Exposed conductive parts

Second letter (denoting the relationship of the exposed conductive parts of the installation to earth)... 

T-Direct connection of the exposed conductive parts to earth, independently of the grounding of any point of the source of energy ... 

TT system All exposed conductive parts of an installation are connected to an earth electrode, which is electrically independent of the source earth. 

Indirect contact. Protection of personnel from hazards that may arise in the event of an insulation failure (single fault) between live parts and exposed conductive parts. 

Earthing is the connection of the exposed conductive parts of an electrical installation to the main protective earthing terminal of the installation. 

Exposed conductive parts are the metalwork of the electrical installation the conduit, trunking, metal boxes and equipment that make up the electrical installation. 

Electric shock is normally caused either by touching a conductive part that is normally live, or by touching an exposed conductive part made live by a fault. The... 

Circuit protective conductor (CPC) - a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal of the installation. This is the green and yellow insulated conductor in twin and earth cable. 

Exposed conductive parts - the metalwork of an electrical appliance or the trunking and conduit of an electrical system which can be touched because they are not normally live, but which may become live under fault conditions. 

Two types of contact will result in a person receiving an electric shock. Direct contact with live parts involves touching a terminal or line conductor that is actually live. The regulations call this basic protection, indirect contact results from contact with an exposed conductive part such as the metal structure of a piece of equipment that has become iive as a resuit of a fauit. The regulations call this fault protection. 

There are five methods of protection against contact with metalwork which has become unintentionally live, that is, indirect contact with exposed conductive parts recognized by the lET Regulations. These are ... 

The installation earthing arrangements are indicated by the second letter, where T means the exposed conductive parts are connected directly to earth and N... 

The trunking installation must be treated as an exposed conductive part and be properly earthed in accordance with Regulation 411.3.1.1. Care must be taken to provide reliable earth continuity and an adequate earth fault current path by making all joints electrically and mechanically secure. 

A protective conductor connecting exposed conductive parts of equipment to the main earthing terminal. 

Electric shock is normally caused either by touching a conductive part that is normally live, or by touching an exposed conductive part made live by a fault. The touch voltage curve in Fig. 6.8 shows that a person in contact with 230 V must be released from this danger in 40 ms if harmful effects are to be avoided. Similarly, a person in contact with 400V must be released in 15 ms to avoid being harmed. 

---