Voltage Transients

So far, we have taken a look at electrical power and reviewed some of the basic laws of electricity. We have also established that there are a lot of undesirable things going on with that power. If we could summarize what those things are, we might put them into three categories.

Category one, we might call outages. By this, we mean a voltage excursion that is so low in voltage that the computer interprets the event as a complete loss of power and it goes down as a result. Of course, a total loss of power, like a blackout, would also fall into this category. This category will be dealt with extensively in later sections.

Category two, we might call slow-moving averages. When we say slow here, we mean an event that lasts for longer than one half cycle. The term average refers to the RMS voltage during the event. These events might be either surges or sags. As we have seen by the evidence, a sag is much more frequent than a slow-moving surge.

Voltage regulation to counteract surges and sags is sometimes, but not often, a concern in the PC environment. We will talk about this later and learn when we need voltage regulation and when we don’t. This is important. Many users misunderstand the issues of this category and make uninformed buying decisions.

The last category, we will call voltage transients. These are momentary events of high magnitude that might be disruptive to operations or might even cause damage to equipment. Transients are sometimes called impulses or spikes. We would not call them surges since this is a misuse of the true meaning of the term surge. Voltage transients or impulses are extremely short-term events. This, as well as the following sections, will build logically toward an understanding of transient suppression and power conditioning. More money is wasted in this area of the market than anywhere else. We will see why as an outcome of our study of transient voltage excursions and will learn how to protect PCs from them.

In order to more clearly discuss this subject, let’s break it down into three more meaningful subcategories. These three subcategories are each variations of one another. They are: lightning, static, and noise. We will explore the cause and effect of these three phenomena. An understanding of each will prepare us to move on and see what some products are trying to accomplish in protecting our PC.

In a way, both lightning and static are forms of electrical noise. We might define noise as any undesired signal appearing in a circuit. But not all noise is the result of lightning or static. In this way, noise is a somewhat imprecise concept. But, as we shall see, it manifests itself in two distinct forms or modes. These modes are at the heart of the lore and the religion of protective product designs.

On the other hand, static and lightning are basically the same thing. Lightning is nothing more than an electrostatic discharge on a planetary scale. Static, as we typically think of it, can be no less destructive in certain circumstances. Well, let’s try and sort this out.

Lightning

There are two kinds of electrostatic discharges that we call lightning. One is a discharge from cloud to cloud. The other is a discharge between earth and the clouds above. It might seem logical that the only type we need be concerned about, from a protective standpoint, would be the cloud to ground discharge. As a matter of fact, this is not always true. Obviously, the amount of energy which might potentially be injected into a power system by a lightning strike from above (to the ground) is significantly higher than a cloud to cloud discharge. But the amount of energy released during each kind of event is so enormous that even cloud to cloud discharges can damage equipment several miles away.

Let’s take a look at how a lightning strike is generated. This will give us a clearer understanding of why precautions against lightning strikes must be taken. Scientists theorize that the lightning process begins when regions of the cloud itself develop differing electrical charges. As shown in ill. 4-1, a cloud develops a positive charge at its base while the area above it takes on a negative charge. The buildup of charge between the two areas eventually leads to a discharge within the cloud itself. This process frees more electrons in the negatively charged region that might otherwise have been trapped in water droplets or ice particles.

ill. 4-1. The lightning process.

These free electrons overrun the positively charged base of the cloud, neutralizing its charge and continuing toward the ground below. it's estimated that this trip takes about 20 milliseconds. This trip is broken down into discrete steps less than a microsecond long, known as the leader process. Luminous, rapid steps of lightning, which are approximately 165 feet long, move toward the earth, one after another, with less than a millisecond separating each step.

As the step leader, which is made up of a large negative charge, moves toward earth, an opposite charge builds up on objects below. This opposite (positive charge) is especially large on objects that project above the surface of the earth, like trees, antennas, or tall buildings. Since the two charges are attracted to one another, the positive charge attempts to move toward the step leader.

This causes an upward moving charge to move toward the negative charge in faint luminous steps. Eventually, this results in a meeting of the two charges as they move toward one another. This meeting point is called the strike point. A channel for current flow between the earth and ground is now established. From the strike point, electrons move toward the ground with great violence. This creates the initial bright flash. This brightness moves up the channel, forming a return stroke. Electrons flow toward the earth as current and luminosity move skyward.

