RECOMMENDED GUIDE FOR THE PROTECTION OF EQUIPMENT & PERSONNEL FROM LIGHTNING
Abstract: Workable special protection methods to prevent lightning damage to equipment and possible harm to associated working personnel are presented. This document covers: protection methods for Ground Potential Rise (GPR), isolation, shielding, and grounding from lightning. Warning: Protecting a structure from lightning does absolutely nothing to protect the equipment within that structure, and the housed equipment is normally worth many times the value of the structure.
Lightning damage to equipment results in losses exceeding twenty-six billion dollars annually in North America, and nearly three times that worldwide with more than 150 strikes per second. Insurance payout resulting from lightning damage, accounts for approximately 6.5% of all property and casualty claims. Ironically lightning damage to equipment can be all but totally prevented.
Special protection methods to prevent lightning damage are simple, very reliable, and inexpensive, particularly when compared to the cost of equipment repair and replacement, as well as the possible consequences of harm to personnel. However, methods for lightning special protection cannot be found in the code books, i.e.; National Electrical Code (NEC) or the National Electrical Safety Code (NESC). Per the scopes of these two well known codes, lightning is not covered whatsoever, yet they are relied upon for practically all general construction in the United States.
Don’t expect the Lightning Protection Standard (NFPA 780) to provide guidance either, for the prevention of lightning damage to equipment. It is not within the scope of this document either. The scope if this document covers the protection of structures only.
Thus, documented methods for the special protection of equipment from lightning cannot be found in the two main codes or the Lightning Protection Standard that are systematically referred to for practically all general construction in the United States. This is in part the reason why there is so much needless lightning damage. This guide is dedicated to providing special lightning protection methods for equipment and filling the vacuum that currently exists today.
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5. Lightning-A major source of Ground Potential Rise...........................4
6. Divide and Control...........................................................................5
7. Tower Location...............................................................................5
8. Single Point Grounding.....................................................................5
9. Bulkhead Panel or Waveguide Hatch.................................................5
10. Isolate Wire-Line Communications..................................................6
11. AC Power Surge Protection............................................................7
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This document presents recommended engineering design practices for the prevention of lightning damage to equipment within structures. It follows that if equipment is protected from damage by lightning, then personnel using or associated with the equipment will also be protected. Protection of maintenance personnel is not covered in this guide. The following topics are included in this document:
1) Lightning grounded towers, buildings, equipment
2) Divide and control lightning strike energy
3) Tower location in respect to equipment building, electromagnetic radiation, need for Faraday Cage
4) Coordinate the coax cable entry with building equipment grounding
5) Voltage divider circuit from lightning traveling down a tower
6) Lightning-A major source of Ground Potential Rise
7) Bulkhead or wave guide hatch
8) Single point ground location
9) Isolate wire-line communications from remote ground
10) AC power surge protection and UPS at the power entrance facility
11) Standard telephone pair protection is worthless in a GPR
Electrical equipment damage from lightning may be placed into two major categories: (1) improper or insufficient grounding, and (2) no special protection from ground potential rise (GPR). Improper or insufficient grounding will result in the equipment being stressed and or damaged (potential difference) from nearby equipment, metal objects, misdirected current flow, etc. No special protection from a GPR will result in the equipment being stressed from its attachment to a remote ground at some distant location through communication wire-lines or power supply wiring.
Standard protection, in the industry, for the termination of communication wire-line services is gas tubes. Gas tubes are shunting devices and can be found on virtually every telephone pair terminated in homes, buildings, etc., in the country. They are designed to shunt (connect to ground) "incoming energy" and protect equipment and personnel from harm.
However, no shunting device ever made will protect electronic equipment from a GPR or "outgoing energy", whether induced from lightning or from a faulted power line. Shunting devices which are now connected to an elevated ground (outgoing energy) during a GPR, merely offer an additional current path off the site to remote ground (the other end). Thus, these devices guarantee a connection of the communication path in the reverse direction from which they were intended to operate if there is a GPR.
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This single fact (outgoing energy from a GPR) places most telephone and power installations in danger of equipment damage and personnel harm from lightning. One of the most dangerous locations to personnel is the 911 PSAP. The typical 911 center (PSAP) is a relatively small building beneath a very large radio tower. This tall tower is for the dispatch of emergency services and is also a very likely target for lightning. Personnel taking emergency calls coming into the PSAP must be at the phones at all times and do not have the luxury of remaining off the phone during lightning storms, as recommended in virtually every telephone book in the country!
Whether it is a 911 PSAP or a cellular telephone antenna on top of a mountain, special protection methods are available to prevent lightning damage to equipment and associated working personnel. Methods will be presented to enable engineers to incorporate them into the general construction design.
