Okay. I`ll give it a whirl. TT. Under correct operational conditions the L, N & E are quite seperate within the installation. So far, so good
Carry out an RCD test downstream of the device.
What does the tester actually do - dumps some of the incoming line current to the CPCs.
So, even tho the impedance thru a TTs earthing system (thru the cpc, earthing conductor, down rod,then thru the mass of earth) back to the supply transformer will be huge compared to the Z of L/N, the Potential Difference between L & E is enough for current to flow & take the only path it knows - the one of least resistance.
Important to remember here, that the Neutral in our functional installation has been left out the loop. Its seperated from the PE - as it should be - but now its not carrying all the incoming Line current back out, cos its being leaked to earth...
Guess what, that ****es the RCD off, so it trips - just as we`d want it to.
However, as you stated, with the cct in question connected, you had 0 Mohms - a dead short between N & PE. For now lets not worry quite where, how or why - just that we know we`ve a path of negligible impedance between the two.
Now when the RCD tester leaks a bit to earth, what does that current want to do. It`ll jump to earth ok, cos its at a lower PD. But so is the Neutral. And now they`re interconnected within the installation. And the neutrals path back to the TX is one hell of alot less impeded than all that soil & stuff. So which way does it head? Not down the CPCs to the rod. No Sir. Straight down the Neutral conductors & back along the main to the centre tap, where it then heads back t`ther way along the Line.
So now we`ve basically got our Earth Loop path as; Line from trannny > thru L pole of RCCB > along R1 to RCD tester > which dumps it to the CPC > which is now interconnected to the Neutral > so fault current then heads back thru N pole of RCCB > back to TX (& not thru Ze)
The residual current device senses no apparent imbalance, so holds. Just as it should in the circumstances - it`s not faulty.
But we do still have a fault - a short between N & PE. With the test current removed, the cct will once again function. Not safely. And not reliably either. Because in this condition it`ll be susceptible to transients in the earthing system. And when a neighbours rod, in close proximity, dumps enough to earth, instead of heading thru the earth back to the centre tap, as its designed to, now its got the nice low ohm Neutral to piggyback...and the RCD won`t like that.
So it trips. The user closes the contacts again. The small reversed earth fault current it sensed is now attenuated, it holds once again. Until the next eddy heads back upstream...
Hopefully, that makes some sense of it for you mate.
Al