This does not end the process. New charges may exist at the base of the cloud, causing the whole process to repeat itself. In fact, this may happen several times in less than a second. The human eye can't perceive these rapidly changing flashes, and we may see them as one very bright event. Scientists have recorded the whole process on film.

The Effects of Lightning

Of course, what is important to us in this whole process is the amount of current that flows in the channel. Peak stroke current averages about 20,000 amperes, but may it be as high as 270,000 amperes.

It would seem that some tall metal structures would be the likely point at which a strike would terminate. This is true but not always. The thunder showers preceding lightning dampens everything, increasing the relative conductivity. Under these circumstances, other objects, like trees, buildings, and concrete structures, may become sufficiently conductive to permit the flow of charges upward. But, because they are still insufficiently conductive to allow a large current flow without significant resistance, extreme heating and enormous mechanical pressures will result.

Some sources state that temperatures reaching 60,000 degrees Fahrenheit can occur after a few microseconds. Metal structures can be warped, and , of course, the current flow can create sparks between metal members that can ignite fires. Flammable material is a grave potential hazard. it's little wonder why lightning is the usual source of forest fires. Wooden buildings can sustain heavy damage since structural members might liter ally explode from the pressure of inside moisture turning to steam. Bark vaporizes on trees, bricks explode, huge blocks of concrete are demolished, and even the ground is sometimes furrowed for several hundred feet. Clearly, the effects of lightning are akin to the detonation of several hundred pounds of TNT.

Superheating of the type we are describing expands the air around the current channel, creating a shock wave. This expanding shock wave is the source of thunder as well as the cause of other widespread damage to the roofs of buildings.

The attractive forces that go to work when so much current flows can cause the fusing of parallel conductors and the squashing of hollow conductors. Large current flows exert a mechanical force as it travels down a conductor, similar to the flow of water through a garden hose. When this current flows through a wire that has tight bends in it, this mechanical pressure has a straightening effect. The bending force can be as high as 5000 pounds. it's recommended that ground conductors, which are meant to serve as a path for lightning-generated current, have no sharp corners.

Lightning Frequency

ill. 4-2 is an isoceraunic map of the United States. This shows the mean annual number of days with thunderstorms. The numbers are recorded by each weather facility making a tally each time they hear thunder. Looking at the map, what we see is that the weather man in Tampa, Florida, hears thunder an average of 90 days each year. The weather man in Seattle, Washington, on the other hand, hears thunder an average of only 10 days per year.

ill. 4-2. Annual isoceraunic map of the United States showing mean annual number of days with thunderstorms. (Courtesy National Weather Service.)

This may seem like a funny way to calculate how often lightning will strike, but from this data, along with other information, a statistical inference can be made that will predict the probability of when a given building will be struck by lightning. Even the current-carrying size of the stroke can be predicted. This information can be used in designing the building and /or the protective equipment that would be used in providing protection from the effects of lightning.

This information on lightning-strike density, along with factors like the safety of building personnel, possible production losses, damage and repair costs, insurance premiums, as well as the cost of protection, are part of the decision-making process that determines the strategy for designing lightning protective equipment.

Building Protection

You may be wondering what kind of strategy has been used to protect your building. Most buildings don't have protection built in as design criteria. In fact, it's often the case that many building elements act together to direct lightning-generated energy directly into the building. To understand this hazard, let’s look at what is commonly called a lightning rod.

There are three requirements for protection against a direct lightning strike that call for the use of a lightning rod. First, a conductor must be established that deliberately attracts the step leader. In other words, we want the lightning rod to be the tallest most-prominent point of the building, and not the piping, e.g., of the building fire-sprinkler system.

Next, a low-impedance path to ground must be provided. What we mean here is that the lightning rod must present the lowest impedance path to earth ground. Remember, earth is the return path in the lightning circuit. We don’t want a satellite dish to be the low-impedance path—not unless we want to direct massive amounts of current into sensitive equipment.

Lastly, the connection to the earth itself must be of low resistance. There are a variety of ways to accomplish this, with the most common being a copper rod driven into the ground. However, this should be dictated by practices which are common to your area, and which take into account the soil content, moisture, and other considerations that are beyond the scope of this guide.

If care is not taken to provide a low-impedance path to ground, an action known as side flash may occur (ill. 4-3). This can be prevented by providing two parallel ground conductors. At lightning frequencies, a ground conductor looks like an inductor. When we place inductors in parallel, the inductive reactance is greatly reduced. For this reason, two, and sometimes more, ground conductors are used.

ill. 4-3. Lightning may find a lower-impedance path by arcing to another conductor. This is called a side flash.

