Abstract:
A system and method for detecting the presence of a voltage on a ground conductor which is coupled to a protective Earth (PE) terminal associated with a power source. The power source is used to apply a voltage to a device. The system may use a high impedance device coupled across a switch, with the switch being in communication with a portion of the device. A control system may monitor a voltage across the high impedance device and determine if the voltage across the high impedance device exceeds a predetermined threshold. Exceeding the predetermined threshold indicates that an unsafe voltage from the power source is present on the ground conductor. A natural high impedance path between the device and the Earth is used to provide an Earth connection through the device to the Earth during operation of the system.

Description:
FIELD 
     The present disclosure relates generally to Earth leakage circuit breakers, and more particularly to a system for detecting a fault in a Protective Earth (PE) fault without the need for a physical connection to the Earth as a reference point, and without the need to produce a tangible current flow in a component. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Existing Earth Leakage Circuit Breaker (ECLB) systems are divided into two types: 1) voltage operated ELCB (vELCB) and 2) current operated ELCB (iELCB). As the name implies, a vELCB operates based on the electrical potential difference (voltage) between the monitored grounding terminal/metal part and a locally Earthed conductor. vELCB acts to break a circuit when the detected voltage reaches or goes beyond a predetermined limit. Because essentially no current is involved with a vELCB detection system, the major advantage of the vELCB is that the devices are able to detect a ground fault with a negligible leakage current. The biggest disadvantage for vELCB detection systems is the necessity of having a functional Earth connection. By “functional Earth connection” it is meant a conductor that is physically connected to Earth. 
     The second major type of ELCB detection system, an iECLB system, is often referred to in North America as “Ground Fault Circuit Interrupter” (GFCI) system. An iECLB operates by measuring the sum of current flowing into and out of an electrical device. In normal operation this sum will be zero, which means there is no current leakage in the system. Put differently, the current flowing into a device is equal to the current flowing out of the device. An iELCB detection system operates by detecting when the magnitude of current flowing into some device on a hot line of an AC power source becomes different from the current flowing on the neutral line of the power source (i.e., the sum of current flowing into and out becomes “non-zero”). When this non-zero current reaches a predetermined upper limit, then an iELCB detection system trips a breaker to interrupt current flow to the component or device. A principal advantage of an iELCB detection system is that no real Earth connection is required. However, a principal drawback, at least in some industries and/applications, is that there needs to be some tangible level of current flow to component or device that is coupled to the AC power source, in order to be able to detect if there exists a predetermined magnitude of difference in the current flowing to the device and the current leaving the device. 
     One specific application where an ELCB system is required is in connection with Electric Vehicle Supply Equipment (EVSE) used to recharge the batteries of an electric vehicle. Safety standards associated with EVSE equipment require that such equipment must continually monitor a protective Earth (PE) connection and interrupt the power to the electric vehicle if a PE connection fails. Two specific International Electrotechnical Commission (IEC) standards require that the power outlet PE terminal (i.e., receiving power from an AC power source) shall not be connected with the vehicle PE terminal until the power outlet PE grounding is verified. But with an electric vehicle, the vehicle itself does not present a direct, conductive ground path to the Earth (i.e., a PE connection). And still, IEC standards do not allow power to be coupled to the vehicle until a proper PE connection is verified, so an iELCB detection system cannot be used because no tangible current flow is permitted to be passed to the vehicle with first verifying the PE connection. 
     SUMMARY 
     In one aspect the present disclosure relates to a system for detecting the presence of a voltage on a ground conductor which is coupled to a protective Earth (PE) terminal associated with a power source. The power source is used to apply a voltage to a device. The system may include a high impedance device, for example a resistor, coupled across a switch, with the switch being in communication with a portion of the device. A control system may be included for monitoring a voltage across the high impedance device and determining if the voltage across the high impedance device exceeds a predetermined threshold. Exceeding the predetermined threshold indicates that an unsafe voltage from the power source is present on the ground conductor. A natural high impedance path between the Earth and the device is used to provide an Earth connection through the device during operation of the system. 
