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Substation Bonding cable/conductor size

M 90

Hello everyone. I'm working on project where the power source (Transformers) supplying the project loads has its neutral point earthed through a resistance of 20 ohm. Considering a 400V, 3ph system (230V, 1ph), this will lead us to a maximum line to ground of fault of 11.5A. This shall enable us to go with the mimimum CSA of protecive earth conductor as per BS7671 (4mm2 for non mechanically protected conductors). However, for the main bonding conductor within the substation, my understanding that it shall be at least half size of the earthing conductor as per 544.1.1. 

In the light of the above, is there a minimum CSA for the main earthing condutor or it can also follow the same regulation of the 4mm2 for non mechanically protected conductors. Please also note that the transformers and medium voltage switchgears are already located within the very same room and will be bonded to the same bonding conductor (maximum LG fault at HV side shall be 700A)

Thanks in advance for your help.

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  • 0

    earthed through a resistance of 20 ohm.

    IT system then? Not necessarily disconnecting on 1st earth fault?

    maximum line to ground of fault of 11.5A.

    But could be higher for a 2nd fault?

       - Andy.

  • 0 in reply to AJJewsbury

    Yes, we can consider it as an IT system. Actually this 20 ohm resistance will be inserted for only a duration of 10 seconds to give the chance to some loads where RCD is equipped to disconnect. Then , the neutral point will be theoritically isolated throughy a very high resistance.

    Second fault shall be line to line fault I assume.

  • 0 in reply to M 90

    Be careful with thoughts that flow from the idea that "the neutral point will be theoritically isolated throughy a very high resistance." in practice once isolated the voltage that it all floats too will be set by the uncontrolled potential division between the different capacitances  between various live or neutral conductors to ground formed by the witing and its installation. As the installation grows the current available through the capacitance values rises, and indeed beyond a certain range, IT can become can be more dangerous than the conventional method, or indeed resistively earthed neutral

    Mike.

Reply
  • 0 in reply to M 90

    Be careful with thoughts that flow from the idea that "the neutral point will be theoritically isolated throughy a very high resistance." in practice once isolated the voltage that it all floats too will be set by the uncontrolled potential division between the different capacitances  between various live or neutral conductors to ground formed by the witing and its installation. As the installation grows the current available through the capacitance values rises, and indeed beyond a certain range, IT can become can be more dangerous than the conventional method, or indeed resistively earthed neutral

    Mike.

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