Grounding is probably the source of the greatest confusion in the understanding of electrical power distribution in general, and in DBS systems in particular. According to the National Electric Code® (NEC®) the purpose of grounding is:
"To limit voltages due to lightning, line surges or unintentional contact with higher voltage lines...to stabilize voltages…(and) to provide a path in order to facilitate the operation of over-current devices".
Note that nowhere is there any mention of protecting equipment. The intent of the grounding provisions of the NEC® is to insure that electrical systems are as safe as possible to the humans that use them.
For our purposes, the most relevant of the three goals listed above is "To limit voltages due to lightning, line surges or unintentional contact with higher voltage lines". These three sources of over-voltage are, in a properly grounded system, provided with an alternative path around the electrical system of your home or workplace by intentionally connecting the system to the earth. In practice this only minimizes damage from such occurrences. The NEC® provides only minimal standards for lightning protection. If lightning is a common occurrence in your area additional provisions must be made to protect your property. We will discuss the subject of lightning protection later. For now, let’s look at the places a DBS system can be grounded and what each step buys us in the way of protection from over-voltages.
The Downlead (Coaxial Cable)
The next place that a typical DBS system will be grounded is at the "grounding block" that lives somewhere along the coaxial cable run from the dish to the receiver. This ground point serves several purposes. The point of coaxial cable is to trap any electromagnetic interference in the outer "shield", preventing it from reaching the inner conductor. Grounding the shield serves to nullify the effects of stray signals by "grounding out", or reducing to a zero potential, the voltages which are picked up by the outer shield. This prevents much of that signal potential from reaching the inner wire core.
The second purpose of the grounding block is to dissipate static electricity. Your satellite sits outside where wind will create a static charge on the dish itself and the wire attached to it. This action is similar to the grammar school science experiment of rubbing a glass rod with a piece of silk. Electrons are "rubbed off" the silk and collect on the glass rod, creating an electrical charge. In the case of your dish, the dish is the glass rod, and the air is the silk. This charge will build up on the dish and the cable until it reaches a high enough potential that it is able to jump across an air space to ground. Unfortunately this will usually follow a path the leads through the electronics inside your receiver, or, more commonly, in the LNB on the dish. More LNBs have been lost to static discharge than any other cause. Most mysterious failures of LNBs after months or even years of operation can be attributed to static discharge. Grounding the coax serves to safely dissipate this static charge.
Lightning
The third reason to ground the coaxial cable is to prevent lightning from striking the dish. This is NOT to carry the voltage from a lightning strike safely to ground. In order to do that, the ground wire would have to be the width of a tree trunk to carry the millions of volts and thousands of amps contained in a lightning strike without melting to slag. Grounding the dish and the coaxial cable effectively turns the dish into a lightning rod. This requires separately grounding the dish itself, in addition to the coaxial cable.
Contrary to popular opinion, lightning rods are not designed to attract lightning, but rather to repel it. To understand this more clearly, let’s take a look at what causes lightning and how lightning arrestors work.
The earth is an enormous sponge for electrons. This is caused by ionization of materials deep within the mantle (as their electrons are stripped off and become either involved in other nuclear reactions or attached to other materials, iron and silicon in the core become positively charged). So the earth can be thought of as holding an extremely large positive charge. This is why grounding works – the negatively charged electrons flow into the earth in a futile attempt to balance the charge. A lightning strike consists of such a discharge of opposite charges of electrical energy. A negative charge or build-up occurs in the bottom part of the cloud closest to earth (caused by the same wind action that causes the static charge on your satellite dish) and a positive charge of energy accumulates directly underneath in the ground. Separating these two opposite charges is the non-conducting dry air belt separating cloud and earth. As the two opposite charges continue to build up and the dry air belt becomes moist, lightning starts down toward earth. At the same time the positive ground charge is attracted upward, leading to the "pop-science" statement that lightning strikes go up (not really true, the discharge or dissipation of a lightning strike travels up, but the lightning itself travels down).
