Patent Application
20060092585
1. Field of the Invention
The invention relates to electrical supply systems with arc fault detection and/or protection circuits.
2. Background of the Invention
In an electrical system, arcing may occur where the distance between two lines of large voltage difference is small enough. Practically arc fault usually occurs in worn power cables where the high voltage lines have the chances becoming very close to each other. As a result, there may occur a tremendous temperature rise at the point of arcing, which may ignite substances around and cause a fire. Such an arc fault may not be detected by traditional over-current circuit breaker since the arcing current may be well below the magnitude of over-current but sufficient to cause high temperature rise.
Various arc fault detection and/or protection circuits have been proposed. For example, U.S. Pat. No. 4,931,894, assigned to Technology Research Corporation and entitled “Ground Fault Current Interrupter Circuit with Arcing Protection,” discloses an additional metal shield at neutral potential simultaneously enclosing line and neutral conductors that senses the arcing current between the line conductor and the neutral potential metal shield. The arc current as the result of the arc fault passes through an additional winding on the same core of the differential transformer for ground fault detection and interruption (making use of the same transformer core for construction of a current transformer for arc fault interruption). A protection triggering circuit responds to arc current occurring between line conductor and the additional metal shield as controlled by a series of resistor.
U.S. Pat. No. 6,292,337, assigned to Technology Research Corporation and entitled “Electrical system with arc protection,” discloses an additional sensing conductor between line and neutral conductors (all line, neutral and sensing conductors are unshielded), which senses the arc current between the line conductor and the sensing conductor during a positive supply cycle, or between neutral conductor and the sensing conductor during a negative supply cycle. The voltage at the sensing conductor is nearly the same in magnitude as the supply voltage, e.g., within a diode drop difference. A resistor is used to convert the arc current into trigger voltage for activation of the supply interruption circuit. An alternative to use metal shields individually for both line and neutral conductors and the shields simultaneously as a sensing conductor is also disclosed. The arc fault interruption mechanism is similar to that of U.S. Pat. No. 4,931,894, i.e., by making use of same core of the differential transformer for ground fault detection and interruption.
In U.S. Pat. No. 6,198,611 and U.S. Pat. No. 6,229,679, both assigned to Pass & Seymour, Inc. and entitled “Arc fault circuit interrupter without DC supply,” sensing of arc fault is conducted by generating short pulses due to di/dt as the result of each arc. Specifically, an integrator in connection with the sensor sums up energy of pulses generated due to detection of arc that generates voltage. When the voltage exceeds a pre-determined threshold, an interruption circuit is activated to disconnect the power supply. This is an indirect method and requires extra care to prevent mis-triggering of interrupter by devices having large transient switching current.
Nevertheless, it is desirable to enhance the protections to the electrical supply system, especially the power line at hazardous voltage.
Therefore, it is an object of the present invention to provide an improved electrical supply system with enhanced arc fault protections, or at least provide the public with a useful choice.
According to an aspect of the present invention, an electrical supply system firstly includes a power cable electrically connected to a power source. The power cable has a power line and a first conductor; the first conductor is isolated from the power line and acts as a sensing line. The power cable also has a second conductor and the second conductor is isolated from the sensing line and acts as a neutral line; the power line, the sensing line and the neutral line are electrically connected to the power source. The system further includes an arc fault circuit interrupter including a sensing circuit in electrical connection with the sensing line for detecting an arc fault and a voltage source being in electrical connection with the sensing line. The voltage of the voltage source is designed to be below the hazardous voltage according to the electrical safety rules.
According to a second aspect of the present invention, an electrical supply system firstly includes a power cable electrically connected to a power source. The power cable has a power line and a first layer of conductor surrounding the power line; the first layer is isolated from the power line and acts as a sensing line. The power cable also has a second layer of conductor surrounding the sensing line, and the second layer is isolated from the sensing line and acts as a neutral line; the power line, the sensing line and the neutral line are electrically connected to the power source. The system further includes an arc fault circuit interrupter including a sensing circuit in electrical connection with the sensing line for detecting an arc fault.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which description illustrates by way of example the principles of the invention.
