What is the difference between TN-S, TN-C-S, and TT earthing systems?

Your home's earthing system is the way electricity can safely return to earth in a fault. There are three types used in the UK. TN-C-S (also called PME) is the most common — your earth is provided by the electricity network's own conductors. If the connection is good, this works very well. TN-S is older and found in properties near older substations — it has a separate earth conductor (usually the metal sheath of the supply cable). TT systems are found in rural properties where there's no easy connection to the network earth — these properties have a metal rod or plate buried in the ground as their earth, and they rely on RCDs for safety. You can usually tell which type you have by looking at where the earth wire from your consumer unit goes: into the meter tails (TN-C-S), into the metal sheath of the supply cable (TN-S), or to a separate earth rod outside (TT). An electrician can confirm this during an inspection.