size of meb on a tns supply

[17thman]

Hi.

Quoting for work in conservatory. At present, the meb on a quote is 6mm.

Now if i calculate the required csa of the main earthing conductor using S= square root of (I squared x t) all divided by K

I have I= mpefc=230/0.56(ze) =410A
t = 10 seconds for an 80A BS1361@ 410 A
K = 143

Required S therefore = 10mm


So, using 544.1.1 The 6mm meb will be ok, won't it?

or should i calculate using the max efli that could occur of .8,

which comes out at 16mm, and therefore 10mm meb is needed.

Thanks

 

[Zambezi]

On a TN-S the main earth should be 16mm (tails should be 25mm)
Main protective bonding conductors should be 10mm

 

[ricicle]

On a TN-S the main earth should be 16mm (tails should be 25mm)
Main protective bonding conductors should be 10mm

I suggest you have a read of 544.1.1

 

[17thman]

ZAmbezi, it does not say that.

Basically, elfi is .56 giving mpefc of 410A

But should i take the max elfi of 0.8 ohms as a worst case scenario. giving a need for 16mm earthing conductor and then leading to having to upgrade the meb. Or is it o.k to use the reading of .56 ohms and the reading of 410A.

 

[Zambezi]

The figures I quoted were from the OSG.

 

[Zambezi]

I have just read 544.1.1 and it confirms what I said.
If the supply neutral is 35mm or less then the minimum cross sectional area of the main protective bonding conductor should be 10mm
Have I interpreted it correctly?

 

[rebuke]

Haven't got it in front of me, but I believe the 10mm min is for PME, not TN-S. Having said that though, I believe there's a reg that states that the MEB must be at minimum half the size of the main earthing conductor - what size is that, as if it's 16mm, then you'll have to use 10mm for the MEB...

 

[Zambezi]

544.1.1 is for anything except PME

 

[17thman]

that is why my question states tns supply, I am choosing to claculate the main earthing conductor rather than select it, and then to use 544.1.1 to see if the presently fitted meb of 6mm is sufficient. Using my measured ze of .56, leads to a main earthing csa requirment of 9mm. so nest one up is 10 and using 544.1.1. 6mm meb is ok.

What i am asking is, can I use the .56 ohms or must I use the maximum .8 ohms.

Any advice is appreciated

 

[17thman]

544.1.1 is for a tn-s

table 54.8 is for pme

 

[Zambezi]

Have a look at Table 54.7 on P130, does that help?

 

[Spark123]

Have a look at Table 54.7 on P130, does that help?

Read 543.1.1 first

If you want to do the math then you may find an earthing conductor of 10mm is adequate, hence a 6mm MPB is the minimum size acceptable providing the conductor doesn't exceed 0.05ohms in total - in reality to avoid the math table 54.7 is generally used.

 

[17thman]

cheers spark123, would you use the measured or .8ohms.

 

[Spark123]

I'd use the measured as this will give a higher current than the max allowable efli.

 

[17thman]

cool. thanks for your input.