Abstract: This application relates to methods and apparatus for handling a fault associated with a voltage source converter (VSC) for exchanging electrical power between an AC system (101, 102) and a DC system (106-1, 106-2). The VSC (104) is connected to the AC system via an interface apparatus, comprising a transformer (107) with a set of primary windings (202) for coupling to a plurality of AC phases (A, B, C) of the AC system. In embodiments of the disclosure the set of primary windings having a neutral point (N) and the interface apparatus includes a fault module (301) having an energy storage element (302) connected in parallel with a resistive element (303) between the neutral point of the set of primary windings and a reference voltage, such as ground.

Abstract: A system and method for fault location and isolation in an electrical power distribution network, where the network includes a plurality of switching devices provided along a feeder. The method includes detecting an overcurrent event in the network from the fault and interrupting the overcurrent event by opening and then immediately locking out or subsequently reclosing and testing the fault. A count value is increased in each switching device that detected the overcurrent event. A count and current (C&I) message is sent from each of the switching devices that detected the overcurrent event and then detected the loss of voltage upstream to an upstream neighbor switching device. Current measurements in the C&I messages, measured current by the devices and the counts values in the devices determine what devices are opened to isolate the fault.

Abstract: A direct current electrical network includes nodes linked by branches and protection elements mounted on the branches, each protection element includes a central processing unit, a current sensor, a current limiter device and a circuit breaking device, wherein: the current sensor is configured to determine the direction of the current in the branch relative to the node with which said protection element is associated; the central processing unit is configured to select, as a function of the direction of the current, a selected threshold value as one of a first value or a second threshold value greater than the first threshold value in absolute value, and to compare the value of the intensity of the current to the selected threshold value; and the current limiter device is bidirectional and is configured to limit the current passing through the branch to the selected threshold value.

Abstract: Provided is a backflow prevention circuit including a backflow prevention transistor as a p-channel MOS transistor interposed in series between an input terminal to which a power supply voltage is supplied, and an output-stage transistor as a p-channel MOS transistor, configured to supply an output voltage from an output terminal, and a backflow prevention control circuit configured to turn off the backflow prevention transistor if the output voltage exceeds the power supply voltage. The backflow prevention control circuit includes a first transistor, a first current source circuit, and a level shift circuit.

Abstract: Messages on controller area network (CAN) buses are communicated over subsea links. Messages are sent as electrical or optical signals. The present invention provides a subsea CAN BUS electronic distribution unit (EDU) for transmitting, receiving, converting, and routing electrical or optical signals sent over a subsea CAN BUS network. The CAN BUS EDU of the present invention is contained within a single housing and combines the functions of transmitting, receiving, converting, and routing electrical or optical signals sent over a subsea CAN BUS network that would typically be handled by multiple devices.

Abstract: A waveform shaping circuit includes a first parallel circuit including a first capacitance element and a first resistance element coupled in parallel with each other, a positive pulse voltage being applied to a first terminal of the first capacitance element and a second terminal of the first resistance element, a first rectifier circuit disposed between a point of coupling between a third terminal of the first capacitance element and a fourth terminal of the first resistance element and an output terminal.

Abstract: The present invention discloses a method for identifying the fault by current differential protection and a device thereof. The method comprises: measuring the full component currents of the two terminals of a two-terminal line system and calculating the corresponding fault component current vectors; obtaining an operate value by calculating a first difference between an absolute value of the sum of the fault component current vectors and Iset1; obtaining a restrain value by multiplying a second difference with a control factor, in which the second difference is calculated between the maximum of the absolute values of the fault component currents and Iset2 or between the absolute value of the difference of said fault component currents and Iset2; and identifying a fault as an external one or internal one by comparing the operate value with the restrain value. The solutions of the present invention achieve better reliability, sensitivity and faster speed than existing products.

Abstract: A location of a broken electrical conductor of an electric power delivery system may be detected by monitoring a rate of change of phase voltage and/or a rate of change of zero-sequence voltage at various points on the conductor. Intelligent electronic devices (IEDs) such as phasor measurement units may be used to obtain measurements and calculate synchrophasors. The synchrophasors may be used by a central controller to determine which two continuous IEDs measure rates of change of voltages of opposite polarities, where the broken conductor is between the two continuous IEDs. The synchrophasors may be used by a central controller to determine which two continuous IEDs where one exhibits a zero-sequence voltage magnitude that exceeds a predetermined threshold for a predetermined time, wherein the zero-sequence voltage magnitude of the other of the continuous IEDs does not exceed the predetermined threshold.

