Power Quality and Your Computer
Copper Applications in Electrical Area
Did you hear the one about the car that was designed to run on a PC? Guaranteed to crash at least once a day.
Just kidding. But since you use your computer regularly, you're probably used to the occasional crashes, hang-ups, etc. that come with the territory. It happens, you re-boot, mutter a few words in the general direction of Redmond, Washington, and off you go again until the next glitch comes along.
It's usually a software problem, maybe accompanied by an ominous message like "YOUR SYSTEM HAS PERFORMED AN ILLEGAL OPERATION " or "INSUFFICIENT MEMORY TO RUN THIS APPLICATION". But sometimes it's not the software's fault. It could be a question of inadequate power quality. Power quality? Since when does power have "quality"? It's either on or it's off, right? Well, yes and no. Back in the dark ages BC (before computers), it usually was sufficient that the power turned on whenever you flipped the switch. And it usually did, except during summertime brownouts when too many people turned on their air conditioners at the same time or after the occasional blackouts when lightning (or some motorist) struck a pole. Other than that, electric power quality was unremarkably adequate.
That's no longer the case, because the way we use electric power has changed significantly. Just a few years ago, most of the electrical uses in our homes (and in our factories and offices, for that matter) involved lighting, electric motors and a few transformers. We still need those things today, but we also increasingly rely on what we might call "electronic" applications of electric power. Electronic applications include not just computers and their peripherals, but all of the appliances, tools, instruments, thermostats, burglar alarms, audio/video equipment, copiers, fax machines and other gadgets that contain some form of microchip or "logic circuits". It's those electronic gadgets that are so finicky about power quality. The Institute of Electrical and Electronic Engineers (IEEE) even has a name for them; they call them "Sensitive Electronic Equipment," and have published special recommended practices to deal with them correctly. These things simply can't tolerate the sort of lapses in power quality that light bulbs and electric motors routinely shrug off.
Since more and more sensitive electronic equipment is finding its way into our homes - especially as home offices become more popular - we've prepared this article to explain the concept of power quality and how it can affect you. We'll also explain how the proper use of copper wiring can ensure that your sensitive electronic equipment functions properly. And, if you're considering building a new home or thinking about re-wiring your current one, we'll mention a few pointers that will "build in" good power quality from the start.
Continuity, Voltage and Frequency/Waveform
Because of the various ways we use electricity, power quality means different things to different people. Still, there are three basic attributes that we can use to "rate" power quality, at least qualitatively, from the point of view of the homeowner or home office user. These attributes are:
- The continuity with which the power is supplied,
- The voltage seen at the point of use, and
- The power's frequency and waveform. (We'll define these terms in a moment.)
Continuity, or how continuous the supply is, means more than that the power should simply turn on and stay on whenever you flip the switch. It means that there can be no gaps, even momentary ones, when power is flowing. That's important because sensitive electronic equipment requires almost total continuity, literally down to the millisecond. For example, you've probably experienced something like this:
- There's a thunderstorm brewing. Miles away, lightning strikes your utility's power grid. Your lights flicker off and on and electric motors hesitate. But just as quickly, everything returns to normal, except that your computer has re-booted, the satellite TV is asking you to reprogram its available channels, the thermostat has to be reset and all your digital clocks are blinking "88:88".
Or maybe you've seen this situation:
- Your computer is plugged into a wall outlet that also feeds your monitor, printer, scanner, zip drive, copier and maybe a fax machine; a typical home office setup. You hit the "Print" button and everything shuts down because you've tripped a circuit breaker. (Did you save your work?)
The first case wasn't your fault, the second one was. Either way, you've experienced one form of poor power quality - an interruption - and your sensitive electronic equipment couldn't cope. What can you do about continuity problems? The quick-and-dirty solution is to install an uninterruptible power supply, popularly known as a UPS, between your equipment and the wall outlet. A UPS is essentially a battery that is continuously charged by line power, its ac voltage suitably stepped down and rectified to dc. The battery's dc power output is re-converted in the UPS back to ac power by a device called an inverter, whose "synthetic" ac power is then output to your equipment. When line power is interrupted, the battery acts as a reservoir, providing a few minutes to a few hours of power so your equipment can ride out the outage. There are many types and sizes of UPS units. The better ones have built-in surge protection devices. They're cheap insurance: UPS units that are adequate for home offices retail for about $100-$200. If your power grid has frequent outages, consider a conservatively rated (long runtime) UPS.
On the other hand, if you've overloaded the circuit that feeds your computer, you should think about installing a UPS and distributing your electrical loads by adding additional circuits. This is the correct long-term solution to frequent circuit breaker trips. There are several benefits to adding more circuits, but maintaining continuous power is reason enough to give the idea some thought. The cost of the few additional yards of copper wire and a few hours of an electrician's time can be inconsequential compared with the cost of lost data.
