GB2229053A - Protection against arcing in cables - Google Patents

Description

t 1 IMPROVEMENTS RELATING TO THE DETECTION OF ARCING IN CABLES This

invention relates to circuit arrangements for the detection of arcing in electric cables which may be due to arc tracking or mechanical damage sustained by the electric cables.

The present invention is especially, but not exclusively, applicable to the detection of arcing in aircraft electric cabling where the cabling may be normally inaccessibly located within cable trunking or behind bulkheads, for example, so that the arcing may not be detected until complete cable failure occurs and/or a serious fire breaks out.

One of the difficulties in detecting such electric cable arcing which produces a succession of current spikes of variable amplitude in the cable current is that the peak current flow in the cable durin,,- arcing may, remain belo,,,,., the level required for producing tripping of the usual circuit breaker associated with such cabline. Moreover. it is necessary for certain current transients to be permitted in the cabling during start-up of electrical equipment (e.g electric motors c without tripping of the circuit by the operation of the circuit breaker.

According to the present invention therefore there is provided a circuit arrangement for detecting arcing in electric cables comprising discriminator means effective for discriminating between normal permitted relatively short current transients (e.g. due to start-up of electric equipment) occurring in the cabling and relatively, prolonged or continuous current transients due to arcing in the cable and preferably solid-state switching means responsive to an output from the discriminator means when cable arcing is detected to cause 2 disconnection of the power supply to the cable and also responsive to short circuit conditions or prolonged high overcurrent conditions in the cabling to disconnect the cable supply.

In carrying out the present invention the discriminator means may comprise a differentiator circuit followed by an integrator circuit whereby current transients in the cable circuit are positively identified and resultant output pulses from the differentiator circuit are integrated or added by the integrator circuit to produce a step function output representative of the detected current transients in the cable. In this way normal overcurrent transients which occur for very short periods due to start-up of electrical equipment. for example, will not cause the integrator circuit of the discriminator means to produce a sufficiently high output to effect tripping operation of the switching means.

To facilitate tripping operation of the switching means m-hen the electric cable is short-circuited or when prolonged over current conditions exist the discriminator is by-passed by a short-circuit detector and alternative switching means tripping arrangement.

In carrying out the invention the switching means for disconnecting the cabling from the supply may comprise an active fuse-like switching device which automatically restores the cable supply circuit when the fault conditions causing arcing in the cable are removed and such a device may be used alone or in conjunction with a conventional circuit breaker.

Alternatively the switching means for disconnecting the cabling from the supply may comprise a solid-state switch which may be used alone or in conjunction with a conventional circuit breaker.

i 1 3 By way of example the present invention will now be described with reference to the accompanying drawings in which:- Figure 1 shows a block schematic diagram of one circuit arrangement for detecting arcing in electric cables; Figure 2 shows diagrams of discriminator outputs for various cable operating conditions; Figure 3 shows a block schematic diagram of an are detection arrangement according to the invention using an existing circuit breaker; and, Figure 4 shows a circuit diagram of a DC implementation of the invention employing low cost digital logic and MOSFET solid state switching elements.

Referring to Figure 1 of the drawings this shows an exemplary circuit arrangement for detecting arcing in electric cables and for disconnecting the cable supply under cable arcing conditions whilst allowing certain transient current conditions (e.g start-up current transient condition) to prevail without disconnection of the cable supply, In the case of short-circuit or continuous high current conditions the cable supply will also be disconnected.

In the particular circuit arrangement illustrated by way of example in Figure 1 an electric cable 1 which may be one of a multiplicity of cables of a cable form in an aircraft supplies DC or AC current from a DC or AC supply 2 to a load 3 (e.g. electric motor). The load circuit also includes a solid state switch 5 for connecting or disconnecting the load 3 to or from the supply 2. The voltage across a resistor 6 in the load supply circuit provides an indication of the current flowina in the electric cable 1. This voltage is applied to a & c 4 discriminator 7 for discriminating between cable transient current conditions which require disconnection of the load from the supply and those during which the load 3 can be permitted to remain connected to the supply. In particular, the discriminator 7 is utilised for discriminating between normal transient conditions and those which occur during arcing of the cable due to arc tracking or mechanical damage sustained by the cable. The voltage across the resistor 6 after amplification by an amplifier in an input circuit 9 to the discriminator is applied to a short circuit detector 8 which compares the amplified voltage across the resistor 6 with a reference voltage in order to detect short circuit or prolonged unduly high load current conditions and, if necessary, a comparator in the detector provides an output for operating the solid state switch 5 to cause tripping of the load current.

The arcing detector of the discriminator 7 comprises a capacitor/resistor differentiating circuit 10 including a decay, resistor and a capacitor/resistor integrating circuit 11 which integrates pulse outputs from the differentiating circuit 10 in order to produce a step function output. The output from the integrating circuit 11 is applied to a comparator 12 having a suitably adjusted reference input which establishes a "tripping" level for operating the solid state switch 5.

Referring now to Figure 2 of the drawings it can be seen that the electrical outputs at the points A, B, C, D and E shown in the circuit of Figure 1 are represented for various cable conditions involving normal current, permissible transient overcurrent, short circuit and cable arcing conditions. The output at point C in the circuit has a characteristic which gives a risine characteristic due to c r k- integration action which decays due to the action of the deca.' resistor. It will be appreciated from Figure 2 that only cable arcing conditions will produce an output from the comparator 12 following the integrator 11 that causes tripping of the solid state switch 5. This circuitry is not necessarily responsive to short circuits since the tripping signal is initially derived from differentiation of the current increase and when integrated may not reach the trip level. However, under short circuit or unduly high current overload conditions the short-circuit detector 8 will provide an output which by-passes the differentiatorlintegrator/comparator in order to produce operation of the solid state switch 5 and the resultant disconnection of the load 3 from the supply 2.

