RCD and TN-C Earthing system

The question is unanswerable. British Standard 7671 calls for RCDs to be fitted to domestic circuits feeding:

• outdoor circuits
• cooker circuits
• kitchen sockets
• various other areas of heightened shock risk

The method of earthing does not affect the above.

Dear all,

Surely, TN-S is much preferred to TN-C system.

Vector sum of Line, Neutral and earthing current is always zero.

This means Line current + Neutral current + Earth current = 0.

Thus, Line current + Neutral current = - Earth current.

RCD sees only Line and Neutral current sum.

If Line current + Neutral current = 0, Earth current = 0. Thus there is no earth fault.

If Line current + Neutral current is not 0, Earth current is not 0, Thus there is an earth fault.

This scheme works well, only if Neutral does not have earth current.

TN-C system, PE and N are combined to form a PEN conductor. RCD should see Line and Neutral current only, but some earth current flows into RCD. This make the RCD less sensible. That is why TN-C is not prepered for RCD.

Regards

<...TN-C system, PE and N are combined to form a PEN conductor. RCD should see Line and Neutral current only, but some earth current flows into RCD. This make the RCD less sensible. That is why TN-C is not prepered for RCD....>

Eh? What?

RCDs are wired to live and neutral only, and detect a current imbalance. The earthing method for the installation is not at all significant. For example, RCDs work just as well on TT earthing, and would work on a hypothetical system that was not earthed at the distribution board.

Dear all,

TN-C system, there are Line and PEN conductors.

As mentioned by PWSlack, RCDs are wired to "line and neutral only".

If Line and PEN is connected to RCD, RCD will read Line + Neutral + Earth current.

In case of an earth fault, the earth current is distributed among PE and other conductive materials (human body, concrete floor etc) between earthed point and system earthed point (transformer or generator neutral). If the earth leakage current is 100mA, the earth current in PEN may be 80 mA and other 20 mA at concrete floor.

In this case, the unbalanced current at RCD is only 20 mA, because this 20 mA does not go through the RCD. 30mA RCD will not trip with 100mA earth leakage.

I hope everybody understand "TN-C system will decrease the RCSs sensitivity."

Regards