Abstract: A power supply protection system including a first metal oxide field effect transistor (MOSFET) having a first source, a first drain coupled to an input node, and a first gate; a second MOSFET having a second source coupled to the first source, a second drain coupled to an output node and a second gate coupled to the first gate, whereby the first MOSFET and the second MOSFET are coupled back-to-back; a zener diode coupled between the first and second sources and the first and second gates; and charge pump circuitry coupled across the zener diode, whereby a voltage on the first and second gates is boosted with respect to the voltage on the first and second sources.

Abstract: Techniques for improving reliability of a superconducting fault current limiting system (SCFCL) are provided. In one particular exemplary embodiment, the technique may be realized as a method of improving a reliability of a superconducting fault current limiting system (SCFCL), the SCFCL system comprising a superconductor provided in a container. The method may comprise providing one or more sensors capable of detecting a fault current proximate to the superconductor; determining a change in the condition of the superconductor as a result of the fault current; and estimating the lifetime of the superconductor based on the change in the condition of the superconductor.

Abstract: A system and method includes a circuit for protecting a photovoltaic string. A bypass switch connects in parallel to the photovoltaic string and a hot spot protection switch connects in series with the photovoltaic string. A first control signal controls opening and closing of the bypass switch and a second control signal controls opening and closing of the hot spot protection switch. Upon detection of a hot spot condition the first control signal closes the bypass switch and after the bypass switch is closed the second control signal opens the hot spot protection switch.

Abstract: In an example embodiment, systems and methods for detecting and handling faults in a DC bus system (“system”) can comprise detecting a fault in the segment, isolating the segment from the system, waiting for a period of time, connecting a probe power unit to the system at the isolated segment, and determining if a fault still exists on the isolated segment, wherein the determining is based on whether power is discharging from the probe power unit when connected to the system. The system can comprise one or more controllers and a probe power unit.

Abstract: A DC voltage circuit breaker includes a first to fourth nodes, a first interrupter disposed between the first node and the fourth node, a second interrupter disposed between the fourth node and the third node, a pulse generator circuit disposed between the fourth node and the second node, the pulse generator circuit having a capacitor connected in parallel with a series circuit of an inductor and a switch, and a first energy absorber disposed between the third node and the second node.

Abstract: Nuisance tripping is reduced in a power protection and distribution system including an upstream circuit breaker, a power distribution bus, and one or more downstream circuit breakers. A signal indicative of a current or pending downstream circuit breaker tripping action, such as a Zone Selective Interlocking (ZSI) signal, normally used to delay tripping of the upstream breaker, is used to suppress an optical flash detection system from tripping the upstream breaker in response to an optical flash resulting from the tripping of the downstream breaker. In this way, nuisance tripping of the upstream breaker is avoided when the downstream breaker trips (and the ZSI function is preserved).

Abstract: A direct current feeder protection system includes six conductors, four current sensors sensing four currents flowing in four of the conductors, four circuit interrupters interrupting currents flowing in the four conductors, and a processor cooperating with the current sensors to input four sensed currents, and to output a number of commands to the circuit interrupters. The processor includes a routine detecting a number of: a directional current fault of the sensed currents, a differential current fault of the sensed first and third currents or the sensed second and fourth currents, and a residual current fault of the sensed first and second currents or the sensed third and fourth currents, and to responsively output the commands. The first, fifth and third conductors are electrically connected in series with two of the circuit interrupters, and the second, sixth and fourth conductors are electrically connected in series with the other circuit interrupters.

Abstract: A method is disclosed for selectively triggering circuit breakers in the event of a short circuit, wherein an upstream circuit breaker and at least one downstream circuit breaker on the output side are provided. In at least one embodiment, the upstream circuit breaker monitors as to whether the or one of the downstream circuit breakers is already in the process of opening the switch contacts thereof to interrupt the circuit on the output side, while forming a switch arc. In order to reliably determine whether a downstream circuit breaker is already in the process of opening, according to at least one embodiment of the invention the upstream circuit breaker, so as to detect a switch arc on the output side, checks whether the ohmic resistance of the short circuit loop produced by a short circuit has an exponential curve over time in that an exponent is continually computed and based on the computer exponent the switch arc is determined if the computed exponent exceeds a threshold.