The Electrical Effects of Lightning

Our primary concerns are the voltage transient that might be created by lightning and what we must do to prevent this energy from reaching the computer. There are a number of ways this might happen. Of course, a main worry is that lightning may strike the wire or wires feeding our building directly. If this should happen, enormous energy would appear on the line feeding the building. Some attempt has been made to measure this voltage. The highest lightning voltage recorded on a transmission line was 5 million volts. If that sounds impressive, consider this: The strike point was 4 miles up the line from where the measurement was made. Since the peak voltage was reached in less than 2 microseconds, data indicated the voltage rose on the order of 10,000,000 volts per microsecond.

Another way that these impulses might reach the computer is via ground loops. When lightning seeks a path to ground, the potential between different ground reference points may be substantial. The resultant loop current can easily damage equipment. We will cover the topic of grounding and ground loops in detail later.

Induction is yet another way that lightning-generated energy might be injected into critical circuits. As large currents flow through conductors, the expanding magnetic field causes induced current to flow in nearby conductors that could then carry damaging impulses to computers. Even lightning discharges between clouds can induce transient-voltage impulses of several thousand volts in conductors that are several miles away.

We discussed side flashing earlier. This arcing can couple energy into data lines as well as power lines. It might seem strange that lightning would flash over to a current-carrying conductor. But, the voltages and currents that result from a lightning stroke are so large that lightning sees a 120-volt power line as ground potential.

Conductors can also act like antennas for radiated lightning energy and , thereby, can feed transient overvoltages into the power line. All these voltages have the capability of breaking down insulation, bridging the conductors of printed-circuit boards, and shorting the turns of transformers. Underground cable is a favorite path for lightning.

Obviously, the effects of a direct hit would be catastrophic to a PC. But this event would be infrequent. The effects of transient voltages caused by lightning many miles away are far more frequent. Ground-potential differences between points along the distribution system create surges in the power line. The firing of lightning arresters can create impulses as well as the inrush transient that follows momentary sags. Even remote strikes create huge fast-changing magnetic fields that have been known to induce impulses of several hundred volts into data links. All of this undesirable energy can damage equipment, confuse processors, and create disk errors even when the power is turned off.

So what are we to do? We will talk about the use of lightning arresters later in the section on “transient suppressors.” For now, let us make the point that the potential damage from lightning is so great and the energy released is so powerful, that at or near the computer is the last place in the world to try and deal with it.

As alternating current increases in frequency, the current has a tendency to travel along the exterior surface of the conductor. This is called skin effect. Lightning is a mixture of very-high-frequency rapidly changing current. The physics of this is such that lightning tends to travel around, beside, and on top of the path it's following. Thinking that we can actually harness this energy and put it through some circuit in a box and , therefore, protect equipment from it, defies logic. There are places where we can deal with lightning effectively—they are far away from the computer!

Static

Static is short for “electrostatic discharge,” or ESD. As we stated earlier, lightning is a form of ESD. But what we more commonly think of as static is that zap we get when we grab something metallic after walking across a carpet in the winter time. Most of us have a vague notion that the static charge came from movement and some kind of fuzzy material like the carpet. Many of us remember high-school experiments where a glass rod was rubbed with a certain cloth. The result was a charge that was built up on the rod and which could be discharged somewhere else.

In fact, this is called triboelectricity. The term comes from the Greek word tribo or tribos, meaning the action of rubbing. Triboelectricity, then, is an electrical charge that is built up by friction. The action of rubbing your shoes against a carpet creates a charge that is capacitively coupled to you through the soles of the shoes. You then carry this charge with you until it's either dissipated by discharging into the air or back to the floor, or, more likely, discharged through a low-impedance path much in the same way as lightning is discharged, like to a door knob or another person. In fact, the discharge may be so large that a spark may be visible for an instant.

It works like this. The two basic charged components of the atom are the proton and the electron. An electron carries a negative charge while the proton carries a positive charge. The proton is a permanent part of the atom in solid matter and will move with that matter. The electron is free to move from one atom to another as long as some energy is expended to move it. This energy might be thermal, chemical, or mechanical.

Static electricity is generated when two unlike materials are rubbed together and then separated. When the materials are in contact, electrons move from one material, interfacing with electrons from the other material. Meanwhile, the associated protons must stay behind. An attractive force is built up between the two materials as equilibrium is established. As the materials are separated, stray electrons produce a negative charge on the surface causing a potential difference between the two surfaces.

