Hi guys,
During power restoration (either the RCD being turned on again or the power being reapplied after an outage), there is usually a small amount of Earth inrush as power is seldom applied at the zero crossing of the waveform (not to mention that Live often connects first followed by the Neutral!). This inrush resulting from the natural (and unnatural) capacitance existing on the system.
RCDs need to ignore such inrush else one would simply never be able to restore power to a circuit (many who remember the very early RCDs will remember this problem especially as the "cure" was to trip all sub circuits, restore the main supply, and then, with fear, reenergise each circuit in turn praying the RCD would remain on!).
This anti-inrush-tripping is very simply achieved in the "non-electronic" versions by having two secondary windings on the RCD core. The first winding powers the operating solenoid (as shown in most explanations on RCDs). The second winding also feeds the solenoid, but is wired in reverse (i.e. the two windings cancel each other out). However, the second winding is fed via diodes and a capacitor meaning the solenoid can only operate once the capacitor has charged and thus removes the cancelling current.
At low currents the capacitor has very little charging to do as well as the diodes, because they have voltage drop, allow most of the main secondary current to flow in the solenoid with little cancelling current being allowed through - hence the faster operating times at lower currents.
Make sense?
M.
