How do you tell what earthing system (TT, TN-C, TN-S, TN-C-S) a domestic electricity supply has?

*International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN, TT, and IT.

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):
T : direct connection of a point with earth (French: terre);
I : no point is connected with earth (isolation), except perhaps via a high impedance.
The second letter indicates the connection between earth and the electrical device being supplied:
T : direct connection with earth, independent of any other earth connection in the supply system;
N : connection to earth via the supply network.

*TN−S : PE and N are separate conductors that are connected together only near the power source.

*TN−C : A combined PEN conductor fulfills the functions of both a PE and an N conductor.

*TN−C−S : Part of the system uses a combined PEN conductor, which is at some point split up into separate PE and N lines. The combined PEN conductor typically occurs between the substation and the entry point into the building, whereas within the building separate PE and N conductors are used. In the UK, this system is also known as protective multiple earthing (PME), because of the practice of connecting the combined neutral-and-earth conductor to real earth at many locations, to reduce the risk of broken neutrals - with a similar system in Australia being designated as multiple earthed neutral (MEN).

Tt Earthing System

Tncs System

As the others have said, there may be a lable on the incoming supply fuse box. If there isn't a label then don't guess. It may even be different for your neighbour. You could be horribly wrong Call your electricity suppler.