If the two materials are conductive, this potential difference will equalize before the materials are completely separated. In good conductors, the electrons can return easily during separation. In poor conductors, the electrons become trapped and can't return. They remain and the charge remains. When the electrons come in close proximity with a good conductor, they move toward a discharge path. In most instances, the charge is sufficient to ionize the air that insulates it from its low-impedance path and the charge arcs across.

The speed of separation, the characteristics of the material, and the area of contact all have an effect on the possible buildup of a static charge. Perhaps the largest effect on the magnitude of the ESD is the relative humidity. Moist air is a much better conductor than dry air. Walking across the carpet in 10% - 20% humidity might build up a charge of 30,000 volts.

The identical movement in 85%—90% humidity might build up a charge of less than 1000 volts. In low humidity, just raising one arm can generate a charge of 100 volts.

Gas, steam, or air passing through an opening can produce static. Chutes, belt drives, or conveyors can be a source of static. These charges can then be passed onto the operators who might then pass these charges to the PC they are operating.

What does this mean for PCs? As a static charge discharges into a conductor, peak voltage and current occur extremely fast—in picoseconds or nanoseconds. it's this rise time more than anything else that causes concern. Very fast rise times of high magnitudes, like 30,000 volts, produce an expanding magnetic field that can induce undesired current flow in nearby circuits. it's not unusual for a trace on a logic card to pick up 3 volts or more from an ESD traveling through a conductor 30 centimeters away. This undesired signal might be interpreted as data, causing soft errors in processing. These extraneous bits cause memory errors, pollute program results, and may cause an action to be taken, like the false starting of a disk motor. Suddenly you’re in hyperspace—lost forever.

Damage to hardware can also occur. ESD has the capability to punch micron-size holes in the oxide of IC gates and cause junction or metalization burnout. Even if the ESD does not blow a chip, it may have stressed it to a point where it will have its life significantly shortened. A potential of 25 volts for 100 nanoseconds will destroy some memory chips and microprocessors.

The rise time of the ESD is affected by the position of a finger when it contacts a metal chassis. If the side of the finger touches, the rise time is significantly slower than if the tip of the finger touches. Fingernails create an even faster rise time, and a sharp instrument or tool is even faster. In an office of a dozen people, several hundred discharges of a potentially damaging magnitude may occur each day. Ironically, with the exception of JFETS and bipolar transistors, all semiconductors fail at 3 kilovolts or less. Our ability to detect or feel the static discharge in our own bodies starts somewhere around 6 kilovolts.

All of the preceding factors have been under observation by professional DP people for many years. Grounding wrist straps, special work surfaces, and protective packaging have been used by service personnel for many years. Mainframe operators have long been accustomed to using humidified air conditioning, ionizers, conductive floors and /or footwear, special clothing, and topical antistatic sprays. These techniques, however, are rarely applied in the PC environment. There are products on the market that plug into the safety ground conductor of the receptacle where the PC is connected. The operator is supposed to touch this device first before touching the computer, thereby dissipating any static charge. The principle of this idea is a good one in those parts of the country that experience chronic low humidity. The habit of touching some other metal surface before touching the PC is a good one. However, whenever possible, a far better approach is to encourage the use of carpeting materials that have a lower propensity for static buildup, as well as the use of humidification, sprays, and antistatic mats. If none of these precautions are taken, the PC is at the mercy of those charges carried by the operator.

Noise

Now, our discussion gets a little more relevant to the PC. Noise, as we described earlier, is a signal appearing in a circuit that is other than the desired signal. This may be the result of lightning or of ESD. More often, it's the result of other events happening in the environment. Certainly, lightning creates the kind of high-magnitude noise that we must ensure does not reach our computer.

As pointed out in the section on power quality, any number of things, both inside the building as well as outside, will produce unwanted noise impulses, along with surges, sags, and outages. ill. 4-4 shows how noise might appear on a sine wave. We have turned on the cursors of the oscilloscope to show the magnitude of the sine wave and the relative magnitude of the noise impulse. ill. 4-5 shows this same noise impulse, but it's photographed through a special filter that eliminates the sine-wave waveform and shows only the noise that exists above about 5 kHz.