Abstract: A power supply system S comprising a power processor 1, having a power input 2 and an AC power output 3 with a nominal voltage VN and a nominal current IN, and a power supply safety circuit 6 connected in parallel to said power processor 1 between said power input 2 and said AC power output 3. The purpose of the power supply safety circuit 6 is to provide an additional current in case of a short circuit condition downstream of the AC power output 3, so as to trigger the opening of an individual circuit breaker 4 connected to the short circuited load 5.

Abstract: A Compact Secondary Substation including a fault protection system and a method for fault protection in Compact Secondary Substations (CSS). The CSS includes a Ring Main Unit (RMU), a transformer and a Low Voltage (LV) switchgear. The CSS includes a number of detectors, which detectors can be placed at least in one of the RMU, the transformer or in the LV switchgear, which detectors can be connected to a control unit, which control unit by fault detection by the detectors can activate a fast operating switch for grounding. By using the fast operating grounding switch and controlling this on the basis of the detectors placed inside the CSS, an active arc fault protection is achieved. The control unit also controls the fast operating grounding switch so that it works as a working grounding switch.

Abstract: The present invention relates to a transformerless photovoltaic grid-connecting inverting device and an inverting control method thereof. The inverting device comprising a boosting unit, an inverting unit, a grid-connecting unit and a control unit. The boosting unit is connected to a solar cell, for boosting an output voltage of the solar cell and then outputting a direct voltage. The inverting unit is used to convert the direct voltage output by the boosting unit into an alternating voltage. The grid-connecting unit is connected between the inverting unit and an alternating power grid, and closes or breaks up the electrical connection between the inverting unit and the alternating power grid based on a control signal.

Abstract: There is described a peripheral module which in addition to the connection terminals for the process data supply has connection terminals for the supply of voltage. Furthermore, the peripheral module has a changeover switching device to disconnect the peripheral module from an upstream load group. The peripheral module can assume the function of a supply group or a power module when a supply voltage is applied to the terminals. In a peripheral system made of several peripheral modules the voltage within a load group is supplied via an internal self-constructing voltage bus.

Abstract: A method is disclosed for selectively triggering circuit breakers in the event of a short circuit, wherein an upstream circuit breaker and at least one downstream circuit breaker on the output side are provided. In at least one embodiment, the upstream circuit breaker monitors as to whether the or one of the downstream circuit breakers is already in the process of opening the switch contacts thereof to interrupt the circuit on the output side, while forming a switch arc. In order to reliably determine whether a downstream circuit breaker is already in the process of opening, according to at least one embodiment of the invention the upstream circuit breaker, so as to detect a switch arc on the output side, checks whether the ohmic resistance of the short circuit loop produced by a short circuit has an exponential curve over time in that an exponent is continually computed and based on the computer exponent the switch arc is determined if the computed exponent exceeds a threshold.

Abstract: In an electrical storage apparatus including an electric storage device connected between a main power supply and a load, a first bypass FET and a bypass diode which are connected in series between the main power supply and the load are provided, and first and second main path FETs connected in series between the electric storage device and the load are provided. A controller judges that the first bypass FET suffers an open-circuit failure if the voltage (Va) of the load is equal to or smaller than the first threshold value Vth1 or if the voltage (Vc) at the connection point between the first bypass FET and the bypass diode is equal to or smaller than the second threshold value Vth2, when the first bypass FET is turned on and the first main path FET and the second main path FET are turned off.

Abstract: An arc crowbar with electrodes separated by a gap in a protective case. Each electrode is connected to an electrically different conductor of a circuit. A sensor detects an arc flash condition on the circuit and signals a trigger circuit to send an electrical pulse to an arc-triggering device in the arc crowbar gap. The triggering device ionizes a portion of the gas between the electrodes, initiating a protective arc between the electrodes that absorbs energy from the power circuit and trips a breaker, eliminating the arc flash condition. The triggering device may be a plasma gun, especially one that injects plasma of an ablated material into the gap. The sensor may signal a circuit breaker to open in the power circuit. Arc flash sensor types may include a differential current sensor and/or an optical sensor.

