Adiabatic equation calc

[rick.oshea]

can anyone help

I want to calculate a main earthing conductor for a three phase supply using the old adiabatic equation.

I'm struggling to work out what value of I to use. If you use 230V/0.35 (for a TN-C-S) then you get 657 amps.... if it turns out that Ze is lower, the fault current is going to be higher, so what do you do?

And again with time, do you use 0.1 or 0.4?

Any light shed would be greatly appreciated....

Cheers

 

[pushrod]

With regard to time i would go with 5s as it is a distribution circuit.

With 3 phase, pscc will be larger than pefc . edit : Think you need measured values or use table 54.7

 

[rick.oshea]

With regard to time i would go with 5s as it is a distribution circuit.

With 3 phase, pscc will be larger than pefc . edit : Think you need measured values or use table 54.7

Thanks Pushrod....... wanted to try and calc if it was possible (as a paperwork exercise) so I had it clear in my head rather than use 54.7.....

Q. would a Pscc fault use the earth path / earth conductor? Got in my head that it would follow 'line - neutral' or 'line - line' and then not come into the equation? Your thoughts?

 

[pushrod]

Thanks Pushrod....... wanted to try and calc if it was possible (as a paperwork exercise) so I had it clear in my head rather than use 54.7.....

Q. would a Pscc fault use the earth path / earth conductor? Got in my head that it would follow 'line - neutral' or 'line - line' and then not come into the equation? Your thoughts?

Max PSCC would be L-L as then it would be 400V/Zs, whereas a pefc would be L-E so would just be 230 V/Zs

 

[IQ Electrical]

On a TN-C-S system, the earthing conductor must also meet the requirements of 544.1.1 and so must be selected from 54.8 rather than calculated.

 

[pushrod]

On a TN-C-S system, the earthing conductor must also meet the requirements of 544.1.1 and so must be selected from 54.8 rather than calculated.

thought 54.8 was just for the main protective bonding conductors in a PME.

 

[rick.oshea]

On a TN-C-S system, the earthing conductor must also meet the requirements of 544.1.1 and so must be selected from 54.8 rather than calculated.

I thought that was for bonding?

 

[IQ Electrical]

Yes but on a TN-C-S system, the earthing conductor must also meet the requirements of a main protective bonding conductor.

 

[IQ Electrical]

Check out 542.3.1

 

[pushrod]

Check out 542.3.1

To my mind that is "543 and 544.1.1" so it is just imposing a minimum size of 10mm²; as 54.7 doesn't specify a min size. It just says S = protective conductor if below 16mm². Table 54.7 is a more stringent requirement (ie requiring a larger size) on earth size than 54.8 would be. So IMO i would still in a real life situation size by 54.7