NEMA Surge Protection Institute: For Engineers & Specifiers (Industrial)

What Should You Know About Purchasing or Specifying a Surge Protective Device?

Safety and performance. While there are many varying criteria to be considered during the decision-making process for selecting an appropriate surge protective device (SPD), if the design engineer neglects the importance of either of these two areas for the products being specified, there can be serious implications for the client.

Before an SPD is purchased or specified, however, one must first determine whether the power quality issue is caused by surges (transients).

Power quality problems are easily pinpointed by methodically following four steps:

Investigation
This requires facility staff members or power quality consultants to conduct a structured approach to site survey that progressively identifies problems. The power path must first be traced from the affected load back to the incoming source of power; wiring must be checked for proper sizing and breakers checked for correct rating. It must be determined that transformers are not overheating, and that the facility's ground system is intact and performing as specified. Isolated ground receptacles must be properly installed. Use of appropriate and carefully calibrated test equipment is crucial for site surveys: a true-RMS voltage and current meter is required to accurately measure sinusoidal voltages and current. Be aware that sine wave distortion may cause some test instruments to falsely indicate correct operation of some systems.

Determination
This requires monitoring of the power supply to the affected loads. A microprocessor-based power disturbance analyzer is required to examine transient disturbances present on the line. By sampling the sine wave thousands of times per second, these highly sophisticated devices record many different types of events, including transients and high frequency noise that occur too rapidly to be detected by a voltmeter. Power disturbance analyzer results may be printed out and saved for further review and comparison.

Analysis
Following data collection, analysis of both recorded information and environmental parameters is necessary. Don't overlook natural causes such as humidity or condensation, radical temperature differentials, radio frequency interference (RFI), radiated electromagnetic interference (EMI) or magnetic fields produced by transformers.

Elimination
When the source and type of power quality disturbances have been identified, the real dilemma of selecting the proper solution begins. The most reliable, most cost-effective power quality solution depends on the unique requirements of each facility. What works best for large high-rise buildings with massive HVAC, telecommunications and computer networks may not be the wisest approach for a busy textile mill where highly sophisticated manufacturing equipment is constantly switched on and off.

In retrofit or troubleshooting situations, the nature of problems being encountered often indicate the type of equipment required. Table 1 below briefly details today's most typical power quality issues and solutions.

Loss of facility or load power	Uninterruptible power supply (UPS)
Printed circuit board damage	Suppression filter systems, transient voltage surge suppressors (TVSS), voltage regulators, fuses
Microprocessor lock-up	Suppression filter systems, noise filters, isolated ground receptacles
Equipment shutdown resulting from voltage sags	UPS, voltage regulators

Table 1: Common power quality problems and solutions.

Product Solutions

Power quality solutions are predicated on the individual requirements of each facility. What works best for large high-rise buildings with massive HVAC, telecommunications and computer networks may not be the wisest approach for a busy textile mill or food processing plant.

An Ounce of Prevention

The most logical approach to a power quality environment is prevention. When electrical transient protection is properly specified into the facility's blueprints along with lighting, HVAC and other mandatory systems, tenants are saved the unnecessary headache and expense of transient episodes, which can cost production-critical operations -- such as major airlines, refineries, television networks and telecommunications operations -- as much as one million dollars per minute in damage and downtime. Compared to this staggering figure, the price of a facility-wide network of integrated power protection is truly a justifiable expense in the long run. Human life is also at risk without power protection in place. Imagine the peril of a night airplane landing without the guidance of control tower systems, or the tragedy of a medical misdiagnosis due to a transient-related data error on an MRI.

For complete coverage, a facility-wide approach is required. The commercial configuration typically combines uninterruptible power supplies (UPS) with SPD or TVSS products. Voltage regulators may be included to maintain minimal 60 Hz voltage fluctuations.

In addition to the products listed above, industrial facilities such as textile mills or automated production plants may require specialized protection for motor control centers or electrical busway, both of which provide electrical power for many different pieces of equipment. Suppression systems designed specifically for these applications are available for configuration into a facility-wide protection plan.

