Main Earthing Conductor Regulations 2024
There is often a lot of confusion surrounding main earthing conductor regulations and how they should be applied when conducting 18th edition inspection & testing.
Please note that this guide deals with the Main Earthing Conductor and the parts of BS7671 (mainly section 4 on protection) which apply to that. Protective bonding is a separate issue altogether and is often confused between earthing and bonding.
The differing types of earthing arrangement also play a particular role in determining the size of the earthing conductor itself (more on this later)
Basics On Main Earthing Conductor Regulations
The Main Earthing Conductor (MEC) is a cable which connects the specific earthing arrangement with the Main Earthing Terminal (MET)
The MET can be located either externally from any consumer unit/distribution board, or particularly on smaller installations, the MET may simply be part of the consumer unit.
The specific cable colour for protective earth cables is green/yellow striped. This is easily identified and no cables of this colour may be used for any purpose other than earth connections/bonding
The Different Earthing Arrangement Types
The main earthing arrangement is generally one of 3 types in the UK (although others do exist for more particular installations, these will be covered in a separate post):
- TNS – This stands for Terra-Neutral Separate = The neutral conductor and the protective earth conductor to the installation from the transformer are distinct and separate. In this instance the protective earth connection is provided by, and maintained by the DNO.
- TN-C-S – This stands for Terra-Neutral Combined Separate = The neutral conductor and the protective earth conductor are one and the same from the transformer to the incoming service head at the installation. Here the neutral and protective earth are separated out. This is often called PME (Protective Multiple Earthing) as the incoming neutral cable has multiple connections to earth electrodes along it’s route to the consumer. In this instance the protective earth connection is provided by, and maintained by the DNO.
- TT – This stands for Terra-Terra. There is simply a Line & Neutral supply to the premises. A protective earth connection is not supplied by the DNO. These installations have an earth electrode of some kind, be it a spike, a mat or tape in the foundations. In this instance the protective earth connection is provided by, and maintained by the end user.
When having a new supply installed, the DNO is under no requirement to provide a means of earthing, you may have to TT the installation yourself. In the instance of DNO supplied protective earth connections pre-existing, the DNO is bound by the ESQCR (Electricity Supply Quality & Continuity Regulations) to maintain that means of earthing.
In most modern installations the earthing type is now generally TN-C-S (PME). This provides a good quality, low impedance path to earth. However it is not without it’s problems, particularly in the case of ‘lost neutral conductor’ situations.
How To Size Main Earthing Conductors
The main issue surrounding earthing conductor regulations is the issue of correct sizing of the conductor. This can be done in one of 2 ways:
- Selection – Table 4??? gives a series of instructions for sizing earthing conductors according to the size of the incoming neutral conductor of the supply. If the installation earthing arrangement is TN-C-S (PME) then this method of sizing the earthing conductor is the only way permitted.
- Calculation – An alternative way to size earthing conductors is by calculation using the Adiabatic Equation. This equation uses the prospective fault current encountered at the installation, together with the time it would take the OCPD to blow and finally using a K figure for the different types of cable which could be used (copper/aluminium). This method is only allowed for main earthing conductors on TNS or TT earthing arrangements.
It is best, where possible, particularly in the case of new installations, to always follow the guidance laid out in table???? as this will cover the installation even if the main earthing arrangement is ever changed to TN-C-S in future.
Certainly with regards new supplies by the DNO, most will now insist on a 16mm main earthing conductor being supplied as a minimum on domestic and small scale commercial supplies.
Protected Against Corrosion
The wiring regulations make requirements for different size main earthing conductors when calculating based on whether the conductor is protected against corrosion or not.
Sheathed cables, ie: those which have protection against corrosion, can be thinner than the required cross sectional area of a conductor which does not have protection against corrosion