1) IEEE Std 487-2000, IEEE Recommended Practice for the Protection of Wire-Line Communication Facilities Serving Electric Power Stations
2) The "Grounds" for Lightning and EMP Protection, by Roger R. Block, PolyPhaser Corporation
3) Lightning Parameters for Engineering Applications, by Anderson-Eriksson
1) Lightning: A current of approximately 30kA (50% probability), that has an approximate frequency range from dc to 1MHz, with a minimum current value of approximately 9kA and a maximum current value of approximately 400kA.
2) Ground Potential Rise (GPR): The voltage that develops on a grounding system from electrical current flowing through the impedance of that grounding system. One source of this current is from lightning discharging down into the grounding system.
3) Lightning GPR: A minimum voltage of approximately 7.5kV in the earth at the point of a 30kA lightning strike.
4) Ground Grid or Ground Ring: The grounding system built under a building, antenna site, etc., in which all metallic equipment, plant structures and hardware are bonded.
5) Remote Ground: The distant end of a wire-line communication circuit that is at a different ground reference with respect to the near end.
6) High Voltage Interface (HVI): The physical separation and isolation of wire-line communication's conducting paths using optical isolation or magnetic coupling.
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5. Lightning-A Major Source of Ground Potential Rise
There is a 50% probability that a lightning strike will be approximately 30kA. If the self-inductance of the earth is estimated very conservatively at .5x10-6H, and lightning takes the form of a pulse which has a typical rise time of 2x10-6S, then from V=Ldi/dt; the estimated GPR of a 30kA strike will be 7.5kV. Values of GPR could easily triple for higher current lightning strikes or strikes passing through higher inductance.
If the inductance of a grounding system is 10-6H, then a GPR of 15kV may result on a grounding system from a 30kA lightning strike. Thus, any grounded equipment that is connected to wire-line communication pairs is in jeopardy from outgoing currents seeking remote ground.
If we are considering a very large structure with many (1000+) communication pairs, such as a central office, the effect will be greatly reduced with the many multiple paths to remote ground, because of current division. However, if we are considering a small structure with relatively few communication pairs to remote ground then we must consider isolating all wire-line conducting paths.
As discussed in the introduction, gas tubes, MOV's, SCR's, SAS's, etc. are ground shunting devices and thus, will not protect equipment from a GPR. Also, the firing speed of these devices is of no consequence. These devices merely offer an additional path to remote ground through the communication pairs for any and all outgoing currents. In fact, they guarantee a connection to the communication path in the reverse direction from which they were intended to operate!
The only GPR solution for protecting equipment connected to wire-line communications is through isolation, using optical isolators or isolation transformers. These devices isolate and thus prevent current flow. If there is no path for outgoing currents to flow, there will be no current flow. If there is no current flow there will be no harm to equipment or associated working personnel.
6. Divide and Control
The control of dissipating lightning strike energy requires division. This is an absolute must for success, because of the magnitude of the current and the resulting surge impedance of any single dissipation path. Ten radials connected to a ground ring bonded to an antenna, will divide lightning current up into ten smaller segments. This will help insure that the lightning will more likely follow the designated paths for dissipation into the earth and lower the resulting GPR to the adjacent equipment building grounding system.
The optimum length of these ten radials is approximately 80 feet each with interconnecting 10 foot ground rods, spaced every 20 feet. Longer length radials will offer little dissipation improvement, because the lightning strike energy will not remain on the radials for much over 80 feet. In very limited spaces, the recommended minimum grounding system is at least 200 feet of buried bare ground conducting wire composed of five radials, each 40 feet in length, with interconnecting 10 foot ground rods, spaced every 20 feet.
A greatly improved copper wire grounding system can be easily achieved by the use of conducting cement placed around the radials at the time of installation. The cement will harden into concrete both protecting the grounding system (giving it many years of additional life), and making the system a much better (lower) ground resistance.
7. Tower Location
Design engineers attempting to keep transmission loss low along with the real-estate group usually dictate that the associated equipment building be as close to the antenna tower as possible. This current practice couldn’t be more wrong in the design of a reliable and robust equipment system to lightning.
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First, equipment buildings associated with nearby antenna towers must be far enough away (thirty feet minimum) to minimize the magnetic field associated with lightning and the resulting (microwave oven) damage to equipment circuits. Magnetic field strength drops off as the square of the distance. If real-estate prohibits the building from being more than thirty feet from its antenna tower, consideration must be given to engineering a Faraday cage (wire mesh) around the interior of the building. Without a Faraday cage, equipment damage cannot be prevented no matter how well the equipment is grounded or isolated from remote ground.
Second, equipment buildings associated with nearby tower antennas must also be far enough away (thirty feet minimum) to keep the lightning GPR at the tower base from saturating the building grounding system, before a majority of it can be dissipated. These two grounding systems must be bonded together at one single point. However, a bond of thirty feet or more will significantly reduce the resulting GPR at the equipment building due to the impedance of this lengthy bond. This is one of those rare exceptions in which a lengthy bond is an advantage in supporting a robust grounding system to lightning.