Abstract: A circuit breaker includes separable contacts, an operating mechanism structured to open and close the separable contacts, and a trip mechanism. The trip mechanism includes a sensor structured to sense current flowing through the separable contacts, a processor cooperating with the sensor to determine a plurality of different trip conditions responsive to the sensed current, a nonvolatile memory operatively associated with the processor, and a trip actuator cooperating with the processor and the operating mechanism to trip open the separable contacts responsive to one of the different trip conditions from the processor. The processor is structured to save in and retrieve from the nonvolatile memory cause-of-trip information for the different trip conditions including the one of the different trip conditions.

Abstract: An alternative power source provides alternating current (AC) through a proper connector into a first outlet of a pre-existing electrical distribution system which is normally powered by an alternating current electrical power supply through main breakers and a breaker box. At least one of the breakers which controls power from the normal AC electrical power supply on the electrical distribution system is opened prior to connecting the alternative power supply. The alternative power supply provides electricity through the proper connector into the first outlet, or node, to provide alternating current to a selected portion of the electrical distribution system, and possibly to both halves of the pre-existing electrical distribution system if the proper connector is connected to a 240 volt outlet, or if two proper connectors are utilized and connected to first and second outlets with the first and second outlets on each of the two phase (power) wires of the pre-existing electrical distribution system.

Abstract: A virtual closed loop power distribution system couples a parallel source to a feeder upon an initial indication of a fault existing on a distribution feeder If the fault is persistent, a fault protection system including fault protection devices segmenting the distribution feeder operates to isolate the fault segment of the distribution feeder from each of the coupled sources. The coupled sources provide substantially uninterrupted service to the non-faulted segments of the distribution feeder until a circuit reconfiguration and return-to-normal function operates to restore the system upon repair of the fault.

Abstract: The invention relates to a method for error location in branched low voltage and medium voltage networks and to an evaluation network used therefor.

Abstract: A method of operating a relay device, in an electrical circuit comprising first and second main circuit breakers, and a bus tie circuit breaker connecting the first and second main circuit breakers, the method comprising computing a first positive sequence current phasor for the first main circuit breaker and a second positive sequence current phasor for the second main circuit breaker computing a first positive sequence voltage phasor for the first main circuit breaker and a second positive sequence voltage phasor for the second main circuit breaker; computing a correction of the first positive sequence voltage and second positive sequence voltage to a line-to-neutral phase angle reference if there is any rotation in a potential transformer connection; determining a fault has occurred when a magnitude of a current through at least one of the first main circuit breaker or the second main circuit breaker exceeds a predetermined threshold; determining a phase angle difference by comparing the phase angle of the

Abstract: An installation is disclosed which can be switched on again by way of a common switch-on key. A pulse processing device integrated into the respective emergency off device is required for putting the emergency-off device for predetermined period of time after the switch-on pulse of the switch-on pulse of the switch-on key into an activation mode in which it can be switched on when no switch-off signal is available. It is thus possible to take the reaction times of the individual devices or actuators required for the cascading of emergency-off devices for switching on into consideration for a common switch-on operation.

Abstract: A method of operating a relay device, the method comprising the steps of: providing main circuit-breakers and a bus-tie-breaker connecting the main-circuit-breakers; defining a partial-differential-zone comprising a main circuit breaker and the bus-tie breaker; assigning a first value to the direction of the fault current flowing into the partial differential zone;—assigning a second value to the direction of the fault currents flowing out of the partial-differential-zone, wherein the first value is not equal to the second value;—comparing the values assigned to the fault currents; determining if the fault currents are flowing into the partial differential zone; and determining if at least two of the fault-currents is flowing in a different direction with respect to the partial-differential-zone wherein one of the fault currents is flowing into the partial-differential-zone and one of the fault currents is flowing out of the partial differential zone.

Abstract: An over-voltage protection system including a phase bus connected to a phase conductor of an electrical system and one or more transient-suppressing lines connected to the phase bus. Each of the transient-suppressing lines includes a contactor and a transient-suppressing element. The contactor of each transient-suppressing line is selectively opened and closed by a processor, thereby protecting the transient suppressing element from excessive currents.

Abstract: A system for bypassing a power cell of a power supply, the system including a multi-winding device having a primary winding and a plurality of three-phase secondary windings, a plurality of power cells, wherein each power cell is connected to a different three-phase secondary winding of the multi-winding device, and a bypass device connected to first and second input terminals of at least one of the power cells and to first and second output terminals of the at least one of the power cells.

