Surge in electricity means a sudden increase of current or voltage, and/or a violent oscillation, which is originated mainly from lightning and from switching. Surge has much energy in spite of it's life time very short. When the surge is introduced to circuits of electronic equipment or IT equipment, those equipments are probably malfunctioned, or something go wrong, and or shorten those life time. And sometimes voltage of the lightening strike is so high that could destroy electronic equipments completely. Such a damage of surge increases as high as the electronic equipment is composed more compact and highly integrated of its circuit composition, and nowadays because all of electronic equipments are going to be made of very precise circuit elements, such damages are increasing the more. In general, the surge in electrical line caused from lightning has high voltage so that break down the dielectric material and make flash over certain electrical path, or the surge could be one of reason to induce over₋current in electronic circuits inside equipments, so that circuits come to burn out.

There are various paths of the lightning surge flowing into power₋line(or signal₋line). Generally the surge is considered as two cases, direct and indirect lightening surge. Direct lightning surge, the lightning could struck a point of the electrical system directly, would be thoughted not so often because most of building is built with steel frame and lightning₋rods over the building working as electric shield, but once the direct lightening surge introduced in electrical lines, there might be little of equipments could be avoid a damage at all because this is the most destructive case. The indirect surge might be generated in electrical or signal lines when the system(line) is near the direct₋stroke lightning point, even though the lightning₋rod is. Even if the electric system or earth line is well₋isolated from that point and could be considered safe, when huge current with high current passes through certain path(even if it is intentional or incidental) in very short time, magnetic field variation caused by that current induce lots of surge in neighbor circuit which is coupled in magnetically with this current path. This is often called as induced lightning surge and could appear in a level nearly the one that is arose from direct stroke lightning. Another surge could be found in a electrical or communication line between the lightning spike point and the normal point that is not opened to the that lightning. The reason of this kind of surge current is mainly earth potential differences of each earth points. Damages of these two cases are the most case of damages in general, and levels of the damage is different depending on it's environmental conditions. Even if this case is it's environment of earth’s conductivity is bad, and so the lightning current can't be disperse easily around the earth plane, but the current that is flowed into it's earth line could damage equipments that are connected together by earth line.

In IEC 61643₋11, the statics of wave₋form of surge is defined for test. 8/20㎲ wave₋form for current and 1.2/50㎲ wave₋form for voltage is used for test. The sign slash(/) is the datum. The front one means the rising time when it rises to peak value, the following one means the decay time, the time from starting point to the point of the level is reduced half of maximum amplitude.

SPD(Surge Protective Device) is a device to protect electric or electronic equipments from electrical surge. It is installed on the electric line or signal line, and is to disperse the surge energy to earth level quickly in several nano₋seconds. Surge current is diverted its path from lines to earth plane and then surge voltage is reduced to certain suppressed level. So the sensitive electric or electronic equipments can be protected from surges.

SPD is one portion of LPS(Lightning Protection System). Especially, if there is not well₋designed earthing system, the SPD installed can't be expected to work well. Therefore, build well₋designed earthing system in conformity with IEC has to be preceded before SPD installing. The choice of SPD is important also for protection. SPDs satisfying IEC(International Electro₋technical Commission) Standard is recommended too. IEC61643₋12 and IEC61643₋21 will help you for selecting and appling SPD.

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