Supply Types
TT supplies are often found where the incoming wires are overhead, or in rural areas.
TT Supply
This type is the easiest to identify. There is no earth connection to the supply network at all. The earth connection is the responsibility of the property owner, and is normally provided by a metal earth rod in the ground.
The two T's signify Terre, or Earth. One of which is at the supply transformer, and the other one is the customer's earth rod. Other than the soil, there is no direct connection between them.
In the event of a live-earth fault, the current which flows via the earth rod may only be a few amps, and not enough to blow the fuse or operate the circuit breaker. For this reason, all TT supplies have RCDs fitted to disconnect the supply in the event of a fault.
Older installations (prior to 1981) may have a voltage operated earth trip instead of an RCD. These can be identified easily, as they will have an earth connection as well as live and neutral. They are no longer acceptable and should be replaced.
New colours
This diagram shows the same TT installation but with the new wiring colours. It is also possible that the tails between the cutout, meter and consumer unit will have a grey outer covering and coloured inner insulation.
Earth rod
The earth rod or electrode is typically a copper clad steel rod driven into the ground. However other options are available including metal mesh, copper tape and other metallic objects. The connection to the electrode must be accessible for testing and inspection purposes.
In some installations, the main erath may be the incoming water pipe. This is no longer acceptable, as underground water pipes are frequently replaced with plastic ones.
