In general a surge is a transient wave of current, voltage or power in an electric circuit.  In power systems in particular – and this is likely the most common context that we relate surges to – a surge, or transient, is a subcycle overvoltage with a duration of less than a half-cycle of the normal voltage waveform.  A surge can be either positive or negative polarity, can be additive or subtractive from the normal voltage waveform, and is often oscillatory and decaying over time.

Surges, or transients, are brief overvoltage spikes or disturbances on a power waveform that can damage, degrade, or destroy electronic equipment within any home, commercial building, industrial, or manufacturing facility. Transients can reach amplitudes of tens of thousands of volts.  Surges are generally measured in microseconds.

Every piece of electrical equipment is designed to operate at a specified nominal voltage such as 120 Vac, 240 Vac, 480 Vac, and so on. Most equipment is designed to handle minor variations in their standard nominal operating voltage however, surges can be very damaging to nearly all equipment.


Sources of Surges/Transients

A common source for surges generated inside a building are devices that switch power on and off. This can be anything from a simple thermostat switch operating a heating element to a switch-mode power supply found on many devices.  Surges that originate from outside the facility include those due to lightning and utility grid switching.


60-80% of surges are created within a facility
Transients can originate from inside (internal sources) or outside (external sources) a facility:


Internal Sources:

  • Switching of Electrical Loads

    The switching (on and off) and operation of certain electrical loads – whether due to intentional or unintentional operations – can be a source of surges in the electrical system.  Switching surges are not always immediately recognized or disruptive as larger externally generated surges but they occur far more frequently.  These switching surges can be disruptive and damaging to equipment over time.  They occur as part of every day operations.

    Sources of switching and oscillatory surges include:

    • Contactor, relay and breaker operations
    • Switching of capacitor banks and loads (such as power factor correction)
    • Discharge of inductive devices (motors, transformers, etc.)
    • Starting and stopping of loads
    • Fault or arc initiation
    • Arcing (ground) faults
    • Fault clearing or interruption
    • Power system recovery (from outage)
    • Loose connections


  • Magnetic and Inductive coupling

    Whenever electric current flows, a magnetic field is created. If this magnetic field extends to a second wire, it will induce a voltage in that wire.  This is the basic principle by which transformers work.  A magnetic field in the primary induces a voltage in the secondary.  In the case of adjacent or nearby building wiring, this voltage is undesirable and can be transient in nature.

    Examples of equipment that can cause inductive coupling include: Elevators, heating ventilation and air conditioning systems (HVAC with variable frequency drives), and fluorescent light ballasts, copy machines, and computers.

  • Static electricity

    Electrostatic discharge (ESD) phenomena, or static, can generate electromagnetic fields over a broad range of frequencies up to low gigahertz range. The term ESD event includes not only the discharge current, but also the electromagnetic fields and corona effects before and during a discharge.  ESD results in a sudden transfer of charge between bodies of differing electrostatic potentials. ESD induced onto the electrical distribution contains a great deal of high-frequency noise.

    An electrostatic discharge event can cause equipment malfunction as well as physical damage. Equipment malfunction might include corruption of data and equipment lock-ups. Physical damage might include equipment damage and even loss of life. In order to achieve meaningful ESD immunity, the design of an
    entire system must be considered, both for direct discharge and for fields.

    The minimum voltage necessary for a person to be aware of his or her involvement in an electrostatic discharge is approximately 3000 V. Nevertheless, electrostatic discharges that occur below this threshold of human perception can contain sufficient energy to cause upset or damage to electronic equipment. In fact, the faster initial slopes of current waveforms that result from ESD events at these low voltage levels can make such discharges even more disruptive than ESD events originating at higher voltages.

    The voltage on a human body or on a mobile object can vary widely from one environment to another. It can remain well below 5 kV in controlled humidity situations involving only antistatic or static dissipative materials. It can range from 5 kV to 15 kV in low humidity environments with synthetic materials. The equipment victim is in close proximity to the ESD event and can be upset or damaged by the electromagnetic fields generated by the discharge between the intruder and the receptor.

    [Source IEEE Std C62.47-1992, IEEE
    Guide on Electrostatic Discharge (ESD): Characterization of the ESD Environment


External Sources:

The most recognizable source of surges generated outside the facility is lightning.  Although lightning can be somewhat infrequent in certain regions, the damage it can cause to a facility can be catastrophic.  Other areas are subjected to thunderstorms and lightning much more frequently.

The surges that are the result of lightning can either be from direct contact of the lightning to a facilities electrical system or, more commonly, indirect or nearby lightning that induces electrical surges onto the power or communication systems.  Either scenario can be immediately damaging to the electrical system and/or the connected loads.

Other external sources of surges include utility-initiated grid and capacitor bank switching.  During the operation of the electrical grid, the utility may need to switch the supply of power to another source or temporarily interrupt the flow of power to its customers to aid in clearing a fault from the system.  This is often the case in the event of fallen tree limb or small animal causing a fault on the line.  These interruptions of power cause surges when the power is disconnected and then reconnected to the customer loads.

Power quality disturbances can be delivered during the normal operation of the electric power system.  Electric utilities produce electricity from a number of power-generation facilities and allocate the power to specific grids of users.  Because the equipment used to produce power runs most efficiently at a constant speed, the utilities adjust the allocation of power, rather than making constant adjustments to the power facility’s generation equipment. As utilities switch the supply of power from one grid to another, power disturbances occur, including transients or spikes, and under- and over-voltage conditions.  These activities will cause transients to be introduced into a system and may propagate into end-user equipment and may cause damage or operational upset.

For additional information on these and other topics that are important to consider for surge protective devices, see IEEE Std. C62.41.1-2002 and IEEE Std. C62.72-2007 which are shown on the Regulations and Standards page of this website.