adiabatic equation

[ses]

Can someone please tell me if i have done this correct.
Got a fuseboard change and the tails are 16mm sinlges fed from an 800kmf with a 6mm single main earth.

Board is other side of house to meter so a pain to upgrade main earth.

Done the adiabitic and it comes out as follows.

Supply is tnc-s with an 80amp bs 1361


So : S = Square root of I(squared) x Time divided by k factor

230/0.2 = 1150 ( I)

1150 x 1150 = 1322500 ( I squared)

1322500 x 0.2 = 264500 ( I squared x Time for 80amp fuse)

Square root = 3.59

So 6mm main earth is fine

 

[Guest123]

Hi

Where is the main bonding connected...the DB or the MET???

 

[ses]

gas is at the met water is from the db will be installing 10mm bonds from met to gas and water

 

[ses]

just read the post below so i think i will run the 10mm bonds from the met in the meter cupboard and the 6mm cpc for the db leave in place

 

[pushrod]

Can someone please tell me if i have done this correct.
Got a fuseboard change and the tails are 16mm sinlges fed from an 800kmf with a 6mm single main earth.

Board is other side of house to meter so a pain to upgrade main earth.

Done the adiabitic and it comes out as follows.

Supply is tnc-s with an 80amp bs 1361


So : S = Square root of I(squared) x Time divided by k factor

230/0.2 = 1150 ( I)

1150 x 1150 = 1322500 ( I squared)

1322500 x 0.2 = 264500 ( I squared x Time for 80amp fuse)

Square root = 3.59

So 6mm main earth is fine

Sorry don't follow your calc at all - k(for copper) usually 115

but anyway, if it is a tn-c-s the earthing conductor should be at least 16 mm and the protective bonding conductor should be at least 10mm (see table 54.8 BRB)

 

[Sav]

Hi,
I think the adiabatic equation is for cpc cross sectional areas (csa's) in a circuit.

It does not apply to the cpc csa for main protective or bonding conductors.

You should use 16mm main earth and 10mm for gas/water

Sav

 

[shane]

yeh adiabatic is for cpc's mate, and you will have to up grade the tails to 25mm2(obviously) as well as the earthing conductor to 16mm2, the gas and water can be 10mm2.....

 

[ses]

I dont agree with you on that.
Where in the regs does it say that the main earth should be 16mm for tnc-s supply?
It says that you can use either calculation or the table to choose the size for any earth except main bonds.
If i run the main bonds back to the incoming met then dont see a problem.
And yes sorry used the wrong k factor so comes out at 4.7 mm

 

[ses]

Single core 16mm is rated to 87amps clipped direct

 

[pushrod]

I dont agree with you on that.
Where in the regs does it say that the main earth should be 16mm for tnc-s supply?
It says that you can use either calculation or the table to choose the size for any earth except main bonds.
If i run the main bonds back to the incoming met then dont see a problem.
And yes sorry used the wrong k factor so comes out at 4.7 mm

Reg 544.1.1 leads you to table 54.8 which tells you the minimum size of a main protective bonding conductor for a tn-c-s is 10mm.

with regard to the earthing conductor i think it is more a usual requirement of the distributor wanting a minimum size for both tn-s and tn-c-s. The OSG page 28, note 2 tells you this .

Hope this helps.

 

[ngoodson1969]

Can someone please tell me if i have done this correct.
Got a fuseboard change and the tails are 16mm sinlges fed from an 800kmf with a 6mm single main earth.

Board is other side of house to meter so a pain to upgrade main earth.

Done the adiabitic and it comes out as follows.

Supply is tnc-s with an 80amp bs 1361


So : S = Square root of I(squared) x Time divided by k factor

230/0.2 = 1150 ( I)

1150 x 1150 = 1322500 ( I squared)

1322500 x 0.2 = 264500 ( I squared x Time for 80amp fuse)

Square root = 3.59

So 6mm main earth is fine

Where did the 0.2 come from, resistance presumably but why 0.2?

 

[Sintra]

The 0.2 would be the Ze value.

 

[ses]

the first 0.2 is the ze and the second one is for disconnection time of an 80amp fuse