Explanation Of Terms

14.17.1. ELECTRICAL EARTHING OR CIRCUIT PROTECTIVE CONDUCTORS (CPC)

Conductors installed to provide a low impedance path for the current which flows under fault conditions to the general mass of earth. Normally the CPC is connected directly to any associated metal work of the equipment.

14.17.2. ELECTRICAL BONDING

Conductors installed to establish continuity between adjoining metal work and the armouring of separate cables to ensure that, under fault conditions, all metal work and cable armouring are maintained at the same potential.

14.17.3. EXPOSED CONDUCTIVE PARTS

Exposed conductive parts include the metal work of switchboards, enclosures, motor frames and transformer tanks.

14.17.4. EXTRANEOUS CONDUCTIVE PARTS

My metal work associated with the plant, for example pipe work which can be touched at the same time as a metal switch board cover or motor frame, will be deemed extraneous conductive parts.

Letter classifications are used to denote the earth system used.

The first letter indicates the supply earth type.

T - Terra

At least one point of the supply is directly earthed. (Terra is the Latin word for dry earth)

I - Impedance

There is a deliberately inserted impedance in the supply, or an infinitesimal earth impedance (i.e. open circuit) designed to limit the fault current.

The second letter indicates the installation earthing system. T Terra

All exposed conductive metalwork is directly connected to earth.

N - Neutral

All exposed conductive metalwork is directly connected to the Electricity Supply Companies earth conductor.

The third and fourth letters indicate the arrangement of the earthed supply conductors.

S - Separate

Neutral and Earth are separate systems.

C - Combined

Neutral and Earth are combined into one system.

This gives the following systems IT, TT TN-S, TN-C-S, TN-C.

Note: the TN-C system is no longer allowed as a new installation with the introduction of ESQCR. (The Electricity Safety, Quality and Continuity Regulations 2002). However, national standard apply.

14.17.5. POTENTIAL EQUALISATION

In order to prevent different voltage potentials occurring in the metal work of plant in hazardous areas, potential equalisation will be necessary. This applies to TN, TT and IT systems where all exposed and extraneous conductive parts are required to be connected to the equipotential bonding system. The bonding system may comprise protective conductors, metal conduits, metal cable sheaths, steel wire armouring and metallic parts of structures, but not neutral conductors. The security of connections must be assured by non-loosening devices.

14.17.6. STATIC REQUIREMENTS

The risk of incendive sparks from non-metallic installation material (plastic coated cable tray, plastic weather protection shall be controlled by the following.

Material IR not to exceed 1GΩ or

Limitation of surface area of non-metallic parts.