Arc fault circuit detector device detecting pulse width modulation of arc noise

• US 6,362,628 B2
• Filed: 04/21/1999
• Issued: 03/26/2002
• Est. Priority Date: 12/21/1998
• Status: Expired due to Term

• Current Assignee: Pass & Seymour Incorporated (Legrand SA)
• Original Assignee: Pass & Seymour Incorporated (Legrand SA)
• Inventors: Packard, Thomas N., Macbeth, Bruce F.
• Primary Examiner(s): Brown, Glenn W.
• Assistant Examiner(s): KERVEROS, DEMETRIOS C
• Application Number:
  US09/295,219
• Publication Number:
  US 20010040458A1
• Time in Patent Office: 1,070 Days
• Field of Search: 324/536, 324/509, 324/508, 324/514
• US Class Current: 324/536
• CPC Class Codes: G01R 31/12 Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing G01R31/08, G01R31/327 and G01R31/72 take precedence; measuring in plasmas G01R19/0061; measuring dielectric constants G01R27/2617; ESD, EMC or EMP testing of circuits G01R31/002
• CPC Class Codes: H02H 1/0015 Using arc detectors

Abstract

An arc fault detecting circuit using a pulse width modulation, or PWM, technique is used to sense series or parallel arc faults. When arc faults occur, typically an arc step in current is followed by broad band arc noise which is caused by the random fluctuations in arc column resistance. In this invention, a logic signal is created which has a duration or width that corresponds to the time interval during which the broad band arc noise generated by the arc is present. Typically an arc fault randomly starts, generates broad band arc noise, and then extinguishes as the next current zero cross of the AC load current is approached. The random start causes PWM of the logic pulse whenever the start of the arc randomly moves with respect to the current zero cross. An arc fault that randomly starts, and then randomly stops, will also cause PWM with respect to the zero cross. The PWM logic pulse is coupled to the input of a microprocessor which measures and stores the length of the present PWM pulse under measurement, and compares that pulse to the pulse length of the last stored PWM pulse, and increments a counter if the difference in pulse length is greater than a predetermined amount. When the counter number is greater than or equal to a predetermined number held in the microprocessor memory, an output signal is sent to a controller which activates the circuit interrupter. Another input to the microprocessor, corresponding to the magnitude of the arc current, is used to adjust the predetermined number value down or up depending on the intensity of the arc fault for faster or slower interrupts respectively.

Claims

1. An arc fault detecting device for protecting electric power lines, the device comprising:

a sensor coupled to a power line for detecting broad band arc noise caused by an arc fault in the electric power lines;

a pulse generator connected to the sensor for generating an arc start pulse at the start of the broad band arc noise;

a power line zero cross detector generating a zero cross pulse;

a time interval generator connected to the zero cross detector which is triggered by the start of the zero cross pulse and which is reset by a transition of the arc start pulse, and which generates a time interval proportional to a delay time between the zero cross and the start of an arc;

a memory for storing the time interval;

a comparator connected to the time interval generator and the memory for comparing the delay time of a present arc start pulse with the delay time of a previous arc start pulse, wherein the delay times are of random duration corresponding to the broad band arc noise generated by the arc fault in the electric power lines;

a counter connected to the comparator, the counter being incremented if the difference between the present delay time and the previous delay time is greater than a first predetermined value; and

a second comparator for generating an arc fault detection signal when the counter reaches a second predetermined value.

2. The arc fault detecting device of claim 1, in which the second predetermined value is determined by the magnitude of the arc fault noise.

3. The arc fault detecting device of claim 1, in which the second predetermined value is inversely proportional to the magnitude of the arc fault noise.

4. The arc fault detecting device of claim 1, in which the power line zero cross detector senses voltage zero crosses.

5. The arc fault detecting device of claim 1, in which the power line zero cross detector senses current zero crosses.

6. An arc fault detecting device for protecting electric power lines, the device comprising:

a sensor coupled to the power lines for detecting broad band arc noise caused by an arc fault in the electric power lines;

a pulse generator connected to the sensor for generating an arc start pulse at the start of the broad band arc noise;

a power line zero cross detector generating a zero cross pulse;

a timer width generator connected to the zero cross detector which is triggered by the start of the zero cross pulse and which is reset by an edge of the arc start pulse, and which generates a delay time interval;

a memory for storing the delay time interval;

a comparator connected to the timer and the memory for comparing the present delay time interval with a previous delay time interval, wherein the delay time intervals are of random duration corresponding to the broad band arc noise generated by the arc fault in the electric power lines;

a counter connected to the comparator, the counter being incremented if the difference between the present delay time interval and the previous delay time interval is greater than a first predetermined value;

a second comparator for generating an arc fault detection signal when the counter reaches a second predetermined value; and

a third comparator for lowering the second predetermined value if the difference between the present time interval and the previous time interval is greater or less than a third predetermined value.

7. The arc fault detecting device of claim 6, in which the second predetermined value is determined by the magnitude of the arc fault noise.

8. The arc fault detecting device of claim 6, in which the second predetermined value is inversely proportional to the magnitude of the arc fault noise.

9. The arc fault detecting device of claim 6, in which the power line zero cross detector sense voltage zero crosses.

10. The arc fault detecting device of claim 6, in which the power line zero cross detector senses current zero crosses.

11. A method for protecting electric power lines from an arc fault, comprising the steps of:

sensing a load current;

determining a first time interval between a zero cross of a power frequency of said electric power lines and a start of a high frequency signal within said load current during said first interval;

determining a second time interval between a subsequent zero cross of said power frequency and a start of a subsequent high frequency signal with said load current during said second interval;

determining a number of instances wherein said second interval differs from said first interval by a predetermined amount; and

producing an AFCI trip signal if said number of instances within a specified time period exceeds a predetermined number.

12. A device for protecting electric power lines from an arc fault, comprising:

means for sensing a load current;

means for determining a first time interval between a zero cross of a power frequency of said electric power lines and a start of a high frequency signal within said load current during said first interval;

means for determining a second time interval between a subsequent zero cross of said power frequency and a start of a subsequent high frequency signal with said load current during said second interval;

means for determining a number of instances wherein said second interval differs from said first interval by a predetermined amount; and

means for producing an AFCI trip signal if said number of instances within a specified time period exceeds a predetermined number.