TT systems Table 54.1 P127

[pompeygit]

Assistance please gentlemen with this table. Earthing conductor from MET through a buried duct into a pit with a lid. According to this table 2.5mm2 is adequate.........Tell me where I'm going wrong

 

[Spark123]

Nowhere, that is what the table is telling you!
What sort of duct? Does it provide adequate mechanical protection?

 

[pompeygit]

Thanks for reply Spark123. This is a hypothetical. Lets assume that the earthing conductor and the electrode have as much protection as Fort Knox. What mystifies me is that our main protective bonds into the METare 10mm2, then we run 2.5mm2 to the electrode.....it just doesn't seem to sit well together

 

[Spark123]

The resistance of the electrodes and the general mass of earth will mean in a fault only a limited amount of current will flow. This is why you tend to RCD protect all circuits in a TT system. If you look at your regs you will probably find you can use 6mm² for a main protective bonding conductor in a TT system. (iirc at least half the size of the earthing conductor, 6mm² min)

 

[RF Lighting]

You certainly could use 6.0mm for your MEBs under the 16th edition for a TT supply, not sure about 17th, but I havn't seen anything to the contrary.

I still install 10mm MPBs for TT supplies in readiness as and when a PME conversion becomes available.