The Wiring Unlimited is a valuable document that deserves to be maintained and developed. I hope to understand parts of it better and possibly help to improve. Throughout Chapter 7 there are several considerations about RCDs that I am getting thoughtful over.
Overall the message in the document seem to be that RCDs are useless unless the neutral is grounded. This should in fact mean that RCDs are useless in IT-networks.
Am I right to say that RCDs are not useless in IT networks? In fact I claim they are even more important than in TN systems. RCDs in IT networks provides early detection of second earth fault and more importantly may save a life if the system already contains one fault and a person introduces the second.
There is also one paragraph on page 62 that says that pressing the test button of an RCD is useless if the neutral-to-earth link is missing and that you will get a false impression that the RCD is operational. I would disagree. It was explained just a few paragraphs above how the test button works and this has nothing to do with the earth link. Then the main point is that the RCD will work as long as e.g. 30mA or more is leaking from the circuit.
In installations with a lot of components and numerous terminations, it is difficult to speculate in the likelihood of multiple semi-earth faults creating leakage currents. There is also a capacitance between earth and the live parts in IT networks that serves as a path for the first fault current. This current may be small in compact off grid networks, but may get significant enough in larger networks to enable the RCD to trip on the first fault in IT networks.
I would very much like to see Wiring unlimited to be more precise on this topic and make it more clear what earthing system a paragraph relates to. Also I would like to see a full drawing of an IT system together with the other drawings on page 64.
In an off grid network, is there really any reason why you should not use IT network?
I wonder if there are other perspectives on this topic? Please feel free to elaborate.
