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pushrod
An RCD works on the current balance between L and N and the subsequent magnetic fields being balanced. A residual current difference of 30mA+ will generate a sufficient magnetic force to cause a trip. In the event of an earth fault the balance between the L current magnetic field is no longer balanced by the N current's mag field (some of the current is now travelling direct to earth) and so the DP switch is tripped. If it is a short (between L and N) then a larger current will flow in L due to the lower resistance but it will return via the N and so the balance is maintained and the device does not trip. However the larger current flow should trip the mcb.
If what you said was true there would be no point in having mcbs as well as rcds on the same circuit!!! An rcd is an earth fault device are you thinking of an RCBO?
