Abstract:
The present invention discloses an improved arc fault detection circuit comprising a light detector for generating a light detector signal, a differentiator circuit for receiving a light detector signal and creating a differentiated light detector signal, and a comparator circuit for comparing the differentiated light detector signal and a reference signal and generating a comparator output signal for indicating when the differentiated light detector signal exceeds the reference signal.

Description:
TECHNICAL FIELD 
     This invention relates to electrical switchgear. More specifically, it relates to electrical switchgear with arc fault light detectors. 
     BACKGROUND OF THE INVENTION 
     In the past, several products have been designed to detect the light caused by an arc in electrical switchgear and terminate that arc as quickly as possible. These products operate by detecting a magnitude of light above a fixed threshold to determine the presence of an arc. When the presence of an arc is detected, the circuit indicates such and a mechanism is actuated to remove the source voltage. 
     Unfortunately, an arc does not produce light with a high content of radiation in the infrared frequency range. This is the range most easily detected by conventional light detectors, such as photodiodes and phototransistors. Therefore, the signal detector has to be set so as to be very sensitive in order to detect an arc before it becomes too destructive. 
     However, flashlights, sunlight, and building light sources have a much greater infrared content. Even though the absolute level of light from these sources is typically much lower, when conventional light detectors are subjected to these sources it can cause an erroneous detection of a arc. The source voltage is then erroneously tripped and must be reset. This erroneous detection is exacerbated by the need to set the detector to be very sensitive. 
     Therefore, there is a need for an improved switchgear arc fault detection circuit. The present invention is provided to solve this and other problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved electrical circuit for detecting an arc fault in electrical switchgear. To this end, there is provided a circuit which comprises a light detector which generates a light detector signal in response to the amount of light detected. A differentiator circuit receives the light detector signal and differentiates the light detector signal over time to generate a differentiated light detector signal. A comparator circuit then compares the differentiated light detector signal to a reference signal and generates an output signal when the differentiated light detector signal exceeds the reference value. 
     In an alternative embodiment, a second comparator circuit and a logic circuit are added to the circuit. The comparator circuit compares the light detector signal to a second reference signal and generates a second comparator output signal when the light detector signal exceeds the reference signal. The comparator output signal and the second comparator output signal are connected together in such a way as to provide a logical AND operation. The comparator and the second comparator outputs generate a trip signal only when both comparator outputs are activated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an electrical circuit according to a first embodiment of the present invention. 
     FIG. 2 is a diagram of an electrical circuit according to a second embodiment of the present invention. 
     FIG. 3 is a diagram of the electrical circuit of FIGS. 1 or  2  incorporated within electrical switchgear. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention. It is to be understood that the present disclosure is to be considered only as an example of the principles of the invention. This disclosure is not intended to limit the broad aspect of the invention to the illustrated embodiments. The scope of protection should only be limited by the accompanying claims. 
     Referring to FIG. 1, there is provided an electrical circuit  10  having a light detector  12  and a resistor R 1  connected in series between a voltage source Vcc and ground G. The light detector  12  may be any commercially available light detector and may have an internally adjustable or fixed signal gain. The light detector  12  generates a light detector output signal  14  which corresponds to the amount of light to which the light detector  12  is exposed. The value of R 1  is selected to deliver a voltage across R 1  that is suitable for processing by the remaining circuitry given the range of light to be detected. 
     A buffer U 1  is connected at a point between RI and the light detector  12  to receive the light detector output signal  14 . Because the buffer U 1  has unity gain, its output and input are identical. A differentiator  16  receives the light detector output signal  14 . Preferably, the differentiator  16  is a RC high pass filter comprising a resistor R 2  and a capacitor C 1 . RC high pass filters are well known in the art. The resistor R 2  and the capacitor C 1  are selected such that the cutoff frequency of the filter is 1.5 kHz. This type of differentiator is preferred due to its simple and inexpensive structure. However, there are numerous types of differentiators, and any type of differentiator may be used in accordance with the present invention. The output of the differentiator  16  is a differentiated light detector signal  18 . 
     A comparator U 2  is connected to the differentiator  16  output to receive the differentiated light detector signal  18 . The comparator U 2  compares the differentiated light detector signal  18  to a reference voltage Vref. The proper value of reference voltage Vref will vary depending on the values of the resistor and capacitor chosen for the differentiator and the desired sensitivity of the circuit to changes in the light detected. When the value of the differentiated light detector signal  18  is larger than the value of the reference voltage Vref, the comparator will generate a comparator output signal  20  indicating that an arc fault has been detected. 
     In a system incorporating alternating current, arc faults are not continuous arcs, but are intermittent arcs. An arc generated by a sinusoidal alternating voltage source will occur whenever the voltage is near its peak. Therefore, the light created by an arc fault is not continuous, but rather a short pulse of light which occurs every half cycle of the alternating voltage source (i.e. an arc occurs at the voltage&#39;s sinusoidal peak and at the voltage&#39;s sinusoidal trough). As a result, the comparator output signal  20  will pulse in unison with the detected light from the arc fault. However, it is normally desired that the comparator output signal  20  remain active during an entire arc fault episode and not return to an inactive state during points of the voltage source&#39;s sinusoidal zero-crossing. As a result, a pulse extender circuit  21  may optionally be used with the present invention. The pulse extender circuit  21  is preferably a capacitor C 2  having a large value of capacitance which is connected between the comparator output signal  20  and a ground point G of the circuit. The capacitor C 2  charges during peaks of the comparator output signal  20  and discharges during troughs of the comparator output signal  20 , thereby holding the comparator output signal  20  active during the period between arc faults. 
     Referring to FIG. 2, a circuit  22  is provided. The circuit  22  comprises the circuit  10  of FIG. 1 with two additional components—a second comparator U 3  and a logic circuit  23 . The second comparator U 3  is connected to the output of the buffer U 1  and a second reference voltage signal Vref 2 . When the output of the buffer U 1  (the light detector signal  14 ) exceeds the reference voltage Vref 2 , the second comparator U 3  provides a second comparator output signal  24  which indicates a threshold light value has been reached. 
     In the circuit  22  of FIG. 2, the comparator output signal  20  and the second comparator output signal  24  are attached across a resistor R 3  to the voltage source Vcc to form a trip indicator output signal  26 . If the comparators U 1 , U 2  are open collector operational amplifiers, when only one of comparator U 1  or second comparator U 2  is high, the voltage of the trip indicator output signal  26  is low. When both comparators U 1 , U 2  are high, the voltage of the trip indicator output signal  26  is high. As a result comparators U 1 , U 2  and resistor R 3  act to form the logic circuit  23  which performs a logical AND operation. While in the present embodiment open collector operational amplifiers are used with resistor R 3  as the logic circuit  23 , the logic circuit  23  could be implemented in many other ways by one of ordinary skill in the art, such as with truth table logic circuits. 
     With reference to FIG. 3, the circuit  10  of the present invention is located inside a switchgear enclosure  30 . The light detector  12  is situated inside a switchgear enclosure  30  such that light from an arc fault, if one occurred, would be cast upon the light detector  12 . More than one light detector  12  and/or light detector circuit  10  can be placed within the switchgear enclosure  30  in locations prone to arc faults. An electrical switch  32  inside the switchgear enclosure  30  receives the comparator output signal  20  of the circuit  10 . In response to comparator output signal  20 , the electrical switch  32  interrupts power coming into the switchgear enclosure  30  through an electrical line  34 . The circuit  10  and the comparator output signal  20  can be replaced by the circuit  22  and the trip indicator output signal  26 , respectively. 
     While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.