METHOD, APPARATUS AND SYSTEM FOR DETERMINING A CAPACITANCE VALUE OF AN EARTHING CAPACITANCE OF AN UNEARTHED POWER SUPPLY SYSTEM

The invention proposes a method for determining a capacitance value (CE') of an earthing capacitance (CE) of an unearthed power supply system (1), in which, in order to monitor an insulation resistance (RE) of the power supply system (1) with the aid of an insulation monitoring device (8), a periodic voltage (10) with a variable period duration (T) dependent on the earthing capacitance (CE) is applied to the power supply system (1), wherein a measurement capacitance (CM) with a known capacitance value (CM') is electrically connected to the power supply system (1), and the capacitance value (CE') of the earthing capacitance (CE)) is determined on the basis of a change in the period duration (T) or a temporal variable (~T) proportional thereto, which change is established on account of the connection of the measurement capacitance (CM). An apparatus and a system for carrying out this method are also proposed.