8. Single Point Grounding
Single point grounding (ground window) is an absolute must to prevent equipment damage from lightning. Single point grounding is an absolute must, because the resulting GPR from lightning is a wave of voltage rise, or energy surge that passes through a grounding system. This demands that all equipment be bonded to the grounding system, at one location (single point), to insure that every metallic object rises and falls in potential together.
Personnel working at equipment that is susceptible to GPR must be protected by a single point grounding system to guarantee that they are out of harms way to different equipment’s potentials. This is also known as touch potential.
9. Bulkhead Panel or Waveguide Hatch
The ground window where coax cables, waveguide, antenna wires, etc. penetrate the wall of the equipment building is called a bulkhead panel or waveguide hatch, and is an absolute must to prevent equipment damage from lightning. The bulkhead is made out of solid copper. The installation and proper engineering design of the bulkhead will insure that lightning does not enter the equipment building on any entrance cables coming from the antenna tower.
The bulkhead must be bonded to the building grounding system at the single point grounding location. This is also the same single point ground where the tower grounding system is bonded to the building grounding system.
The height above ground that the tower cables pass into the building through the bulkhead is comparable to a voltage divider circuit. The approximate voltage over the height of a tower struck by lightning is approximately 250kV. Thus, at one tenth the tower height, the voltage on the tower cables will be 25kV.
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The best entrance location for the bulkhead is at tower base level to insure the lowest level of voltage on the entrance cables. This cable entrance at ground level also enables all equipment in the building to be grounded at the base or floor level. This results in minimum equipment voltage stress and maximum safety to personnel.
If the bulkhead is engineered high above tower base (15 to 20 feet), then all equipment grounding within the building must be made at this height. Thus, equipment racks must be isolated from the floor and grounded at ceiling (bulkhead) level to prevent lightning current from passing through the equipment in order to get to ground. Grounding of the bulkhead to building ground requires wide copper strap.
10. Isolate Wire-Line Communications
A lightning strike to a grounding system produces an elevated ground or GPR. Any equipment bonded to this grounding system, and also connected to wire-line communications, will most likely be damaged from outgoing current seeking remote ground. Personnel working at this equipment are susceptible to harm, because they will be in the current path of this outgoing current.
The equipment damage from a lightning strike may not be immediate. Sometimes equipment is weakened by stress and primed for failure at some future time. This is called latent damage and leads to premature ‘mean time before failure’ (MTBF) of the equipment.
The best engineering design, for open ended budgets, is the use of all dielectric fiber optic cable for all communications. Obviously, a fiber optic cable is non-conductive, provided that it is an all dielectric cable with no metallic strength members or shield, and isolation is no longer a requirement. This is obviously because physical isolation is inherent in the fiber optic product itself. This all dielectric fiber optic cable must be placed in PVC conduit to protect it from rodents.
However, if budgets are not open ended, the engineering design solution to protect this equipment is to isolate the wire-line communications from remote ground. This is accomplished using optical isolators and or isolation transformers. This equipment is housed together, mounted on a non-conducting surface in a non-conducting cabinet, and is called the High Voltage Interface (HVI).
The HVI isolates the equipment during a GPR and prevents any current flow from a higher potential grounding system to a lower potential grounding system. This totally protects any equipment from damage or associated working personnel from harm.
As mentioned in the introduction, no grounding shunting device ever made, no matter how fast acting, will ever protect equipment from a GPR. Ground shunting devices are connected to the elevated ground during a GPR, and offer an additional current path in the reverse direction from which they were intended to operate. Obviously, this flow of current, even away from the equipment, will immediately cause equipment damage and harm to working personnel.
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11. AC Power Surge Protection
The building AC power supply is also susceptible to the effects of a GPR. Since the neutral and ground wire of a power entrance facility must be bonded (by code) to building ground, a rise in potential of the grounding system will place a surge on the neutral and ground wire. This surge will radiate, not only throughout the building, but also away from the building on the incoming power cables. In some cases, the elevated potential of the neutral and ground wire may actually be greater than the potential of the power (phase) wires.
The resulting surge on the power wires may damage building equipment power supplies, other powered equipment parts, or weaken equipment parts for future failure. This is known as latent damage, and was discussed in section 10. However, the facility entrance power supply is much more robust a system than the communications system, and its protection using a shunting system is very effective (in most situations) in protecting the associated building equipment. Obviously, the best protection would be to power the building with fiber optic cables, but that technology is not here yet, so second best (a shunting device) will have to do.
In addition to a protected power entrance facility, there may also be the need to protect secondary power panels throughout the building. This is to minimize the magnitude of the power surge that may get past the main power panel.
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