Main Earth Size

[New2Game]

Is it ok to have a 10mm main earth if the phase and neutral are 16mm on a TNCS system?

 

[Adam_151]

What does 54H say?

 

[New2Game]

Yeah i've checked that and it says acceptable, but other tables for phase conductor state 16mm

 

[Adam_151]

Yeah i've checked that and it says acceptable, but other tables for phase conductor state 16mm

As long as it meets 54H then you may size it as you would do any CPC or a main earth on TNS; 54G or the adiabatic

 

[RF Lighting]

Table 54H is for sizing the MEBs.

I assume by 'main earth' you mean the wire from the cutout / MET to the CU?

 

[Adam_151]

Table 54H is for sizing the MEBs.

I assume by 'main earth' you mean the wire from the cutout / MET to the CU?

Despite there being nothing concrete in BS7671 to say so, I always assumed the earthing conductor should meet 54H as well as the bonding conductors, otherwise you could if you used the adiabatic to size the earthing conductor end up with bonds of a greater CSA than the earthing conductor which seems a tad silly to me

 

[New2Game]

Table 54H is for sizing the MEBs.

I assume by 'main earth' you mean the wire from the cutout / MET to the CU?

Yeah i mean the main earth from cutout to MET, so is 10mm acceptable?

 

[Spark123]

What does your distribution network operator say about it?

 

[GaryMo]

543-01-01 says that CSA of every protective conductor other than equipotential bonding conductors shall be

(i) calculated in accordance with 543-01-1 (adiabatic equation: S=sqrt(I²t)/k

or

(ii) selected in accordance with 543-01-04:

54G says that if the phase conductor is less than or equal to 16mm2 then the protective conductor should be the same size as the phase conductor.

 

[Spark123]

How do you calculate the current flowing in the main earthing conductor in the event of a network fault to calculate the size using the adiabatic?

 

[GaryMo]

Measure Ze then calculate (I=V/R) or pfc at the origin.
That should be the maximum current which can flow under fault conditions for the installation.

 

[Spark123]

As it is TN-CS then the main earthing conductor is connected to the neutral conductor.
The neutral conductor will almost certainly be at a different potential to mother earth, hence current may be flowing through the MEBs from extraneous conductive parts etc. Also, I have read on here someone who disconnected one of the main equipotential bonding conductors only to find the network had a fault on it and the network was using his earthing conductor, MEBs, metallic water/gas pipe, neighbours MEBs, their earthing conductor for continuity!! I think he said he saw a big flash!!!
For the calcultation, I cannot see how the current nor the time can be calculated in this instance.

 

[EdwardCurrent]

Hope you weren't in a rush New2Game !


Ed

 

[GaryMo]

For the calcultation, I cannot see how the current nor the time can be calculated in this instance.

Nor do I, I was just stating BS7671's take on things.
In this instance I would go with table 54G and upgrade to 16mm2.

 

[GaryMo]

Hope you weren't in a rush New2Game !


Ed

If he's unsure then a metre of 16mm2 6491X isn't going to kill the budget!