Need for neutral grounding resistance in HV system
As the system voltage level increases the cables dielectric strength should be increased to offer higher level of insulation. This will in turn increase the mutual capacitance of the cables connected to the generator terminals. The mutual inductance of the cables and mutual capacitance together act like an LC oscillator circuit. Under steady state conditions this effect is of no consequence. When transient fault occurs in the system the current limiting action is done by alternator sub transient reactance and the cable reactance.The fault current is usually 10 to 12 times the rated current of the alternator. The system mutual capacitance effect causes oscillating voltage and will tend to increase the voltage surge that occurs at the instant of fault. This will damage the insulation of the breakers and cable to a large extent. If NGR is provided it will not only limit the fault current but also damps the voltage oscillations produced due to system’s mutual reactance and cable capacitance. In low voltage system the effect of mutual reactance as well as fault levels are much lower compared to HV system. Hence NGR is not recommended or in other words it is superfluous for LV system.
What is TN-C-S system? These have been designated in the IEE Regulations using the letters: T, N, C and S. These letters stand for:
T – terre (French for earth) and nmeaning a direct connection to earth.
N – neutral
C – combined
S – separate.
When these letters are grouped, they form the classification of a type of system.
The first letter denotes how the supply source is earthed.
The second denotes how the metalwork of an installation is earthed. The third and fourth indicate the functions of neutral and protective conductors.
A TN-C-S system is nothing but the supply cable sheath is also the neutral, i.e. it forms a combined earth/neutral conductor known as a PEN (protective earthed neutral) conductor.
The installation earth and neutral are separate conductors.
This system is also known as PME (protective multiple earthing).