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GROUNDING,

LIGHTNING PROTECTION AND

SURGE PROTECTION

For

Telecommunications Sites

Author: Rohit Narayan

BE Electrical: CPEng. MIEAust

Date: 6 February 2006

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Introduction

In the field telecommunications, many companies and personnel, talk about having grounding problems or about the efforts that are being made to improve the grounding of a site. In many cases, despite many efforts, problems associated with grounding and lightning, continue to affect the network reliability and indeed cost a lot in repair and lost revenue. In this paper, attempt has been made to help the designer of the grounding and lightning protection system, design a practical and cost effective system.

In summary there are 5 aspects of lightning protection, surge protection and grounding that are looked at.

1. Ideal Indoor Grounding Arrangement

2. Ideal Outdoor Grounding Arrangement

3. AC Surge Protection

4. MDF Surge Protection for Telephone Lines

5. Direct Strike Lightning protection.

Generally, the order of importance of points 1 to 4) will remain the same for all sites powered by mains power. However, points 5 may go up in the order of priority for sites that have tall masts or located on a mountaintop.

Ideal Indoor Grounding Layout

Figure 1, below shows the ideal arrangement of the grounding system inside a telecommunications equipment room. This arrangement is not always possible due to certain constraints at the site or if the site is existing. Alternative layouts, can be implemented. However when that is done efforts should be made to minimize ground loops between the AC Power ground, telecommunications building earth and telecommunications tower earth.

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Figure 1 : Ideal Layout for the Indoor Grounding in a Telecommunications Equipment Room

Notes:

1) A Service Ground Bar, SEB shall be installed in close vicinity to the MDF, and the AC Main Switchboard. The case study at the end of this paper discusses, examples which demonstrate improvised yet effective designs, whereby the SEB is not in close vicinity of the AC Main Switchboard. The Service Ground Bar shall be a minimum of 50mm x 5 mm.

2) Bonding Terminal

3) Telecom Ground

Electrode 4) AC Power

Ground Electrode

Communications Racks

Communications Racks

AC Distri -bution

DC AC

DC Power to Racks

AC Power to Racks

Rectifiers

MDF

1) Service Ground Bar, SEB

AC Main Switch Board

UPS

5) CEB

6) MDF

7) Battery Ground

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2) A bonding terminal, CET shall be provided as a means of connecting the SEB to the ground bar inside the AC Main Switchboard for equipotential bonding. This terminal, should prefebrably be inside an enclosure out the AC Main Switchboard and clearly be labeled as “ Telecommunications Bonding Terminal” or “Communictations Earth Terminal”. The equipotential bonding conductor shall be a minimum of 35 mm2 and less than a total length of 5 metres in an ideal layout.

3) Telecommunications Ground Electrode. The resistance to ground for the telecommunications ground shall be less than 5 ohms. For larger repeater sites the telecommunications ground resistance shall be less than 2 ohms. For a large telephone exchange or switch the ground resistance shall be less than 1 ohms. The recommended layout of the telecommunications ground is discussed later.

4) AC Ground Electrode. The resistance of this electrode shall be as specified by the local electricity authority or local standards. Many electricity authorities do not specify a maximum value. If the electricity authority or the standards allow the telecommunications ground electrode to be used as common grounding, for AC Power, than this electrode is not required. Most authorities not do allow the use of common grounds for telecommunications and AC Power.

5) CEB, Communications Ground Bar. For the ease of installation, it is a good practice to have a communications ground bar close to equipment racks. All the equipment can be grounded to the CEB and a single run of grounding conductor can be run to the SEB. THE CEB can be installed below a false floor or and top of equipment racks on cable trays. In the absence of the CEB, it is an acceptable practice to run individual ground cables from the equipment to the SEB. The minimum size of the grounding conductors shall be 35 mm2. Flexible conductors of equal to or greater than 70 mm2 CSA are preferred.

6) The ground conductor from the MDF to the SEB shall be a minimum of 35mm2. Flexible conductors of equal to or greater than 70 mm2 CSA are preferred.

7) It is common practice in telecommunications to have positive grounded. The arrangement will be no different of negative ground is used, other than the change in polarity. If multiple battery banks exist with opposite polarities they can still be grounded at the SEB. Figure 2, shows the arrangement of grounding if batteries are opposite in polarity. The size of the battery grounding conductor is depended on the Ampere-Hour rating of the batteries and consultation with relevant standards or standards of battery manufacturers shall be used as a guide to choosing the conductor size.

