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Why RCD is unallowed in TNC system? I think it is useful for preventing indirect electric shock

Former Community Member
Former Community Member
Why RCD is unallowed in TNC system? I think it is useful for preventing indirect electric shock. For example when a fault happens (phase to metal body) and a person contacts the metal body, current flows from his body and goes to the earth. So Unbalancing current of phase and PEN inside of RCD make it to be triped.
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    Do you mean in TNC system, There is unwanted tripping of RCD so because of this we can not use RCD?




    yes -

    In a TNC circuit with a singly earthed PEN, it will quite correctly only trip on faults to terra-firm earth, and not trip on faults to PEN (as these are the same as legitimate currents L-N)


    In a TNC system with a PEN earthed in more than one place ( for example the distribution side of a PME or MEN  installation) then PEN and live currents are always out of balance, so any RCD will operate, even if there is no fault . This is why when there is a power cut and the DNO bring in a hired genset to back feed the network, their first act is to disable the earth leakage trip


    This may or may not be unwanted.

     


Reply
  • 0

    Do you mean in TNC system, There is unwanted tripping of RCD so because of this we can not use RCD?




    yes -

    In a TNC circuit with a singly earthed PEN, it will quite correctly only trip on faults to terra-firm earth, and not trip on faults to PEN (as these are the same as legitimate currents L-N)


    In a TNC system with a PEN earthed in more than one place ( for example the distribution side of a PME or MEN  installation) then PEN and live currents are always out of balance, so any RCD will operate, even if there is no fault . This is why when there is a power cut and the DNO bring in a hired genset to back feed the network, their first act is to disable the earth leakage trip


    This may or may not be unwanted.

     


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