSM base
stations that were suffering the most. The first base
station that was protected has suffered continues and
excessive damages from the beginning of its operation
and was in Akarnanika mountains (East Hellas with Td =
50 days per year). The LPS design and the statistical data
that will be described in this paper were based on this
base station, which is successfully protected since 1996.
In total more than 200 GSM base stations were also
successfully protected by applying similar design.
II. GENERAL LAYOUT OF A GSM BASE STATION AND
REPORTED DAMAGES
In general the GSM base stations are quite complex but
for the purpose of this paper only parts that require
protection will be described.
The exterior of a common GSM base station covers a total
area of approximately 200 m2 and is composed out of
three main parts, which are all surrounded by a concrete
fence. The three main parts are: the antenna tower, the
diesel generator shelter and the main shelter, which
contains all the necessary electrical and electronic
equipment for the receiving and transmitting of the signal.
The antenna tower height varies according to the purpose
and the usage of the station; usual heights are between 15
and 45 meters. The antenna and the shelters are
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supported on a reinforced concrete base, which can be
either one large base for all or smaller individual for each
one (see Figure 1).
Rod for antenna
protection
Figure 2 shows a damage caused by poor equipotential
bonding inside the main shelter. In the event of a direct
lightning strike on the tower of the station the local earth
potential will raise causing high potential difference
between earthed and non earthed equipment. Due to the
small area inside the shelter the equipment were installed
near to each other. If the equipment were only earthed
through the PE conductor having as a reference point to
earth the equipotential bonding bar, which is usually
installed at the electric supply meter, the time needed to
equalize the potential between two neighbouring
equipment will be high and therefore the probability of a
flashover between the two is high.
Cellular Antennas
LPZ0B
LPZ0A
LPZ0B
Building containing
equipment for the
operation of the site
MV/LV substation
Wave-guides
LPZ2
LPZ0B
LPZ1
detail analysis of the surge protection is explained further
is the paper.
LPZ0B
Interconnected earthing grids for tower footing support and foundation earthing
for the equipment cabinets building (additional electrodes may be used)
Fig. 1. Layout of the exterior of a common GSM base station
The tower supports all the antennas, which usually are
covered by a plastic enclosure to protect them from ice
and snow. Wave guides are running along the tower
connecting the antennas with the receiving / transmitting
equipment, which are installed in the main shelter.
In the main shelter all the necessary electrical and
electronic equipment that are used for receiving and
transmitting are installed. Generally inside the main
shelter the equipment that are installed are, the receivers
and transmitters, the multiplexers, alarm panel – EXALI,
A/C units, main electric board and various secondary
electric panels such as fire protection, security lights etc.
Coaxial, telecom and power cables are running in metallic
ducts interconnecting the antennas with the equipment.
The GSM base station is usually connected to the MV
electric distribution network through an isolated MV line
having a local MV/LV substation supplying only the
station. The isolated part of the MV line supplying the
station may experience lightning flashes also causing
damages to the electrical and electronic equipment of the
station.
The main electric supply is connected via a standby diesel
generator, to DC power supply units, since most of the
receiving / transmitting equipment are using DC supply.
Common operating DC voltage is 48 VDC.
All the equipments have sensors, connected to a main
alarm panel, known as EXALI that transmits the
operating conditions of the equipment to the main control
which is based in the headquarters of the GSM company.
The most sensitive to lightning surges equipment in a
GSM base station are, the standby diesel generator, the
DC power supplies, the EXALI board and the input
output of the receiving and transmitting equipment.
Figure 10 shows a schematic configuration of the cabling
routing with the equipment and the position and type of
the surge protection devices that were used. A more
Fig. 2. Damages due to poor bonding and incoming lightning surges
III. EXTERNAL LPS OF GSM BASE STATIONS
In rural areas the risk is high since the site is exposed to
either direct lightning stroke on the tower or on the
connecting service facilities. The fact that rural areas
may have difficulty to be approached especially during
winter months requires a higher efficiency LPS design.
Usually the most common protection level of GSM base
station is level I with additional measures [2].
Figure 1 shows a general layout of the external LPS for a
simple GSM base station. The vulnerable points of the
tower are the antennas, which can be protected either be a
metallic ring surrounding the antenna or if this is not
allowed due to operating limitations a protection rod may
be installed according to IEC 62305 – 3 [3]. The shelters
situated underneath the tower in most of the cases do not
require any external LPS since the protection angle of the
tower is sufficient to protect them.
Rod
Collar
To Rx - Tx
To Rx - Tx
Fig. 3. Protection of microwave antennas
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Epsilon is composed have much lower impedance than
typical rod type electrodes.
