Home Power Quality

Despite your electric utility's best efforts, electricity supplied to your home may occasionally have power quality problems, such as voltage surges, swells and sags, and interruptions. These transient events can be caused by weather, accidents, animals, utility equipment malfunctions, a neighbor's equipment, or even appliances and tools at your own home.

Ideally, alternating current (AC) switches from positive to negative in a smooth, even fashion. Instruments would display this even switching as a smooth sine wave (above left). However, power is seldom perfect as shown in the two drawing to the right. These variations, called transients, had little effect in years past.

Today, modern electronic equipment is very sensitive to power quality problems, which can slowly damage or destroy semiconductor devices, such as microprocessors and dedicated-purpose electronic circuit chips. These chips are the "brains" in many home appliances such as computers and computer peripherals, telephone answering devices, stereos, televisions and VCRs, fax machines, copiers, coffee-makers, bread-makers, washing machines, dishwashers, and power tools. Growing numbers of "intelligent" appliances need to be protected from power quality problems.

The transient voltage surge suppressor (TVSS) is a product designed to limit and divert spikes and surges in the electric power supplied to your sensitive electronic equipment. However, there is often more to protecting sensitive appliances than purchasing a good surge suppressor.

The first section concerns power quality problems which a surge suppressor may not solve or protect against. The second section provides information on what to look for in a good surge suppressor.

Basic Precautions

The best way to protect your equipment from power quality problems is to unplug it! Power line transients are most likely during lightning storms, high winds, wet, heavy snowfalls or freezing rain, or utility service work in the neighborhood. Anything that might cause a power interruption or outage in your neighborhood can also produce power transients and power quality problems. Turn your vulnerable electronic equipment off during these situations if you can.

Your own household appliances can generate damaging transients on your home's electrical system. Keep sensitive electronic equipment plugged into a different circuit than electrical appliances and equipment with motors (inductive loads like refrigerators, freezers, air conditioners, washing machines, dryers, shop tools), and high-amperage appliances (switching loads like portable heaters, electric frying pans, toasters, etc.). If your house wiring is not flexible enough to allow separating inductive and switching loads from sensitive loads, you may have to add a dedicated, isolated ground circuit for electronic equipment only.

Your house wiring may not be supplying power of correct polarity or providing proper grounding. Never plug a 3-prong cord into a 2-prong outlet, or use an adaptor for this purpose. If your house is older than the early 1970's, you may have a 2-wire system that has had 3-prong outlets installed. Your house system ground may have high impedance, illegal neutral-ground bonds, or ground loops that allow unpredictable voltages to "float" on the grounding circuit. These house wiring problems will make your electronic equipment more likely to act strangely or be damaged, and may prevent even the best surge suppressor from protecting your equipment.

Surge Suppressors

A good surge suppressor will have these minimum specifications:

The Underwriters Laboratories (UL) TVSS 1449 listing clearly noted on the package or device. Some outlet strips look like surge suppressors, and the advertising on the box shows them connected to a computer, and they even show a UL label, but they are actually UL listed as an extension cord. Use only UL 1449 listed devices as surge suppressors.

A good quality surge suppressor will have two indicator lights on it. One shows whether the house electrical circuit the suppressor is connected to is properly grounded and polarized, and the second light shows whether the protection circuitry is still functioning. Most surge suppressors will still deliver power after the protection circuitry has been destroyed. You have to check the lights yourself to be sure you still have a surge suppressor, and not just an outlet strip. Be aware that most surge suppressors fail eventually, as a result of repeated "hits" by high energy surges. Some surge suppressors have an audible alarm to notify you that they have been damaged, and the best ones cut off the power through them when they are ready to fail.

Insurance and a lifetime guarantee to cover the replacement cost of your equipment comes as part of the purchase price with most good quality suppressors. The dollar amount of insurance and length of guarantee noted on the package is a good indicator of how much faith the manufacturer places in their product.

If you want to protect a TV, VCR, or cable channel box, or a phone-connected appliance such as an answering machine, fax, or computer modem, you need to protect the cable/antenna wire, or the phone cable going into the appliance also. Buy a surge suppressor that has jacks to plug in these types of connections.

To identify the highest quality surge suppressors, look for these specifications:

The highest Joule rating. A suppressor rated for use with a computer will have at least 750 Joules capacity, and can be rated into the thousands of Joules. This indicates how much energy the suppressor can absorb before passing a transient through to your equipment.

Clamping response time of 1 nanosecond or less. (There are 1000 picoseconds in I nanosecond, so I picosecond is faster than one nanosecond. In fact, 999 picoseconds is faster than one nanosecond.) The faster the surge suppressor responds, the better.

Voltage clamping level of 330 Volts or less. The very best suppressors will perform active tracking of the sine wave and provide 10 to 12 Volts let-through. The lower the amplitude of transient voltages reaching your equipment, the lower the chances of damage are.

Both common-mode and normal-mode protection. Full protection provided hot to ground (H-G), hot to neutral (H-N), neutral to ground (N-G).

Electromagnetic interference (EMI) and radio-frequency interference (RFI) filtering of incoming power.

For maximum protection, look for these specifications:

Keep the cords and cables between the surge suppressor and your electronic equipment shorter than 3 feet, or as short as possible. Lengths of wire act as antennas for undesirable radio-frequency interference, and may suffer damaging induced voltages during lightning storms. For this reason, circuit breaker-type suppressors that install into your home's breaker panel may not be the best stand-alone choice for expensive appliances.

Combining a breaker-suppressor at the panel, and a plug-in type surge suppressor at the appliance offers an excellent combination of protection capabilities. The breaker-type suppressor often has superior energy absorbing capacity, while the plug-in suppressor can react faster and protect against induced transients.

Using a surge suppressor does not guarantee that your sensitive electrical equipment will not be damaged by power quality problems. It simply improves the odds that your equipment will escape such events undamaged while the surge suppressor diverts the extra energy in the supplied power. Do not forget that most surge suppressors will fail eventually. Generally, the more expensive the surge suppressor, the better the protection it offers.


Surge suppressors don't help if the power goes out altogether. For that you'll need an Uninterruptible Power Supply (UPS). These devices connect to critical electronic devices, such as computers, and keep them running long enough to prevent data loss. Generally, UPSs are sized to give 10 to 20 minutes of operation--long enough to save data and shut down the system safely.

A UPS offers the additional advantage of superior surge protection and noise filtering. If you live in a high-risk area for power quality problems, or want to protect very expensive or critical equipment, the UPS is a good choice. Briefly, four critical parameters of UPS performance are sensing time, transfer time, length of backup service, and output quality. Residential UPS systems start at about $150.

Copyright Alan Van Zuuk

Source: Oikos

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