In a TT system the source neutral is earthed at the substation and the installation's exposed metalwork goes to a local earth electrode, with no metallic return path to the source. Earth fault current has to pass through the soil between the two electrodes, so it is often only a few amperes, far too little to trip a fuse or breaker quickly. RCDs are therefore the main protection, and the electrode resistance must keep touch voltage under 50 V at the RCD rating.
Farm supply is TT, earth rod tested at 87 ohms, so a 30 mA RCD covers it with plenty to spare.
Also heard as
- TT earthing
Do not mix it up with
- TN system: In a TN system fault current returns through a metallic protective conductor, not through the soil.
Related terms
Earthing system
Arrangement describing how a supply's neutral and an installation's exposed metalwork are connected to earth.
RCD
Residual current device: trips when the current going out on the live conductors does not all come back.
Earth fault loop impedance
Total impedance of the path a fault current takes from the source, through an earth fault and back again.
Touch voltage
Voltage a person bridges between a hand on faulted metalwork and their feet or other hand during a fault.