It's nearly done.  After all those months, weeks, days and nights of hard work, the wireless system is almost up and running. Tomorrow, the local telephone service provider will be bringing in the T1 lines, the wireless will be connected to the wire-line and the money will start to go on the plus side of the ledger instead of the negative.

Every possible means to have the best, most competitive, cost-efficient wireless system has been used. The best hardware, the best software, communications experts, tower experts, grounding experts - the works. New towers built when necessary and existing towers used wherever possible.   In fact, even space on electric transmission line towers was purchased from the local electric utility.   In other words, this is one state of the art system that will give you the edge.

But only if it all works.

Why wouldn't it work?  Everything has been checked and re-cheeked, tested a zillion times.  It's all perfect.   But what about the high voltage protection? Towers - transmission line or otherwise - are a high voltage environment. And these can be pretty hostile places.  One bad storm, a lightning strike, and thousands of dollars worth of equipment are gone. Not to mention those dreaded words: down time. That's why so much attention is given by grounding experts to install the most up-to-date equipment and shunt to ground any surge coming into the site, away from the equipment.  That takes care of the enemy from outside, but what about the enemy from within? The strike that causes a surge to come up through the ground instead of coming in on the wire-line?

This type of surge is called a ground potential rise (GPR) and our purpose here is to explain what this is. how it can cause damage and how to protect against it, using high voltage wire-line isolation devices.

The basic premise behind the use of wire-line isolation devices in high voltage environments is simple: when phone service is required at a site that may be subject to high voltage surges, special protection measures are required by various national standards to ensure personnel safety and prevent damage to equipment. In classic W5 fashion, let's look at who needs to be aware of these special protection measures, what type of equipment is required to achieve this protection, where this equipment is installed, when should the equipment be installed and, most importantly, why it is required.  So that, when the night docs grow dark and stormy, the network stays safe and reliable,

Ground Potential Rise

As the old saying goes, "Know thine enemy." The "how's" and "whys" of GPR must first be understood, before designing and implementing a safe and effective protection scheme. In a nutshell, when a fault or lightning strike occurs and a current reaches a ground grid (like a tower site), (he result, according to Ohm's Law, is a potential rise. V equals R * I where I is the surge current, R is the impedance of the ground grid and V is the resulting potential rise.

also terminated to a Central Office (CO) by copper pairs.  This CO is the remote earth, and the copper wire-line is a conductor lied between two ground planes. Therefore, a difference in potential between the two ground planes will cause a current to flow up from the ground at the tower site, through the equipment and out on to the wire-line. This is dangerous to personnel and can damage the site equipment.

Using an analogy, we can compare this situation to two glasses filled with water, one representing the ground plane at the tower site, the other, the ground plane at the CO.  Imagine one glass up on a shelf and the other lower on the table.  If there is no connection between the two, then no matter what happens to the water levels in the glasses (comparing variations in potential), no water will flow between the glasses (meaning no current will flow). 

However, if the two glasses are connected by means of a straw (i.e. connecting the two ground planes by means of a copper phone line), then sudden increases in the water level of the first glass will mean that water will flow down the straw (i.e. current on the wire-line) to the second glass. Anything tied to that straw would get wet.   In the same way, anything tied lo the wire­line will see the current.  The only way to prevent this is to put a barrier in the straw. This is what isolation devices do.

While proper grounding is essential and standard communication protection methods, used properly, are critical at these sites, they are unfortunately ineffective in protecting equipment from GPR.  For example, shunting devices normally are placed at each end of a cable communication facility and are designed to direct foreign voltage impulses into a grounding system. During a GPR, these devices merely offer an additional path to remote ground reference and actually provide a path for current to flow in the reverse direction from which they were intended to operate.  Thus, no matter how good standard protection devices are, equipment or cable facilities will become part of an electrical path between the GPR and remote ground. The only effective protection scheme against GPR is an isolation device.

The next step is defining what tools are available to help solve GPR problems.  A series of field-proven national standards provide methods for protecting people and equipment from GPR. The most important and useful standards include:

•  ANSI/IEEE Standard 487-1992 - Guide for the
  protection of wire-line communication facilities
  serving electric power stations;
   

•  ANSI/IEEE Standard .167-1996- Recommended
  practice for determining the electric power station
  ground potential and induced voltage from a power fault;
   

• ANSI/IEEE Standard 80-1997 -Guide for safely in AC substation grounding;
   

• NFPA 70-lyyfi-National Electrical Code (NEC).

