Since the purpose of any coax protector is to equalize the center conductor potential with the shield potential to minimize current flow through the equipment input, how much lightning current will actually be on the center conductor of your coax cable? To answer this often asked question, we need to determine two things:
The total current available from a direct strike is a given amount. Typically, the 50% occurrence strike will have 18kA or less, and only 10% will have 65kA or more. We will use the 65kA figure for this discussion.
Electromagnetic Pulse (EMP) is an intense electromagnetic field that can instantly overload electrical circuits and damage microelectronics, electrical control and communications systems as well as electric power distribution networks. Produced in the atmosphere by the gamma ray pulse of a nuclear explosion,it is referred to as High-Altitude Electromagnetic Pulse (HEMP) or Nuclear Electromagnetic Pulse (NEMP), which affects electronic equipment over a wide area. Other sources of EMP include nuclear explosions on the ground, man-made weapons like High Power Microwave (HPM) or Directed Energy Weapons(DEW) as well as natural causes like solar storms and lightning.
The sudden release of gamma rays (high energy rays) in a nuclear explosion will cause almost instant ionization (the removal of electrons from atoms) of the atmospheric gases that surround the detonation. Free electrons are driven outward. Gamma rays can travel great distances ionizing the atmosphere. This forced movement of electrons, which will again recombine with atmosphere atoms (Compton Recoil Effect), creates a pulsed electromagnetic field (EMP), or “Electromagnetic Pulse.” This is also referred to as “Nuclear Electromagnetic Pulse” (NEMP). About 99% of the NEMP is radiated in a broad spectrum between 10kHz. and 100MHz. Most of the energy is at frequencies below 10MHz. For comparison, lightning’s power density spectrum is from dc to 1 MHz (for the -3dB point).
Outdoor Closed Circuit Television (CCTV) Security Cameras can be a prime target for lightning. They are usually mounted on a building or other vantage point such as a metal o r wood pole. A lightning strike can not only destroy the camera, but can damage your control console due to energy flowing back through the coax and camera power wiring.
To ensure the safety of trains, passengers and track workers as well as guarantee uninterrupted traffic flow, the US Rail Safety Improvement Act of 2008 mandates the adoption of a Positive Train Control (PTC) system by December 2015 by any mainline rail corridor sharing freight and passenger operations.
An entrance protector is required for telephone or control lines for the same reason an entrance protector is needed for power lines. The local telephone company usually supplies the building entrance station protector as part of their installation service. This is usually a single gas tube per wire type protector. Ground it to your perimeter ground system. The telco supplied protectors prevent surge currents from entering the building and radiating a magnetic field inside the equipment room.
RF lightning protectors are designed to protect RF equipment by equalizing the potential difference between the center pin of the coax cable and its shield. The majority of RF protectors do not feature indicator lights to show whether or not they are still functioning. Since power is not readily available at some installation points, an indicator light is not practical and would add significant cost to the protector. So how do we know if the protector is still functional? Since the IS-50 and IS-B50 are the most widely used PolyPhaser RF protector series, finding their application in Land Mobile Radio, SCADA systems, amateur radio and many others, this paper focuses on their specific testing condition.
There are volumes of information available on what we believe lightning is and how we think it works, most of it beyond the scope of this modest textbook. We will indulge in a form of pragmatism focusing on a practical approach to equipment protection at a communications site during a lightning “event.” The science of grounding (earthing) for lightning events encompasses both the laws of physics and RF design. Throughout this textbook are proven concepts, which will protect your valuable equipment from direct or induced lightning damage. Whether your equipment is at radio site, pipe line, utility sub-station, telephone central office, maritime, military, or sensitive security installation, the same requirements apply for protection devices, proper device placement, and earth grounding.
Protecting tower mounted electronics requires a protector at both ends of the feed cable. If coaxial cable is used, the protector should dc block the RF path, yet provide a separate, protected path for dc Voltage and control signals. If twisted pair data cables (CAT 5) are used, each pair should be protected in a differential and common mode at both ends.
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