by David Fiedler
If current plans for digitizing and automating the Army are to succeed, a more effective electrical ground system than currently used will be essential for every tactical-operations center shelter, communications vehicle, equipment installation, antenna mast and power source in the architecture.
A good earth-grounding system is necessary for both tactical and fixed-station equipment to:
|Maintain electrical contact with the earth so that non-current-carrying metal components are maintained at ground or zero volts potential for soldier safety;|
|Provide multiple direct-current paths to earth for high currents caused by lightning, short circuits, atomic weapons or power-system surges to dissipate unwanted energy into the earth as quickly and safely as possible before surges damage equipment or endanger personnel; and|
|Provide a low-impedance path to the earth for low-level radio signals on power lines, control lines or antenna lines caused by radio-frequency interference, static or cosite interference before it disrupts system operation.|
Reason No. 3 is perhaps the most critical operational reason for good electrical grounding of tactical equipment in the modern digital Army. It’s important to remember that any electrical conductor with an alternating current traveling through it is essentially an antenna that will radiate electromagnetic energy. If the current has a frequency in the range that will interfere with nearby automation or communications equipment, the effects can vary from minor – such as static heard on a telephone – to major – such as causing a computer to dump its memory.
Poor electrical grounds, bonds and shields are a major contributor to the generation and propagation of broadband noise into TOC-mounted communications equipment such as combat-net radios. Once in the receiver, this noise affects the signal-to-noise ratio and "desensitizes" the radio receiver. This in turn reduces the distance over which the radio can communicate. One look into any of our modern TOC facilities, particularly those that are shelter/armor vehicle-mounted – and their mass of radios, telephones, computers, electronic displays, monitors, printers, routers, etc., and associated cabling all packed into a small space – should tell you they are a stray RF nightmare. The monumental self-inflicted cosite-interference problems TOCs present if not handled properly can degrade operations just as severely as an enemy electronic or physical attack.
The Army’s project manager for TOCs takes great pains to ensure that grounding provisions internal to shelters and vehicles are done correctly during manufacture. Most grounding problems usually occur when multiple shelters, antennas or power generators are cosited to form a TOC. Poor connections between shelters, internal equipment and the earth are usually the beginning of grounding-related system problems.
The first step in building a good ground system is to bond together electrically all electrical equipment in the TOC with each other and with the earth. This will keep all equipment at the same (earth) electrical potential and direct any stray RF/DC currents present on equipment structures to the ground, where they will dissipate. These connections inside the TOC shelter are done via a master ground bus, which is usually a heavy copper conductor designed for this job.
Depending on the specific TOC-shelter configuration, the MGB can also be the "ground" conductor of the power system or the shelter structure itself. MGB will safely conduct any fault current likely to be imposed on the TOC shelter or equipment to the earth ground system. Due to the nature of TOC functionality, however, many operators fail to assure that electrical continuity between equipment and MGB is well maintained. In many cases, the connection between MGB and the equipment is via compression connection (grounding screw) and a wire that’s often forgotten or lost as the equipment in the TOC is configured or moved during operations. For tented TOCs, a MGB is almost always not installed, which leads to an array of interference and safety concerns.
To avoid both electrical shock and RFI problems, operators must assure that an MGB exists, frequently test connections to it for continuity and inspect the connection for breaks or corrosion. This should be done both visually and with an ohm meter. Operators must also be certain to connect equipment located in shelter-tent extensions to the MGB inside the shelter via the shelter’s ground terminal.
Another consideration is the physical length of the electrical ground connection. Some TOC components are notorious RF radiators. These include computer monitors, processor clocking circuits and large screen displays. If the distance between a radiating-device ground connection and the ultimate ground point (usually the earth ground) approaches ¼ wavelength at the radiating frequency, the ground connection will act as a classic antenna. Since the grounded end of the wire is at a very low impedance point and the equipment end is at a higher impedance, RF "hotspots" are created around the TOC. While usually not strong enough to cause physical "RF burns" to personnel, this radiation adds to background radio noise that degrades tactical communications and may cause sensitive circuits in computers and other equipment to malfunction.
