The purpose of main protective bonding is to create an earthed equipotential zone. All exposed and extraneous conductive parts within this zone are connected to the Main Earth Terminal (MET) by means of the circuit protective conductors or the main protective bonding conductors.
Main protective bonding conductors are required to connect the MET to extraneous conductive parts such as:
* metal water service pipes
* metal gas installation pipes
* other metal service pipes (inc oil, gas, ducting)
* metal central heating and air conditioning pipes
* Exposed metallic building structure
* Lightening protection systems
If the water or gas supplies are plastic but the installation pipework is metal then main protective bonding conductors are required to connect the metal pipework to the MET. If both the supply and the installation pipework are plastic then no main protective bond is required.
Main protective bonding should be applied using copper conductors (other metals are acceptable, seek further advice). They should be as near as possible to where the service enters the building on the consumers side of any meters or insulating section, before any Tees, within 600mm of the meter or within 600mm of entry if the meter is external.
Main protective bonding conductors should be as short as possible and their resistance from bonded part to the MET should not exceed 0.05Ω.
Main protective bonding conductors should be continuous from the MET to the point of bonding without any breaks. They may however be looped between services providing the conductors remain unbroken.
The required size of the Main protective bonding conductors is dependent on the supply type.
The main protective conductors should not be less than half the size of the main earthing conductor for the installation and should not be less than 6mm². (The main earthing conductor is the cable between the DNO supply and the MET.) As an example a normal 100A TN-S service the meter tails will be 25mm², the main earthing conductor is sized as 16mm² in line with table 54.7 meaning the main protective bonding conductors will need to be 10mm.
For PME the size of the main protective bonding conductors need to be selected in line with table 54.8 (all sizes are given for copper cables);
| CSA of the supply neutral | Minimum CSA of MPB conductor |
|---|---|
| ≤35mm² | 10mm² |
| 35mm²> ≤50mm² | 16mm² |
| 50mm²> ≤95mm² | 25mm² |
| 95mm²> ≤150mm² | 35mm² |
| 150mm²≥ | 50mm² |
Note, the distribution network operator may require larger than the sizes stated above and should also be consulted during the selection process.
Similar to TN-S the main protective bonding conductors should not be less than half the size of the main earthing conductor which in this case is the cable between the electrode and the MET, the main protective bonding not having a CSA of less than 6mm².
TT supplies often get upgraded by the supplier to TN-C-S, so if you are installing or renewing main protecvive bonding cables you might as well use 10mm² or 16mm² as a form of future proofing.