Due to the way it is connected to the ground, networks with voltage range II are divided into TN systems, TT i IT.
The first letter in the network layout type symbol means correspondingly:
T - direct connection of a specific point (most often neutral) or points of the network to earth I - isolation of all parts of the network from earth or connection through a resistor (impeder) a specific point of the network with the ground.
The second letter means:
T - connection of the PE protective terminal of the device (receiver) with ground N - connection of the PE protective terminal of the device (receiver) with the net neutral point
TN networks, with direct earthing of the neutral points N, and the accessible conductive parts are connected to the neutral point of the N network by protective conductors, they are further divided into:
— TN-C systems, in which the function of the protective conductor PE and neutral N is performed by one protective-neutral conductor PEN,
— TN-S systems, in which separate conductors perform the function of the protective conductor PE and neutral N,
— TN-C-S systems, in which the first part of the network works in the TN-C system, and the other in the TN-S system.
TT networks are networks, with direct earthing of the neutral points N, and the accessible conductive parts are connected with protective conductors to the earthing electrode independent of the earthing of the neutral point of the network.
TT grid system; E - ground wire, N - neutral wire, PE - protective conductor, 1C - conductive parts available.
IT networks are networks, no point of which is directly connected to the earth, or network, in which the neutral point or other point in the network is connected to earth via a resistance (impedance) of sufficiently high value (Lynx. c), and the accessible conductive parts are directly connected to earth.
IT network system: a) isolated network, b)network with open earthing, c) network with indirect earthing; E - ground wire, PE - protective conductor, B - spark gap fuse, K - conductive parts available, R - resistor.