This video explains the three main types of supply earthing - TT, TN-S and TN-C-S.
These are the three types typically used in the UK and other countries.
Although other types exist as described elsewhere in this section, TN-C is not permitted in the UK, and IT is rarely used - certainly not for normal supplies to dwellings and other buildings.
Here, the danger of a broken combined protective earth and neutral is explained. With the majority of supplies this is somewhat unlikely, particularly where concentric cable is used for the supply - a cable where the PEN conductors surround the inner line conductor.
Where the supply is provided with two separate conductors, such as overhead, a break in one only is more likely.
The most important point about this situation is that the only thing wrong is the break in one of the supply conductors - there is no fault with any of the equipment or cabling within the installation, and any RCD will not trip.
TN-C-S supplies are not therefore used for high risk situations such as portable caravans and temporary outdoor supplies which have exposed metalwork.