Toroidal cores

       

Sometimes it is desired to have the current on only one out of two or more parallel conductors. This may be the case e. g. as a provisional solution in an "unclean" TN-S system where there are several interconnections between the neutral conductor and the earthing system, which should not be the case because this makes the advantages of the TN-S system against a TN-C system void. But sometimes it turns out fairly difficult and time consuming to spot and remove the superfluous connections. A toroidal core - or several of these, if required - slid onto the PE conductor may bring back most of the advantage of the TN-S system (until an adequate opportunity for the final refurbishment arises): The core blocks any great alternating current where said current is driven only by a minute voltage, as is the case when two conductors of great cross sections act in parallel. The split of the current across any parallel conductors is inversely proportional to the impedances of these parallel conductors. A toroidal core on one of the conductors can make its impedance many times higher; albeit this is only true as long as the voltage across the conductor with the core is very low, otherwise magnetic saturation will occur, and the core will thus hardly impair the tripping conditions in the case of an earth fault. Here you see an arrangement adequate to figure out where this point of beginning saturation lies: It uses an ordinary toroidal core transformer with a sufficiently wide "residual hole". The primary winding is energized via a variable transformer, while the secondary is ignored and a makeshift secondary winding with a very thick wire or cable and only one or very few turns mantled - and next week you will find the test results here.