It is not unusual for this magnitude of noise to exist on a sine wave (off and on) throughout the day. Obviously, we want only the sine wave to enter the computer. The noise is unwanted interference. Noise of a low magnitude will not bother our PC, but noise or impulses of a high magnitude can interfere with data processing or can damage internal components. Here, we are using the term noise interchangeably with the terms impulse, spike, and voltage transient. But, at some level, we might not consider noise an impulse or voltage transient. On the one hand, we have the constant source of low-level interference from a spinning motor with wearing brushes. We might call this noise. On the other hand, we have a huge voltage transient or impulse that is caused by lightning. We might call this a spike. The difference between the two is subjective, but their behavior in signal circuits is the same. Let’s see how this works.

ill. 4.4. A 60-Hz sine wave with noise riding on it.

Noise in Circuits

ill. 4-6 shows a simplified circuit diagram. Let’s go through the circuit and identify each part of the circuit and explain what it's doing there. First we have our noise generation, which appears across terminals A and B. In the diagram, this looks like an intentional connection to a noise source. it's not. Noise enters a circuit in many different ways. What we are trying to do is to reduce this circuit diagram to something very simple to see something that is really the heart of this guide. The noise generator might be lightning impulses coming down a power line, or a noisy piece of equipment, or an infinite number of noise sources such as are encountered in the environment everyday.

ill. 4-6. A simplified view of noise in a circuit.

Take a look at L1 and L2. Unwanted noise in a circuit is going to have a high frequency, much higher that 60 Hz. As frequency goes up, the value of the inductance of the wires between components goes up. In other words, at 10 MHz, the noise signal may see a straight piece of wire as an inductive element of high impedance. We call this lead inductance. The value of R is a combination of the simple, purely resistive nature of the wire and the other components of the circuit.

Capacitance C, in the diagram of ill. 4-6, is perhaps the most significant circuit element of them all. We are showing C as stray capacitance. What does that mean? At some noise frequency, two wires (or any other circuit elements) placed close together might take on a value of capacitance such that a noise impulse might see it as a conductive path. For instance, two tracings on a printed-circuit board might, at some frequency, have a capacitive value that completes a circuit between the two. These are unplanned and unexpected paths where high-frequency signals can pass with little impedance. This is called stray capacitance.

We also have a current, I, in the circuit. This is the current flow in the circuit. We have shown that current as flowing just one way, but the current could be flowing both ways if the noise signal alternates. The important point is that current flows in the circuit. Voltage E, then, is the voltage or electrical potential that the noise presents across the terminals of the circuit.

Now, we come to an important point. This is the circuit that the noise signal sees. The actual components that make up this circuit may be nothing like what is shown in our diagram, but because of the frequency of the noise, those circuit elements—ICs, boards, coils, wires, connectors, etc.—take on inductive and capacitive values.

Frequency

Let’s clarify frequency. ill. 4-7A shows an alternating noise signal. Notice that this is the kind of noise waveform that we might expect. It looks like noise, alternating rapidly up and down. Its frequency is dependent on the number of oscillations that occur over a given span of time. (Line voltage has 60 oscillations per second, so we call it 60-Hz, or 60-cycle, voltage.) If a noise signal oscillated 1,000,000 times a second, we would call it 1-MHz (megahertz) noise. Our signal in ill. 4-7A shows a varying signal that may oscillate over a broad spectrum of frequencies.

ill. 4.7. Two different concepts of frequency. (A) An alternating noise signal. (B) A noise impulse.

The noise impulse in ill. 4-7B shows something entirely different. This is a single noise impulse. It might have been sliced out of the waveform shown in ill. 4-7A. There is no oscillation occurring here. However, the face of the slope of the impulse is rapidly changing. Near the bottom of the slope, the rate of change is slow, while at the top of the slope the rate of change is extremely fast. If we wish, we can study the rate of change of the slope of a curve by using higher mathematics. But we don’t need to do this to understand that the rate of change is synonymous with the term frequency.

If our noise impulse in ill. 4-7B appeared across terminals A and B in ill. 4-6, then capacitance C would appear to be a low-impedance path at some frequency which allows current to flow. Let’s see how this works by simplifying our circuit even more in ill. 4-8.

ill. 4.8. Simplified noise circuits. (A) A simple Circuit. (B) Resistance in parallel.

