Abstract: A method and apparatus is disclosed for three phase sensing of a polyphase distribution system to provide true RMS signal monitoring capabilities. Each current transformer is provided with a rectifier and a separate burden to not only provide accurate signal representation of the true RMS currents in each phase, but also provide a DC voltage supply to thereby self-power the circuit. Two configurations are disclosed, one in which two CTs sense all three phases. In this configuration, two phases of the three phase system are used as a single primary winding in each current transformer such that each current transformer produces a voltage representation of a sum of two phase currents. A second configuration discloses the use of a current transformer for each phase. In both embodiments, by providing a separate burden and rectifier for each CT, true RMS signal characteristics can be extracted from the produced discrete signals, while at the same time providing a DC voltage supply for self-powering the circuit.

Abstract: Problems with inaccuracies in selecting the relay trip current in an overload relay are eliminated in an overload relay employing a custom potentiometer having a resistance that varies at multiple rates to set the trip current value. The dial markings on the trip current adjustment dial are sufficiently spaced to allow for a more precise selection of the relay trip current at the higher ampere settings of the relay.

Abstract: A device for protection against overload of the switching contacts (22, 23) of a switching device (6) has an energy transducer (5) which is arranged near the switch contacts (22, 23) and is exposed to electromagnetic radiation emanating from an electric arc occurring at the switch contacts (22, 23). If the switch contacts (22, 23) are separated electrodynamically due to an overload, the energy transducer causes the circuit to be shut down by tripping solenoid (1) of a power circuit-breaker (6), for example.

Abstract: A ground fault protection module is attached to a circuit breaker employing a thermal-magnetic trip unit to separate the circuit breaker contacts upon occurrence of a ground fault. The ground fault module contains a logic circuit and a giant magnetic resistor for detecting the ground fault.

Abstract: A current sensor has a closed loop magnetic hoop with a gap and a central aperture for receiving a conductor. An IC chip incorporating a Giant MagnetiResistive (GMR) sensor is positioned in the gap to provide a measure of the current in the conductor. A relatively small current transformer provides the necessary information about the direction of the current to an electronic trip unit for a circuit breaker as well as power supply for the IC chip and the trip unit. The trip unit operates in the presence of AC and DC faults. In a three phase power system, a single GMR chip in the proximity of three closely spaced phase conductors, together with a current measurement from the neutral line, enables trip units to operate on the occurrence of AC and DC faults.

Abstract: A multi-pole circuit breaker assembly for interrupting AC power to a load upon the detection of an earth leakage fault or an overcurrent condition includes a pair of circuit breaker units mounted adjacent to an earth leakage module. The earth leakage module includes an electronic circuit that senses a current imbalance between the power line lead and the neutral line lead using a differential transformer. In response to a current imbalance greater than a predetermined value, the electronic circuit periodically energizes a switching circuit that provides power across a solenoid. The solenoid, when actuated, engages a common trip mechanism to open concurrently the contacts of the circuit breakers units. The common trip mechanism includes an elongated bar that extends through the casing of the earth leakage module and the circuit breaker units. The elongated bar engages a U-shaped trip member pivotally disposed in each circuit breaker.

Abstract: A circuit breaker interconnect arrangement and a method for selective interruption of electrical circuits includes at least one upstream circuit breaker having an upstream electronic trip unit and a microprocessor and a plurality of downstream circuit breakers connected to the upstream circuit breaker. The microprocessor transmits a trip command signal to the downstream electronic trip units at the occurrence of a predetermined load condition, such as an overload condition, through the upstream circuit. One or more downstream circuit breakers are tripped after the predetermined time delay for tripping according to a predetermined priority until the load condition through the upstream circuit is reduced below the predetermined load condition whereby the upstream circuit breaker is deterred from interrupting current in the upstream circuit.

Abstract: A ground fault/rating plug for molded case circuit breakers is inserted within the circuit breaker cover for electrical connection with the circuit breaker electronic trip unit. A rating resistor within the ground fault/rating plug enclosure sets the circuit breaker ampere rating. Rotary switches on the outer surface of the ground fault/rating plug enclosure configures the trip unit for ground fault protection.