Voltage variations are another common source of problems to home computers and other sensitive electronic equipment. Voltage variations can be positive (higher than normal) or negative (lower). Variations can be huge, reaching thousands of volts, but even small ones can cause problems. Most computers can withstand variations of ï¿½10% (ï¿½12 volts on a 120-V circuit), although they can detect, and may be affected by, smaller variations. The human eye can distinguish a lamp flicker caused by a ï¿½1.5v change in voltage, so if you can see the effect of a voltage change, chances are good that your computer can feel it. An important point to remember is that the computer manufacturers base their voltage range assuming a true sine wave voltage waveform. If the voltage waveform is flat topped or distorted, your computer may not operate within the stated voltage range. We'll explain sine wave distortion in a moment. Let's get back to that overloaded branch circuit we described earlier. This time, we won't quite trip the circuit breaker.
- Everything's running well. Your spouse turns on the dishwasher. Your desk lamp dims but doesn't go out. Still, your computer hangs.
- You decide to move your home office to the other end of the house because it's quieter there. You don't add any equipment; you just move what you have. Your equipment is now more than 50 feet (15 m) from the electrical service entrance panel. You begin to notice that your computer doesn't work as reliably as it used to, and you start having problems with your monitor.
The problem in these cases may be low voltage. In the first case: Switching on a high-startup-current appliance (usually one containing a powerful motor) will momentarily reduce line voltage in that branch circuit. Everything else connected to that circuit would feel the reduction. In the second case: Your standard 20-amp circuit may have reliable 120v at the electrical panel, but after running through 50 ft of wire, what your sensitive electronic equipment sees at the outlet is a voltage that may be reduced by as much as 5%. This voltage reduction (or "delta v") can have several effects:
- First, low voltage can cause the dc power supplies in your equipment (all electronic equipment has them) to run hotter than normal.
Here we have to introduce a little algebra to explain our point. The power (P) these power supplies draw is equal to the line voltage (E) at which they operate (nominally 120v) times the current (I) they draw, or P=EI. The power drawn is constant, fixed by the needs of the equipment; so any decrease in voltage must be compensated by a proportional increase in current. Now, the amount of heat dissipated by the power supply (specifically, by resistive (R) elements such as resistors, wires, etc.) is proportional to the square of the current flow, P=I2R. Therefore, the end result of a drop in voltage at the wall outlet is a rise in the temperature in the equipment. At best, this excess heat is represents wasted energy; at worst, it can shorten the equipment's life.
- Second, low voltage can cause components to fail because the increased current exceeds the components' ratings.
Power supplies in computers are usually more powerful than they need to be because manufacturers understand that owners often add equipment (additional hard drives, tape backups, etc.) after the computer has been placed in service. As an incidental benefit, such oversized power supplies can accommodate the extra current drawn when line voltage drops. Power supplies in monitors, on the other hand, are sized just large enough for the existing load, there being no need to expect any later additions. High currents caused by low line voltage can damage such power supplies.
- Finally, a drop in line voltage may interfere with your computer's operation, particularly if the computer is networked. The power cable that supplies your outlet contains three conductors, a phase (or "hot") wire, which usually has black, red or blue insulation; a neutral wire, usually white, and a ground wire, conventionally green. If your computer's plug has three prongs, the round "third" one is the ground.
When we speak of voltage in the conventional sense, we refer to the voltage between phase and neutral conductors. However, a voltage can also exist between the neutral and ground conductors at distances well removed from your house's service entrance panel (it has to be zero at the panel because that's where the neutral and ground conductors are joined). This neutral-to-ground voltage is normally zero or close to it, but when the phase-to-neutral voltage drops, the neutral-ground voltage rises. In fact, it rises by exactly one-half as much as the drop in phase-to-neutral voltage, e.g., reducing the phase voltage from 120v to 110v incurs a 5-v increase between the neutral and ground conductors.
Most desktop computers can tolerate as much as ï¿½10% variation in phase-to-neutral voltage, but they are quite sensitive to small changes in neutral-to-ground voltage. This is particularly true for newer computers whose logic circuitry may operate at less than one volt. (Laptop computers aren't as sensitive to this situation because they generally don't have an external ground connection.)
Moral of the story: play it safe, play it smart, use a separate circuit, or add one, for your sensitive electronic equipment.
Positive voltage variations can be even more troubling than negative ones. If powerful enough, they can destroy components in sensitive electronic equipment. Such positive variations, or "transient voltage surges" as they're called, can arise outside your home. Lightning striking power lines is a frequent cause, as is load switching (re-routing power around the grid) by your utility. Voltage surges can also be caused by equipment in your home. Refrigerator motors, air conditioners, vacuum cleaners and other electrical loads can generate voltage surges and electrical noise.
