Overvoltage protection of low voltage networks

SPD (Surge Protection Device) selection criteria.

  • permanent operational voltage Uc
  • voltage protection level UP
  • ability to absorb energy (Joule rating)

The network type designations given have the following meanings:

a) The first letter – expresses the relationship between the network layout and the earth potential

  • T – direct grounding of a given point of the operational current circuit, usually a node (zero point)
  • I – insulation of all live wires from the ground or direct connection of the grid system point to the ground through high impedance

b) The second letter – expresses the relationship between the non-live conductive parts and the potential of the earth

  • N – direct connection of non-live conductive parts to a grounded point in the grid system
  • T – direct connection to the ground of non-live conductive parts

c) The third letter, if present, indicates the relationship between the neutral conductor and the protective conductor

  • C – the function of neutral and protective conductor is combined in one PEN cable
  • S – the function of protective conductor is carried out by a separate wire independent from the neutral wire

TN-S the supply grid has the neutral conductor connected with the grounding conductor only in the power supply transformer

TN-C neutral and grounding conductor functions are combined (PEN) and the grounding is on the transformer or in its vicinity

TN-C-S the neutral conductor is grounded at the transformer and in other points of the grid

TT neutral point of the transformer is directly grounded and the non-live parts are grounded independently from the distribution network

IT in this system there is no direct contact between active parts of the grid with the ground, however available conductive parts of installation components are grounded