     In another aspect the present disclosure relates to a system for detecting the presence of a voltage on a ground conductor which is coupled to a protective Earth (PE) terminal associated with an AC power source. The AC power source may be used to apply a charging voltage to a switch system of a charger associated with an electric vehicle. The system may include a high impedance device coupled across a switch of the switch system, with the switch being in communication with a chassis of the vehicle. A control system may be included for monitoring a voltage across the high impedance device and determining if the voltage across the high impedance device exceeds a predetermined threshold. Such an instance indicates that an unsafe voltage from the AC power source is present on the ground conductor. A natural high impedance path between the Earth and the vehicle&#39;s chassis is used to provide an Earth connection through the vehicle&#39;s chassis during operation of the system. The control system may control opening and closing of the switch system in response to the sensed voltage across the high impedance device. 
     In still another aspect a method is disclosed for detecting the presence of a voltage on a ground conductor which is coupled to a protective Earth (PE) terminal associated with a power source, and where the power source is being used to apply a voltage to a device. The method may include using a natural high impedance path including at least one of capacitance and resistance between the device and the Earth, to form a connection with the Earth. The method may also include sensing a voltage across a high impedance device coupled across a switch while the switch is open, with the switch being in communication with the PE connection between the device and the Earth. A control system may be used which is in communication with the high impedance device. The control system may operate to determine when a voltage across the high impedance device indicates the presence of an unsafe voltage from the power source being present on the ground conductor. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The drawing described herein is for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a high level schematic diagram of one example of a ELCB detection system in accordance with one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     Referring to  FIG. 1 , one embodiment of an ELCB detection system  10  is shown in accordance with the present disclosure. In this example the system  10  is coupled to a switching system  12  of an Electric Vehicle Supply Equipment (“EVSE”) component. The switching system  12  is used to apply a Protective Earth connection to load  14 . The load  14  in this example is an electric vehicle, and will hereinafter be referred to as “vehicle  14 ”. An AC power source  15  supplies electric power to the vehicle  14  through a suitable connection wall socket  16  having a PE ground terminal  16   c , a first phase terminal  16   a  and a neutral or second phase terminal  16   b . A switch K 3  of switching system  12  is used by the vehicle charger circuit (not specifically shown) to couple the PE ground terminal  16   c  to the vehicle  14 . The vehicle  14  effectively has a natural capacitance and resistance, represented by capacitor symbol  18  (C 3 ) and resistor symbol  17  (R 3 ), between it and the Earth. It will be appreciated, however, that no separate physical electrical conductor is used to make a direct connection between any portion of the vehicle  14  and the Earth. 
     An important recognition that helps to form a basis of operation for the system  10  is that, while no separate physical component (i.e., conductive cable) is coupling the vehicle  14  to the Earth, the natural capacitance and resistance of the vehicle  14 , relative to the Earth (i.e., the natural capacitive/resistive path between the vehicle and the Earth), can function as the Earth connection for the circuit. Because the impedance between the vehicle and the Earth is high, essentially only an intangible leakage current flow is produced in the vehicle  14  (such as through its chassis) when the system  10  is checking for a proper PE connection. 
     With continued reference to  FIG. 1 , the PE ground terminal  16   c  of the connection wall socket  16  is normally connected to Earth, and thus forms a proper PE connection. However, if this PE connection is faulty, such as through a defective terminal  16   c , or possibly because of mis-wiring, and voltage from the terminal  16   a  or the terminal  16   b  is present at reference point T 3 , then the possibility exists for the chassis of the vehicle  14  to receive all or a portion of the AC line voltage. This condition can also be difficult for a traditional iELCB detection system to detect because none of the three inputs (i.e., first phase, neutral/second phase or PE ground line) can be assumed to be a safe reference point with respect to the Earth potential. 
     In any electric circuit, there may always be a point that can be considered as a reference point. In the circuit of  FIG. 1 , the point labeled “T 3 ” may be considered as the reference point, or in other words the point in the circuit that the potential of all other points are measured against. 
     To continually sense and monitor whether point T 3  is at Earth potential, the system  10  may make use of a high resistance resistor  20  (R 1 ), which may have a resistance of typically at least about two megaohms, a voltage measuring device  28 , and a control system  24 . For Electromagnetic Compatibility (EMC) purposes, the system  10  may include two Y-capacitors  26  (C 1 ) and  22  (C 2 ). A control system  24  is responsive to an output of a voltage measuring device  28 . The resistor  20  (R 1 ) is coupled across switch K 3  of the switching system  12 , which places point “a” of the resistor effectively at point T 3 . The control system  24  may be used to control switches K 1 /K 2  via a control signal  24   a  and switch K 3  via a control output signal on signal line  30  so that no one of switches K 1 -K 3  is closed until the system  10  verifies that a PE connection at point T 3  exists. Also, the control system  10  may operate to generate a signal that directly or indirectly acts to open the switches K 1 /K 2  and K 3 , for example by signals on signal line  30  and  24   a , if a faulty PE connection at T 3  is detected at any time during a normal charging cycle. It will also be appreciated while the voltage measuring device  28  has been shown as a separate component in  FIG. 1 , that the voltage detection across the resistor  20  (R 1 ) may be performed within the control system  24  by an integral voltage measuring subsystem of the control system. However, for the purpose of helping to describe the operation of the system  10 , the voltage measuring device  28  has been shown in  FIG. 1  as an independent component. 