The best defense against voltage surges is to install a transient voltage surge suppressor (TVSS) between your equipment and the wall outlet. As with uninterruptible power supplies, surge suppressors are cheap and widely available in a variety of styles and sizes. You may already have one: it's the "power wand" containing about half a dozen outlets into which you connect you computer, monitor and other equipment. Check the label to see if it has TVSS protection. If you need to purchase a TVSS make sure it is UL 1449 listed. This assures the product has been tested and meets industry standards.
No amount of additional copper house wiring will protect you against voltage surges, although a good lightning protection and grounding system will isolate your sensitive electronic equipment - along with the rest of your home - from nearby lightning strikes. We'll talk about grounding and lightning protection in a future article.
Frequency & Waveform
In North America, household electric current and voltage alternate (it's the "a" in ac) or switch back and forth between positive and negative poles at a frequency of exactly 60 alternations each second. Engineers call that the fundamental frequency; it's usually expressed as 60 cycles per second (cps) or 60 Hertz (Hz). In most other parts of the world, the fundamental frequency is 50 Hz.
Alternating voltage/current alternations take the form of a sine wave. That is, if you could see the current changing over one ac cycle (which takes 1/60th of a second or 16.7 milliseconds) the shape of the alternations, the sinusoidal waveform, would look like this:
Voltage and current begin each cycle at zero (the horizontal line), rise to a maximum in one-quarter of a cycle, fall back to zero at the half-cycle point, then reverse polarity and fall to a maximum negative value at the 3/4 cycle point before rising to zero again. All of this repeats every sixtieth of a second. If you've ever touched a live electric line, the vibrating shock you feel is this sixty-time-per-second voltage reversal.
In ordinary house current, 120 volts is what engineers call the effective voltage, which is actually a sort of average (the root mean square value) of the varying voltage depicted by the sine wave. The actual peak voltage in a 120-V circuit can be as high as nearly 170V. Except for simple things like incandescent light bulbs, most of the electrical and electronic devices we use today are designed for ac power that follows the sine wave curve. To these devices, the right voltage plus a perfect sinusoidal waveform add up to ideal power quality. By the same token, anything that disturbs either the power's frequency or its waveform detracts from power quality, and in so doing, reduces the ability of electrical devices - especially sensitive electronic equipment - to operate properly.
What can disturb the frequency? Not much, so far as power supplied by the utility is concerned. In fact, electric utilities go to great lengths to make sure that their generators operate at precisely 60Hz and that they're all synchronized as closely as possible. When the current produced by a power station in Bangor, Maine is at the exact maximum point of the sine wave, current coming out of a power station in Los Angeles - and out of your wall outlet - is there, too.
Most frequency disturbances actually originate inside buildings. Ironically, it is mainly the sensitive electronic equipment that is at fault. Remember those dc power supplies we keep mentioning, the ones that power most of our "electronic" equipment? These days, most such power supplies are of a type (called "switched mode power supplies") that draws current in sharp pulses rather than in a smoothly flowing sine wave. As they do so, they echo those pulses back into the power line, where they are superimposed on the normal sine-wave current flowing there. Other common household sources of these erratic disturbances include:
- dirty switches or relays that arc when they make or break contact;
- worn dc electric motors (you know you have a problem when you can see sparks in the commutator);
- portable arc welding machines;
- certain UV lights, and
- many electronic fluorescent light ballasts.
Add them all up and the result is a messy waveform circulating in your house current.
Normally, waveform disturbances are not a problem in a home-office situation. Really serious problems arise in offices and commercial buildings where there may be hundreds of computers, electronic lighting ballasts and similar equipment on the same floor. The power disturbances generated by all those switched mode power supplies can add up to wreak havoc with computer reliability, not to mention fire hazards and other problems.
The number of computers and peripherals installed in a home office is far to small to cause that level of disturbance. On the other hand, there is a real possibility that by concentrating all of your sensitive electronics on one branch circuit, you can degrade power quality enough to provoke the occasional electronic hiccup and crash your computer.
The main cause of waveform distortion in modern "electronics"-loaded circuits, including the ones in your home office, is an electrical phenomenon known as harmonics. All commercial power is based on a pure sine wave waveform (remember physics class?). The utility generates 60-Hz sine wave power and transmits it to your home. Your home wiring distributes the sine wave power to your wall outlet and your computer power supply is designed to run on it. The assumption is that the power maintains the correct sine wave shape from generation to utilization. This assumption is not always correct. It is now understood that the very power supply that is in your laser printer and computer can cause a distortion of the voltage waveform, superimposing electrical jiggles that can be expressed as multiples of the standard 60-Hz frequency, i.e., 120Hz, 180Hz, 240Hzï¿½etc. These jiggles are called harmonics.
