Distribution systems

Ground connections in our alternating current systems (single-phase, three-phase, rotary current systems), these systems can be put into three separate categories (IEC 60364):

System name

Type of ground connection to the energy source (first character)

Type of ground connection to the electric operation (second character)

TN system

A certain point on the neutral conductor, normally close to the feeding current source, is grounded directly.

The body of the electrical equipment is connected with the ground point using a ground conductor.

Variations

TN-S system

TN-C system

TN-C-S system

Depending on the application of the N conductor, TN systems are split into three different types:

  • S: Separated neutral and equipment grounding conductors

  • C: Combined neutral and equipment grounding conductors (PEN)

  • C-S: System with TN-C section(s) and TN-S section(s)

TT system

A certain point on the neutral conductor, normally close to the feeding current source, is connected with a ground connection.

The body of the electrical equipment is connected with other grounding elements, independent of the neutral ground.

IT system

No point in the system is grounded directly.

The body of the electrical equipment is grounded.
NOTE: Security stipulations for these various systems (cut-off conditions for example) are found in IEC 60364-4-41.

Character definitions

The letters have the following meanings:

Letter

Origin

Meaning

T

French: terre (Earth)

Direct connection of a point to earth

I

isolated

Either all active parts are separated from earth, or one point is connected through an impedance with earth

N

neutral

Body is connected directly to the ground point of the system

(In alternating current networks, the grounded point is normally the zero conductor or if none exists then the external conductor.)

S

separate

A conductor is provided for a protective function and is separated from the neutral or the external conductor.

C

English: combined

Neutral and protective function combined in one conductor (PEN conductor)

First and second letter assignment

The identifying letters for the current distribution system are assigned as follows:

  • First letter: Indicates the ground connection to the energy source (Transformer for example)

  • Second letter: Indicates the ground connection to the electrical equipment