Think secondary when defending against power surges

Aug. 1, 2002
Primary and secondary surge-protection devices team up to provide layers of protection for your sensitive electronic equipment

Primary and secondary surge-protection devices team up to provide layers of protection for your sensitive electronic equipment.

A Belkin Components ( white paper Need for surge and UPS protection, says, "on average nationwide, your home or office will receive over 264 power disturbances per year with certain areas sustaining more than 400 disturbances."

If you thought power protection only concerns professionals in the high-voltage wiring industry, you are wrong. Professionals involved in low-voltage cabling should make themselves aware (see "Power protection: Whose business is it?" April 1998, page 32).

A surge is a short-term increase in voltage, typically lasting at least 1/120 of a second, but it can cause long-term damage. Even smaller surges can cause gradual deterioration of your expensive electronic equipment's internal circuitry. Microprocessors (chips) are common in this equipment and have been known to be sensitive to voltage fluctuations.

Surge-protection devices (see table, page 66) act like an electrical sponge, absorbing excess voltage and preventing most of it from reaching your sensitive equipment. And like a sponge, surge protectors have a limited capacity to absorb. Primary and secondary surge-protection devices must work together to absorb excess voltage.

Lines of defense
Primary surge-protection devices should be able to be hardwired. Hardwired products will have a higher surge rating than secondary surge-protection devices because they are in an environment where larger surges can happen. "Your plug-in (secondary) protectors will see a lot less action than a hardwired device on a panel," says Warwick Beech, an Erico Inc. ( surge protection product manager.

This illustration shows the proper location of primary and secondary protection devices. Your telephone company typically installs primary protection devices at the demarcation point. Primary devices are also installed in campus applications where lines are leaving one building and entering another outside building. Secondary protectors are placed between the building entrance and the electronic equipment.
Click here to enlarge image

"If you are running off of a T-1 line, for example, the telephone company may have primary protection at the demarcation point (building entrance)," explains Jeff Miller, vice president of sales, marketing, and engineering at The Cylix Corp. ( "From a business standpoint, this is where the telephone company hands off the responsibility for the wires to the building owner."

The primary protector will not stop surge events once they penetrate the building. Adding a secondary protector to the system is the only way to defend against these surge events.

Secondary surge-protection devices are designed to protect telecommunications and networking equipment, including telephones, computers, printers, cables, modems, and fax machines. Manufacturers of these devices are aware that a customer will have a variety of products that need to be protected, so a broad product line is often available. "You can't walk into your customer's door with one type of footprint and one type of protector," says Dennis Mozingo, general manager, Bravo Communications Inc. (

Capacity (voltage)
After making sure that your surge-protection device is Underwriters Laboratories Inc. listed (see "UL listings: A common misconception," above), you should check the voltage of the device you intend to protect. Surge protection devices are designed to "clamp" a transient voltage down to a manageable level.

Some small amount of transient will still reach the equipment-even if you have both primary and secondary protection. This is acceptable provided that the "let-through voltage" is within the range that the protected equipment can withstand.

There are many different terms used to identify this "let-through voltage" rating, including: breakover voltage, clamping voltage, protection level, and protection voltage.

"When an AC power surge protection device has a rating of 120V, this means it has a rating of 120V nominal," says Beech. "There are some [AC power] devices on the market that if the voltage gets to 130V, the device will attempt to clamp the voltage before it fails. Most products on the market now have a maximum continuous operating voltage (MCOV) of 150V."

You want to have a surge-protection device that will not blow a fuse after it is used. Mozingo says a quality device will recover from a transient surge. Within the range of capacity of the surge-protection device, the unit should be able to take the "hit," get rid of the damaging energy out through the building systems' ground, and automatically recover to a standby state.

Equipment protection
As far as telephone and data lines are concerned, traditionally, equipment protection programs are only given with secondary, and not primary protection devices. The surge-protection device vendor will often only compensate your loss if you can prove that primary protection is properly in place while using secondary protection.

How would you know it is in place? "An experienced electrician or telephone service technician can find where the primary protector should be installed outside a home or business," says Eric Sadler, manager, engineering sales, Porta Systems Corp. (

When you plug into a secondary protector, the vendor will give you the warranty because the primary protector will take the hit.

"On the electrical side, all of your computer appliances will have AC-to-DC power converters," explains Sadler. "When an electrical disturbance comes down the line, it is the converters that get fried. The surge does not make it to the more expensive computer hardware."

Most often, electrical disturbances will not get to your equipment if you use secondary and primary protection properly. But buyer, beware: It does happen. Sadler compares surge protection to an airbag in a car. "No matter how good an airbag is, if you have enough speed, it still won't prevent a fatal car accident," he warns. "Lightning bolts have a lot of energy."

Ryan Cliche is a former assistant editor for Cabling Installation & Maintenance.

UL listings: A common misconception


People believe that just because something has an Underwriters Laboratories Inc. ( mark on it, it must be safe to use the product. The highest UL rating for surge-protection products is UL-497. This can be confused with UL-497A and UL-497B. Although the listings look similar, the testing procedures for each are extremely different. The truth about Category 5/5e standards, a white paper by Donald Sweeney, a sales engineer at ITW Linx (, discusses the differences:

  • UL-497 (primary protection): The primary protector is used at building entrances and is capable of withstanding the highest level of over-voltage surges-lightning. These devices are required by National Electrical Code (NEC) Article 800 to protect buildings and humans. The NEC Article 800 requires all communications cabling entering a building to be protected by an agency-listed primary protector.
  • UL-497A (secondary protection): The secondary protector is intended to protect buildings, humans, and sensitive electronic equipment. The TIA/EIA-568 standard does not allow fuses and positive thermal coefficients (PTCs) to enable secondary protection due to the 100-W impedance mismatching.
  • UL-497B (isolated loop protection): Isolated loop protectors are designed to handle over-voltage events that are introduced within a building. This protected loop must not be exposed to the outside world. The protector is designed for low-voltage protection-not lightning-type surges.


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