SISTEMAS DE DISTRIBUIÇÃO DE PODER

16.1

AC POWER DISTRIBUTION SYSTEMS

In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. Note that regulations for earth (grounding) systems vary considerably between different countries.

A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It also ensures that in the case of an insulation fault, a high fault current flows, which will trigger an over current protection device (fuse, MCB) that disconnects the power supply.

International standards require that electrical installations implement two types of protection of person against the dangers of electrical currents.

•

Protection against direct contact with conductive parts that are intentionally live. This protection is provided by barriers or enclosures for which the design and type define the corresponding IP degree protection index.

•

Protection against indirect contact with conductive parts that are not normally live but which may become live accidentally. This protection is provided by disconnecting the supply of power when the potential difference of these parts becomes dangerous.

International standard IEC 60364 distinguishes three families of earth arrangements using the two-letter codes TN, TT, and IT (three letter codes only for TN systems)

FIRST LETTER

SECOND LETTER

THIRD LETTER

(only for TN systems)

connection of the Neutral

earthing of the equipment

protection conductor

T

Direct connecting to the earth of the power distribution

T

Connected to a local earth independent from the power distribution earth

C

Neutral and protective conductor (PEN) are combined in a single conductor

I

Isolated from the earth

N

Connected to the earth of the power distribution

S

Neutral and protective conductor (PE) are distributed separately

TT system

The power distribution system have one point directly earthed:

•

the neutral is connected directly to earth

•

the masses are earthed independently of power earthing

IT