The cable arcing detection arrangement described in this application can be applied to both AC and DC circuits and can be implemented in relatively, simple circuitry.

As previously mentioned the primary? object of this invention is to provide protection from arcing and tracking faults. It is possible to provide an are detection circuit arrangement according to this invention incorporating existing circuit breakers so that this.invention adds the arcing breaking capability to the existing protection afforded by a conventional circuit break-er. Such an arrangement including a circuit breaker 13 is shown in Figure 3. In this case it can be beneficial to arrange that an auto-reclosing feature is incorporated in this arrangement so that in the event of the fault clearina the circuit can be re-established. This can be of particular advantace in aircraft subject to damage of cables.

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6 Referring now to Figure 4 of the drawings, in this low cost DC implementation of the invention not requiring amplification, in order to determine the instantaneous value of the current flowing in the load circuit, the voltage drop across a MOSFET type switching device PSI) when in the conducting state, is used. The output of a logic NOR gate GAI drives the control gate of the power switching device PSI). According to the logic truth table FOR TBE NOR gate GAI, both inputs a and b need to be at a logic 0 voltage to produce a logic 1 voltatpe- at c to switch the control gate of the power switching device PSI). The inputs to the NOR logic gate GAI, via resistors/capacitors R2/C3. R3/C6 are applied to point d and are derived from a further NOR gate GA2 which itself can be controlled by a manual switch s or can be controlled from a system source.

The operation of the circuit will be described by, considering the closing of the controller s on to normal full load conditions.

A logic 1 voltage level will be applied at d from the lom. impedance source from the previous logic NOR gate GA2. To close the power circuit a logic voltage 0 will be provided at point d and by virtue of the capacitors C3 and C6 this voltage level will be initially sustained at points a and b. The application of two logic 0 volta4-,-es at a and b will cause a positive logic 1 voltage to appear at the output c and the gate of the output switch PSI) which will therefore close.

In the event of a short circuit developing in the system. the voltage at points g will rise steeply, capacitor C3 will charge rapidly and point b will rise to a logic 1 voltage which will in turn produce a logic 0 voltage level at c which will open the power switch PSI). In order to reclose it will be necessan, to reset the input to logic gate 1 4 7 GA1 to a positive value. If closure on to a short circuit is attempted, the voltage at point g will be sustained sufficiently long to charge capacitor C3 to a logic 1 level and will trip out or actually prevent closure of the power switch PSI). If cable arcing transients are present (with the power switch PSI) closed) then the variations of current will be detected at point h, inductor L preferably being included to serve to increase the voltage due to back EMY. These arcing voltages will be differentiated by, capacitor C10 and rectified by rectifier RC. The resultant undirectional pulses are used to charge up capacitor C8 which will act as an integrator to produce a voltage V1. When this voltage rises to a level exceeding a logic 1 then point a will be charged to a logic 1 voltage level and the power switch PSD will open.

In order to achieve optimum operation of the circuit it can be of advantace to alter the potential of point i instead of makine, a direct connection to the negative pole. It may also be beneficial to insert a resistor R12 which may be of non-linear for compensation purposes, The circuit described can be adapted to operate at higher voltages both AC and DC. This may require the addition of additional components in line with the block diagram of Figure 1.

The arrangement of Figure 4 can be exploited to be used in circuits having existing circuit breakers in order to provide arc detection and more rapid short circuit protection. In such applications an automatic reclosure feature may be included so that the power switching device will reclose after a defined period and for a predetermined number of attempts to simulate an automatically renewable fuse.

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Claims (8)

1. A circuit arrangement for detecting arcing in electro cables comprising discrminator means effective for discriminating between normal permitted relatively short current transients occurring in the cabling and relatively prolonged or continuous current transients due to arcing in the cable and switching means responsive to an output from the discriminator means when cable arcing is detected to cause disconnection of the power supply to the cable and also responsive to short circuit conditions or prolonged high overcurrent conditions in the cabling to disconnect the cable supply.

2. A circuit arrangement as claimed in claim 1, in which the discriminator means comprises a differentiator circuit followed by an integrator circuit whereby current transients in the cable circuit are positively identified and resultant output pulses from the differentiator circuit are integrated or added by. the integrator circuit to produce a step function output representative of the detected current transients in the cable.

3. A circuit arrangement as claimed in claim 1 or claim 2, in which the discriminator means is by-passed by a shortcircuit detector and a switching means tripping arrangement.

4. A circuit arrangement as claimed in any preceding claim, in which the switching means for disconnecting the cabling from the c supply comprises an active fuse-like switching device which C t 1 9 automatically restores the cable supply circuit when fault conditions causing arcing in the cable are removed.

5. A circuit arrangement as claimed in claim 4, in which the active fuselike switching device is used in conjunction with a conventional circuit breaker.

6. A circuit arrangement as claimed in claim 1. claim 2 or claim 3, in which the switching means comprises a solid-state switch.

7. A circuit arrangement as claimed in claim 6, in which the solidstate switch is used in conjunction with a conventional circuit breaker.

8. A cable arcing detection circuit arrangement substantial]., as hereinbefore described with reference to the accompanying drawin,-s.

c Published 1990a The Patent Otlice. State House. 6671 High Holborn. IondonWC1R4TP.FUrther copies maybe obtainedfrom The Patent Office Sales Branch, St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray. Kent, Con. l.87