Earth Cable Size for TT system

[LalindaPrasad]

I have used ecostructur power design to size my project. But earth size is half of phase conductor as usual. Why is it not used adiabatic equation for this and size PE accordingly. If so cable size will be very less. 

Please advise

[Genghiz]

Hello,

Thanks for your query and interest on the 'EcoStruxure Power Design Ecodial application. 

Yes, there is an adiabatic equation for conductor sizing is available.

However, this can be applied during the following conditions only.

• When calculating the ‘fault current capability/rating’ of the cable…It is generally assumed that the duration of the ‘fault current’ is short, and there is no heat is dissipated by the cable to the surrounding.
• This is applicable only for the ‘disconnection time of the protection devices is not exceeding 5 seconds.

Limitations of adiabatic equation:

• If the application of the formula produces ‘nonstandard sizes,’ then the conductors of the ‘nearest standard cross-sectional area shall be considered.’
• It is necessary that cross sectional area; so calculated be compatible with the conditions imposed by the fault loop impedance.
• The maximum permissible temperature of the joints should be taken in to account for the further decisions.

[LalindaPrasad]

Hi,

Yes. but why is it exactly half size of conductor. If we calculate 3 ohm earth loop impedance(TT system) it will be very less value. 

Please advise. 

Can't they include it to the software ?

[Genghiz · Accepted solution]

Thanks for your comments.

As we are respecting the IEC 60364-5-52 and 5-54; We are ensuring the minimum permissible cross sectional area of the protective conductor, even with the following sub condition respected.

[LalindaPrasad]

Thanks. I am having earth loop impedance 3 ohm. Calculation gives me less than 2.5 mm2 for main earth cable size at main switch board. So if i use 4 mm2 cable will be satisfied right. 

Please advise. 

[LalindaPrasad]

Actually phase size is 240 sqmm Cu/XLPE/PVC. Calculation says PE size very less for mentioned impedance. 

[Genghiz]

Thanks for your comments;

It would be better to discuss (over a call) with our support experts by sharing the screen and explaining the project scenario.

they would prefer to understand the "calculated fault level" and the complete network.

So, kindly raise a support case through the following link, and our representative would be able to reach you for the further analysis.

https://www.se.com/in/en/work/support/customer-care/contact-schneider-electric.jsp