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Suppress Those Surges
Jon Titus, Contributing Editor -- Design News, February 4, 2008
In addition to protecting exterior sensor and communication lines from transient pulses caused by nearby lightning strikes, you should protect inside equipment from transients or surges that can travel through power lines. Standard 12- and 14-gauge wires used for 120V ac circuits have a guaranteed minimum breakdown voltage of 600V. A few thousand volts on the wires from a nearby lightning strike will cause arcing between the conductors that limits most surges to a 6,000V pulse that can last from nanoseconds to microseconds.
Many companies sell plug-in surge suppressors, or more formally, transient voltage surge suppressors (TVSSs) that you can connect between a power outlet and sensitive equipment. “The Underwriters Laboratory UL 1449 rates performance in the form of 'TVSS ratings' and the best rating is a maximum 330V let-through voltage,” says Patrick Donovan, product line manager at American Power Conversion. “So buyers should ensure their surge suppressor meets that standard. Some suppressors offer a lower let-through voltage.” The TVSS rating applies across each conductor; line to ground, line to neutral and neutral to ground.
The smaller the let-through voltage, the better. This voltage depends on the protective devices — usually metal-oxide varistors or MOVs — a TVSS manufacturer chooses. When a MOV experiences a voltage above its preset limit, it changes from an insulator to a conductor in a nanosecond or two. Manufacturers often use other suppression components, too, to lower the let-through voltage and ensure long-term reliability.
Vendors may highlight joule and ampere specifications. But, according to Donovan, the let-through voltage of a TVSS provides the best measure of protection capability. “People see a joule rating and think the more joules, the better, but that's not always the case,” says Donovan. Neither the IEEE, IEC nor NEMA recommend using joule ratings when you specify or compare surge suppressors because they can provide misleading and conflicting information. According to “Eaton's Guide to Surge Suppression,” a TVSS on a 120V ac circuit could use a 150 or a 175V MOV. Although the 175V MOV could have a higher joule rating, the 150V MOV has a lower let-through voltage and offers better surge protection.
MOVs can ground multiple surges, but sometimes they fail. So, look for TVSSs that include fuses and indicators that alert users to a failure. APC's Donovan noted most power supplies provide some protection against surges and in most cases a TVSS with a 330V rating will work fine. But, if you have equipment in places that experience many lightning strikes, a TVSS with a lower let-through voltage will provide better protection over time. Keep in mind that when an unprotected power supply dies due to a transient surge, it can take other circuits with it. A TVSS provides inexpensive “insurance” for sensitive equipment.
| Author Information |
| Jon Titus, a former designer and chief editor of EDN and Test & Measurement World magazines, remembers when storage scopes used Polaroid instant film to capture transient signals. |
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 This graph shows how let-through voltage varies for circuits with and without a surge-suppression device. |
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