Installing a foundation earthing in every part that is
composed out of reinforced concrete will provide a low
earthing system impedance (see Figure 4). Usually parts
of the station that are made out of reinforced concrete are
the fence of the station and three bases (tower, main
shelter and generator shelter). To succeed a better
equipotential and to reduce the total earthing impedance
all of individual foundation earths shall be interconnected
having at least four connection points with each other.
Antenna
Mast
Additionally the foundation earthing will help to have
awaiting conductors at the points where a direct
connection with the earthing system is required.
Common points where direct bonds with the earthing
system are: the tower base and the metallic stairway, inner
equipotential bonding rings and generally wherever an
equipotential bonding bar is required.
Standby diesel generator
shelter
Equipment with metallic
enclosures installed inside
the main shelter
Equipotential bonding bars provide a star bonding method
for the waveguides, which should be earthed at various
points along their body. Important points of waveguide
earthing are at the base of the tower, before entering the
main shelter and before connection to the equipment (see
Figure 5 – Figure 7).
All the metallic equipment situated in the shelters are
bonded with each other either direct or through the inner
equipotential bonding ring performing a meshed bonding
method, which has one bond with the foundation earthing
system. The inner ring will have an easier access if it is
installed about 0,5m above the equipment of the shelter
since if it is at the bottom after the installation of the
equipment access will be almost impossible. All the
metallic elements such as cable ducts, doors, air condition
ducts are also bonded to the inner equipotential bonding
ring in order to minimize any potential difference between
metallic surfaces.
Main Shelter with all the
transmitting / receiving equipment
IV. EARTHING AND BONDING OF GSM BASE STATIONS
M
1:1 Isolation Transformer
(delta – star) with local
Neutral
Local MV/LV substation
Neutral is left O/C
30 x 3mm St/tZn tape used for
foundation earth electrode
Ø8mm St/tZn or Aluminium
conductor used for interconnections
of the individual earthing systems
30 x 3mm Cu tape used
equipotential bonding inner ring
Clamp used for connecting
conductors/tapes/metallic surfaces
1.5m long rod earth electrode
ETM Plate earth electrode
Fig. 4. General layout of the earthing system
It is important to avoid the generation of long earth leads
connecting equipment to either to the inner ring or to an
equipotential bonding bar. Therefore if two equipment
are at a close distance with each other, instead of having
as a common reference point to earth the inner ring,
which can be 1m away from both, it is better to have an
additional bond between them with a sort length lead.
For earthing system shown in Figure 4 and at a soil
resistivity between 300 – 400 Ωm a DC resistance of 30
Ohms may be possible to be achieved. But in Hellas the
soil resitivity may be significantly higher and for that
reason the DC resistance may well be above 100 Ohms.
It is therefore required to improve the foundation earthing
system. By using awaiting conductors additional earth
electrodes can be connected to reduce further the earthing
system impedance. The Epsilon (ETM) earth electrode is a
flexible and effective solution. It is equivalent to six 1,5m
rod earth electrodes and additionally the plates that the
Fig. 5. Waveguide earthing at the base of the tower
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In electrical installations there is common problem to
have multiple earthing systems (i.e. LPS earthing system,
electrical earthing system – PE, telecom earthing system).
All the earthing systems should have one common
reference point, unless due to operating issues one should
remain isolated (i.e. telecom), then this should also be
bonded through a low sparkover voltage isolating spark
gap.
A common mistake that was found in numerous cases was
that the SPD to have as an earth the LPS earthing system
where the equipment had a reference to earth only the
electrical earthing system through the protective
conductor. Obviously this means that the SPD will not
protect the equipment unless the two earthing systems are
bonded at the SPD and only one earth conductor is then
connecting the SPD and the under protection equipment.
Fig. 6. Waveguide earthing before entering the main shelter
As a practice is useful to have at the electric supply main
distribution board one equipotential bonding bar, where
all the individual earthing systems should be bonded (see
Figure 9)
Equipotential
Bonding
Bar
Fig. 7. Waveguide earthing after entering the main shelter
PE
Earthing
system
Under
Protection
Equipment
LPS
Earthing
system
Fig. 9. Effective bonding of individual earthing system
Fig. 8. Bonding of metallic equipment inside the main shelter through
the inner equipotential bonding ring
V. INTERNAL LPS OF GSM BASE STATIONS
Bonding of all the metallic equipment through the
shortest path is very important as mentioned in the
previous paragraph. Additionally cable screening is also
important and therefore all cables shall be driven inside
metallic ducts, which should be bonded with the inner
equipotential bonding ring. Cables and waveguides that
change from one LPZ to another LPZ should have a
reference to the earthing system either direct or through
Surge Protection Devices – SPDs.
