Though many think wireline and wireless networks are distinctly separate, that is false.  PCS (Personal Communications Services) provider’s MSC (Mobile Switch Center’s) connection to their local BTS’ (Base Transceiver Stations) using high bandwidth digital communication links.

These digital links, referred to as BACKHAUL, provide transmission paths within the PCS network and access to the PSTN (Public Switched Telephone Network) while also performing complicated user call authentications, completions, handoffs between cells, and billing functions.

And when a pesky thing called lightning gets in the way of that important backhaul connection, service interruption costs providers big bucks.

That’s why there are four major reasons to use fiber optic systems for radio tower communications:

• 1 – Fiber optic links eliminate transferred potentials during lightning induced Ground Potential Rise (GPR) conditions.
• 2 – Fiber optic links are immune to high energy Electro Magnetic Pulse (EMP) induction from lightning; both within and external to the radio site.
• 3 – Fiber links provide total dielectric isolation for all circuits (backhaul) between the tower radio and the serving telco copper based network.
• 4 – Fiber optic links provide the highest level of worker safety and service reliability.

This article addresses these four major reasons for using fiber, and the basic principles for providing overall electrical protection for stand-alone radio towers.

A vast majority of PCS cell sites are located on stand-alone radio towers or atop various private buildings and are not associated with the power network. In order to understand radio site electrical protection you must first understand the stress effects of lightning. After all, a radio tower is nothing more than a metallic lightning sucker. Once it sucks in high frequency lightning currents the objective of protecting all electronic components (including workers) at the site is to insure the majority of lightning current passes around, rather than through, the facility into a low impedance grounding system.

Lightning protection configurations do not provide guarantees of any kind, only lower probabilities of being struck and receiving damage during any given storm. The NFPA (National Fire Protection Association) recommends, in their Lightning Protection Code 780, the 150 foot rolling ball concept to provide effective protection to tall structures. (See Figure 1.) Everywhere the sphere touches the structure there is a 100% probability it will be hit with lightning, and below the sphere the probability is reduced by 96%. In other words there will always be a small chance (4%) that regardless of the protection, facilities can and will be hit with lightning over a period of time.   In my opinion, there are two types of radio towers:  those that have been hit with lightning, and newly built towers that will be hit in the near future.  The objective is to minimize the level of lightning damage over the life of the site.

Figure 1.

The NFPA rolling ball concept is based on lightning step leader jumping distances. These jump distances are a function of the length of the stroke, its current configuration and amplitude, and the ground impedance of the structure. Due to the random characteristics of lightning, the length of these step leader jumps will change with each individual strike.

On a tall tower above 150 feet, coax line grounding kits should be spaced every 75-100 feet. This will prevent side flashes between the tower and coax lines. If grounded guy lines are used to support a tower, these additional grounding kits may not be required if the antennas fall inside the 4% area created by the down guys.

Lightning has a wide variation in rise times and decay times:

• Rise time = the time in micro seconds it takes for the stroke to reach its maximum or crest value
• Decay time = the time it takes it to drop to half crest value

Running Hot and Cold
In Figure 2, an 8x20 (eight by twenty micro second) stroke would have a lot less energy, for a given amount of time, delivered to a structure. This type of lightning stroke is commonly referred to as cold lightning. Strokes of this type leave little traces of burning or fusing, but do develop explosive pressures in materials that are saturated with moisture. Strokes of this type turn water into steam rapidly and produce tremendous internal pressures (32,000 pounds/square inch typical). Examples of this are wood transmission towers and trees, and small concrete foundations that are not combined with external grounding systems. High intensity shock waves have been known to destroy the roofs of large flat buildings with the sonic high pressure waves they develop in a short time frame.

Figure 2.

Hot lightning generally has a lower crest value with extended half crest times. As can be seen also in Figure 2, a 10x70 stroke would provide a lot more energy over a given amount of time. These strokes do not produce the explosive effects of cold lightning, but will ignite flammable materials (trees, wood towers and buildings) and fuse conductors that do not have sufficient current carrying capacity (cables, coaxes, ground leads, power conductors).

Inductance stores energy in the form of a magnetic flux or field that is created as current passes through the conductor. Inductance is related to the magnetic flux lines and measured in Weber’s (Wb). One Weber per Amp equals the inductance unit H (Henery).

We’re usually working in small units of Inductance such as a μH. The transfer of energy into a magnetic field represents work performed by the power applied. Since power is current multiplied by voltage, there is a voltage drop in the circuit while energy is being stored in the flux lines. This voltage drop, or difference, is the result of an opposing voltage induced in the metallic structure components while the field is building up to its crest value.

The amplitude of the voltage difference is proportional to the rate at which the current changes in time (Rise to Crest), and the length and configuration of the conductor. This value is expressed as inductance in Henrys (L). The voltage difference between any two points, expressed as a function of lightning rise time, can be determined with the relationships found in Figure 3:

Figure 3.

