Voltage Surge or Lightning Strike?

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Tony Catalfomo, M.S.E.E., P.E.

Voltage Surge or Lightning Strike... Would you consider Voltage Sag?


Many times electrical engineers are confronted with trying to classify damages to electrical and electronic equipment as the result of a lightning strike. More specifically, was the damage the result of a surge or a direct lightning strike? The answer is “maybe neither.” The chances are the damage may be the result of voltage sag.

First off, we need to clarify an issue that may be the cause of some confusion. The term “power surge” is used on a regular basis as the culprit associated with electrical and electronic damages. It should be noted, the Institute of Electrical and Electronic Engineers (IEEE) is careful in using the term “power surge”. In fact, power quality definitions are avoiding the term in many instances. The power quality definitions used by IEEE address overvoltage transients (impulses), voltage swells, voltage surges, voltage dips, voltage sags, flicker, harmonics, and interruptions. Notice the use of the term “voltage” rather than “power”. Although this may appear to be a matter of semantics, it is a more accurate description of the electrical anomaly that may be causing an issue.

A direct lightning strike is somewhat easy to identify, as the damage will be localized and visible. Lightning strikes may result in a short duration overvoltage transient, or impulse, with significant magnitude causing damage to electrical or electronic equipment. Lightning may also result in a ground potential rise or induced voltages on transmissions lines, which will propagate as traveling waves in either direction from the point of a strike. In some cases, the traveling waves may result in damage to transmission and distribution equipment.

In most cases, the faults resulting from overvoltage transients are cleared by the circuit protective devices, such as circuit breakers, which are designed to handle such events.

The problem arises when the faults are cleared and the electrical distribution systems are responding as they come back online. As the system is coming back on-line, voltage transients are imparted on the electrical distribution systems, causing voltage anomalies. The issue is identifying electrical anomalies imparted on electrical transmission and distribution systems that propagate through the electrical systems to industrial and consumer electrical and electronic equipment. These electrical anomalies may cause damage or inconsistent operation of electronic components. These anomalies may be incorrectly identified as a “power surge.” A common anomaly is a voltage sag. A voltage sag is a reduction in voltage for a short period of time. The voltage range of a sag is between 10% and 90% of the nominal voltage at 60 Hertz (Hz). The duration of a voltage sag event may be 8 milliseconds and up to 1 minute.

The symptoms of a voltage sag:
  • Inconsistent operation, faulty operation, or failure may occur in sensitive electronic equipment
  • Computer shutdowns or failure as a result of damage to power supplies
  • Capacitor damage on circuit boards 
  • Blown fuses at the inputs of Variable Frequency Drives (VFDs)
Common causes of a voltage sag include:
  • Phase to ground faults from various conditions – animals, downed tree limbs contacting distribution lines, etc.
  • Starting of large loads (industrial facilities)
  • Transformer energizing: This is caused by normal system operation including manual energizing of transformer and reclosing actions.
  • Reclosing actions as a result of clearing faults in the electrical distribution system
  • Utility equipment failures

The extent of damage as the result of voltage sag is related to the magnitude and duration of the sag. Some equipment will be designed to ride through short voltage sag conditions (10% nominal voltage drop for less than ½ cycle, up to 1 minute). Extreme sags of large voltage decrease over a longer period of time (greater 10% nominal voltage drop for up to 1 minute) may result in damage to sensitive electronic equipment.

Most household appliances are unaffected by voltage sags. Sensitive electronic equipment is more susceptible to voltage sag conditions. This may explain why refrigerators and heating ventilation and air conditioning (HVAC) compressors appear unaffected, yet equipment control devices, computers, and televisions, which use electronic circuit boards, may be damaged.

Voltage sags in industrial facilities are a more common occurrence. Voltage sags may be the result of storms imparting disturbances on the incoming electrical service. A more common cause is due to the starting of large loads in, or near, the industrial facility. The resultant sag may cause programmable logic controllers (PLCs) or VFDs to exhibit inconsistent operation during the sag. The voltage sag may not cause damage to the equipment but may result in shutdowns due to fault conditions within the PLC or VFD. As a result, production lines may be shut down impacting production rates or quality of the product being produced. The production line shutdowns will be a financial burden to the operation as additional time and services will be required to clear, check, and re-start the lines.

Most manufacturing facilities may have the ability to observe the voltage waveforms of the incoming electrical power. For these locations, a study can be conducted to identify and quantify the voltage anomaly. If the facility does not have the information recorded, the local electrical service provider may be able to assist as they will monitor the power quality to a facility. It may be useful to record the power quality into a facility after an incident has occurred in order to establish baselines or determine if the power quality concerns were a transient event or inherent to the incoming electrical service.

Although some symptoms of voltage sags and other anomalies are similar, voltage sags have some distinct characteristics that will differentiate them from other voltage transient events. Being able to recognize these characteristics is imperative in evaluating and assessing voltage anomalies that may result from lightning strikes, poor power quality at the point of delivery, or other sources. 


About the Author

Tony Catalfomo, M.S.E.E., P.E. is a Consulting Engineer in our Charlotte, North Carolina Office. Mr. Catalfomo provides consultation in the areas of electrical engineering, with an emphasis in industrial power distribution and controls. His expertise includes power quality and harmonics, NEC and NFPA compliance, instrumentation and controls, drive systems, circuit protection, and rotating machinery. Services include electrical failure analysis, damage assessment, and repair/replacement analysis. You may contact him for your forensic engineering needs at tcatalfomo@edtengineers.com or (704) 523-2520.

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