As shown in
Such cable 101 as described in this preferred embodiment is used to replace traditional ones to achieve detection of arc faults in the cable by the arc fault circuit interrupter 105. The power and neutral wires 205, 201 deliver power to appliances from the power source 103 through solenoid-controlled contacts. The sensing wire 203, which is connected to a sensing circuit 107 (See
Furthermore, the sensing wire 203 is connected to a low voltage (not necessarily constant) derived from a voltage reference circuit 113 of the circuit interrupter 105. The low voltage is designed to be below the hazardous voltage that electrical safety rules apply. The supply for the sensing wire 203 provides both electric current sinking and sourcing capabilities, i.e. bi-directional current flow capabilities.
Such an exemplary system offers an extra advantage to users as the power wire 205 at hazardous voltage is firstly protected by a conducting shield 201 at neutral potential, followed by another metal shield 203 at voltage that is safe. By applying a voltage below that which safety rules apply with both current sourcing and sinking capability to the sensing wire 203, both arc current between the power wire 205 and the sensing wire 203, and the are current between the sensing wire 203 and the neutral wire 201, can be detected for the whole supply cycle.
Now referring back to
The voltage detector circuit 111, also in electrical connection with a voltage reference circuit 113, detects whether the deviation of the sensing voltage exceeds a reference voltage defined by the voltage reference circuit 113. Further more, the circuit interrupter 105 includes a timing circuit 115 electrically connected to the voltage detector circuit 11, and the timing circuit 115 defines a time constant for filtering accidental triggering of the voltage detector circuit 111. The circuit interrupter 105 also includes an output driving circuit 117 electrically connected between the timing circuit 115 and the power resource 103 for disconnecting the power source 103 with the power line 205 and neutral line 201 when the voltage detector 111 has detected the are fault. A latching circuit 119 is provided in the circuit interrupter 105, electrically connected between the timing circuit 115 and the output driving circuit 117 for turning on the output driving circuit 117 in the arc fault situation and for holding the output driving circuit 117 in an off state if no arc fault has occurred. In addition, the circuit interrupter 105 has an under-voltage-lockout circuit 121, which will be discussed in details below.
As shown in
The voltage reference circuit 113 includes zener diode Z1, resistors R2, R3, transistor Q2, and zener diodes Z2, Z3. It supplies three reference voltages for the voltage detector circuit 111, namely, Points B, C and D. The voltage at Point C, together with the voltage developed across resistor R9, contribute to the voltage at Point A, which appears at the sensing wire 203 of the power cable 101 when it is open. Therefore, these voltages are chosen such that the open voltage at the sensing wire 203 is below safety rules. The voltage at Point A is chosen to be at the mid-point between those of Points B and C. It is the voltages at Points B and C that are used by the voltage detector circuit 111 for detecting the potential deviation at Point A when an arc fault occurs. Also, they are chosen such that the potential deviation at Point A is sufficient to be detected (i.e. below voltage at Point C) when the sensing wire 203 is shorted to the neutral potential. A supplementary reference voltage at Point D is supplied for an accompanying fault detector circuit's use such as ground fault circuit interruption.
The voltage detector circuit 111 includes two individual analog voltage comparators U2A, U2B, which form a so-called window comparator such that whenever the voltage at Point A is either above that at Point B or below that at Point C, the comparator output will be active; otherwise it will be inactive. Therefore, when a hazardous voltage appears at the sensing wire 203, either positive or negative with respect to the neutral line 201, the voltage at Point A deviates, either in a positive or negative amount and will activates the comparator output. For simplicity, comparators U2A, U2B are chosen to be of open collector output type so that these outputs can be wired directly together to form the window comparator output without additional components. When active the comparator output becomes shorted to the circuit ground potential. When inactive, it behaves like an open circuit, neither sinking nor sourcing any current.
The timing circuit 115 includes resistors R14, R15, capacitor C3, and transistor Q3. One end of resistor R14 is connected to the output of voltage detector circuit 111. Initially, both transistors Q3 and Q4 are not conducting. When the voltage detector output is inactive, no current flows through resistor R14 or R15. The voltage at Point E is brought to VCC by resistor R15. As the emitter voltage of transistor Q3 is lower than VCC, transistor Q3 is held in the off state. When the voltage detector output becomes active, it shorts to ground potential and sinks current through resistor R14, pulling the voltage at Point E low. Because of the presence of capacitor C3, the voltage at Point E only goes low gradually according to RC charge/discharge phenomenon. Therefore the time constant formed by resistors R14, R15 and capacitor C3 determines the time required to bring the voltage at Point E from VCC to a value low enough to just turn on transistor Q3. The use of the timing circuit 115 is to filter out any spurious triggering of the voltage detector output due to, for example, electrical interference that exists in practical environment.