Although most of these standards address protection from GPR due to 60 Hz fault currents, lightning strike energy applications are basically the same when considering higher frequency impedance. Both currents generate a GPR and can potentially harm personnel and damage or destroy communication facilities.

The above standards define when high voltage interface (HVI) device is necessary for wire-line protection.   In general, an HVI should be installed when the calculated GPR is above 1,000 V peak asym-metrical, or the service performance objective (SPO) is for Class A, always requiring protection.

It should also he stressed that failure lo comply with national standards can have serious legal repercussions should a GPR incident cause injury to personnel or damage to property.  Safely issues must be considered when designing and installing communication systems.

In summary, there are three issues that must be considered before a protection scheme can be designed and implemented:

Is the site a likely candidate for GPR? The answer is yes if a wire-line communication link enters a high voltage area or one that is prone to lightning.

What is the calculated level of GPR at this site?  If it is evaluated at greater than 1,000 V peak asym­metrical, then high voltage isolation is required by

IEEE standard 487, rather than using shunting devices.

What level of service performance objective is required at the site? If it is Class A, then again, HVI protection should be installed rather than using shunting devices

The Wire-Line Isolator Concept

The basic objectives for the protection of wire-line facilities are to ensure personnel safely, protect the telecommunications plant and terminal equipment, maintain reliability of service, and accomplish these in the most economic way.
  
   A good wire-line isolator will use state-of-the-art technology to isolate and protect telephone facilities and personnel from the hazardous voltages associated with GPR's.  Inserted into the wire-line link at the terminal end, the wire-line isolator unit breaks the copper continuity of the telephone line as it enters the high voltage site, thus eliminating the conductive bridge, which links the high voltage site and the CO ground planes.  Basically, it acts as a dam between the local site services (also called station side) and the Central Office (called CO side), allowing all of the communication signals to pass through transparently but preventing any fault currents from passing on the phone lines.  It accomplishes this either through an isolating transformer or a fiber optic link. This basic concept is illustrated in figure 1.

Design and Installation of High Voltage Wire-Line Protection

The high voltage isolation device is installed on the wire-line telecommunication link that feeds into a high voltage area, such as a PCS or cellular site, where Telco wire-line service is required. This type of device is should be installed when the calculated GPR at the location where service is to he provided is over 1,000 V peak asymmetrical or when the service performance objective (SPO) calls for Class A service.

Once the requirement for use of this device is determined and the device purchased, then proper installation procedures must be followed.  If the installation is not done properly, the equipment will not perform as specified. The installation of a High Voltage Interface (HVI) is a detailed procedure.  It is also a simple procedure

necessary steps are done in the right sequence and safely regulations are observed. The different elements which constitute an typical HVI installation are: the telephone cable junction (splice point) outside the zone of influence, the dedicated cable in PVC conduit. The lightning arrester, a non-metallic splice case, the high voltage isolation device proper, and the secondary protection to which the local site services are connected.  Figure 2 shows a typical high voltage protection installation using wire-line isolation devices.
   Some safety precautions: All of the local communications connections at the tower site should be made first and tested before connection to the dedicated cable. The reason being that since a ground potential rise (GPR) may occur at any time when the wire-line cable is connected to both the communications at the tower site and remote ground planes, the connection to the dedicated cable should be made last.  Additionally, it Is recommended that a single crew, who can see each other at all times, should perform the installation.  Work should always be done while standing on a 20kV insulating rubber mat, and wearing insulating rubber gloves and boots, Work should be accomplished on a clear day when lightning strikes are less likely to occur.
  In figure 2, the wire-line from the central office comes into the HVI through the 4-inch PVC conduit. This is then routed through a lightning arrestor and spliced lo the shelf (the red box). This shelf is a central part of the isolation device. It is where the CO and the station cables are kept separate by an isolation gap. The station cable is connected to the upper right hand side of the shelf, and feeds through a secondary protector before going on to the local services. The CO cable comes in from the lower left hand side. These two sides are kept separate From each other at all times, to maintain the integrity of the isolation. Power is supplied to the shelf when needed.
  This describes the bones of the system.  The meat of the system is how to get the telecommunication signals across the isolation gap.  Isolation circuit cards accomplish this. They are the heart of the system. These cards plug into the shelf and are the telecommunication bridge between the CO and station sides of the assembly. The cards are designed to suit just about any communication requirements. Figure 3 shows some examples of types of circuits that can be isolated with available isolation devices.
   To design a system, the type and number of lines that require protection need to be determined, such as POTS, four-wire AC data, T1 carrier, etc.  Then the size and quantity of shelves to house the cards is determined. An installed system can be as small as about 13 by 11 by 6 inches for isolating a single circuit. The entire assembly shown in figure 2 would take up about 35 by 40 inches of wall space and protect up to eight circuits.   At this stage the user should keep in mind any future expansion needs when selecting the shelves. It is sometimes useful to reserve one or two empty slots in a shelf for this purpose. Powering requirements need to be determined and the appropriate power supply selected.  Finally, other equipment needs should be considered, such as lightning arrestors, mutual drainage reactors, etc.
  The end result will be a comprehensive high voltage wire-line communication assembly that will protect site communication services from damage caused by GPR.