It should be remembered that uninterruptible power supplies require power protection to ensure completely reliable operation. Also worth mentioning is the minimal value of isolation transformers and lightning arrestors in a comprehensive power quality environment. Frequently installed at the electrical service entrance, lightning arrestors are designed to limit the surge voltage magnitude to a level that is not damaging to transformers, switchgear or other service equipment; in other words, they must reduce magnitude events to 2,000 - 3,000 volts. While this voltage range is within the withstand capability of most service entrance equipment, it will likely still be damaging to electronic loads within the facility unless additional electrical transient protection is installed at strategic electrical distribution points inside.

Isolation transformers, while attenuating common mode noise, fail to provide cross-interference protection for loads connected on the secondary side. Because a majority of system-upsetting noise is load generated, safeguarding one load from affecting another becomes even more critical as increasingly sensitive electronic loads are implemented into a noisier distribution system. Transient voltage surge suppressors with capacitive filtering offer broad-based, more reliable protection.

Sorting Through the Solutions

In past decades, power quality solutions were limited. Specifying electrical transient protection into new buildings was almost unheard of, and the data processing manager or plant supervisor who identified a power problem had little difficulty making a selection from the handful of options available in the marketplace. But power problems have expanded with the electronic age, and today's decision-maker is often confused and overwhelmed by the dozens of power protection products marketed as comprehensive solutions.

To provide consumers with a logical and methodical means for selecting electrical transient disturbance protection, the American National Standards Institute (ANSI) and Institute of Electrical and Electronics Engineers (IEEE) developed C62.41-1991 as an electrical transient exposure level/surge severity categorization guideline.

By identifying the various levels of potential transient exposure in a given facility and then specifying products developed in accordance with the ANSI/IEEE categories, today's purchaser is assured of a cost-effective and reliable power quality environment.

IEEE Category C	High	Large ampacity service entrance Service entrance in high lightning area Service entrance near utility substation Service entrance on grid with other large industrial users
IEEE Category C/B	High-to-Medium	Lower ampacity service entrance Service entrance remotely located from utility power factor correction and grid switching High-lightning area distribution panels feeding roof-top loads
IEEE Category B	Medium	Large distribution panels Non-service entrance distribution switchboards Heavy equipment located near unprotected service entrance Panels feeding variable speed drives Non-service entrance motor control centers utilizing drives, PLCs, soft-start or electronic starters
IEEE Category B/A	Medium-to-Low	Branch panels with heavy sensitive equipment loads Branch panels with combination of "dirty" and sensitive loads Branch panels without up-stream protection Busway feeding sensitive loads Bus riser feeding multiple floors with critical or sensitive loads
IEEE Category A	Low	Branch panels with upstream protection Branch panels with primarily sensitive electronic loading Branch panels deep within a facility

Table 2: ANSI/IEEE Exposure Level Categories

Although the industry's leading manufacturers are proponents of standardized product testing and performance ratings, many electrical transient protection products are marketed with performance ratings that may have been obtained using questionable test methodology.

The prudent specifier or end-user should request detailed comparative data (using the NEMA LS 1 specification format) and verification of listing or recognition from all manufacturers being considered from a nationally recognized testing laboratory ( NRTL), such as Underwriters Laboratories (UL), Canadian Standards Association (CSA), or MetLabs, etc.

How will you know if the electrical transient protection installed in a facility is performing as it should? Routine testing and evaluation of the TVSS, UPS, suppression systems, harmonics cancellation devices or other power quality products is mandatory to ensure that sensitive systems are adequately safeguarded.

Doubtless, a preventive power quality environment specified into a facility at the design stage prevents costly damage and downtime down the road. Facility-wide protection may be purchased at a cost that is often less than the sales tax for the equipment to be protected. Commercial insurance companies will often discount commercial policies for customers with an adequate power quality protection network in place.

Additional resources

NEMA LS 1, Low Voltage Surge Protection Devices

The National Electrical Manufacturers Association (NEMA) has published LS 1-1992 (R2000), a standard publication which provides engineers, specifiers, and end-users (consumers) with the information they need to select an appropriate SPD. Download NEMA LS 1 (PDF).

UL 1449 Second Edition, UL Standard for Safety for TVSS

In addition to safety testing, UL provides Suppressed Voltage Ratings (SVRs) that are posted on the UL label of each and every UL Listed or Recognized TVSS. Engineers and specifiers can request proof of UL certification from TVSS manufacturers for their records.