It’s not just something from science fiction. High-altitude electromagnetic pulse (HEMP) is a threat that government organizations, military agencies, communications leaders, and network engineers continue to research, developing systems, standards and equipment to address this ever-present danger. But what is HEMP?
An Electromagnetic pulse or EMP is a burst of radiation created by a high-altitude nuclear explosion. The range and extent of damage this type of destructive event would cause to commercial networks including utilities, 911 responders and transportation hubs is being evaluate globally by government leaders, military agencies, and scientists.
The incidence of damage to equipment in general is higher from power line surges than by any other I/O port. This is not to say more energy comes through the power line, just that the damage is more visible there. Since the coax connection to the tower is the source of the largest surge current in the building, power line port damage is usually due to improper grounding techniques and lack of surge protection devices. There are two probable ways power line caused equipment damage occurs.
Antenna manufacturers utilize shunt-fed dc grounded antennas as a means of impedance matching and providing some form of lightning protection to their customers. It has been proven that these antennas do work and should be used as a means of diverting a portion of the direct strike energy to the tower and its ground system. Unfortunately this protection is designed to help the antenna survive and not the equipment. A direct hit, or even a near hit, can “ring” an antenna whether it is grounded or not since it is a tuned (resonant) circuit. The ringing waveform will contain all resonances that are present in the antenna and its coax phasing lines. This means both “on frequency” ringing and other frequencies present will be propagating down the transmission line towards the equipment. The “on frequency” energy will not be attenuated by a high Q duplex filter or a 1/4 wave grounded stub being used as a protector. In both instances, the “on frequency” energy will pass right through. Also, if we look at a typical dc grounded/ shunt-fed antenna at the top of our 150-foot tower example, both the center conductor and shield will be at the same 243kV potential above ground at the antenna feed. Although the grounded antenna will help prevent arc over of the transmission line, it will have a 6kA peak current traversing its length. The same parallel tower segment will have 12kA. The shared strike current, between the tower and the coax, will contain mostly low frequency components.
Skin effect is a physical phenomenon that relates to the limited penetration into a conductor of a RF signal, according to its frequency. This effect is present in coax cable, keeping the RF signal inside and any coupled outside interference on the shield’s outer surface. The effect begins to fall apart as the frequency is lowered and the penetration, which is a gradient, begins to mix the shield’s outside interference energy with the desired inside energy. A ground loop, which imparts 60 Hz onto a desired signal, is due to dissimilar ground potentials causing ac current flow between points on the coax shield and is low enough in frequency to couple energy through to the center conductor With lightning, the main frequency range is dc to about 1 MHz (-3dB). This is in the range that affects the coax transfer impedance. The thicker the shield material, the less the effect to these low frequency currents.
Corrosion Protection for Tower Structures. Most people have a tendency to use copper as for grounding because it is a good conductor, and one of the more noble metals. However, it does have a significant drawback. Since it is near the upper end of the table of noble metals, copper when put in direct contact with most common metals, which are lower in nobility, will cause accelerated corrosion of the lesser metal. The significance of being more noble means any other metal buried and connected to your copper ground system will become sacrificial. (Also see Topic: Dissimilar Metals.)
Any properly applied lightning protection device is only as effective as the ground system attached to it! Ground resistance (impedance) is usually measured using the 3-stake fall of potential method. Theoretically, the final measurement achieved on the completed ground system is the same resistance (impedance) to any other ground system on earth. A good ground system measurement would be between 5 and 10 Ohms. A well designed 5 Ohm ground system is usually considered optimal for a lightning ground system.
Fuses react very differently to transient surge current as opposed to sustained overload current conditions. Fuses and fusing concerns become complex when contained within surge suppression equipment. Utilizing current limiting fusing in any SPD can quickly become counterproductive because the device’s overall performance is hindered. These fuses greatly limit the total surge current capacity of a SPD to their own individual capacity. The causes of the hindrances are related to the various current ratings and the operational characteristics of specific types of fuses. While there are legitimate ways to compensate for this troublesome drawback, many suppressor manufacturers choose to misrepresent the facts and further complicate the issue.
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