Basic Information about Surges

Definition of Surges

Very simply, surges are very short-term spikes in voltage. They are not easy to understand and practically impossible to explain to a customer. However, there is a similar phenomenon that everyone can understand.

Spikes occur in the pressure of a water supply system. Almost everyone has turned off a water faucet too fast and heard water pipes rattle. The same happens in an electric circuit, except you can’t hear an electrical spike.

How high does the voltage get during a spike? It turns out that for building wiring, the maximum voltage is limited to 6,000 volts or 6KV. The voltage is limited to 6KV because that is when standard wiring insulation breaks down.

If the voltage can reach 6KV, why isn’t everything electrical destroyed? Because spikes are fast and only last for a very short duration of time. 

Although the voltage is high, the duration is so short that traditional electrical equipment is unaffected. Sensitive electronics can be damaged or destroyed because not much energy is required to damage solid-state parts.

The duration of most spikes range in the 50 to 500 microsecond range. This is 50 to 500 millionths of a second (extremely fast). A good rule of thumb is that very high voltage transients are usually of very short duration and lower voltage transients are of longer duration.

The two major characteristics of transients are extremely short duration and very high voltage.


How much energy can a transient contain? The first reaction to this question is that they must contain a lot of energy since they do so much damage. A little more analysis can be surprising.

Since energy is related to current (which is driven by voltage) and time, the actual energy delivered by a transient can be calculated knowing the voltage and duration of the transient.

We know that transients may involve very high voltages, but is the current delivered to building wiring very high? Records indicate that delivered current can be about half of the value of the peak voltage. A 6,000 volt transient can deliver a 3,000 amp current pulse.

Although this sounds like a lot of current, remember it is only delivered for a duration of around 20 microseconds. The amount of energy, in watts, could be as much as 6,000 volts x 3,000 amps x 20 millionths of a second. This is actually 360 watt-seconds, the equivalent of switching on six standard 60-watt light bulbs for one second.

Such low energy would not damage a motor or resistance heater. However, it could do a lot of damage to sensitive electronics. To propagate the mystery of transient suppressors, manufacturers do not typically list the energy capability of a suppressor in watt-seconds or watt-hours. This is because it wouldn’t impress the average consumer. There is another term that can be used and is not so easily recognized: the Joule.

A Joule is defined as one watt for one second. Therefore, 360 watt-seconds becomes 360 Joules, a term that sounds a lot better from a marketing standpoint. The key to remember here is that transient protectors are all rated in Joules and therefore cannot handle much energy. Even the most damaging transients contain very little energy. They do deliver that energy very effectively!


Customers recognize not the surges, but rather the symptoms and problems caused by them. Mostly the symptoms fall in the area of equipment damage involving sensitive electronics.

While the customer may not be able to determine that the damage was caused by a surge, what the customer understands is the repair cost and the inconvenience and loss of use of the equipment. The customer may also know that the damage is occurring in a particular part of the sensitive electronics. Usually, damage to sensitive electronics occurs either in the power supply or in a data circuit.

It would seem that lightning events cause most surge damage. However, lightning is NOT the most frequent source of surges.

Damage can happen in normal weather conditions. The most common source of surges is switching of power. Large loads such as motors and variable speed drives, cause the majority of surges.

Electronics that are wired to both electrical power and external data or communication lines are especially sensitive to transient damage. This is because the communication line frequently links two computers together that are powered from different electrical power systems.

Usually, this means that different grounds are connected through the electronics to the data lines. Different grounds usually have different ground potentials and connected ground currents can flow. This alone can cause damage. However, the ground current becomes extreme if either ground becomes elevated in potential.

High ground potential typically occurs when lightning strikes near one of the two grounds. The resulting ground current instantly destroys sensitive electronics. It may be expected that I/O circuits are destroyed, however, the ground currents must flow through the electronics’ power supply and sometimes this current is enough to destroy the power supply before destroying the I/O circuits. Surge damage can be prevented by the proper application of transient protection. 

Less frequently, transients cause erratic operation of sensitive electronics. This can occur by slow degradation of electronic circuits or damage to data. Slow degradation happens to all electronics. Over time, transients slowly erode circuitry until the device becomes less than reliable in operation. When this happens, it is usually because it is time to upgrade to newer electronics anyway, so most people consider the electronics have lived their useful lives and new equipment must be purchased.

Sometimes this aging occurs much more rapidly. In this case, the unreliable operation occurs sooner, and repairs are much more than expected. Often when this starts to occur, transient protection can prevent continued degradation and repairs.

Lastly, some erratic operation of electronics is caused by transients. Usually, this takes the form of corrupted data within the electronics that cause the system to lock up. Sometimes, memory is lost, or the system is reset and reboots. Occasionally, transients cause corruption of data saved on magnetic media. This can usually be stopped by installing transient protection.

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