Here we have two circuits. Let’s look at the circuit in ill. 4-8A first. At some frequency, a noise-signal potential appears as voltage E across terminals A and B. The very fact the this potential is present causes current to flow through resistor R1. The amount of current present depends on the value of R1, according to Ohm’s law. (We might place a battery or a 60-Hz AC current across terminals A and B, and current would also flow in this simple circuit.) The point we are trying to make is that, at some frequency, a circuit like the one in ill. 4-8A may suddenly appear when a noise signal presents itself to that latent noise circuit. This happens instantaneously. At some other lower frequency, the circuit quite literally might not exist as a path for current to flow, and our diagram would no longer be a valid representation.

What we are saying is that the components of a circuit designed to direct the flow of electricity in a certain way at one frequency may be seen as a completely different electrical path to a signal of a much higher frequency. We might never dream that a data cable can suddenly turn into a capacitor at some very high frequency, but it can. A power supply is designed to do one thing at 60 Hz, but, at 10 MHz, it looks entirely different to a noise signal. The physical characteristics of a material displays different properties as the frequency of a signal is increased. Wires become inductors, capacitors become wires, and coils disappear.

The circuit in ill. 4-8B shows us that the noise current splits between the values of R1 and R2. The amount of current that flows in each is determined by their ohmic values. If R1 has a low ohmic value relative to R2, then more current will flow in R1.

This is a profound truth that underlies much of the discussion in this guide. When we want to keep current away from something, we must give it an alternate lower-impedance path, so the current will flow through the alternate path. Inadvertently creating undesired signal paths is every engineer’s night mare, and they are not always easy to avoid.

Normal-Mode Noise

Noise as it appears on power lines comes in two varieties: normal mode and common mode. As shown in ill. 4-9, normal-mode noise is noise that can be measured between the phase wire (or hot wire) and the neutral wire. Sometimes, the phase wire is called the “live” side of the line.

The root cause for normal-mode noise is usually heavy step load changes of the variety that we talked about earlier. Turning large loads off or on, or shunting utility power-factor correcting capacitors across the line, creates a ringing or oscillatory impulse. ill. 4-10 shows the impulse created by a spinning motor or other such device, since we see the impulse appearing every one half cycle or 8.34 milliseconds. ill. 4-11 shows this impulse on a shorter time base so we can see its typical waveshape.

ill. 4.9. Normal-mode noise appears between the phase wire and the neutral wire.

ill. 4-10. A normal-mode impulse created by a spinning load.

Common-Mode Noise

Common-mode noise is a noise impulse that can be measured between the ground wire and the neutral wire, and at the same time is measured between the ground wire and the phase wire. That is why it's called common-mode noise, since the impulse is common to both lines with respect to ground. This is shown in ill. 4-12.

Common-mode noise is caused by such things as lightning and the tripping and re-closures of utility breakers. it's also caused by poor grounding techniques, ground faults, radio transmitters, time clocks, and machine tools. In addition, normal-mode noise traveling along a phase wire can induce noise in the neutral wire that can be detected as common-mode noise (ill. 4-13). At times, this issue gets a little confusing. The important thing to remember about common-mode noise is that it's measured between the neutral and ground wires, even though the impulse is technically on both the live and neutral lines.

ill. 4-11. The impulse of ill. 4-10 enlarged for a better view.

ill. 4-12. Common-mode noise appears simultaneously between the phase wire and the ground wire, and between the neutral wire and the ground wire.

Noise about Noise

Noise is electricity just like the 60-Hz sine wave. Electricity, even one positive-going impulse, must follow the laws of electricity and flow in a circuit. What flows to a point also flows away from that point. Electrical energy doesn’t just magically appear and disappear. The concept that electricity, even noise, moves in circuits or loops is so basic that we might forget it. We will discuss this more in Section 5, when we discuss grounding.

The point we made in ill. 4-6, as well as in ill. 4-8, is profoundly simple when we think about noise; it helps us understand what might at first appear to be complex. If a voltage is applied across an impedance, current will flow. If there is infinite impedance, no current will flow. If there is no potential across the impedance, no current will flow. The relationship we learned in an earlier section serves us well when thinking about noise: E = IR. Clearly, if the impedance (Z) is 0, then E = 0. If the impedance is some extremely high value, then I will be some minute value, If a noise potential, E, appears across an impedance that is not extremely high, then an increasing current, I, will flow as the impedance decreases. Note: Normally, the letter “Z” is substituted for the letter “R” when we talk about impedance instead of a pure resistance.

ill. 4-13. These two traces show normal-mode impulses being induced in the wiring and converted to common-mode noise.

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