What is a surge and how to protect against it

Author: Ing. Jaroslav Dostál

To better understand the function of the surge arresters and the reason why we are installing them, it is necessary to start with the fact what the surge (or the overvoltage if you wish) actually is. A voltage is a difference of potentials between two points, which allows an electrical current to be generated. If such a voltage is within the permitted limits, this state is desirable because it generates electrical energy. If, however, the difference of potentials is higher, we are talking about a surge. This state is undesirable and can cause disturbances in electrical distribution system or even cause damage to electrical installation and end devices. In other words, it is any voltage whose peak value exceeds the corresponding peak value of the highest operating voltage of the electrical distribution system.

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If we want to prevent any difference of potentials, we have to ensure their equalization. The most effective way is a mutual connection that results into a permanent conductive connection between two points and thus the undesirable condition cannot arise. Unfortunately, this common principle of protection cannot be applied on live wires as it would cause a short-circuit failure, and the electrical installation would not be operational. This is why Surge Protection Devices (SPD) are installed. These are used for short-term (only for the duration of a surge) connection of live parts with non-live parts or between two live parts. Thus surge protection devices help to eliminate any present overvoltage to a permissible value. SPDs change their internal impedance when the limit value of the voltage is exceeded in the order of nanoseconds, thus ensuring the desired equalization of potentials.

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SPDs are divided into three basic types – T1, T2 and T3, according to their discharge capabilities given by the cause of a surge. There are several of causes but the most important ones in terms of surge protection are atmospheric and switching surges. Atmospheric surges are caused by lightning strike and are accompanied by the most energy-intensive pulse, causing destructive effects. There are many cases where such surges caused not only destruction of the end devices but also ripped cabling from walls, burned switchboards or whole buildings, and unfortunately also caused even a loss of human lives. The switching surges do not reach such a high energy values as the atmospheric ones, but they are much more frequent and affect each of us, every household, every industrial object. The paradox is that we create it ourselves by switching devices connected to the power supply system. There are many electrical devices that, when switched on, trigger a voltage spike which threatens sensitive electronics such as TV sets, control systems, computers, data storage, etc. We will look at the breakdown of SPDs in more detail in the next article: What are the types of surge protection devices and how they are tested.