Abstract: A digital measuring instrument which samples at discrete points in time the analog input signals emitted by a measurement transducer of an electrical supply system in a sample and hold circuit. The sampled values are digitized in an analog/digital converter and are then used to determine a system specific quantity such as, for example, the root mean square value or the system frequency. This measuring instrument is suitable for solving the essential measurement value processing tasks occurring in an electrical supply system in a simple manner and with projectable measurement accuracy. This is achieved by the fact that the digital processing unit includes a block for shifting the fundamental and harmonic frequencies to a direct current signal which is proportional to the system specific quantity and a subsequent filter arrangement with a digital low-pass filter for suppressing the frequencies deviating from the direct current signal and a digital band-stop filter for suppressing aliasing frequencies.

Abstract: A method for error-protected switching operation of switching devices of a high-voltage switching station. The current operational state of the station is determined from the signals provided by position indicating contacts (K.sub.I, K.sub.II, K.sub.III) of the switching devices. Using the obtained operational state, switching operation request commands (for example KA) are checked for release or blocking status by means of error-protected switching interlocking rules. In the case of changes in the topological configuration of the high-voltage switching station and/or in the case of changes of the error-protected switching interlocking rules, it is possible to adapt the basic method to these changes in a simple manner and without losing operational reliabilty. This is achieved by decoupling the topological configuration of the high-voltage switching station from the error-protected switching interlocking rules.