Abstract: A sectionalizer assembly includes a mounting structure having upper and lower electrical contacts, a sectionalizer tube, an actuating member mounted to the tube and movable from a non-actuated to an actuated position, and a pivotal latching mechanism having a pivot member, latch arm and catch. The pivot member is mounted to the lower electrical contact and to a lower end of the tube for undergoing pivotal movement relative thereto between locking and releasing positions. The latch arm fixedly attached to the pivot member extends outwardly therefrom and undergoes pivotal movement therewith between the locking and releasing positions. The catch is carried on a lower end of the actuating member and undergoes movement therewith. The latch arm has an outer end fittable within a recess in the catch that is open at least at one side thereof.

Abstract: A module for use with the circuit breaker, the module being arranged to be mounted on an end of the circuit breaker and to monitor and/or control the circuit breaker and/or an electrical circuit control thereby in use. The module can be active or passive, and, where circuit breakers are ganged in side-by-side relationship, the module can be equal in width to the width of the gang breaker unit to which the module is mounted. The invention also resides in the combination of a module and a circuit breaker.

Abstract: An electronic circuit interrupter is protected against thermal damage upon loosening of the lug connectors that connect the circuit interrupter with the load circuit. Thermal sensing elements attached to the lugs connect with the circuit interrupter trip unit to initiate circuit interruption when the temperature sensed exceeds a predetermined value.

Abstract: An overcurrent trip device includes a capacitor 13 that stores power for driving a trip electromagnet 7, and a charging current generating means 18 that starts charging the capacitor 13 upon detection of an overcurrent and that completes the charging of the capacitor 13 until a trip control circuit 5 outputs a trip signal. The operation of the trip electromagnet 7, which is driven with the charges of the capacitor 13 by causing a thyristor 12 to be turned on by the trip signal, is continued until the charges of the capacitor 13 are lost, and even after the power supply from a rectifying circuit 4 is stopped due to the opening of a circuit breaker 8, it is made impossible to reset and re-close the circuit breaker 8 for a predetermined time.

Abstract: A power converter protective apparatus for protecting an AC-DC converter against an excess fault current by preventing the fault current of a power system from flowing into the AC-DC converter. The protective apparatus comprises a series transformer, whose primary winding is connected in series with a power system and whose secondary winding is connected to a voltage-type AC-DC converter, first current bypass means of a normally open-type connected in parallel with the primary winding, a current transformer for detecting a fault current in the power system, and a control unit for outputting a bypass control signal to the first current bypass means when the current transformer detects the fault current, whereby when the fault in the power system is detected, a fault current flowing through the primary winding is bypassed by the first current bypass means.

Abstract: An electronic trip device for a circuit breaker comprising at least one current sensor supplying a current to a first power supply circuit. A distribution line connected to an output of the first power supply circuit distributes an electrical voltage regulated by a first chopping regulator to electronic circuits of the trip device. The trip device comprises a second power supply circuit connected to an external power source and a second chopping regulator connected between the second power supply circuit and the distribution line. The second chopping regulator operates in synchronized manner with the first chopping regulator.

Abstract: Each pole of a multi-pole circuit breaker is equipped to individually separate the circuit breaker contacts within each pole upon overcurrent occurrence. A digital processor within the circuit breaker trip unit determines the occurrence of zero current crossing of phase current within the individual poles and initiates contact separation at zero current values.

Abstract: A circuit breaker thermal-magnetic trip unit for arcing fault detection includes a current transformer having a gapped magnetic core arranged around the load side of the trip unit. The bimetal within the trip unit is positioned abutment with the gap in the transformer core to thereby heat the bimetal causing the bimetal to trip and separate the circuit breaker contacts.

Abstract: A motor starter includes separable contacts; an operating mechanism for operating the separable contacts; an overload relay controller including a memory for storing the settings for a plurality of selectable operating features, and a microprocessor for retrieving the settings from the memory and employing the settings at least for controlling the operating mechanism; and a programmer module temporarily engaging the overload relay controller for changing the settings in the memory.

Abstract: A circuit interrupter arrangement for interrupting current in a circuit path includes a hysteresis control circuit arranged to control a power supply charge-accumulation switch and an undervoltage trip-lockout circuit in parallel. More specifically, the arrangement includes a current inducer circuit for inducing a current signal having a magnitude corresponding to the current in the circuit path; an energy accumulator for accumulating a magnitude of energy to cause an interruption of the circuit path; an energy accumulator control circuit controlling the magnitude of energy accumulating; a trip lock-out circuit, responsive to an insufficient magnitude of accumulated energy, for preventing an interruption of the circuit path; and a range control circuit, responsive to the current inducer circuit, for operating a power-related signal between a first level and a second level to control the energy accumulator control circuit and the trip lock-out circuit.