In extreme cases, it may be better to have no ground connection at all or a local ground, rather than deal with RFI problems created by antenna-like radiating equipment grounds. In some cases, grounds that form antennas can be detuned into inefficient radiators by lengthening or shortening the ground conductor or by putting tuning components in the ground circuit (sometimes called artificial grounds). The effect is the opposite of tuning a radio antenna for peak efficiency with an antenna-matching unit. When using these techniques, RF currents are dissipated in the ground conductor or the circuit components, not the antenna field, so interference is eliminated.
Current Army TOC designs don’t include provisions for detuning RF radiators. In extreme cases of RFI, constructing shields may be necessary on conductors carrying an RF current. A shield is simply a conductor that surrounds an RF radiator and is connected to the earth ground (DC) system. Currents are induced in the shield by the radiation field and taken to the earth ground before the energy can reach sensitive locations in the TOC. If the offending radiator is wiring, shielding is usually a simple metal pipe, or more commonly a wire braid, similar to the outer conductor of a coaxial cable grounded on both ends.
When constructing shields, care must be taken to get the shield up to the connectors, since even a small length of uncovered wire will radiate effectively, particularly if the interference’s RF is very high. Sometimes, metal connector covers called "backshells" are needed to cover the space between the wire shield and the cable connector. Backshells are commonly found in aircraft wiring and may not be seen normally in TOCs. Other common RFI producers such as computer monitors require a more complicated wire screen shield. This gets difficult, since the screen must be very fine to allow the monitor to be viewed and yet also be an effective RFI reducer.
An easy method of locating RFI in the TOC is to use a cheap amplitude-modulation transistor radio as a detection device. Cheap radios have no filtering on the receiver so a broad band of energy is detected. Simply by tuning between AM stations and then moving the radio near equipment, interference can be detected and heard as an increase in radio-noise level as the receiver approaches the source of interference. Some may argue that checking electrical bonding and tracking down sources of RFI is not the job of TOC Signal personnel. I maintain that keeping the TOC operational is the job of the G-6/S-6, and these actions must be taken as part of normal installation and operations. Obviously, once a source of interference is identified, it then needs to be suppressed.
Traditionally, the method for providing an earth ground for TOC facilities (to include communications and power equipment) has been to insert a 6 ½-foot ground rod, usually made of copper-plated steel, at each TOC component (shelters, radio-antenna masts, power generators) individually. The ground rods themselves were always as long and as thick as practical, since physical contact between the surface area of the ground rod and the earth provided the low-impedance electrical path for both DC and RF currents to be dissipated into the soil. Connections to the ground rod were made with a heavy copper cable, copper strap or braided cable and wingnut fasteners. This worked well enough when TOCs contained much less electronic equipment than today in areas where the soil was uniform and fairly conductive. The problem with a rod system is the installation process. As rods are driven into the earth, the copper plating is chipped or scratched. The steel underneath usually corrodes, reducing the rod’s ability to conduct current.
In Vietnam, Signal soldiers of 1st Signal Brigade devised more elaborate grounding provisions to improve the poor conductivity of the grounds in many areas. In 1969 inadequate grounds were significantly affecting the quality of communications the brigade provided. Poor soil continuity and a separate ground rod at each communications shelter caused equipment to be at different potentials even on the same tactical site. This degraded equipment performance and in some cases became a safety hazard.
These more elaborate provisions usually consisted of five 6 ½-foot ground rods interconnected with heavy copper cable in a star configuration. The earth around the rods was usually packed with rock salt and kept wet to make it more conductive. Attached to the star was a cable that ran around the entire facility forming a grounding ring or halo. To this cable ring all equipment grounds, shelter grounds and power grounds were connected, also with heavy copper cable and compression nuts. If possible, the cable was buried to provide more surface contact with the earth.