Waveguides are usually bonded as described in the
previous paragraph and RF type SPDs are installed at the
end of the waveguide before the input of the equipment
that they are connected. The earth leads of the SPDs
should be as short as possible and the earth of the SPD
should be the same with the earth of the under protection
equipment.
The selection of the appropriate SPDs is also very
important in order to have technical and economically
effective solution. ELEMKO has started installations of
SPDs in GSM sites before the implementation of the new
IEC and EN standards 61643 series [1], [4]. Therefore
many of the SPDs that were and are still in service are
only tested with the 8/20µs waveshape. But with the
introduction of the new IEC and EN standards the design
was adjusted of course to the new requirements.
Figure 10 shows a schematic layout of the cabling the
refers to the SPDs that are installed at various points of
the installation. Additionally Figure 4 shows a detail
regarding the isolation transformer that is used in many
GSM base stations in order to isolate the PE coming from
the electricity supply and also the Neutral.
Having the configuration of Delta to Star connection and
by having SPDs in both primary (25kA, 10/350µs) and in
the secondary (40kA, 8/20µs) it is possible to have a local
Neutral conductor isolating the one coming from the
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electricity supply. The earthing system in every GSM
base station was set to be TN – S after the secondary of
the isolation transformer.
MV/LV
Electric
Supply
Standby
Diesel
Generator
DC Power
Supply
Units
-
The test station is in Akarnanika mountain and it is one of
the highest lightning activity areas of Hellas facing the
Ionian sea on the west coast of Hellas on an altitude of
approximately 1000m.
Iimp = 25kA, 10/350µs
Uc < 2,5 kV
Imax = 40kA, 8/20µs
Uc < 2 kV
Imax = 6kA, 8/20µs
Uc < 1,5 kV
+
EXALI
Imax = 10kA, 8/20µs
Uc < 150V
Inputs from
various
monitoring
sensors
To GSM
headquarters
Receiving
&
Transmitting
Equipment
Receiving
&
Transmitting
Equipment
For the purpose of this paper the results of one of the first
GSM base stations will be presented for the first two
years (1996-1998) since during that time this station was
set by TIM-STET Hellas as a test station for the lightning
protection companies to prove the efficiency of their
design and equipment since the IEC and EN standards
were not yet available.
In the first two years (January 1996 – December 1997)
and after various alteration of the initial design the
efficiency of the LPS of the Akarnanika GSM station was
very high since it has not suffered any damage even after
57 surges having an magnitude between 500A and 100kA,
which were recorded by an analogue surge counter. In the
beginning of 2006 the counter was reading 290 surges,
which is an average of 30 surges per year!
The only SPDs that needed replacement after these two
years were some MOV type (50kA, 8/20µs) since at that
time such SPDs were used for primary protection as well.
But none of them was ever exploded or excessively
damaged due to high lightning or surge current in order to
justify the requirements for the current IEC and EN
standards [1], [4]. None of the data or RF type SPDs has
ever been damaged in any of these 212 stations over the
period of ten years.
Imax = 20kA, 8/20µs
Uc < 150V
Microwave
Antennas
&
Security
lights
Fig. 10. Schematic configuration of the cabling network in a GSM base
station
A major problem that was and still occurs is the high
TOVs arriving from the electricity supply. Due to the
poor earthing system of the electricity grid especially in
rocky rural areas in case of a short circuit the generated
overvoltages and the time needed to clear the fault is long
enough to cause high TOVs in the LV network causing
damages LV equipment. A temporary solution was to use
spark gaps for primary protection and high MCOV
(550V) for MOV type SPDs for secondary protection.
Additionally all SPDs should be sealed in a metallic
enclosure to protect the other equipment in case of a
failure.
VI. LPS EFFICIENCY RESULTS
In total 212 GSM base stations all over Hellas are
protected by similar designs as described in the previous
paragraphs for about ten years now. Depending on the
station alterations are always necessary. Every one or two
years there was an inspection on the site and any damages
or failures of SPDs were recorded.
The extensions of the foundation earthing system needed
to be replaced after five years since the material that was
used was galvanized steel and has suffered corrosion
although protective actions have taken when connecting
them with the foundation earthing in order to avoid
electrochemical corrosion.
In such high technological installations the equipment can
be upgraded or replaced by new ones quite frequently,
which require inspection of the equipotential bonding.
VII. REFERENCES
[1] EN 61643 – 11, Low voltage surge protective devices
– Part 11: Surge protective devices connected to low
voltage power distribution systems – Requirements and
test
[2] EN-IEC 62305 -2, Protection against lightning – Part
2: Risk management
[3] EN-IEC 62305 -3, Protection against lightning – Part
3: Physical damage to structures and life hazard
[4] IEC 61643 – 1, Low voltage surge protective devices
– Part 1: Surge protective devices connected to low
voltage power distribution systems – Requirements and
test
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