     In normal operation the control system  24  must first verify that a PE connection exists at point T 3  before closing the switches K 1 /K 2  and K 3  respectively. Put differently, this means that T 3  must be at Earth potential. If a PE fault is detected, switches K 1 -K 3  will not be closed. 
     First consider the situation where a PE fault exists, and when the connection plug  16  is connected to the terminal associated with the AC power source  15 , that point T 3  is not at Earth potential. This may occur if PE (Protective Earth) line has been inadvertently mis-wired so that point T 3  is receiving at least some tangible quantity of AC line voltage. In this instance the voltage present at T 3  (relative to the Earth potential) may be viewed as the “Vin” (voltage input) to the system  10 . The impedance between T 3  and the Earth will be high, meaning essentially that only an intangible leakage current will flow into the vehicle  14  chassis along line  32  through resistor  20  (R 1 ), and to the Earth ground by way of the natural capacitance/resistance of the vehicle  14 . 
     During the operation with switch K 3  open, resistor  20  (R 1 ) forms one leg of a voltage divider and the other leg is formed by the vehicle&#39;s capacitance  18  (C 3 ) and resistance  17  (R 3 ). The voltage across resistor  20  (R 1 ) can be denoted as “VR 1 ” and the natural capacitance  18  (C 3 ) and resistance  17  (R 3 ) of the vehicle  14  provides a substitute for the Earth connection. The voltage VR 1  across resistor  20  (R 1 ) will thus be a function of Vin. VR 1  may be represented by the following formula:
 
 VR 1 =V in*( R 1/( R 1 +ZC 3 ∥R 3))
 
     where ZC 3  is the impedance of C 3  at the AC frequency, and “ZC 3 ∥R 3 ” means ZC 3  in parallel with R 3 . 
     The voltage across resistor  20  (R 1 ) will be detected by the voltage measuring device  28 , which in one embodiment may be a voltmeter, and analyzed by control system  24 . If this voltage is above the predetermined first limit, then the control system  24  determines that a faulty PE connection exists and switches K 1 -K 3  will not be closed. If the detected voltage is less than the predetermined second value, then the control system  24  concludes that the PE connection is intact and closes switches K 1 /K 2  and K 3  accordingly. 
     If the wall socket  16  PE ground terminal  16 C becomes floating (i.e., has no connection to any one of the first phase, second phase or the Earth), then the point T 3  will be half of the voltage between the first phase and the neutral/second phase lines due to the exists of the Y-capacitors  26  (C 1 ) and  22  (C 2 ). As described above, T 3  is not the same potential at the Earth potential. The potential difference between T 3  and the Earth creates a voltage across resistor  20  (R 1 ), which is higher than the first predetermined limit, from which the control system  24  determines that a faulty PE connection exists. As a result, the control system  24  will not close switches K 1 -K 3 . 
     If point T 3  is close enough to the Earth potential, then the voltage across resistor  20  (R 1 ) will not exceed the predetermined second limit discussed above. In this case, the control system  24  considers that the PE is good and closes switches K 1 /K 2  and K 3  accordingly. 
     The system  10  thus does not require a separate physical connection (i.e., an electrical conductor) to be used to couple the vehicle&#39;s  14  chassis to Earth in order to have the needed functional Earth connection between the vehicle and the Earth, such as with vELCB detectors. As a further advantage, the system  10  does not require that an appreciable current be flowing into the vehicle&#39;s chassis in order to perform its voltage sensing operation, such as with iELCB detectors. This arrangement meets the requirements of present day standards for checking/monitoring an AC source connection to a vehicle when using a charging system associated with the vehicle. The system  10  provides the advantage of a traditional iELCB detector (no physical PE connection required) with the advantage of a vELCB (no current flow required to flow to the end device). 
     While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.