The inductance of coax cables and single wire gauges in μH can be calculated using the formulas shown in Figure 3. These values are controlled by two variables; the length of the coax or wire, and their diameter in inches. The formulas become somewhat more complicated for grounding straps due to the skin effect and higher aspect ratio (thickness to width) at higher lightning frequencies.

The Shocking Reality
Towers are struck by lightning more than any other communication site. In order to
minimize lightning currents from damaging equipment in a building it is imperative that the tower, and its associated electronic equipment, have an equalized low impedance path to true earth.  It’s also important to note that reactance – not resistance - is of primary importance at lightning frequencies (DC to 100MHz).

In simpler terms, let’s look at lightning as if it were a bucket of water being poured on top of a radio tower.  If this is the case, the lightning current will move outward from the tower grounding ring along its associated radials when it is first hit. This includes the single bonding connection to the BTS ground ring.

On a properly installed grounding system, the lightning energy will initially spread out along the building side where the main coax bulkhead panel is located.  At this point in time there will be a high potential at the bulkhead and a low potential on the opposite side of the structure being protected. This is where ground loops develop high voltage differences that can damage equipment and present a hazard to workers.

If the tower ground field is designed and installed to provide very low impedances for the soil conditions at the site, most of the lightning energy will saturate into the earth around the tower before it traverses the radio building. Again, this is the primary objective.

By the time the lightning current surge has surrounded the radio building, the tower grounding ring and associated radial systems should have spread out or distributed much of the current.

With the site surrounded by a lightning surge it becomes obvious that feeding the site with copper based cable facilities becomes an electrical protection issue. Thankfully, a considerable amount of safety and reliability can be built into a radio site by providing a fiber optic link for all backhaul circuits.

Whenever practical, metallic telecommunications lines should be eliminated from all radio towers. Metallic (copper based) telecommunications/data lines provide a conductive path out of and into the facility for lightning energy. It can come from either direction. Eliminating metallic telecommunication lines with the use of fiber optic cable provides isolation from lightning-induced ground potential rise (GPR) and lightning energy. (See Figure 4.)

Figure 4.

During lightning and or power fault conditions it is impractical, and often impossible, to design a ground grid based on the touch voltage created by external transferred voltages. Hazards from these external transferred voltages are best controlled by using electric power protection devices that equalize, ground, or isolate through properly designed interfaces.

Ten Steps to Kill the Lightning Sucker
The following notes correspond to the small numbers shown on Figure 5A depicting a side and Figure 5B overhead views of a stand-alone radio tower protection system:

Figure 5A.

Figure 5B.

• 1 – Radio tower footings. At these specific locations it is recommended that an additional #2 AWG bare solid tinned copper wire ground ring be placed at a 24” depth and bonded to each tower leg with a listed bond. If a counterpoise is required, due to poor soil resistivity, extend a #2 AWG solid tinned wire approximately 30-50 feet from each corner with ground rods (if possible) placed at each end and at 20' intervals. The recommended depth of the counterpoise wire is 24” and shall not contact any other metallic components at the site (i.e., fences). This will reduce touch and step potential.
• 2 – Ice bridge bond. When the BTS radio is placed to the side of the power tower the ice bridge should not be bonded to the tower structure here. It should only be bonded at the bulkhead for equalization purposes. This will reduce touch and step potential.
• 3 – BTS grounding ring. Place a #2 AWG bare solid tinned copper wire within 3 ft (+/- 15% tolerance) from edge of the concrete pad, elevated metallic platform, or building at a maximum depth of two feet. Ground rods must be placed a minimum of 3 m (10 ft) apart and or at each corner of the ground ring. This will reduce touch and step potential when the ring is bonded to the mat in item four below.
• 4 – Wire mesh safety mat. It is recommended at joint use power towers that a wire mesh safety mat (6” on center) be bonded to the ground ring and extended a minimum 6' from the edge of the pad or tower foot print, whichever is the greatest distance. This will reduce touch, step, and mesh potential when covered with gravel fill as described in item 5 below.
• 5 – Gravel fill. For worker safety and to decrease step potential, a layer of clean crushed gravel a minimum of 3–6” deep should be placed over the entire grid/mesh area. When a security fence is in place the clean crushed gravel should be placed within the total security fence area. See IEEE-80 for design details. For worker safety reasons do not use conductive asphalt for this application as conductive asphalt will increase touch and step potential.
• 6 – Bulkhead ground bar. The bulkhead is the single point ground for the installation. All equipment or secondary protectors that require a ground or ground reference shall be bonded to this single point, either directly or with the use of a Master Ground Bar (MGB) located within three feet of the bulkhead. Use individual listed grounding kits for each coax cable entering the BTS at this location. This will reduce touch and step potential for workers and provide voltage equalization for equipment at the site.
• 7 – Concrete pad, elevated metallic platform, or stand alone building. If a concrete pad contains rebar and or wire mesh it shall be equipped with external bonding connectors and bonded to the ground ring at a minimum of two opposing corners. If the BTS is placed on an elevated metallic platform or stand alone building it must also be bonded to the ground ring at a minimum of two opposing corners. The bonding wires must be a minimum #6 AWG copper wire. This will reduce touch, step, and mesh potential and provide voltage equalization for equipment at the site.
• 8 – AC power entrance panel. Commercial ac power service entrance cables must be placed in a PVC conduit (suitable for power cable pulling) at a minimum depth of 3 feet to a point beyond the tower Zone of Influence (ZOI). The entrance panel must be bonded directly to the ground ring at its closes location. If properly installed the BTS ring ground meets or exceeds the NEC Article 250 utility protection ground. If local codes require an additional ground rod, bond the ground rod to the ground ring. All power circuits that enter the BTS must be provided with primary (placed on the line side of the serving panel board) and secondary (placed on the load side of each 20A. circuit) protection. Some BTS manufactures provide secondary protection within their equipment that meet the secondary requirement. All secondary green wire safety conductors must be placed within 3 feet of, and bonded to, the bulkhead or MGB with a copper conductor sized per NEC Article 250-122.
• 9 – High Voltage Protection (HVP). Fiber optic cables must be placed in a PVC conduit (suitable for communication cable pulling) at a minimum depth two feet to a point beyond the tower Zone of Influence (ZOI). Secondary protectors on the copper drop side of the fiber interface at the BTS shall be placed directly on, or bonded within 3 feet of the bulkhead. This will reduce touch potential and greatly decrease lightning caused equipment failures.
• 10 – Fence and gate equalization bonds. Use #2 AWG solid tinned copper wire exothermically welded to the ground ring and attached to each inside or outside corner fence post, and or gate post, with a listed wire clamp or exothermic weld. Place at a minimum 12” depth. Wherever practical, due to magnetic coupling with the tower counterpoise wires (if used), cross at a 90° angle while maintaining a minimum 12” vertical separation. Do not bond these two grounding systems together at crossings. Place a flexible bonding strap from each gate post to the movable gate section(s) with listed clamps. If the metallic posts are not set in concrete place an additional ground rod at each post location. This will reduce touch potential.

