A fault detector for power transformers which quickly indicates an internal fault condition. In one embodiment, the current in the primary and secondary windings are compared by a differential relay which may be restrained by a voltage developed during magnetizing inrush current conditions. This voltage is provided by a pick-up coil which is located between the primary winding and the magnetic core leg. The coil is oriented so that the voltage induced therein is relatively large when the core leg is saturated by an inrush current. In another embodiment, a first voltage from a similarly positioned pick-up coil is integrated and applied to a differential relay. A second voltage which is proportional to the difference between the magnetomotive forces in the primary and secondary windings is also applied to the differential relay. The differential relay compares the integrated voltage and the second voltage and is activated when an internal fault changes the normal ratio of these two voltages.

For filtering off of low frequency and a periodic transient phenomena in relay protection devices for electric high voltage networks, the signals of which occur in the event of a fault in the network and which are superimposed on the signals supplied to measuring relays arranged in the relay protection device, a normally short-circuited filter is included in a normally open alternative path for the measuring signal which is parallel to a part of the normal signal path. A supervisory circuit includes a time circuit, a fast fault indicator and an undervoltage detector. A signal from the fault indicator breaks the short-circuit of the filter and, upon a signal from the undervoltage detector, connects the filter in the path of the measuring signal. The measuring relay is blocked during a time interval which is equal to the difference between the pulse time of the time circuit and the time of operation of the undervoltage detector. This is accomplished by an AND-gate which has its inputs connected to the fast fault indicator and to the undervoltage detector and its output connected to a make-and-break contact for closing the alternative current path.

A current differential protection arrangement for an electric power supply unit with a measured value pre-processing device and an analysis device that checks on the basis of a predefined dependence of a differential current on a stabilizing current (response characteristic curve) to determine whether the differential and stabilizing current values thus formed are in a blocking range or a triggering range, where an additional stabilization range is present in the blocking range. To avoid fixed-time blocking after an external fault with transformer saturation with such a protection arrangement, the analysis device in a stabilizing circuit contains a testing arrangement which checks the current values thus formed to determine whether they describe a point in the additional stabilization range. If this is the case, a blocking signal is produced, which activates the blocking device downstream from the testing arrangement. When a first point of the current values is subsequently detected outside the additional stabilization range, a timing element that is connected at the output to a resetting input of the blocking device is set.

A current ratio-differential relay for protecting a power system comprises a logical circuit which provides the final output upon determination that the operating force signal is active and the suppressing force signal is inactive. The operating force signal is made active when the scalar quantity of the vector summation of line currents flowing into and out of the bus-bar is larger than a certain value, and the suppressing force signal is made active when the maximum value of scalar quantities of the line currents is larger than the scalar quantity of the vectorially-summed current multiplied by a certain constant. Both signals are supplied to the logical circuit through circuit means having a certain time delay in the restoring operation.

The amplitude is detected of both the input and output signals of the circuit (such as an audio amplifier) it is desired to protect. When, for example, a short circuit occurs across the load being driven, there is a sudden change in the ratio of these amplitudes and in response to this change, the signal to the output driver stage of the circuit is shunted to ground.