Page 660 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Earthing systems

Every dwelling will have its own earthing system to protect the owner from a
fatal shock. There are several kinds of system commonly used:
● TT system
● TN–S system
KEY POINT ● TN–C–S system.
The letters in earthing These systems are designed to earth the complete property via the earthing
systems stand for: cables throughout the house. If fitted correctly then any fault should return to
● T = earth (from the earth or a point of safety.
French terre)
● N = neutral TT system
● S = separate
● C = combined. The first ‘T’ stands for terre, and the second ‘T’ means that all exposed
metalwork is connected directly to earth via a large copper rod stuck directly
into the ground outside. This system is commonly used in rural areas, and can
be very hard to find as foliage hides the rod. If you do find it, do not touch or
disconnect the connections for any reason.
Electricity supply LABEL − Safety
(usually overhead) Electrical company isolator electrical
Metal water pipe connection
DO NOT REMOVE
Metal gas
pipe

100 A
16 mm 2
Main earthing terminal
10 mm 2
10 mm 2 Gas meter
LABEL − Safety electrical
connection DO NOT REMOVE
Earth rod
Water Gas
service service
pipe pipe

p Figure 11.6 TT system

TN–S system
This is a very common system, which connects the earth conductor back to the
metallic sheath of the distributor’s service cable as it enters the property. This
sheathing provides a separate neutral and earth route back to the sub-station
supply transformer.

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