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Telecommunications Service Ground Bar

Connect to

• Various Telecom Racks

• MDF

• Lightning Protection Ground

+12 Volts

0 Volts

0 Volts

- 48 Volts

Rectifier or Rack Power system *

12 Volt Battery Charger or Rectifier *

Telecommunications

Ground Electrode

Note:

* The rectifier or the battery charger can be substituted for a Solar Regulator. The grounding arrangement will be exactly the same

Figure 2: Typical Grounding of Multiple Batteries with Opposite Polarities

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Ideal Outdoor Grounding Layout

Figure 3, below shows the ideal arrangement of the grounding system outside telecommunications hut. This arrangement is not always possible due to certain constraints at the site or if the site exists. Where the telecommunication equipment is installed in a large multi functional building or several floor above the ground floors this layout may not be possible.

Alternative outdoor ground electrode system needs designing on a case basis if the suggested layout below is not possible to implement.

SEB

1) Ring Earth

3) Tower Ground

2) Vertical Ground Electrode

4) Feeders

5) Feeder Ground

Communications Mast

6) Feeder Ground Bar, FEB

Telecommunications Hut or Building

Figure 3 : Ideal Layout for the Indoor Grounding in a Telecommunications Equipment Room

6) Tower Ground

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Notes:

1) Ring Earth, A ring earth around the telecommunications building is recommended, as this arrangement allows the ground potential rise or the EPR around the building to be kept as close as possible to equal. Alternative arrangements can be multiple radials or crows foot design, where there is limitation in implementing a ring earth. The recommended conductor for the buried ring ground is 25 x 3 mm tinned copper tape. The recommended depth of burial for the tape is 450 mm below ground level.

2) Vertical ground electrodes, of depth varying from 1.2 metres to, in excess of 10 metres can be used to improve the ground resistance to the recommended values. As a rule of thumb, the spacing between these vertical ground rods, shall be a minimum of 2 electrode lengths. Generally these ground electrodes are driven into the ground. It is not practical to drive very long electrodes in one piece. Sectional rods of 1.2 to 2.4 metre lengths can be used with couplers to join individual sections to achieve greater driven depths. Where the soil is too hard to be driven into or there is a need to apply ground enhancement material, GEM, around the electrode, drilled holes can be made to facilitate the installation. All underground connections shall be CADWELD.

3) Tower ground. The tower ground layout is very similar to the building ground layout. Radials buried at a depth of 450 mm and made of 25 x 3 mm tinned copper can be used to reduce ground impedance. The radial lengths can vary from 5 metres to 50 metres.

4) Feeders running from the tower to the building are either coaxial type or waveguides. These shall be grounded on the top on the mast and at the bottom of the mast using appropriate grounding kits.

5) Where Coaxial feeders are used, they shall be provided with surge protection and grounded to the FEB, at the point of entry. Where coaxial surge protectors are not used, these feeders shall be grounded using appropriate grounding kits, at the FEB.

6) Ideally the tower ground should be connected to the FEB so that it can be disconnected for the purpose of testing. It is common to have the tower ground connected to building ground underground, which is acceptable. However this will mean that the building and the tower ground resistance could not be measured independently.

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Surge Protection for AC Mains

A telecommunications site needs good grounding for the purpose of good reference ground, noise control and dissipation of any lightning energy, in the case of the telecommunications towers being hit by lightning strikes. Surges in the power and copper based telephone lines can also originate from lightning strikes that have struck objects, some distance from the actual site, in many cases, even miles away. Having a good ground alone is not enough to minimize damage due to these surges caused by distance strikes. Surges can also occur in power lines due to switching of circuit breakers in the power systems under fault conditions. It is important to have adequate surge protection on the AC mains and on telephone lines. This section discusses the selection of surge protection of AC power.

The best starting point in the selection of surge protection devices is to look at the following 5 ratings and decide which one is most suitable for the application.

• Maximum Discharge Current, or Imax

• Nominal Discharge Current or In

• Voltage Protection Level, or Up

• Maximum Continuous Operating Voltage, or Uc as defined by IEC standards

• The Voltage Rise Attenuation, or the resultant dv/dt on the application of the voltage waveform in Figure 4 below.

1) Maximum Discharge Current, or Imax

The Imax gives an indication of the amount of surge energy the SPD will be able to handle without getting damaged. The Imax, is the maximum single shot current, the SPD can handle. The current and the voltage of the wave shapes at which this single shot is applied, are defined by IEC standards.

The graph in Figure 1 is taken out of AS1768 2003 Interim Standard and provides a depiction of what these wave shapes look like.

2) Nominal Discharge Current or In

In is an indication of how long the device will last in the power system. It is worth highlighting a common misunderstanding, that SPD’s are only good for one shot of a lightning surge. This is not true and there are many devices available that may withstand thousands if not tens of thousands of surges.

