In the United Kingdom there are three main types of Earthing protection for domestic properties’
The first and most effective are called “PME” (Protective Multiple Earth) systems. These are also known as TNC-S systems.
These systems incorporate the earthing cable and the Neutral conductor to the substation in order to provide a path for any current in the event of a fault.
The second type of system is known as a TN-S. These systems rely on the outer sheathing of the electrical supply cable to carry fault currents to earth.
These systems are not a good as the PME systems as if the outer sheath of the electrical supply cable to the property becomes damaged, the fault current cannot flow to earth.
The third type of system is called a TT system. This type of system is not that effective in dealing with electrical faults. The system is connected to an earth rod generally located external to the property.
The problem with TT systems is that when the earth dries out over prolonged dry periods, the area surrounding the earth rod will crack and expand, therefore the rod is not always in good contact with earth.
The Three systems derive their name from the following:
T Being Earth
TT = Earth Earth, i.e. the earth cable connected to an earth rod.
TNC-S = Earth connected to neutral “N” combined in the ground “C” but separate in the property “S”
TNS = Earth and neutral separate
It is recommended that if you have a TT system in your property ie an earth rod outside,
You should have it upgraded to a TNC-S PME supply, as it will probably not carry the correct current in the event of a fault.