A number of different power distribution systems are employed in various countries around the world. It is important to identify the distribution system in use prior to installation of the AC DC MOS Protectors.

To identify the distribution system in use, consult reputable and knowledge able sources such as:

• The local power supply authority
• Local electrical engineers
• Applicable regulatory bodies or standards associations

In accordance with IEC 364-4-41 a low voltage distribution system is usually characterized from electricity source to terminal equipment with the following:

• grounded power supply source (e.g. low voltage connection of local network transformer.
• grounded system of exposed conductible parts in consumer electrical installations.

Through this three basic types of systems could be defined follow distribution systems:

• TN system
• TT system
• IT system

Used abbreviations have the following meaning:

The first letter describes grounded system of power-feeding electricity source:

T - direct grounding of power supply source single point (primarily connecting point of transformer winding);

I - isolation of all active parts from ground or connection of single source point of electricity to ground over some sort of impedance.

The second letter describes grounded system of exposed conductible parts of electrical installation:

T - exposed conductible part is directly grounded independent of eventual existing grounded feeding point

N - exposed conductible part is directly connected to groun-ding electrode (grounding resistor)

Further letters describe an arrangement of neutral and protective conductors:

S - neutral and protective conductor are separated

C - neutral and protective conductor are connected

Amongst these, the TN-S, TN-C-S, TN-C, TT and IT systems are most commonly encoun-tered.

TN-S system: In this system, a separate neutral and protective earth conductor are run throughout. The protective PE conductor can be the metallic sheath of the power distri- bution cable or a separate conductor. All exposed conductive parts of the installation are connected to this PE conductor.

TN-C-S system: In this system, a separate neutral and protective earth functions combine in a single PEN conductor. This system is also known as a Multiple Earthed (MEN) system and the protective conductor is referred to as the combined neutral earth (CNE) conductor. The supply PEN conductor is earthed at a number of points throughout the network and generally as close to the consumer's point of entry as possible. All exposed conductive parts are connected to the CNE conductor.

TN-C system: In this system, the neutral and protective earth conductor combines in a single conductor throughout. All exposed conductive parts are connected to the PEN conductor.

TT system: A system having one point of the source of energy earthed and the exposed conductive parts of the installation connected to independent earthed electrodes.

IT system: A system having no direct connection between live parts and earth, but having all exposed conductive parts of the installation connected to independent earthed electrodes. AC DC MOS Protectors - Blue Series should NOT be used in IT systems without specialist advice.