One of the interesting things about lightning is that each "leg" of a bolt of lightning (i.e. the straight portions of the lightning) is always 150 feet long. As the negatively charged leader stroke from the cloud continues toward earth, the positive ground charge travels up (through the lightning rod system if there is one). When the negative leader stroke is about 150 feet above the top of a protected building, the positive ground charge starts upward to meet and neutralize the downward leader stroke. When they meet, 150 feet above the ground or the lightning rod system, the two opposite charges are neutralized, emptying the negative charges from the cloud and dissipating the ground charge. This all occurs in about one five thousandths of a second. Without a lightning rod system, the lightning will discharge through whatever happens to be in the way of the ground, whether that is your house, your satellite dish, or you!
So the effectiveness of a lightning rod system depends upon its ability to deliver a large positive charge over the entire area to be protected. This is not possible with only one lightning rod (or your satellite dish). Grounding your dish structure alone (without a properly designed and installed lightning rod system) is worse than not grounding it at all. It will result in the ENTIRE lightning strike being dissipated through the dish, which is guaranteed to fry your LNB. Ideally, you should have a lightning rod mounted every 20 feet along the highest edge of your roof. Chimneys wider than 4 feet require 2 rods, less than 4 feet can be protected with only one rod. The rods should be connected to each other starting at the highest point on the roof and working in a constantly downhill manner. Anything that protrudes from the roof must be connected to the lightning rod system. This includes antennas (including the dish), any cupolas, weathervanes, or roof ridge vents (if metal) and the plumbing vent pipes (if metal). This wire should be connected to at least two ground rods, as widely separated as possible, preferably at diagonally opposite corners, if the perimeter, or distance around the house, is 250 feet or less. If the building perimeter is between 250 feet and 350 feet, then three ground rods are required. If between 350 feet and 450 feet, then four ground rods, etc. Lightning protection systems should be applied to metal covered buildings (like sheds) in a like manner as on buildings without metal coverings. Cables must be kept free of sharp turns and "U" or "V" pockets and the cable should maintain a horizontal or downward course at all times. The lightning protection system must be bonded to the household entrance panel ground as well, to prevent grounding potential differences from developing.
Unified Ground
Whenever installing a new ground rod, it must be connected to all other ground rods connected to the residence or other building in question. Failure to do this can enable your lamp, air conditioner, satellite receiver and bathtub to all be at various potentials when referenced to each other. In other words, keeping everything at one potential prevents you from being able to touch two devices at the same time and getting shocked. Another issue that this practice avoids in ground loops. In these situations, two grounds are employed without interconnection, and they thus have a potential difference between them. If two pieces of equipment, each attached to one of these grounds, are interconnected, some portion of the line current will flow from the "higher" ground to the lower, through the interconnecting cable. Having worked in the sound reinforcement area for many years, I can tell you that ground loops can be fun to deal with. This can wreak havoc on an amplifier (or performer holding a microphone) if they are assumed to be at the same potential. At a minimum, they can introduce annoying hum (a 60Hz signal) into analog audio and video signals.
In the case of DBS systems, this issue of bonding the satellite downlead’s ground to the household ground is an area of great discussion. Some people feel that any ground on the coax is better than none (for the shielding and static prevention reasons discussed above). However, using an independent ground for the satellite coaxial cable is, in fact, very dangerous. Consider the following scenario…
Your satellite is connected to a 120-volt circuit, probably protected by at least a 15 amp breaker but with no current limiting device on the neutral or ground leads. The power transformer in the receiver power supply will have its center tap attached to neutral and the receiver cabinet will be connected to ground, which as we have seen, are tied together at the service panel. If the ground used for the coaxial cable, which is also connected to the receiver cabinet, is a better ground (lower potential) than the one used for the electrical service, then the entire house’s current will, at least partially, flow back through the receiver’s electrical service ground lead. From there, current will flow to the receiver cabinet, through the coaxial cable’s shield, to the grounding block and then to ground. How much current flows depends on the ground potential difference. In any event, besides the effect such a current flow will have on the transformer and voltage regulator in the power supply, the potential danger to anyone touching the receiver is extremely serious. For this reason, ALL local building codes as well as the NEC® require that all grounds be bonded together. Do not fool around here. It can save your life.
