What is a transient?

Welcome to the World of Transients

What are transients?

A transient (surge, impulse, spike) is defined as a temporary rise in voltage and current on an electrical circuit that has been disturbed. They are high energy (with magnitudes in the thousands of volts) and short duration (with rise times in the 1 to 10 microsecond range). Since they are sub-cycle events they should not be confused with longer duration events such as swells or temporary overvoltages. They disrupt, damage or destroy our electrical and electronic equipment in an instant and/or cause cumulative damage over a period of time resulting in unexplained failures.

How Transients are Created


Lightning strikes and utility company grid switching are the major external sources of transients. The ground surge from nearby lightning strikes can ravage electrical systems by entering unprotected circuits through the system ground. Cloud-to-cloud lightning discharges are capable of inducing electrical field intensities in the hundreds to thousands of volts per meter. If this type of transient appears across an unprotected power, telephone, data, or coaxial line the result can be system destruction.
The bad news is that power utility companies have little or no control over the surges induced on their power lines. Noisy electrical neighbors sharing your electrical distribution system, such as welding shops, can also be a major source of transients.

Examples of externally generated surge sources include:

Lightning Strikes
Direct Strikes (Cloud to Ground) – reach as high as 25,000 Volts
Induced (Cloud to Cloud ) – reach as high as 15,000 Volts
Utility Impulse Generators
Load Shedding for Demand – reach as high as 15,000 Volts
Power Factor Correction – reach as high as 15,000 Volts


Any time the flow of electrical energy is altered, transient activity can be the result. The simple act of turning on (or off) a light, motor, copy machine or any other electrical device can disturb the electrical circuit and create transients. In general, the larger the load current the greater the disturbance when the load is switched off or on. The switching of high ampacity loads such as electric welders and electric motors are known to create damaging transients. The simple fact is that the normal operation of most electrical equipment creates voltage surges and spikes. While they vary in magnitude and duration, they are constantly present in all unprotected circuits.

Examples of internally generated surge sources include:
• Local Capacitive Loads – reach as high as 10,000 Volts
• Static Electricity – reach as high as 5,000 Volts
• Loose Wiring – reach as high as 3,000 Volts
• Inductive Load switching – reach as high as 7,000 Volts
• Switch Bounce On Starters/Contactors – reach as high as 10,000 Volts
• Electronic Loads Added – reach as high as 5,000 Volts
• Ground Potential Differences – reach as high as 10,000 Volts

Other sources of internally generated transients include:
• Variable frequency drives Electronic or computer loads
• Electronic lighting ballasts Energy management systems
• Switch mode power supply or SCR’s HVAC Systems
• Voltage Switching Space Heaters
• Copiers Printers

Some of these sources can and do produce immediate catastrophic damage and interruption of your operation. This damage is readily noticeable due to the large volumes of smoke and sometimes the fire that is caused when such an event is allowed to occur unchecked. Others produce cumulative damage. This damage occurs a little at a time over a period of time. Sort of like the damage you do to a block of ice with an ice pick. Each chip doesn’t amount to much but the cumulative affect is devastating.

However, in the high tech world of today, the threshold between cumulative damage and catastrophic damage is swinging more and more to the low end of the spectrum of magnitude and duration. What that means to you, as an engineer, maintenance director, or consumer is that surge suppression is no longer an option, it is a necessity.


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