The latching circuit 119 includes transistors Q3, Q4, and resistors R16, R17 and R18. Initially, transistors Q3, Q4 are turned off by the under-voltage lockout circuit 121 by setting the emitter voltage of transistor Q3 to near ground potential which is insufficient to turn on any base-emitter of transistor Q3 or Q4. Then the emitter voltage of transistor Q3, is raised to near but lower than VCC. As long as the voltage detector output is inactive and since transistor Q4 is turned off, there is no base drive to transistor Q3. Since the only base drive to transistor Q4 is from the collector of transistor Q3, both transistors Q3 and Q4 are thus held, or latched, in the off state. Only when the voltage detector output becomes active, should there be the necessary current to drive the base of transistor W3. When transistor Q3 starts to turn on, its collector outputs current, which drives resistors R17. R18 as well as the base of transistor Q4. Therefore transistor Q4 also starts to sink current through its collector, reinforcing the base drive to transistor Q3. In this aspect, transistors Q3, Q4 drive each other, holding both in the conducting state. Since there is no means to remove the base drive of any of transistors Q3, Q4, the devices are latched in this state. Point F is the output of the latching circuit 119. It is a bi-state output turning on or off the output driving circuit 117. When the latch is in the off state, it sinks current from Point F through R18. When the latch is on, it sources current to Point F through R17.
The output driving circuit 117 includes capacitor C4 and SCR D4, with the gate pin connected to Point F. Normally, Point F is pulled to ground potential by resistor R18 and thus SCR D4 is not conducting. When Point F is pulled by resistor R17 to a voltage high enough to trigger SCR D4, SCR D4 starts to conduct, energizing the solenoid K1 that it drives to trip the power cable 101 from the main power 103. The function of capacitor C4 is to help preventing SCR D4 from mis-triggering by relieving Point F from being influenced by electrical noise
The under-voltage-lockout circuit 121 includes resistors R11, R12, comparator U1B, diode D3 and resistor R13. The circuit 121 is to reset the latching circuit 119 to off state upon powering on. Comparator U1B, being a voltage comparator in this aspect, senses the divided voltage of VAA through resistors R11, R12 at its non-inverting input. On powering up, VCC ramps up is gradually. When VCC is high enough to turn on zener diode Z1, VAA also ramps up accordingly. As VAA ramps up from ground potential, since the inverting input of comparator U1B is already at a higher potential, comparator U1B outputs a voltage near ground potential. It outputs low until VAA ramps up high enough such that the voltage at the non-inverting input of comparator U1B becomes higher than that at its inverting input, at which time comparator U1B outputs high at a voltage near its power supply pin, in this case, VAA. The value of VCC at this time should be higher than the minimum voltage for the whole circuit to operate. Also, the output of comparator U1B pulls its non-inverting input even higher through diode D3 and resistor R13 so that the output of comparator U1B will not change to low again easily until VAA and hence VCC, decays to a sufficiently low value like powering off the whole circuit. Since the emitter of transistor Q3 is driven by the output of comparator U1B through diode D3, the latching circuit 119 is reset to off state at power up and guaranteed to be released to functional state only when VCC is stable for the whole circuit. The presence of diode D3 is to prevent breaking down the base-emitter Junction of transistor Q3 in case the output of comparator U1B momentarily goes low due to electrical noisy condition that usually happens around live mains.
The power supply circuit 109 includes the solenoid K1, resistor R10, diode D2, zener diode Z4 and capacitor C1. Power is drawn from the power wire 205 through solenoid K1, resistor R10 and diode D2 to VCC. Zener diode Z4 is used to limit VCC to a desirable value. Resistor R10 limits the current drawn to a value suitable for the whole circuit to operate. The solenoid K1 helps to reduce the power dissipation of resistor R10 by dropping a certain amount of voltage across ft. Diode D2 rectifies the AC supply into DC voltage, and capacitor C1 helps to smooth the voltage as flat as possible. Since current is drawn through the solenoid, the quiescent current of the whole circuit should be small enough not to mistakenly turn on the solenoid.
An alternative design embodiment is shown in
