Kerry Burdett - Technical Support Advisor
The T-T system uses an earthed neutral at the supply transformer and an earth electrode at the user’s installation. The electricity supplier provides no earth conductor therefore an electrode is used. The earth electrode is connected to the main earth terminal of the installation. T-T systems are often found in remote locations such as rural villages, caravan sites and temporary supplies from generators in applications such as fair grounds.
Fig.1 A T-T earth system (reproduced by kind permission of the IEE)
The IEE recommended figure for external loop impedance (Ze) for a T-T system is up to 200O although current standards define a higher maximum value. Due to Ohm’s law, this means low fault currents can be expected when you have a high resistance or impedance.
Ze = 200O V = 230V
PFC = 230/200 = 1.15A
The IEE recommended figure for external loop impedance (Ze) for a T-T system is up to 200Ω although current standards define a higher maximum value. Due to Ohm’s law, this means low fault currents can be expected when you have a high resistance or impedance.
Although more difficult to make the connections, this is still a valid method for testing an RCD. An additional earth electrode cannot just be put in the ground wherever convenient. Testing will need to take place to ensure the resistance areas do not cross over. If they meet, a lower resistance result will be measured rather than the true resistance value. (see Fig.4)
Fig 4 B is greater than A
As a general rule, the distance between electrodes should be at least the depth of the electrodes e.g., 1 m depth, 1 m distance between them. (see fig.3)
Fig 3 B is less than A
In some locations, the electricity supplier will not connect the installation to the supply unless the earth electrode has been tested.
To do this a dedicated earth tester would be required. Or following connection to the supply an earth loop impedance tester (normally known as a loop tester) can be used. In conclusion, testing on T-T earthing systems offers its own particular problems as a result of the local earth electrode that has to be used.
Other earthing systems such as TN-S and TN-C-S do not require a local earth electrode and so do not have the resultant problems with high resistance earth connections.
For more detailed information on earth resistance testing see the Megger publication ‘Getting Down to Earth’ the practical guide to earth testing available by clicking here