Abstract: A combined bell alarm and lock-out accessory is connected with the electronic trip unit that controls a high ampere rated circuit breaker. The accessory interacts with the circuit breaker operating mechanism to activate the bell alarm upon circuit interruption and to prevent the closing of the circuit breaker contacts until the accessory is manually reset. The circuit breaker lock-out mechanism interfaces between the circuit breaker operating mechanism and the accessory to insure that the contacts remain open until the accessory is completely reset.

Abstract: An electrical receptacle unit includes at least one receptacle body (3) internally provided with a pair of slotted terminals (4) for insertion of a pair of blades (L) of a load-side electrical plug (P), a thermistor (7) encased in a protective tube (9) outwardly projectable from between the slotted terminals (4) of the receptacle body (3), a coil spring (13) for projecting the thermistor (7) out from the receptacle body (3), a relay (14) for enabling and disabling supply of electric power to the slotted terminals (4), a control circuit (19) which operates the relay (14) to cut off supply of electric power to the slotted terminals (4) when the temperature of the thermistor (7) reaches or exceeds a preset temperature, and a buzzer (23) operated by an output signal produced by the control circuit (19) when the temperature of the thermistor (7) reaches or exceeds the preset temperature.

Abstract: A counter indicator for circuit breakers maintaining a running count of the number of times that a circuit breaker has been tripped and providing a visual indication of a tripped condition. The counter indicator employs time delay circuitry so as to allow the circuit breaker to be switched manually for purposes of testing without registering a tripped condition. A two stage binary coded decimal counter visually displays the amount of counts that the breaker has been tripped or alternatively when a switch handle has been placed in an intermediate position. The device is powered by an independent battery source providing a continuous power supply to maintain memory and power to a visual display only upon detection of a tripped circuit.

Abstract: A circuit breaker device for use on an electric vehicle has a high-voltage drive power supply for supplying electric energy to a load to drive the electric vehicle, a circuit breaker for cutting off the electric energy supplied from the drive power supply to the load, a sensor assembly for detecting conditions of the electric vehicle, and a controller for controlling operation of the circuit breaker in response to a detected signal from the sensor assembly. The controller is connected to a low-voltage accessory power supply for energizing accessories on the electric vehicle, and also connected to the drive power supply through a power supply converter for converting the electric energy supplied from the drive power supply, so that the controller can be energized at all times by at least one of either the accessory power supply or the drive power supply.

Abstract: An electrical service installation comprises a plurality of circuit breakers protecting against overcurrent hazards in branch load circuits. An acceleration, bypass circuit is provided for each circuit breaker. Current flow through each of the bypass circuits is controlled by a normally non-conductive SCR. Current flow to each branch circuit passes through a sensing resistor that is connected across the gate and cathode of the associated SCR. When the voltage drop across the sensing resistor reflects a current level indicative of an "arc short circuit", the SCR is triggered to a conductive state. Current then flows through the bypass circuit increasing current flow through the associated circuit breaker, to thereby provide a substantially increased current magnitude for magnetically tripping the circuit breaker. Circuit breakers are thus tripped to provide protection against the hazards of "arc short circuit" currents that do not have a magnitude sufficient to initiate magnetic tripping.

Abstract: A hybrid switch is provided for switching a sink on and off. The hybrid switch contains a first semiconductor switch with a commutation switch. According to the present invention, a magnetic drive includes a first magnetic coil through which short-circuit current flows. A second semiconductor switch is connected in series with a second magnetic coil that series being connected in parallel to the series connection of the first magnetic coil and the sink. Short circuit flows through the first magnetic coil via the commutation switch which bridges at least the first semiconductor switch.

Abstract: A transient voltage surge suppressor comprises a first series circuit formed of a normally-off voltage surge-responsive gas discharge tube and a first voltage clamping device connected across input terminals to which a line voltage is applied. It also includes a second series circuit formed of a first electrical energy absorbing impedance element and a second voltage clamping device connected across the first voltage clamping element, thereby forming a voltage-clamping, voltage divider circuit which clamps the voltage across the first voltage clamping element at a voltage level reduced in comparison with that obtained by the first series circuit alone. When the gas discharge tube turns on, it places both voltage clamping devices in the circuit simultaneously, so that the let-through voltage appearing across output terminals can be reduced to the voltage clamped by the voltage-clamping, voltage divider circuit.

