Imagine for a moment that you are sitting in your home, listening to the rain while reading your favorite novel. Then suddenly you hear a roar of thunder and your home becomes dark. What just happened? You may have just experienced a power surge. Although lightning is not the most common cause of this phenomenon, it is excellent for illustrating what a power surge is and how it can affect your home. A power surge is a term used to describe a condition where the voltage and or current increases beyond its normal levels. Power surges, when due to the elevation of the voltage being supplied by the electrical utility company, have the potential to change data streams, interrupt electrical control circuits, burn out components, cause arcs, create shorts between conductors, overload systems, and cause system failures. The most common causes of power surges are indicated as follows:
In the example of a lightning strike, the lightning may hit near a power line causing the electrical energy to increase the potential by many volts, thus a power surge results. The lightning is introduced into the electrical utility grid. The grid is made up of transmission lines which can be run over head mounted on poles, which are typically 30 to 100 feet to the top or through an underground system. The overhead system, although more economical, is vulnerable to induced energy from direct lightning strikes.
Lightning can also induce energy into the grid by direct conduction or as an electromagnetic pulse (EMP) caused when lightning strikes a grounded object such as a tree that is near the transmission conductor. The direct strike causes a surge which is typically in the mega-voltages and kilo-amperes ranges. The EMP causes voltages as high as in the kilo-voltage range but rarely in the mega-voltage range. The current of the EMP is typically less than a direct strike and therefore it has much less energy. The damage from a direct strike is usually massive in a system, while that of the EMP is normally restricted to the smaller electrical and electronic components of a system.
Lightning strikes are only one cause of power surges. There are many causes that can be attributed to either direct or induced energy being introduced to the system. The two main causes are equipment failure and normal switching operations as the utility company attempts to balance the loads on substations or reroute loads in order to allow maintenance to occur. Many systems have the capability to absorb the energy and prevent its affects on the system. Even more systems have built in devices that are designed to dissipate the energy without damaging the system.
Power surges from utilities can have a period from a few milliseconds to seconds in duration. The voltage can range from a few hundreds of volts to thousands of volts above the normal. The energy delivered is dependent on the supply capability at the point where the system is connected to the supply. This is typically measured in fault current. When equipment failures occur, the electrical status-quo of the network is disrupted as the load has not changed but the supply has changed. With modern technology, equipment failure, such as loss of a transformer at a substation, will result in the rerouting of the power source for this portion of the grid through an alternate substation. During the rerouting process, caused by either the equipment failure, or the adjusting of loads, the load is momentarily disconnected and then reconnected. This happens in a very short amount of time, typically milliseconds. At the point where the alternate supply route is connected, the system will have a surge of voltage as the regulator tries to compensate for the added load. The regulator can cause a ringing effect which can continue for up to seconds depending on various factors within the system. This ringing effect is the same in voltage waves as a bell ringing in sound waves.
Damage to the local electrical power grid is also a failure, which reacts in the same manner as equipment failure. This damage can be caused by component failure, a tree falling across the grid's conductors, a car running into a utility pole, and many others.
Although a less known cause for a power surge, the surge created by factories and commercial facilities utilizing large power loads are very common. In such facilities power to large loads are often switched, especially during shift changes or shift start-ups. This sudden load change causes a rise and then ringing effect of the voltage as the system regulates to handle the new load requirement. Typically these are isolated by their own substation, reducing the effect of surges that have made their way out to the general power grid. However, these still can be a cause of damage to electronic components to the systems connected to the grid. This kind of damage is typically averted by the use of standard surge protectors.
There are also other sources for power surges which have much less energy in the typical occurrence. Broken or cracked insulators on high tension transmission lines can set up a path for arcing between a transmission line and the grounded tower supporting the line. This can continue for extended periods of time and will cause power surges which can damage electronic equipment and cause radio interference. Similar arcing can occur in outdoor lighting systems.
Power surges can have a wide range of definitions varying by the cause, which vary mostly in the amount of energy introduced into a system. Because power surges can be caused by manmade processes or by acts of nature and because evidence from one cause to another often overlaps one another, it can be difficult to define the primary cause. However, in most instances, one will be able to look at all the evidence presented and isolate variables to determine the specific cause there by defining the power surge more accurately.
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