Rooftop Risks
The major difference between roof top and ground level installations is the availability of a ground. There are two additional electrical protection requirements when placing a PCS, or any radio system, on top of an existing building.   And equally as important is that the grounding conductors are low impedance and run in a very short direct path to ground the site.

For this application, we recommend, in their order of preference, these six low impedance and electrical protection grounding conductors:

• a – Steel frames and their supporting concrete or steel pilings provide the lowest impedance to ground and should always be the first choice for the primary grounding conductor.
• b – Concrete based frames and their supporting steel rebar cages also provide low impedance to ground, but should be two point tested for continuity before they are used for the primary grounding conductor.
• c – A = 2” diameter metallic water pipe may be used as a grounding conductor if it is certified to be electrically continuous from the BTS to the building ground or municipal water system.
• d – A grounding conductor may be placed in a schedule 80 PVC pipe (using non-metallic support brackets) if it is not placed parallel to other metallic piping systems.
• e – A grounding conductor may be placed in a rigid metallic conduit if it has “listed” grounding bushings placed where it enters and exits the conduit.
• f – An external grounding conductor may be placed on a non-metallic building face using nonmetallic support brackets. If the face is metallic, and can be certified as electrically continuous, use it like a steel building frame.

Keep in mind that a roof top radio system is in the highest lightning exposure area. The 150 foot rolling ball concept dictates that everywhere the sphere touches the structure there is a 100% probability it will be hit with lightning, while the interior of the building the probability is reduced to 4% or less depending on the building structure. Standard copper based communication cables will be subjected to the same high level of lightning voltage as coax cables on stand-alone towers. Unfortunately they are not protected in the same manner. Fiber optic cables are immune to lightning voltage differences and immune to Electro Magnetic Pulse (EMP) induction as well.

Fiber Hero – Again?
Always provide fiber optic communications to the base of the building in suburban locations, and extended external to the building in rural locations. In suburban areas the building will always be bonded to the massive metallic infrastructure surrounding it. This may not be true for a building located in a rural area. A rural building may have a lightning GPR condition and require an extended fiber cable beyond its ZOI. Another advantage for using a fiber link on roof top installations has to do with the transmission of DS-1, or higher bit rates, over Cat-5 copper cables. A fiber link eliminates these restrictions, and allows backhaul circuit extensions regardless of the building height.

Though complex, these prevention tactics are critical to save restoration dollars and prevent customer churn.  Now, more than ever, the objective in this industry is to keep each and every cell site operational at all times in order to maximize minutes of use.

About the Author
Bill Petersen, Protection Technologies Inc. Bill Peterson has more than 44 years experience in transmission and electrical protection. For more information, visit: www.electricalprotection.com  and www.fiberopticlink.com.

What’s your take on this subject? Leave a comment and get the conversation going.