Abstract: A low voltage switchgear with a single or multipole contact arrangement (1) with a corresponding number of fixed contacts (2) and movable contacts (3), a magnet armature (4) that moves the movable contacts (3), at least one magnet coil (5) that is energized by direct current and a setpoint switch release (9).

Abstract: A circuit breaker comprises a static switch electrically connected in series with a disconnecting switch having an operating mechanism with a handle to close and open a system of main contacts. A voltage sensor ocmprises an electromagnet with a moving core capable of occupying an active locking position and an inactive unlocking position of the mechanism, respectively in the on state and in the off state of the switch. Opening of the disconnecting switch is inhibited if voltage is present on the output terminals.

Abstract: A circuit for controlling the orientation of magnetic fields is provided. The circuit has particular application to a flux shunt trip device in a circuit breaker. The flux shunt trip device has a spring-loaded plunger which when released to its extended position causes the circuit breaker to trip. Under normal operating conditions, the plunger is maintained in a retracted position by the force of a permanent magnet. A trip coil is mounted adjacent to the plunger. When the trip coil is energized a magnetic flux is generated opposing the magnetic flux of the permanent magnet. When the opposing flux neutralizes the flux of the permanent magnet the spring-loaded plunger is released to its trip position. A control circuit is provided to aid in maintaining the plunger in its retracted position under high current conditions close to, but below, the trip value.

Abstract: A protected reversing contactor is disclosed comprising two electromagnets intended to be alternately energized and whose armatures actuate two sets of respective phase switches and two acknowledgement switches mounted respectively in series in the supply circuit of the two coils of the two electromagnets. These two acknowledgement switches are actuated by a transmission system comprising two coaxial rotary shafts disposed end to end and each having an arm adapted to be driven in rotation, in one direction, by one of the electromagnets and a finger cooperating with the acknowledgement switch of the other electromagnet.

Abstract: This invention prevents a circuit breaker from being re-set for a time sufficient to insure safe dissipation of ionized gases. Current monitors and rectifiers generate a reduced voltage direct current output which is fed into an auxiliary rectifier circuit. That direct current output is proportional to current in power conductors controlled by a circuit breaker. A holding circuit has an electrical power source derived from the power conductors and connected to a release coil for the main power contacts in the circuit breaker. A holding relay has a holding coil in the rectifier circuit, and normally closed holding contacts in the holding circuit. A time switch is connected in shunt with the holding coil. When the direct current output reaches a value corresponding to an overcurrent condition in the power conductors, it energizes the holding coil to open the holding contacts and cause the release coil to trip open the main contacts. Concurrently, the time switch activates; after a predetermined time (e.g.

Abstract: An electrical distribution system for factories and offices is assembled from prewired modules. The basic module comprises a section of duct of a standard length with electrical feeder wires mounted therein. The wires are bared of insulation at one end and have connectors at the other to receive the insulation-free wire ends of an adjacent module. Some modules may also include prewired branch circuits with circuit breakers and outlets in a variety of configurations.

Abstract: A current-limiting load cutout includes at least one contact pair (1, 2) with one contact (3, 5) rigidly linked to electrodynamic current limiter (9, 10) that is electrically connected to controllable pulse current supply (17), and at least one emergency state sensor (12, 13, 14) kinematically linked to contact displacement and clamping assembly (11) which is kinematically linked to the other contact (4, 6) of contact pair (1, 2). The current-limiting load cutout is equipped with a pair of power terminals (20, 21, 22, 23) per each contact pair (1, 2) with one power terminal connected to the other contact (4, 6) of the contact pair (1, 2) and the other power terminal connected to contact (3, 5) which is rigidly linked to electrodynamic current limiter (9, 10) equipped with terminal (15, 16) to which a controllable pulse current supply (17) is connected via an emergency state sensor (12).

Abstract: A current interrupting apparatus adapted to be interposed in either a single or multiple phase electrical circuit includes a fuse interposed in each phase of the circuit. A current interrupter is electrically connected in series with each fuse for interrupting the current through the circuit, each current interrupter including a pair of normally closed electrical contacts and structure for opening the pair of contacts in response to a fault current of a magnitude different from the magnitude of current necessary to cause fusing of the fusible element of the fuse. A current interrupter triggering circuit is included in the apparatus for triggering operation of the contact opening structure of all of the current interrupters in response to fusing of the fusible element of any of the fuses to protect the circuit from the effects of single phasing.

