Principle

The path followed by fault current as the result of a low impedance occurring between the phase conductor and earthed metal is called the earth fault loop. Current is driven through the loop impedance by the supply voltage.

The extent of the earth fault loop for a TT system is shown in {Fig 5.7}, and is made up of the following labelled parts.

Fig 5.7 The earth fault loop

l. - the phase conductor from the transformer to the installation

2. - the protective device(s) in the installation

3. - the installation phase conductors from the intake position to the fault

4. - the fault itself (usually assumed to have zero impedance)

5. - the protective conductor system

6. - the main earthing terminal

7. - the earthing conductor

8. - the installation earth electrode

9. - the general mass of earth

10. - the Supply Company's earth electrode

11. - the Supply Company's earthing conductor

12. - the secondary winding of the supply transformer

For a TN-S system (where the Electricity Supply Company provides an earth terminal), items 8 to 10 are replaced by the PE conductor, which usually takes the form of the armouring (and sheath if there is one) of the underground supply cable.

For a TN-C-S system (protective multiple earthing) items 8 to 11 are replaced by the combined neutral and earth conductor.

For a TN-C system (earthed concentric wiring), items 5 to 11 are replaced by the combined neutral and earth wiring of both the installation and of the supply.

It is readily apparent that the impedance of the loop will probably be a good deal higher for the TT system, where the loop includes the resistance of two earth electrodes as well as an earth path, than for the other methods where the complete loop consists of metallic conductors.