GB2177561A

GB2177561A - Electrical arc fault detector

Info

Publication number: GB2177561A
Application number: GB08615957A
Authority: GB
Inventors: Terence Frank Hart
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-07-04
Filing date: 1986-06-30
Publication date: 1987-01-21
Also published as: GB8615957D0; GB2177561B

Abstract

An electrical arc detector, capable of detecting without electrical connection, and providing warning of an electrical arc fault condition in an electrical appliance eg. an electric blanket, has an oscillatory magnetic sensor (S) located close to the mains supply lead of the appliance so that, upon the occurrence of an arc fault condition, an oscillatory magnetic field around the mains lead is detected.The signal generated by the sensor (S) triggers a latching circuit (TR1, TR2) with a positive feedback loop (R4),which actuates a transistor (TR3) connected to audible and visual alarm devices and/or a cut-out. A test switch (T) is provided. <IMAGE>

Description

SPECIFICATION Electrical arc fault detector The present invention relates to an electrical arc fau It detector which is capable of detecting, without electrical connection, and providing warning of, an electrical arc fault condition that can develop in electrical applicances. The present invention is particularly suitable for use with domestic appliances, such as an electrically heated blanket, where an electrical arc hazard may eventually cause afire or consuming the blanket and any surrounding combustible material.

Obviously, such appliances, iffaulty or damaged can be of considerable danger to both life and property.

The electrical are fault condition can occur in an electrical circuit where a current conducting wire is severed and the loose ends are in close intermittent contact, with the arc then developing between the severed ends, especially when the circuit issignifi- cantly inductive and the level of current is sufficient to sustain the arc. Such conditions have been observed in AC mains voltage operated electric blankets, where an arc at the severed strands within an insu la- ted multi-strand conductorinsidethe blanket has eventually burnt th rough the insulating material and ignited the blanket and the surrounding bed linen.

It is the object of the present invention to provide a detection device capable of detecting, without electrical connection, an electrical arc fault condition in an electrical circuit and providing an output when said condition is detected.

According to the present invention there is provided a detection device, capable of detecting,without electrical connection, an electrical are fault condition in an electrical circuit, comprising an oscillatory field magnetic sensor locatable, in use, in close proximity to a conductor of said electrical circuit, the outputof said sensor being supplied to an amplification and latching circuit, the output of which is connectableto actuate a visual or audible warning device and ora safety cut-out device.

The arc detector device is preferably a batteryoperated device, which may be simply attached to the mains supply lead ofthe electrical applicance, e.g. clipped on to or around the mains lead atany convenient position. Since no electrical connection is required to the appliance to be protected, the device can be readily attached and removed when the applicance or blanket is not in use. The arc detectorcircuit preferably utilises transistors in a zero-biased, zero-current mode to enable operation ofthe detector circuitry for a period equal to the 'shelf-life' ofthe battery supply, when switched on in the armed state, and a test switch or button is preferably provided which, when operated, tests for correct operation of the alarm circuit, and which may also serve as a battery level check.

The audio orvisual alarm may be of any conventional type including light emitting diodes or incadescent bulbs and/or mini DC or piezo buzzers or solid state tone generators connected to miniature loudspeakers. Alternatively, provision could be made or includedforacut-out deviceforthe mains supply to switch off the current,when an electrical arc is detected.

The present invention will now be described further with reference to the accompanying drawings in which: Figures la and lb illustrate an oscillatory magnetic field sensor; Figure2 illustrates an oscillatory magneticfield sensor array; Figure 3 illustrates an arc detector circuit; and Figure$ illustrates an audible alarm circuit.

Referring firstly to Figures 1 a and 1 b, the detection principleforthe present invention is illustrated, in which a mains lead 10 is shown with a sensor 11 mounted proximate thereto, the sensor 11 suitably being an iron dust core choke. When a continuous arc fault condition arises, owing to severing of a cur- rent conducting wire ofthe appliance whilstthe loose ends are held in close intermittent contact, an electrical arc thereby produced would be comprised of an oscillatory current flow which generates high frequency oscillatory magnetic fields surrounding the mains supply lead. The sensor 11, placed in close proximity to the mains lead 10 detects the presence of such magnetic fields and a current is induced in the iron dust core choke when orientated as shown in Figure 1 a.In view of the fact that th e m ag n itude of such fields and hence the induced current in the choke are relatively small, the sensitivity of the device can be increased by connecting a number of chokes together in series as shown in Figure 2 to provide a phased array. The signal produced in the oscillatory magnetic field sensor 11 reproduces the wideband signal components ofthe arc caused bythesevering ofthe leads.

