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Basics of Transient and Surge Protection for Coaxial Lines

Basics of Transient and Surge Protection

Coaxial Lines

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.

Commercial Protection

Commercial uses of coax include the ubiquitous cell phone towers. These towers dot our landscape in ever increasing numbers. The towers range from large multi-leg constructions to artificial trees selected for their environmentally friendly aesthetics. Regardless of the type of support there is a significant lightning induced threat on these cables.

The type of tower construction can however increase the threat level on these cables. Use of a nonconductive support tower or pole with a lightning down conductor will increase the energy present at the base of the tower on the cable. This effect is because transient protection at the top of the tower has a higher inductance to ground than with a multi-leg metallic construction.

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.

The multiple arrester approach is necessary for several reasons. The first is that we want to protect the cable sheath and dielectric. Breakdown of the sheath will lead to water infiltration, which can lead to increased VSWR and eventually cable failure. Dielectric breakdown many not be as great a problem for low power circuits but is certainly a significant problem with higher power RF transmitters. Secondly because grounding conductors have a considerable inductance per foot the ability to transition a high power transient is diminished. The greater the length of conductor the higher will be the impedance. The easiest way to reduce inductance is to create parallel paths, particular using flat straps. Because a transient is a high frequency phenomenon, the principle of "skin effect" comes into play. Thus the higher the frequency the less depth is useful for conducting the transient. The greater surface area per ounce of copper of a flat strap is very effective at higher frequencies. Our ability to use flat strap may be impeded by corrosive soils and we may need to transition to multiple large cables upon entering the earth.

Protection for coax run between structures or between separated areas in a single structure may be required at the entrance to structures or even at the entrance to individual pieces of equipment. For additional guidance on this you should consult an engineer who specializes in transient protection for facilities. The engineer can analyze the threat and provide the required protection scheme necessary.

Residential Protection

Conceptually the techniques applied to residential protection are the same as for commercial protection. Residential protection, not including that for "Ham" radio which is the same as commercial protection, may actually involve more protectors because of the need for surge reference equalization.

Most residences are grounded to two ground rods driven close to the entrance of the power line to the home. These rods are required by article 150 of the National Electrical Code (NEC) NFPA-70. In fact, if the first rod driven measures less than 25 ohms to distant earth a second rod is unnecessary. Most electricians find it more cost effective and less time consuming to simply drive the second rod rather than measure the resistance of the first rod to earth.

Residences may have several types of wiring networks that utilize coaxial cables. The most common is a television cable distribution. This may be supplied by either a signal from a cable TV supplier or from a satellite TV antenna. Other possibilities include wireless local area network antennas to facilitate outdoor use of laptops.

The ability to achieve either a single point of grounding or an equipotential plane in a home is severely limited. It is therefore necessary to apply the principles of surge reference equalization at the point of use. The governing principle is to make certain that all ports for a closely located system are referenced together. When equipment is located at a distance all ports to the equipment in a given area must use the same reference.

Since residential circuits are typically limited to 15 amps at 115 volts the power draw of the equipment may also influence our ability to protect multiple pieces of equipment as a system and may force us to protect the individual equipment elements independently.

Protection equipment manufacturers have begun to recognize this need and to supply integrated point of use protection for installations for home theater and computer systems. One example of a particular challenge is a cable modem and home local area network (LAN) wired/wireless router. Typically the ports involved will include at least one coax connection for the cable from the cable system, a 10/100 LAN connection, two power connections and possibly two wireless LAN antenna connections. If the wireless LAN antennas are located directly on the equipment or in the immediate vicinity additional protection is not required. However, if the antennas are remotely located protection is necessary and is typically not provided in the integrated units currently available.

Protection for remote access point antennas must be compatible with the frequencies utilized. 802.11b and 802.11g wireless LANS operate in the 2.4 GHz band (specifically in overlapping channels from 2.412 to 2.462 GHz). Wireless LANS working on the 802.11a standard operate in the 5 GHz with non overlapping channels. Thus the type of protector selected is dependent on the operating frequency of the wireless LAN access point. Once a protector with the appropriate frequency characteristic has been identified the insertion loss should be considered. The insertion loss will directly impact the area of coverage of the access point.

Protection for a residential access point antenna located on a roof should be at three points:

1) At The Antenna. This protector should have a ground routed via the shortest possible path, without sharp bends or kinks to the ground rods referencing the homes power system as discussed above.

2) At The Entrance To The Home. The antenna coax should be routed to enter the home near where the power and telephones enter the home. This is normally referred to as a single point of entry. The protector should be as close as possible to the point where the power system is grounded and routed with the shortest most direct conductor possible to the ground rods.

3) At The Access Point. The access point protector should be referenced to the ground of the power feeding the access point and to the LAN RJ45 ground. By referencing the power, LAN cable and remote access point antenna together surge reference equalization is achieved.

Summary

The principal of multistage protection must be applied for sources external to a facility or residence. Surge reference equalization is necessary at the point but may be achieved through such techniques as equipotential planes or single point grounding in commercial applications. The utilization of whole house/facility power surge protection or signal transient protection is not a sufficient protection; it is simply a very good starting point. Consult a transient or electromagnetic compatibility engineer for specific situations where protection is critical.

Ed Roberts
E. F. Roberts & Assoc.
13906 Bald Cypress Circle
Fort Myers, FL 33907
efrobertsassoc@comcast.net

Lightning and Transient Protection, Grounding, Bonding and Shielding
Surveys - Analyses - Solutions
© Copyright 2005 by E. F. Roberts & Assoc.

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