No. Surges, or transients, do not explain all of the voltage-related power quality issues in the world today.

  • Drops in voltage that last from a fraction of a second to as much as a few seconds are known as voltage sags.
  • Drops in voltage that last from several seconds to minutes are known as brownouts. Both sags and brownouts are more damaging to motor loads than they are to microprocessor based equipment.
  • Sustained over-voltages are known as voltage swells and can last from a few seconds to several minutes. Sustained over-voltage conditions can be very damaging to many types of equipment, but are not nearly as frequent as transients or surges.

Power outages are another type of power quality issue that can last from several seconds to hours or even days. Surge protective devices cannot prevent the affects that can occur with the loss of power. Surge protective devices can address the transient events that accompany power outage occurrences. Any unplanned power outage will have transients events during the process of power being removed and the restoration of power.

The use of either uninterruptible power supplies (UPS systems) or standby power generating equipment can aid in restoring power to the impacted systems. However, either solution has a need for protection using surge protective devices.

UPS systems: Standby generators:
  • Generally powered by batteries.
  • Do not protect equipment from surges. In fact, UPS systems actually contain sensitive electronic equipment that should be protected from transients.
  • Generally powered by diesel fuel, natural gas, or liquid propane.
  • Can generate transients each time they are put in to service or when they are tested.

Many hospitals and businesses will incorporate surge suppression equipment on panels that are being fed by a standby generator or a UPS system. For additional information on the protection of automatic transfer switches, please see the following application note.

Many other power quality issues exist that all users must be aware of. A partial list of these problems includes:

  • Harmonics
  • Circulating ground currents
  • Noise
  • EMI/RFI interference

Power quality products cannot cure all of these issues. Proper diagnosis is the key to good power quality and the minimization of lost productivity.

Since it is impossible to prevent voltage surges from entering a building or occurring inside a building, surge protection was invented. The function of the surge protective device is to stop or limit the effects of less-than-perfect power quality on solid state electronic devices.


Diagnosing Power Quality Problems

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

  • Investigation
  • Determination
  • Analysis
  • Elimination

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 overcurrent protective devices 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. Additionally, it is important to ensure that the neutral to ground bond exists and is properly made within electrical systems which are intended to have a neutral to ground bond. 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.


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.


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.


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.

Power Quality Issue Potential Solutions
Loss of facility or load power Uninterruptible power supply (UPS)
Printed circuit board damage SPDs and EMI/RFI filters, voltage regulators, fuses
Microprocessor lock-up Suppression filter systems, noise filters, isolated ground receptacles