Star grounds were effective when properly constructed but were difficult to employ at other than fixed locations because some shelters, like modern TOCs, were moved often. The rods themselves tended to corrode quickly due to the rock salt.
Over the last 10 years, the Army has modified some shelter grounding provisions to follow a more ring-like concept such as 1st Signal Brigade used. Many TOC shelters now carry a multipoint surface grounding kit containing about 12 15-inch grounding spikes. The spikes are of galvanized steel to improve electrical conductivity and have an X-shaped cross-section to increase surface contact. The spikes are placed at eight-foot intervals around the shelter in a ring and connected with a cable. Ground connections from shelters and equipment are tied to this cable via short lengths of cable and alligator spring clips.
The multipoint surface kit has had both its good and bad points. Inserting/removing multiple shorter rods is easier than trying to insert one long rod in many areas. The rods’ shape does provide more surface area in contact with the earth than single ground rods do. At the same time, the short ground spikes are not optimal conductors because they aren’t copper-plated. Performance is further degraded by the way the spikes are connected to each other and by the use of spring-clip connectors to connect to the shelter and the ground ring. The multipoint ground kits also have no chemical means built in to increase soil conductivity near the ground spikes. This considerably reduces the capability to transfer electrical current from the ground system into the earth in soil that is not naturally conductive.
Only some TOC shelters are currently equipped with multipoint kits. Power generators, tented equipment and communications shelters are all still provided with the old-style long, single-ground-rod system. Both systems will function under circumstances where the soil chemistry and electrical connections are favorable and well maintained; however, under many conditions these systems aren’t adequate and may actually degrade operation of communication and automation equipment within TOC facilities.
An excellent alternative to both types of driven rod systems the Army now uses is what is commercially known as an electrolytic ground-rod system. This type of ground system, while intended for fixed-site operation, can also be easily adapted for use in tactical facilities. The actual ground electrode (rod) is a "hard-drawn" copper pipe that’s filled with a mixture of nonhazardous salts to increase electrical conductivity to the earth (see figure below). When commercially installed, the soil around the electrode is usually packed with dense clay called bentonite. This improves contact between earth and electrode even more, and also prevents the earth around the electrode from drying out and becoming less conductive.
|Common electrolytic grounding system, manufactured by Lyncole Industries for the Defense Department.|
While extensive ground preparation with bentonite may not be practical for tactical operations, it could be useful at many fixed and semifixed site locations the Army now uses. For tactical facilities such as TOCs that are often located over poorly conductive earth, such as arctic permafrost or desert sand, the electrolytic grounding rod is a vast improvement (ground five to eight times more conductive) when compared current rod or spike systems. If just two or three electrolytic rods and a ring-type ground system are installed in a typical Army TOC (see figure below), grounding performance in terms of the ability to provide a low-impedance path for fault DC and RF currents to earth will improve manyfold. The creation of this improved low-impedance path will also provide better protection from lightning, electromagnetic pulses and transient power surges.
This safeguards both the communications and automation equipment and enhances the chances of TOC survival under adverse operational conditions. Also, the more effective ground system will do a better job in keeping all non-current-carrying metal objects such as shelter structures and equipment enclosures at the same electrical potential. This will reduce cosite interference, an ever-present problem in Army TOC facilities.
As the automation of the tactical Army becomes more and more of a reality, we can see from the experiments conducted so far that good electrical grounds for tactical facilities are an important issue. Historically we know that poor grounds degrade system performance even in systems far less complex than those we’re planning. Both the battle labs and development centers need to pursue a better approach to system grounding, such as tactical use of electrolytic ground systems, now before we begin deploying the Army battle-command system on a mass scale. This will assure a better final product to the units in the field.
Retired LTC Fiedler is an engineer and special assistant to the project manager, operational tactical data systems, Fort Monmouth, N.J. He has served in Army, Army Reserve and Army National Guard Signal, infantry and armor units and as an Army civilian engineer. He holds degrees in both physicals and engineering and an advanced degree in industrial management. He is author of many articles in the fields of combat communications and electronic warfare.
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