Earthing

Earthing of equipment refers to the connection of non-current carrying parts of electrical equipment to the earth to maintain earth potential. Usually the earthing circuit is connected to a burried electrode. Done using a low-resistant conductor.

An effective earthing system avoids having dangerous potentials on the equipment even during electrical faults and also ensures the proper operation of electrical protection equipment during fault condition.

Potential of an installation is measured with respect to earth.

Purposes

Neutral Earthing

Limiting the voltage of current carrying conductors forming a part of the system.

This is important because the performance of the system in terms of short circuits, stability, protection, etc., is greatly affected by the state of the neutral conductor. When the neutral is properly grounded, voltages of the phases are limited to near phase to ground voltage.

Equipment Earthing

Limiting the potential of non-current carrying metal work associated with equipment, apparatus and appliances in the system.

Governed by various regulations such as the IEE regulations. The objectives of this grounding are:

• To ensure effective and rapid operation of the protective gear in the event of earth fault currents.
  Otherwise, those currents might be undetected, and cause fire
• Protect against shocks caused by installation metal work having a dangerous potential

Types of earthing arrangements

In the regulations for electrical installations, the types of earthing systems are identified as follows, depending on the relationship of the source (supply authority network) and of the exposed conductive parts of the installation, to earth.

• TN - earthing of the installation is done to that supplied by the supply authority
• TT - supply authority earth and the installation earth is independent
• IT - supply authority has effectively an isolated neutral and the installation has an independent earth

Above the first letter means:

• T - Short for terra. Means the source is earthed at its star point.
• I - Short for isolated. Indicates either:
  1. all live parts are isolated from earth
  2. one point of live is connected to earth through high impedance

Above the second letter means:

• T - Means all exposed conductive parts of consumer installation are directly earthed independently.
• N - Denotes a direct electrical connection of the exposed conductive parts to the earthed point of the supply authority side (which is usually the neutral point)

TT system

Used in Sri Lanka. Supply is earthed at the source end. All exposed conductive parts of the installation are connected to earth (independent earth electrode) at consumer end. Earth serves as the return path for the fault current.

Note

In domestic systems, the earthing circuit is usually earthed through buried metallic pipes. The electrode’s resistance depends on the condition of soil and may have values in excess of . Thus in the TT system of earthing, it is now essential to use an RCCB for protection.

The normal earthing practice is to provide a circuit protective conductor throughout every installation. A circuit protective conductor connects exposed conductive parts of equipment to the main earthing terminal.