Conclusion
So what does this all mean? Basically what it comes down to is protecting your investment.  That's what all the protection, grounding and isolating equipment installed at a site is for.  Wire-line isolation is one more factor to consider when designing and installing a wireless system. The question to ask is: will I have a wire-line communication line coming into my wireless site? If the answer is yes, then you need to look at wire-line isolation.

Lucie Trepanier, E. Eng. (electrical), MRA, is the product manager for Positron's Power Division.   With the Teleline Isolator, Positron has become u recognized industry leader in the engineering and manufacturing of high voltage communication isolation equipment.  Lucie can be reached at 514-345-2200, by e-mail at ltrepanier@positron.qc.ca or through the web site at www.positronpower.com.
Positron engineers and manufactures telecommunications equipment for high reliability, critical service applications, including public safety emergency response systems (E9-1-1) and communication protection for high voltage environments.  Established in 1970, Positron's corporate and engineering headquarters, and 160,000 sq. ft. {14,864 sq. m.) manufacturing facilities, arc located in Montreal. Positron 's U.S. headquarters are in Atlanta, with six sales support offices across the United States.
Positron systems operate throughout North America and around the world, including Asia, South America, the Middle East and Europe. Virtually all major U.S. telephone companies and power utilities have standardized on Positron Products. Positron is registered ISO 9001.

High voltage wire-line isolation:
the basics

Who should use wire-line isolation devices?
Anyone requiring service over a copper wire-line going into a. high voltage .environment. Examples of applications include T1 or HDSL to PCS sites, remote metering, SCADA, and many more.

What is this equipment?
It is isolation device which allows telecom communication signals to pass through transparently, while blocking high voltage surges.

Where is this equipment installed?
In high voltage environment receiving phone service over copper lines, typically PCS or cellular sites (particularly those insulted in transmission line corridors), substations, power plants,  E9-1-1 sites, etc.

When should this equipment be installed?
When the calculated ground potential rise (GPR.) is over '1000V peak asymmetrical (calculated by the electric utility), or when the SPO calls for  Class A service.

Why should this equipment be installed?
To prevent injury to personnel and damage to equipment, and loss of valuable data, when the site is subject to a ground potential rise.
 

How do I protect my antenna from lightning? It says here to be sure to ground the antenna. Hmmm, lemme think about this ... "Ground the antenna." Sounds sorta like drilling holes in a boat, Well, actually, no, it isn't. Although counter intuitive, properly grounding an antenna designed to be grounded actually helps the performance of the antenna and is absolutely necessary to achieve personnel and equipment protection during a direct or nearby strike. Besides, in most cases we are not actually grounding the antenna element support structure.

When we talk about antenna grounding we are actually talking about several different antenna system components. First, the antenna itself: Most antennas are grounded to their mounting support structures by their mounting hardware. For this discussion, we will assume that the antenna is mounted on a tower although the

example of bad tower leg grounding practice

principles apply to any mounting structure or system. The antenna mounting bracket electrically grounds, or more accurately, bonds the antenna to the tower. This assures that the antenna will be at the same potential as the tower, providing electrical continuity for the lightning energy to transition from the antenna to the tower. Without an easy path off the antenna, the resistance and impedance produce heat, which can cause the common Florida malady; antenna disappearance. In reality, the antenna does not actually disappear completely it quickly reappears at the bottom of the tower in a modified form.