Referring now to Figure 3, the arc detector circuit of the present invention is shown, in which the oscillatory magneticfield sensor is designated as Sand the output of such is applied to the base of transistorTR1 and to one end of by-pass capacitor C2. In a 50 hertz AC circuit; the arc current detected when a fault arises exhibits a pulse train effect with a period of 10 milliseconds in which the wideband oscillatory currents peak and rapidly decay. The positive-going signal component in the 10 millisecond interval pulse train bursts applied to the base oftransistor TR1 and will cause corresponding conduction through TR1 when the base/emitter input levels approach or exceed the 0.6 V "turn-on" levels required for silicon transistors.The output oftransistor TR1 is supplied to the base oftransistorTR2, whose collec- tor feeds an RC time constant circu it formed by resis tor R5 and capacitor C3 so thatthe original busts of wideband signals applied to the base oftransistor TR1 at 10 millisecond intervals now appear as "ramping" step increases in the DC voltage across capacitor C3. This positive DC voltage is fed backto the base of transistorTR1 via the resistive divider R4, R2 which when set to a given ratio, e.g. 10:1, provides forward biasto transistorTR1 thus increasing the gain oftransistorTR1 and hence the sensitivity of the arc detector the presence of oscillatory magneticfields. With a continuous 10 millisecond interval "arc" pulse trains applied to the base of trans istorTRl ,the detector circuit rapidly traverses the increased sensitivity state to a point where "latch-up" occurs.When the voltage across resistor R2 rises to a point where transistorTR1 is driven into full conduc tion,taking transistorTR2 also into full conduction, a stable "latch-up" state is achieved via the resistors R4 and R2 positive feedback path. When a "latch-up" state occurs, a sample of the "latch-up" bias on transistorTR1 is used to forward bias transistorTR3 and hence initiate a conventional piezo-buzzeraud- ible pulsed alarm tone generator (see Figure 4) and a visual alarm is produced at light emitting diode D1.

The time interval from detection ofthefirstwideband arc pulse to activation of the audible alarm, given a continuous train of are pulses can be set by the ratio of resistors R4 and R2. Using a 10:1 ratio, the arc detector gives audible warning after app- roximatelyten, 10 millisecond pulses, i.e. approximately 100 milliseconds after detection ofthe first arc pulse.

The on-off switch E, while serving to switch on and "arm" the detector, is also used to "unlatch" or reset the alarm by switching off and then on again which allows the "latch-up" voltage across capacitor C3to discharge through resistors R4 and R2 during the "off" state to a level where transistor TR1 conduction is not sustained. With on-off switch E 'on' and battery supply applied to the arc detector circuitry, no cur- rent is drawn from the batteries when the circuit is in the "armed" and "unlatched" state. This is achieved by zero forward bias operation ofthe transistor circuitry,which allows the arc detector to operate con tenuously in the "armed" stateforthe "shelf-life" of the batteries.

Afurther switch is provided for the detector circuit, namely the test switch Twhich, when operated, provides a controlled level of forward bias to transistor TR1 via the resistor divider R1, R2 and the chokes of the oscillatory magnetic field sensor Sin order to generate a "latch-up" alarm activated state to testthe correct operation ofthe circuit. Further, selection of the value of resistor R1 will determine the minimum battery voltage level below which a "latch-up" state will not be achieved, thus providing a check of satisfactory battery voltage level.

As will be appreciated, a variety of differenttypes of alarm systems could be activated bythe device of the present invention and although not particularly convenient for use with the device as currently envisaged, i.e. as a low cost safety deviceforattach- mentto existing appliances, such circuitry could alternatively be built-in to a control device forthe appliance and could be connected to meansfortriggering a safety cut-offofthe mains supply currents well as annunciating an alarm.

Claims

1. A detection device, capable of detecting, without electrical connection, an electrical fault condition in an electrical circuit, comprising an oscillatory magneticfield sensor locatable in use proximate to a conductorofan applicanceto be protected, the output of said sensor being connected to an amplification and latching circuit which, when a faultcondition is detected, provides an output for actuating a visual or audible alarm and/or a safety cut-out device.

2. Adetectordeviceas claimed in claim 1 in which the oscillatory magnetic field sensor is an iron dust core choke.

3. A detection device as claimed in claim 1 in which the oscillatory magneticfield sensor comprises a plurality of iron dust core chokes connected in series to provide a phased array.

4. A detection device as claimed in any preceding claim in which the amplification and latching circuit utilises a pair of transistors connected in a zerobiased, zero-current mode so that the circuit can remain in the "armed" state for the "shelf-life" of a battery supply therefor.

5. A detection circuit as claimed in any preceding claim wherein the output of the detector circuit is suppliedtoavisual and/oraudiblealarm,deviceand/ orto a safety cut-off device for the supply current to the appliance to be protected.

6. A detectordevice as claimed in claim 1 in which the delay time from detection of a first arc pulse to activation of the alarm is adjustable.

7. A detector device as claimed in any preceding claim in which a test switch is provided which both enables a testof properoperation of the detection circuit and battery voltage level.

8. A detector device substantially as hereinbefore described with reference to the accompanying drawings.