Abstract: A telephone subscriber line is connected to a SLIC via contacts of an isolation relay. Line voltage on the line side of the relay contacts, and line current on the SLIC side of the relay contacts, are detected during opposite polarity half-cycles of an a.c. fault on the line, and the contacts are opened to protect the SLIC if the voltage and/or current is excessive. The interruption of current causes the line voltage to rise to maintain the protection for the duration of a fault.

Abstract: A highly efficient over current circuit breaker for an electric system is provided with a timer which is adjustable and can be set to produce a first control signal a predetermined time after the electric system is turned on, and a load detector in association with an over current control device which produces a second control signal when the load applied to the electric system exceeds a predetermined level. The first control signal and second control signal jointly activate an amplifier to produce a third control signal which activates relays to cause a magnetic switch to open, or "trip", whereby the electric system is turned off. The timer allows an electric surge to occur in the electric system without causing the magnetic switch to trip during the period of starting the electric system. The circuit breaker is particularly useful in systems having induction motors in which starting current surges are common.

Abstract: An electric circuit breaker for protecting an electric circuit of an electrical installation of a building is of such a construction that a substantial proportion of its components are standard for the preferred current ratings of the breaker and that some components having a limited useful life can be readily replaced.

Abstract: Circuit protectors employing a reed switch or electromechanical switching device, has a primary coil connected in series with it. Two resistors are connected across the switch/device and series connected primary coil. A secondary coil is connected across the load to the juncture of the two resistors. The primary and secondary coils are concentric windings. The switch/device is open when there is no power supplied. Thus, the two resistors and the secondary coil are connected to the load. The values of the two resistors and the secondary coil are chosen so that the coil current increases when the power supply is turned ON. When the load reaches approximately 80% of its full level, the magnetic flux generated by a secondary coil winding is sufficient to cause the switch or device contacts to close. As the load reaches its normal state, the secondary coil continues to hold the contacts closed.

Abstract: A magnetically operated circuit protector utilizing magnetizable reed contacts, primarily reed switches, in combination with a network of coil windings which results in positive trip and reset action, arc suppression, local and remote trip indication, overcurrent and overvoltage protection, operable in alternating and direct current systems, remote turn-on and turn-off, trip time of 50 to 100 microseconds, miniature size, automatic reset, and especially suitable for protecting sensitive electronic systems and instruments.

Abstract: An ultra-fast contact breaker is described which includes an ultra-fast cut-off mechanism provided with a repulsion disk fitted with moving contacts (7, 7'), the said mechanism also including fixed contacts (9, 9'), to the input and output terminals (1, 3) of which there is connected an assistance circuit (10) including a capacitor (21) and a coil (23) as well as a group of semiconductors (13, 15, 17, 19), the coil (23) wholly or partly constituting the repulsion coil of the said ultra-fast mechanism, which includes at least two parallel branches (CD, EF), a first branch (CD) including two opposed diodes (13, 15) or similar in series, each directed in the non-conducting direction for the current entering the circuit, a second branch (EF) including two opposed thyristors or similar (17, 19), each directed in the conducting direction for the current entering the circuit, and the LC oscillating circuit (21, 23) which connects the common point of the two diodes (13, 15) or similar and the common point of the two

Abstract: A circuit and apparatus are provided for the protected supply of a load using static and electromechanical switches. A switch apparatus having current limiting properties is opened by remote control means after a very short delay, following disabling of a static switch system placed in series therewith and connected in parallel across a voltage limiting component.

Abstract: A driving apparatus for a DC circuit breaker of reverse current insertion type. The driving apparatus includes a magnetic repulsive coil for instantaneously driving a movable electrode of a vacuum valve directly in an opening direction, and a first coil of capacitor energization type and a second coil for voltage tripping purposes as trip coils for an operating mechanism connected to the movable electrode through a contact spring whereby the magnetic repulsive coil and the first trip coil are respectively energized by capacitors in the case of tripping under faulty conditions and only the second trip coil is energized in the case of tripping under other conditions than faulty conditions.

Abstract: In a high voltage circuit interrupter (30) or recloser, an auxiliary undervoltage lockout mechanism (300) provides a backup lockout means to deenergize circuit contactor closing solenoid (78). Auxiliary undervoltage lockout device (300) provides a backup means to lock out recloser (30) after a period of time longer than the normal functioning time of the recloser to prevent damage to closing solenoid (78) in the event of low voltage or other unexpected conditions on the power line.

