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