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

mapj1:

In a TNC system (unlike a TN-S or TNC-s) you cannot tell what is a legitimate neutral current, and what is an earth fault current using the PEN, as the earth and neutral path is common.

Therefore there will be multiple connections from the PEN to parallel paths (imagine TNC distribution and metal water pipes feeding a row of buildings - at each building the water pipe is earthed by a water heater.) so the neutral current is expected to follow these alternate paths   to some degree, so the live and PEN current are not in balance, even when all is working well.

If however you were very careful to only connected the PEN to earth at one point, then what you say would work, but the second connection to earth - the person, would unbalance the system.

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.