The IEC standards require SPD’s tested for common power system applications, to withstand 15 impulses at In followed by 10%, 25%, 50%, 75% and 100% of Imax. In is a multiple shot rating of a SPD.

Obviously a SPD, with a higher In rating, will withstand more surges and last longer.

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3) Voltage Protection Level, or Up

Typically, SPD’s are connected between the phase and the ground or the neutral in the case of MEN systems. The way a SPD works, is that remains open circuit at nominal voltages, but if the voltage exceeds its clamp voltage, the SPD will temporarily short out to ground. This enables the excess energy to be diverted to ground. This is perhaps the reason that SPD’s are also commonly referred as surge diverters.

Figure 5 shows typical wiring of SPD’s.

The Up characterises the performance of a SPD in limiting the voltage. The Up indicates how well the SPD, clamps an applied surge and hence a SPD with a lower Up, is a better device in terms of limiting the voltage across an equipment. The graph in Figure 3 below shows what the Up is with respect to the applied standard voltage wave shape and the clamp voltage.

4) Maximum Continuous Operating Voltage, or Uc as defined by IEC standards

SPD’s are voltage-limiting devices and it is important to select a SPD that will not attempt to clamp slight over voltages at 50 Hz. Uc is a guide to how rugged the SPD is against over voltages. If the SPD attempts to clamps the voltage continuously, then this can either result in damage to the SPD or even a fire hazard if the SPD get hot.

Figure 4 Standard Voltage and Current Wave Shapes

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Figure 5 – Typical Wiring of SPD’s

Various Loads

SPD

Surge Diverter

Neutral

63 Amps

16 mm2

Ground

Mains Supply

CB or Fuse

Voltage Protection Level, Up

Clamp V

Peak Voltage is approximately 6000V

Applied Voltage

Voltage After Clamping by SPD

Time in micro seconds

Voltage

Figure 6 – Voltage Protection Level, Up

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Example: Comparing Performance Yardsticks of Two Products.

This example demonstrates, that two SPD’s which have identical Imax ratings may have starkly different In, Up and Uc. The CRITEC TDS MT 277 is the better choice in this example.

Figure 7 : Comparing Two SPD’s

It is worth pointing out that there are several schemes that can be used for choosing installation locations of SPD’s. The choice of the scheme depends on cost, the sensitivity of the equipment being protected, the frequency of occurrence of surges, the importance of the systems or the processes being protected.

For example, a simple scheme would have a SPD with low Up and a high Imax and In at the Main Switchboard an no subsequent downstream protection. In a larger installations there may be a need to install a SPD at the main switch board, as coarse primary protection and SPD’s on distribution boards as finer secondary protection.

5) The Voltage Rise Attenuation, or the resultant dv/dt

In critical applications, surge reduction filters are used as finer protection. The yardsticks described above, that is, Imax, In, Uc and Up can still be used to define performance of surge reduction filters, SRF’s. But other than lowering the voltage to which the equipment is exposed, the SRF’s also reduces the voltage rise time or dv/dt. It is widely recognised that electronic equipment is at danger of being damaged, both, from large amplitudes and high rise time associated with power surges. While surge diverters or SPD’s take care of the amplitude factor only, the SRF’s take care of the amplitude and the dv/dt factors both. Figure 8 below, explains the performance of CRITEC TSG SRF.

CRITEC TDS MT 277

Imax = 100 kA

In = 80 kA

Up= 750 V at 3 kA(8/20µs)

CRITEC DSD1100

Imax = 100 kA

In = 50 kA

Up= 850 V at 3 kA(8/20µs)

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It is strongly recommended to use surge reduction filters, or an SRF with good Imax, In, Up, Uc and dv/dt performance in telecommunications applications. In a telecommunications site consuming currents between up to 200 Amps, SRF’s are an economical yet robust solution to comprehensive AC Power surge protection.

Figure 8 : Operation of CRITEC TSG SRF

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CRITEC

SRF

Main Switch

Mains Supply

Various Loads eg, UPS, Rectifiers

Figure 9: Physical Appearance and Wiring Schematic of

CRITEC SRF for AC Power Protection

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Surge Protection for Telephone MDF

Lightning surges can get coupled into telephone lines, in a very similar manner that they couple into power lines. The subject of telephone line protection is a comprehensive subject, much the same as power line protection. For the ease of discussion, two topologies for providing telephone surge protection at MDF will be compared in Figure 10. The two topologies on the left are essentially identical from a protection point of view and have a single gas arrestor between the lines. The topology of the right has a gas arrestor followed by a secondary clamp device, which is usually a semiconductor device.