Abstract: Positive assurance of correct connection, in supplying electrical utilization equipment from a current source, is provided when it is critical to safety and particularly when receptacle outlets are used or a fixed wiring installation required which may be subject to wiring error. An electromagnetic relay is employed to prevent equipment use and to provide audible indication in the event of hazardous connective error such as reversal of ungrounded and grounded neutral conductors and the omission of a grounding connection. The relay may be one that is also otherwise controlled for on-off power switching.

Abstract: A safety device for disconnecting an electrical appliance from a power supply, and thereby preventing electrical shock hazards. A normally closed switch connects the appliance to the power supply. A frangible device, such as a flash bulb, maintains the switch in the closed position. A sensor detects a dangerous condition and actuates the flash bulb, which action fragments the envelope of the bulb, whereupon the switch assumes the open position, thereby disconnecting the appliance from the power supply.

Abstract: A magnetically operated circuit protector utilizing magnetizable reed contacts, primarily reed switches; in combination with a network of coil windings which results in positive trip and reset action; arc suppression; local and remote trip indication; overcurrent and overvoltage protection; operable in alternating and direct current systems; remote turn on and turn off; trip time of 50 to 100 microseconds; miniature size; and especially suitable for protecting sensitive electronic systems and instruments.

Abstract: An electronic sectionalizer mechanism includes upper and lower contact mounting structure, and a sectionalizer tube carrying a logic circuit is telescopically received within the lower contact mounting structure. Once fault current conditions downstream of the sectionalizer mechanism are detected by the logic circuit, an actuator is fired and the tube drops in a longitudinal, non-lateral motion toward an isolated or open circuit position. In preferred embodiments of the invention, a pin of the actuator shifts along an axis parallel to the longitudinal axis of the tube so that immediate drop-out of the tube is assured. The sectionalizer mechanism requires comparatively little space for operating clearances and this is especially adapted for use on a retrofit basis within the confines of an existing subsurface or pad-mounted switchgear cabinet; additionally, the sectionalizer mechanism may also be installed in existing overhead mounting structure for cutouts.

Abstract: A fault protection system for a power circuit composed of a main current path and a plurality of parallel branch paths connected in series with the main current path and each containing a winding of a medium voltage transformer, which system includes: first and second switchable current interrupters for switching the current flowing through the paths and for protecting the paths against fault currents, the first interrupter being connected in one branch path for interrupting fault currents in a first amplitude range and the second interrupter being connected in the main current path for interrupting fault currents in a second amplitude range which is higher than the first range and includes short circuit currents; and current monitoring circuits connected for monitoring the current in the one branch path and for maintaining the first interrupter in the closed state when the current in the one branch is below the first amplitude range, for placing the first interrupter in the open state in response to the pres

Abstract: The specification discloses an injection-molded electric plug having an integral circuit breaker. The plug housing is molded directly onto the power cord and defines a circuit breaker chamber. A circuit breaker is slidably received within the chamber and retained therein by spring fingers. The housing is elongated and the prongs extend from one of the long sides thereof so that the housing lays substantially flat against a receptacle. The prongs are angularly oriented with respect to the elongated body so that the body will not overlie adjacent receptacle outlets.

Abstract: An electronic circuit breaker having a trip solenoid assembly. The trip solenoid assembly includes a permanent magnet, plunger, trip coil and assist coil and is positioned adjacent to a phase conductor. At high current levels the phase conductor generates a flux that may cause the circuit breaker to nuisance trip. An assist coil is wound adjacent the trip coil. The assist coil is energized only when the current through the circuit breaker is of such a level so as to cause nuisance tripping. The assist coil is not energized during that time period when the trip coil is energized.

Abstract: The contacts of an isolation relay are positioned between a line interface circuit and the associated feed resistors, the relay being controlled to open the contacts in response to an excessive common mode alternating current through the feed resistors. The common mode a.c. is detected by a sensing amplifier, and is integrated and compared with a threshold level in a dectector unit. If the common mode a.c. is excessive, a zero crossing predictor enables the detector unit to control the isolation relay and open its contacts at a zero crossing of the a.c. A phase shifter in the zero crossing predictor compensates for the response delay of the isolation relay. As the excessive current is now interrupted, the sensing amplifier responds to the line voltage. When this falls below a predetermined peak level, a delayed resetting of the protection arrangement is automatically effected.