Selector for internal protection

Lightning can fundamentally affect us in two ways, either by direct impact or through an overvoltage, so a complete lightning protection system is made up of two systems; an external system against direct impacts and another internal to counteract the effects produced by overvoltages.


The ITC-BT 23 Technical Application Guide tells us in which situations the use of protection devices against transient overvoltages is mandatory, regardless of the power system. These cases are:

  • Totally or partially overhead low voltage power line or when the installation includes overhead lines.
  • Risk of failure affecting human life (security services, emergency centers, hospitals...)
  • Risk of failure affecting the life of the animals (livestock farms, fish farms...)
  • Risk of failure affecting public services (computer centers, telecommunications systems...)
  • Risk of failure affecting non-interruptible agricultural or industrial activities.
  • Risk of phallus affecting the installations and equipment of the places of public concurrence that have non-autonomous security services.

The 61643-11 standard indicates that there are 3 types of surge protectors: Type 1, Type 2, and Type 3; differentiated by their energy absorption capacity, the speed of response, and the origin of the overvoltage.


To know what type of protector we need, according to the regulations, for each specific case it is necessary to take into account if there are lightning rods, what is the isoceraunic risk of the area, and the point of installation of the equipment.


The following program is a simplified and automated method of surge protector selection based on the aforementioned regulations, in order to make the process more accessible to everyone. If a more precise calculation is required, or more complex structures, please contact us.



The regulations UNE 21186, NFC 17102 and NP 4426 contain a method for assessing the risk of lightning, extracted from the international standard IEC 62.305-2, which determines the necessary protection systems. The following program is a simplified and automated version of this method, in order to make it more accessible to everyone. If a more precise calculation is required, or more complex structures, please contact us.

The aforementioned regulations include 3 parameters for calculating the risk index:

  • The number of dangerous events per year.
  • The probability of damage to a structure.
  • The losses in case of damage.

1. Dangerous events are divided into 4 types, depending on how they can reach the structure:

  • Events in the structure.
  • Events near the structure.
  • Events in power lines that affect the structure.
  • Events near power lines that affect the structure.

2. The probability of damages is divided among three parameters:

  • Direct injuries to living beings.
  • Physical damage to the structure.
  • Damage to electrical and electronic systems.

3. The losses are divided into 4 types:

  • Human losses.
  • Losses of service.
  • Losses of cultural heritage.
  • Economic losses.

Protection against lightning is necessary when the calculated risk exceeds the tolerable risk set out in the regulations.