Figure 10 – Topologies for Telephone Line Protection

The surges that occur from each line to ground, usually do so of the same magnitude at the same time, hence the name common mode. This is an important observation and derives from the fact that these twisted pairs are balanced, and hence noise signals or surge energy is coupled onto both wires equally. The receiving telecommunications equipment is looking for differential signals, and is most sensitive to noise and surges in the differential mode. That is, the telecoms equipment is generally more robust against common mode L-G signals. The following diagram illustrates the idea of a common mode and differential mode surge.

Common Mode Surge Differential Mode Surge

Figure 11 – Common and Differential Mode Surges

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A single stage gas arrestor installed at the MDF ensures that both the lines are shorted to ground momentarily to take bulk of the energy down to ground when a surge occurs. However because it is not possible to short out the two lines at exactly the same moment, a differential mode transient develops which is very damaging to the end equipment. Hence in critical applications, the use of single stage gas arrestor modules at MDF is not adequate. Figure 12, below, shows two topologies, that are designed to reduce the resulting differential mode surge to a minimum. Equipment protected by these protection modules are protected from common and differential mode surges and are strongly recommended for critical applications or for facilities that experience frequent lightning damage.

The series elements are needed to coordinate the operation of the primary and secondary clamping elements. It should be noted that the gas arrestor at the front end can handle a lot more energy than the secondary device and hence the co-ordination is important to ensure that this stage works first. The differential mode surge is lower in energy and the secondary devices can handle these. The secondary devices also have a lot better clamp performance than the primary gas arrestor.

Figure 13 – Physical Appearance of Surge Protection Devices for Krone LSA MDF

Figure 12 – Multiple Stage Protectors – To Provide Common and Differential Mode Protection

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Direct Strike Lightning Protection for Telecommunications Towers

Direct lightning strikes, to telecommunications towers are a reasonably regular occurrence, more so on mountain tops and in certain parts of the world. The traditional approach to lightning protection on towers, is to have a lightning rod on the top connected to the tower. In some cases a dedicated downconductor ,comprising of bare cable or tape is installed on the tower to connect the lightning rod to the ground. In this method of lightning protection, there is no electrical isolation between the tower and the down-conductor. Hence the tower and the antennae mounted on the tower get electrified. This is not a problem in many cases, provided there is adequate, bonding to ensure that everything rises in potential together. The tower legs generally have a low impedance to ground and a dedicated conductor ensures that there is no reliance on incidental connections.

If this method is deployed, then it is critical, that antennae feeders, are grounded at the top and bottom of the mast and that there are surge protectors at the point of entry of these feeders in the building. In absence of surge protectors, the feeders should be grounded at the point of entry as a minimum, in addition to bonding on the top and the bottom of the mast. For exceptionally tall mast, it is recommended that the feeders be grounded to the tower every 20 to 30 metres apart.

While the method described above has been around for many years, it is not totally palatable to accept that it is okay, for the antennae and the feeder system to liven up and electrify to the high voltages that a lightning strike produces. For users who would like to consider newer, methods there are system available which can isolate the lightning energy away from the tower and the antennae itself. An example of this is System 3000 lightning protection system, which is described below.

An enhanced air terminal, with higher effectiveness than conventional lightning rods, the ERITECH Dynasphere is mounted on top of the telecommunications mast on a 3-4 metres long fibre glass reinforced pole, FRP. The FRP provides isolation between the air terminal and the tower and ensures that the lightning does not flash over and electrify the mast or the antennae. A special downconductor, called the ERICORE is routed in the core of the FRP and connects to the bottom of the Dynsaphere via a special HV termination. The ERICORE then runs along a leg of the tower away from the routes of feeders, down to the tower ground. The ERICORE is designed to isolate energy away from the tower and the feeders and keep induced surges on feeders to a minimum. Because magnetic fields that occur during a lightning strike, can cause some surges in the feeders, a separation distance of greater than 2 metres is recommended between the ERICORE and the feeders. It is acknowledged that this separation distance cannot be achieved in certain circumstances. Even in this cases, the amount of lightning energy coupled into feeders, will be far less than in the case when a bare conductor or the tower itself is use as the down-conductor.

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Dynasphere Air Terminal

Fibre Glass Mast (4.6metre) Mounted with suitable brackets.