The amount of distortion is measured in total harmonic distortion, or THD. The IEEE has issued guidelines for recommended levels of THD. Generally anything over 5% voltage THD is unacceptable.
Harmonics have created serious problems in offices that contain a high density of computer equipment. We routinely find a high level of voltage THD in office buildings. It far less likely that you would have a harmonic problem in your home. However, if you have undersized wiring or overloaded circuits, you could have this problem.
What problems do harmonics cause? The most common problem is a flat-topping of the voltage waveform. This causes a reduction of the peak voltage level, which in turn can starve your computer's power supply of energy. To put it in simple terms, running your computer on a distorted 140-volt (instead of the expected 170-v) peak voltage is like running it on about 95 volts AC. You may not see the problem, but your computer does! It takes special equipment to measure harmonics so the problem may very likely go unnoticed.
Thinking about Remodeling? Think Extra Circuits.
If you're considering building a new home or thinking about remodeling your current one, you should be aware that there a many advantages to bringing your wiring system up to current practices. This is especially true if you plan to add more sensitive electronic equipment (which is likely), maintain a home office or create a local area network (LAN) to link your home computers.
The National Electric Codeï¿½ (NEC) governs the way we wire our homes, offices and commercial buildings. The NEC permits up to 13 outlets on a 20-amp branch circuit, the type commonly found in homes. This limitation is based on safety: limiting the number of outlets effectively limits the total load (total current flow) on the branch circuit and thereby eliminating overloading and circuit breaker tripping. Recommended practices for home offices with a number of technology systems include:
- Branch circuits that feed sensitive electronic equipment should be kept separate from branch circuits that feed ordinary loads, such as lighting and motor-driven equipment. If possible, use separate circuits for your laser printer and the rest of your computer equipment.
- In older homes with two-prong, non-grounded outlets, upgrade to three-prong grounded type circuits. The only way to correctly do this is to run a new copper cable from the main service to the office area. Remember that a grounded circuit is required for protection of your computer from static electricity and power surges. Without a grounded circuit your modem may become the ground source.
- Have an electrician check your main service for proper grounding. In the past the metal water main was used to ground the service, but now many communities are using plastic water piping. If your home does not have a ground rod, install a 10-foot long ground copper clad rod to supplement your home's grounding. This is cheap insurance and enhances safety.
- Make sure that your home's telephone and cable (CATV) are grounded and bonded to the main electrical service ground. Without this bonding, lightning and power surges can circulate through your computer system. In fact your computer may be the one piece of equipment in your home that combines power, telephone (modem) and cable (cable modem or video), thereby becoming a path for any type of lightning surge.
- Have an electrician install a main service surge protector (TVSS) ahead of your main service entrance electrical panel. This will protect your home from large power surges. A basic unit will cost about $ 150 installed. Make certain that this TVSS is properly grounded or it won't work when it has to.
- Use TVSS protection between the wall outlet and your computer. Use a unit that protects the power, telephone and cable service.
- For additional power protection consider installing an uninterruptible power system (UPS). Choose a system large enough for your equipment power needs.
- To minimize static electricity consider a hard surface floor covering or a static dissipative mat. Make sure all static mats are grounded to the power outlet. There are also some anti-static sprays that you can treat carpets, chairs, upholstery and desk surfaces.
For the most part, these recommendations will not involve major expenses in homes that already conform to practices permitted under the National Electrical Code. They are especially inexpensive when you compare their cost to the cost of losing your sensitive electronic equipment and all of the data you've stored in it.
You can do something about power quality. Think about it the next time your PC starts acting up.
The company has specialized in the field of power quality, power monitoring, harmonics, grounding, site surveys, and testing for over 18 years. Our in-the-field experience tells us that the rapid evolution of technology has brought forth a multitude of power and grounding problems widely affecting every industry segment. In light of these growing needs, we have developed the following mission statement.
Our mission is to assist companies in "Making Technology Workï¿½" by insuring that the client's infrastructure will support their technology needs.
The company is currently focused in the following areas:
1. Providing consulting and technical services to identify, diagnose, and correct power quality problems.
2. Providing consulting services for the design/build of technical facilities such as call centers, computer rooms, and channel assembly facilities. We work with the client through the entire project from initial concept to design then implementation and acceptance.
3. Providing educational services through power quality and grounding seminars and technical papers and articles.
Industry segments have included financial, insurance, health care, educational, manufacturing, utility, communications, broadcasting, and government.
If you have a Case History to share or would like to receive additional information, contact:
National Program Manager, Electrical
Phone: (212) 251-7206
Fax: (212) 251-7234
Also in this Issue:
- Power Quality and Your Computer
- Property Data on The Copper Page
- Summary of 1999 Detroit International Auto Show and Potential Changes in Copper Content