   Where the antenna is connected to its RF transmission line, the grounded portion of the antenna transitions into the outer conductor of the coaxial cable. The inner conductor of the coax is connected to the antenna elements and actually carries the RF signal relative to the grounded outer conductor. For convenience, the antenna is usually connected to a short, smaller, flexible transmission line jumper, where it makes the required turns at the antenna support structure. The jumper is then connected to a larger, less flexible transmission line for the longer distance to the transmitter, since the line loss is usually less on a larger transmission line. After the transition is made to the larger transmission line, a grounding kit should be installed on the
 

Without an easy path off the antenna, resistance and impedance from a lightning strike produce heat, which can cause the common Florida malady; antenna disappearance.
 

transmission line, grounding, or, again more accurately, bonding it to the tower. Again, the purpose of this bond is to bring the outer conductor of the transmission line to the same potential as the tower structure. If they are at the same potential, there will he no arcing, preventing burn-through damage to the outer insulating coating of the transmission line. A damaged insulation coating on a transmission line can allow the introduction of water and other contaminants in the transmission line, reducing its transitivity. Additional ground kits should be installed on the transmission line as it travels down the tower. Again, the idea is to maintain the same potential on the transmission line and tower structure, eliminating the arcing. There is some disagreement on the maximum distance between these ground kits, with commonly specified distances between one hundred and two hundred fifty feet.

Another grounding kit should be installed at the base of the tower. This kit performs double duty. As with the other kits, it bonds the outer conductor of the transmission line to the tower structure, preventing arcing. However, if properly installed, it also performs a more important role; It encourages any lightning energy coming down the transmission line to go directly to ground, without passing "GO" and causing $200 dollars worth of damage. When lightning strikes a structure such as a tower, the energy divides and takes all paths to ground. The proportion of energy taking any given path is equal to the proportion at the surge impedance of the path relative to the sum of all paths. The surge impedance of a transmission line may be relatively low, particularly as it reaches the ground kit, the preferred path to ground. A creative way to accomplish this is to bring the drip loop of the transmission line as close to grade as possible by providing an oversized loop which almost touches the ground. The curve in the drip loop should be as light as possible without exceeding the bending radius of the transmission line. This creates surge impedance. The ground kit should be attached to the transmission line just after the loop begins. As the lightning energy begins to see the high impedance of the loop, it also sees the lower impedance of the ground kit. Since lightning energy will follow the lower impedance path, the impedance of the loop will encourage more of the lightning energy to follow the grounding kit to ground. The trick here is to provide as direct (low impedance) a path as possible from the ground kit directly to the head of a ground rod you have installed underground between the tower and the equipment building. Benefits do accrue from this technique. However, at many sites, frequent equipment upgrades and transmission line change-outs make this more trouble than it is worth. So consider the tradeoffs and decide accordingly. Finally, the transmission lines are grounded one last lime at the service entrance to the equipment building. This provides one last opportunity for lightning energy to transition to ground. It also provides a reference potential to which all other site services, such as AC power, data and telephone should be referenced, and to which all site equipment should be

Undesirable high-impedance guy cable and anchor grounding.

referenced. This one and only ground reference at the building will eliminate the possibility for current flow though the site equipment caused by the grounding system design.

A final word on ground kits. They are only helpful over the long term if properly installed. Be certain that the correct size and type grounding kit is used. Be certain that it is properly installed, with a good electrical bond to the coax, but not so tight that it deforms the coax. Also, be certain that it is properly sealed, with the correct type of tape used, in the correct order, with the proper number of wraps, with final wrap applied so water does not run under the tape. Finally, be certain that the correct type sealant is applied in the correct manner with the prescribed number of coats. An incorrectly applied grounding kit can cause transmission line damage attenuating RF signal or even rendering the system unable to pass a signal.
   To protect your antenna, transmission line, radios and other equipment from lightning, you do need to "ground" the antenna. You also need to provide transient voltage surge suppressors on your RF lines and other services. Structural lightning protection can also drastically reduce the incidence of direct lightning strike attachments to your antenna, but that's another article.

About Lightning Master Corporation

Lightning Master Corporation, headquarters in Clearwater, FL has been a leading provider of a full range of lightning protection products, services and systems to the antenna industry (cellular/wireless, broadcast, RF communications, aviation and telecommunications) for the past 14 years. The company provides integrated design, engineering, specification writing, manufacturing, installation, and follow-up services. Lightning Master's products and services encompass bonding and grounding, transient voltage surge suppression and streamer retarding structural lightning protection.