ERICORE Downconductor

Bond Downconductor to Mast Base and Ground

Figure 14 – System 3000 Using Dynasphere Air Terminal

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Other Factor to Consider in the design of Grounding and Lightning protection System for Telecommunications

Some other factors that need considering when designing a grounding and lightning protection system for a telecommunications site are:

• Manufacturers of equipment and various standards call for specific values of ground resistance for the telecommunications grounding system. These values range from 0.5 ohms to 5 ohms. It is advisable, that prior to installing a grounding system, a soil resistivity test be carried out. From the results of this test, the size and extent of the grounding system required can be calculated. The soil resistivity test is carried out on the surface of the ground and no deep penetration is needed. Specialised ground testing equipment is needed to carry out this test.

• If the desired ground resistance is not achieved, either in the calculation or after physical installation, the first option would be to seek advise from manufacturers of the telecommunications equipment on the higher value obtained. If there is a need to reduce the resistance, then there are specialized product like GEM, ground Enhancement Material and EGel – Earth Gel. The use of Bentonite is also common . Generally bentonite will not give as much of an improvement as these other compounds would

• The recommendations in this paper are based on an ideal site. In many instances it is not possible to implement all these recommendations, due to site constraints. For example the site may be existing with telephone cables entering on one of the building and terminating at the MDF and the power cables entering the opposite end terminating at the Main Switchboard. It is not possible in the scope of this paper to discuss all the permutations that may exist, however the case study, below demonstrate how an improvised design can still be effective.

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Case Study

The layout below in Figure 15, existed at a small telephone exchange. The power cables, entered the AC MSB or the AC Main Switchboard. The AC MSB then supplied the DB or the AC Distribution board with a power feed. The earth connection between the DB and the telecommunications SEB did not physically exist. However, there was an incidental earth bond, through the rectifiers, which feed DC power to the telecom racks, shown in dotted lines. The problem that exists, is that if there is a lightning surge coming via the power line, it will increase the Earth Potential rise, EPR of the AC Earth Electrode. At that moment the Telecom Earth will be close to zero volts. Due to a large potential different between the AC Earth electrode and the Telecom Earth Electrode, a large current, will flow from the AC MSB, via DB, via rectifiers, via the telecom racks to the SEB and through to the telecom earth electrode. This current will flow through small conductors and possibly through the circuit boards within the equipment and almost certainly cause massive damage. The easy answer to this problem, is that the AS MSB and the SEB should be next to each other and connected via a bonding terminal. In practice it is not easy to move the AC MSB and the SEB around that easily.

AC MSB

DB

SEB

Telecommunications

Room

MDF Foyer

Telecom Racks Rectifier

Figure 15 – A Bad Earth Arrangement Layout

AC Earth Electrode

Telecom Electrode

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Improvisation Number 1.

The immediate improvement that can be done on this site is that a solid conductor of say, 70 mm2 cable be installed to bond the DB to the SEB. While this is not ideal, as the current flow from AC MSB to SEB will still occur as previously, but, there is a solid connection between DB and SEB which will equalize the voltage reasonable quickly.

Figure 16 : Slightly Improved Layout

AC MSB

DB

SEB

Communications Room

MDF Foyer

AC Earth Electrode

Telecom Electrode

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Improvisation Number 2

In Figure 17 the SEB has been physically moved close to the AC MSB and an equipotential conductor of a short length is used to connect the SEB to the AC power earth. The old SEB is now used as a CEB, or communications earth bar where all the telecommunication racks are grounded. However, there is no connection between the CEB and the ground electrode system directly. This connection is via the new SEB. Now if there was an EPR at the AC MSB then the telecom earth potential will be at a similar level and there will be no flow of current through equipment. The other improvisation that has been done is that, while we do not have a ring earth, a partial ring starts near the AC MSB and terminated near the telecommunications room. Hence the EPR in the ground outside is kept as close as possible to zero between the AC MSB and the telecommunications room.

AC MSB

DB

25 x 3 mm Cu Tape buried 450mm UG

2 x 70 mm2 Green Yellow

Main Telecom Ground Conductor

1.5 metre Copper Bonded Roads spaced 2.5 to 3 metres apart

Figure 17: Greatly Improved Layout

Equipotential Bonding using 70 mm2 cable and disconnect link

CEB

SEB

AC Earth Electrode

Telecom Electrode

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Conclusion

The subject of telecommunications grounding and lightning protection is indeed very broad. This paper provides a summarized, yet comprehensive guide to the design and implementation of an effective and practical grounding and lightning protection system. While the paper does not cover every possible scenario that may exist, it does provide adequate discussion to enable the designer to come up with an ideal design or an improvised design if the ideal cannot be achieved, due to constraints at the site.

In summary the paper takes a comprehensive look at the following :

1. Ideal Indoor Grounding Arrangement

2. Ideal Outdoor Grounding Arrangement

3. AC Surge Protection

4. MDF Surge Protection for Telephone Lines

5. Direct Strike Lightning protection.

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