Abstract: There is described a peripheral module which in addition to the connection terminals for the process data supply has connection terminals for the supply of voltage. Furthermore, the peripheral module has a changeover switching device to disconnect the peripheral module from an upstream load group. The peripheral module can assume the function of a supply group or a power module when a supply voltage is applied to the terminals. In a peripheral system made of several peripheral modules the voltage within a load group is supplied via an internal self-constructing voltage bus.

Abstract: A system and method for accomplishing power swing blocking and unstable power swing tripping schemes during disturbances of electrical networks is disclosed. The disclosed system and method eliminates the requirement of stability studies. The no-setting scheme utilizes the swing center voltage of the electrical network to carry out its functions.

Abstract: A safety switching apparatus for safe disconnection of an electrical load has at least one input for connecting a signaling device. The safety switching apparatus further has an evaluation and control unit and at least one switching element which can be controlled by the evaluation and control unit in order to interrupt an electrical power supply path to the load. The evaluation and control unit is designed to carry out functional tests at defined instances of time in order to check at least one switching function of the at least one switching element. Moreover, the at least one input for connecting the signaling device is further designed as an input for supplying a supply voltage required for operation of the at least one switching element.

Abstract: A power converter that can continue to operate after suffering a partial failure. The power converter includes multiple array transformers; normally-on switches connected respectively in series with the ends of each of the primary windings of the array transformers; normally-off current bypass devices connected in parallel with the series connections of the primary windings of the array transformers and the switches at the transformer ends; AC-DC converter units having AC sides respectively connected secondary windings of the array transformers; and mutually independent DC circuits respectively connected to DC sides of the AC-DC converter units. By turning on the current bypass device of the primary winding of a specified array transformer and turning off the switches at the ends of that primary winding, it is possible to isolate the specified array transformer and the AC-DC converter unit connected to it.

Abstract: The disclosure includes improvements in the field of retrofit-detector installation such as adapters that simplify retrofit of addressable detectors onto previously installed zone-type system hardware. Each adapter includes electrical circuitry for electrically connecting new addressable detectors with legacy system once installation is complete. The disclosure includes structures that permit users to efficiently solve a number of problems that may arise during installation of retrofit-detectors into legacy systems. For example, one or more switches may be used to reverse electrical polarity to thereby correct polarity and/or mapping faults. One or more switches may also be provided to disconnect at least one electrical terminal to assist in locating ground faults and/or other wiring problems. One or more test points may also be provided to test one or more electrical characteristics relevant to retrofit installations.

Abstract: An electronic circuit for use with an exhaustible power source and at least one load such as a light bulb, a radio or motor, includes a microchip with an input that transmits a signal to the microchip when the load is activated or deactivated. The input does not form a serial link between the power source and the load. The power switch, by on/off switching, controls energy flow from the power source to the load. The input to the microchip acts as an activation/deactivation user interface. The microchip allows the user to select a specific load, specific functions for the selected load based on the time duration of activation signals, the time duration between activation signals and the number of activation signals at the input.

Abstract: The invention contemplates a technique for providing loop control on a faulted overhead or underground loop system, while preventing restored current from being fed or back fed into the fault. In particular, the invention provides a method, device, and system for restoring power to a faulted loop distribution system. The inventive method includes detecting a fault on the loop distribution system, interrupting current on the faulted part of the loop distribution system, isolating the faulted part of the loop distribution system from an unfaulted part of the loop distribution system, restoring current to the unfaulted part of the loop distribution system, and preventing the current from flowing to the faulted part of the loop distribution system. The method may further include providing the current to a load.

Abstract: A switching circuit includes a microchip which, in response to a signal from a signal switch, controls the operation of a power switch which, when closed, connects a load to a battery. The microchip can monitor the status of the battery and control the power switch to ensure optimum operation of the load and optimum usage of the energy in the battery. The microchip can control the connection of the load to the battery in different ways according to the manner of operation of the signal switch.

Abstract: A preferred embodiment of a system comprises a recloser electrically coupled to a voltage source of a power-distribution network and adapted to isolate a section of a distribution feeder of the power-distribution network from the voltage source in response to an overcurrent condition in the section of the distribution feeder. The system also comprises a sectionalizer electrically coupled to the section of the distribution feeder. The sectionalizer and the recloser are adapted to communicate via a data network and the recloser is responsive to control inputs sent to the recloser from the sectionalizer over the data network.