Surge Protection
So far, everything we have covered has been to protect life and limb (and home) from the dangers of short circuits, over-voltages and lightning. This is the only concern of building codes. They consider any and all electronic equipment to be expendable, so long as it is destroyed in a manner that does not present a danger of shock or fire. To protect your equipment requires steps to be taken that go far beyond the building code or the NEC®. We now move beyond grounding per se, and into power conditioning and surge protection.
The greatest threat to electronic equipment during a thunderstorm does not come from a direct lightning strike. After all, in most parts of the country, a direct hit on your satellite dish is a pretty rare occurrence. Most equipment is, in fact, damaged by voltage surges far below that caused by a lightning strike. These surges are induced in the various cables entering the home by a "near miss". When a bolt of lightning strikes the ground it carries enormous amounts of power. This blast of electricity causes an electromagnetic pulse that will induce a current in any wire that is within the magnetic field. This current can reach hundreds or thousands of volts, with substantial amperage. When it reaches electronic circuits designed for tiny voltage and miniscule amperages, these circuits are quite rapidly destroyed. These surges can be separated into two classes. The first is spikes caused by electric motors’ starting capacitors or similar load related spikes, which are generally of moderate voltage but longer duration (several tenths of a second). The other is a very high voltage surge, caused by lightning or other discharges, which is of very short duration (a few thousandths of a second).
The most common method of dealing with both types of anomaly is through commercial surge protection equipment employing metal oxide varistors or MOVs. MOVs are solid state devices that have virtually infinite resistance at one voltage, but gradually decreasing resistance at higher voltages. In very inexpensive devices there will be a single MOV connected between the hot and neutral leads. This is effective against the first category of spike, since it is caused by a sudden increase in the load on the circuit and shows up as a higher voltage on the hot lead. However, in the case of a lightning strike, the surge will occur on both the hot and the neutral (because the neutral, in addition to being connected to ground in the service panel, is also connected to the center tap of the power utility transformer out on the utility pole). In this case, MOVs are required between ground and hot, as well as between neutral and ground. MOVs employed in this manner are very effective at clamping incoming voltages to a few hundred volts. Since almost all electronics is powered by low voltage (5 to 12 volts) DC, even a 300 volt surge, after passing through a transformer and rectifier, will be reduced to 15 to 30 volts, usually within the range of control of a solid state voltage regulator.
This induced voltage effect is also likely to occur on any cable entering the home from outside. The next most likely source of such a surge is the phone line, since it, like the power service line, is strung over a large area. Here, however, the "normal" voltage is much less than the electrical service (around 40 volts), and input transformers and voltage regulators are not as widely used, so a 300 volt surge is far more dangerous to the connected equipment. MOVs, by themselves, do not provide sufficient protection. More complex, and more costly, circuits that employ other kinds of varistors along with triacs and other solid state components are required.
The final, and most delicate, point of attack is the coaxial cable. Typically the maximum voltage sent to the LNB will around 18 volts. The signal returned to the receiver will be no more than one volt or typically less. Here, MOVs are least effective. Even when designed with a low clamping voltage, their response time may still allow a damaging voltage to pass through to the receiver. Even if effective, they are usually installed at the receiver end of the cable. The surge, however, is induced somewhere in the middle of the cable run, and so is dangerous to the LNB as well. While a coaxial surge protector may protect the receiver, it will not do anything for the LNB.
The single most important thing to understand about MOVs is that they can be used only once. In the process of shunting a large surge away from your equipment, they are destroyed. The gradually deteriorate in the presence of smaller surges. Unfortunately, most inexpensive surge protection equipment has no indication that the protection for which it was purchased is no longer being provided. When purchasing a surge protector make sure that it has an indicator light that verifies protection is being provided (although this will almost always ONLY monitor the AC circuit protection). The presence of an indicator light is also a good marker that the unit was built with some care and probably includes three MOVs on the AC circuit. Of course, the three MOV design requires that the unit be plugged into a properly grounded outlet. A good electrical ground is essential to almost all surge protection systems.
