Coaxial Cables Surge Protection
Coaxial lines are commonly used in both residential and commercial applications. Their uses range from video to RF signals measured in Gigahertz. Their advantages include low noise, convenience and ease of connection. They are, however, an extremely effective conduit for lightning generated pulses. Therefore, they present a considerable threat to expensive electronic equipment such as computers, radio equipment and televisions.
Typical protection for coax cables run to the top of a support structure includes protection at three locations. These locations are:
1) At The Top Of The Tower. An arrester such as a gas tube is grounded to the tower structure if metallic or to a lightning down conductor if the structure is non metallic. The arrester shorts the conductor to the shield during the transient event. The shield is continuously grounded to the tower structure by the construction of the arrester.
2) Where The Cable Transitions From Vertical To Horizontal. This is normally near the base of the tower or other support structure near where the equipment shelter is located. The arresters here must be placed immediately above the vertical to horizontal transition to make use of the increased inductance of the bend and to prevent a dielectric breakdown of the cables insulation at this point. The functioning of this arrester is the same as in location 1). This protector should be directly grounded to the earth electrode established for the support structure. A bus-bar connected to the earth electrode via a wide copper strap may be used to help reduce the inductance to earth for multiple cables in a single location. Additionally, the closer to earth that the vertical to horizontal transition is made the better.
3) At The Point Of Entry To The Equipment Shelter. The arrester is the same type used in locations 1) and 2) and is directly grounded to the earth electrode established for the equipment shelter. For small shelters the establishment of a single point of signal entry and grounding will help to prevent surge reference equalization problems. Reference all equipment signal, power, and control lines to this grounding point to minimize voltage differentials between the ports. In large facilities, such as FAA’s Air Route Traffic Control Centers, this equalization is achieved through distributed protection, referenced to equipotential planes.