Abstract: The invention relates to a method and arrangement for generating a trigger signal according to the current differential protection principle when a fault occurs on a section of an electrical power supply system, in which differential current values and stabilization current values are detected and monitored with regard to exceeding limit values. A trigger signal is generated if positive results of the instances of monitoring are present. The differential current values and the stabilization current values are calculated with instantaneous values of the detected power supply currents as instantaneous values. A first measurement quantity and a second measurement quantity are formed, and a check is made to determine whether the two measurement quantities exceed a predetermined limit value of the differential quotient of the differential current with respect to time. If the instances of evaluation and the instances of monitoring produce positive results, the trigger signal is generated.

Abstract: A method for isolating suites includes the steps of isolating the suites when an alarm is initiated, setting a timer during isolation creating a window of time to clear a short circuit, clearing said short circuit, removing isolation from the suites as the timer reaches completion, determining if there is a short circuit in each of the suites, and isolating suites that have a short circuit.

Abstract: A fault protection system whose trip logic considers power magnitude and direction at first and second points on a conductor is provided. Under normal operating conditions, power flows in the same direction through first and second points on the conductor. Under faulted conditions, a differential power condition exists; that is, the power flowing at a first point on the conductor is of different magnitude and/or direction than the power flowing at a second point on the conductor. The trip logic identifies the differential power condition as the result of a fault between the first and second points on the conductor and causes a protective device on each side of the fault to trip open, thus isolating the fault.

Abstract: The present invention relates to a line module protection method and a device utilizing that method. According to the line module protection method of the present invention, line module protection is performed by switching the connection from a broken line accommodating module to an auxiliary module. A plurality of line accommodating modules are included in a line accommodating unit connected to the lines in a network. The plurality of line accommodating modules are divided into a plurality of groups, and the auxiliary module is placed substantially at the same distance from all the groups. Thus, a large number of line accommodating modules for performing line module protection can be employed with one auxiliary module.

Abstract: A distribution control system includes in each of feeder terminal units a controller that updates a value in a latest current value storage area of a memory, every time a current value is detected at a sampling interval. If a distribution line fault takes place, that is, if the detected current value exceeds a reference value, each controller stores the current value in a fault current value storage area of the memory as a fault current value. The feeder terminal units each supply the information about the fault current to a control center through a substation remote terminal unit so that the control center identifies a fault section from the information for detecting the fault section.

Abstract: In switchgear with superordinate power circuit-breakers and a plurality of subordinate power circuit-breakers, the superordinate power circuit-breaker must be triggered in the event of failure of the subordinate power circuit-breakers. This is accomplished as follows: electrical post-arc currents are detected after current zero, the electrical post-arc currents are compared with a predetermined limit value, and if the limit value is exceeded, a signal is effected for triggering the superordinate power circuit-breaker in the switchgear. This method can advantageously be implemented in particular in the case of vacuum circuit-breakers with at least one vacuum interrupter. The associated arrangement has a device which monitors the subordinate power circuit-breakers and, if appropriate, triggers the superordinate power circuit-breaker.

Abstract: A distribution control system that can isolate a fault section without fail, even if control of load break switches becomes impossible because of a communication error between a control center and a feeder terminal unit. If a communication error takes place between the control center and a particular feeder terminal unit, for example, feeder terminal units associated with the particular feeder terminal unit change the operation mode of their controlling load break switches from non-voltage unopened mode to non-voltage open mode.

Abstract: A vehicular use power distribution apparatus includes a battery mounted in a vehicle; a main control means connected directly to the battery; a first switch means connected to the battery; a power feed line for loads connected to the first switch means and supplying power to loads mounted in the vehicle; a second switch means connected to the battery; a power feed line for control means connected to the second switch means and controlling the loads; and a sub control means connected to the power feed line for control means. In a first state, the main control means sets the first switch means and the second switch means to a connection state so as to supply power from the battery to the power feed line for loads and the power feed line for control means.

Abstract: Short-circuits are liable to occur between the two conductors in a system in which data and/or energy is transmitted to a plurality of receivers/users via two conductors. Transmission is then interrupted and the supply source is inadmissibly over-loaded. The invention provides disconnecting units which recognize occurrence of short-circuits between the conductors and, in response thereto, isolate the part of the system in which the short-circuit has occurred from the supply of data and/or energy, so that the remainder of the system remains operational. After removal of the short-circuit, the isolated part is automatically re-connected to the system. The potential on the conductors ahead of and following a disconnecting unit is monitored via current limiting circuit elements which bridge a switch in the disconnecting unit.