The invention relates to an on-line High Voltage (H.V) coupler for monitoring of electrical discharges in large turbo-generators. In recent years, generators of higher power rating are being introduced to meet the fast increase of power demand. The reliability of such machines is very important, as an unplanned outage of a machine leads to loss of revenue and also high repair cost. Insulation system in a generator is an important component, which needs to be maintained with high reliability. The deterioration of insulation in service is mainly caused by temperature. In large machines, forced hydrogen cooling system is employed to maintain the temperature. However, there may be hot sports in the insulation system where the temperature is more than the environment. This is due to the combined effect of prevailing thermal environment and electrical discharges. These discharges occur in insulation due to (i) presence of gas pocket (ii) absence of conducting paint on the outer surface of insulation in the slot region and (iii) conductor fatigue and breakage. The discharges increase the temperature at a spot, leading to local thermal breakdown and ultimate insulation failure. Hence, it is necessary to monitor the level of such discharges, generally known as 'partial discharges' (PD). Though the effect of partial discharges is depicted by the conventional off-line electrical measurements viz. tanδ, discharge energy, on-line PD detection and measurement becomes absolute necessary especially for large machines in order to maintain high operational reliability. A simple method for on-line monitoring of discharge is to provide an impedance or a high frequency current transformer at the neutral path of the generator. The measurement can be made in the form of time-based pulses or measuring the Radio Frequency Spectrum (RF Spectrum). In general, the magnitude of time based pulses is depending upon the actual discharges. However, because of amplifier bandwidth limitation there is always a cut, off frequency, beyond which the sensitivity of measurement decreases. Recently many manufacturers use amplifiers with very high bandwidth (Up to 300 MHz). In case of RF spectrum a wide range of frequency can be covered and hence the sensitivity of measurement is improved. There are some advantages with the present system of monitoring of discharges. One of the main disadvantages is that even though the neutral current measurement is able to detect "partial discharges', it cannot locate from which phase the "PD1 originates. Therefore the main object of the present invention is to install an on-line H.V. coupler for monitoring of electrical discharges for turbo-generators. Another object of the present invention of an on-line H.V. coupler for monitoring of electrical discharges in large turbo-generators is to provide a low voltage arm (resistance) across which partial discharge signals can be tapped out for measurement. Yet another object of the present invention of an on¬line HV coupler for monitoring of electrical discharges in large turbo generators is to provide a necessary scheme to protect the system and the operating personnel in the event of the failure of the surge capacitor. Still another object of the present invention of an on¬line HV coupler for monitoring of electrical discharges in large turbo generators is to provide control system required for the effective functioning of the system. According to the present invention there is provided an on-line High Voltage (H.V.) coupler for monitoring of electrical discharges in large turbo-generators comprising a Low Voltage (L.V.) arm in the form of a non-inductive resistor connected between surge capacitor and ground, a resistance divider circuit connected to a protection circuit for protection against overvoltage and a control circuit for on/off control and protection against failure output of said resistance divider circuit is connected to the measuring system. In the aforesaid on-line high voltage (HV) coupler, said control circuit is provided with a main relay Rl with coupler "on" switch with Hold-on, (ii) "off" switch and