AU2014221310A1 - Earth leakage detector - Google Patents

Abstract

An earth leakage detection circuit comprising; a current source, capable of drawing current from a sourcing power conductor, a current sink, sinking current to a sinking power conductor. The current source and current sink are each connected to earthed equipment which requires earth leakage detection protection. The current source and current sink enabling the earthed equipment having a low impedance to bias the earthed equipment to a voltage that is above an operational threshold for the current source and below an operational threshold for the current sink. Source Conductor Sink Leak Detected Current Source 12 12 - ~ arthed Components sourcee Leak Detected Sink Conductor 6 Figure 1, Block Diagram Source Conductor 26 -P 32 E arI Sink Conductor 6 Figure 2. Block Diagram

Description

EARTH LEAKAGE DETECTOR BACKGROUND OF THE INVENTION [0001] The present invention relates to leakage current detection and protection devices used in connection with earthed equipment. More particularly the present invention relates to an earth leakage detection circuit for power equipment which can source and sink current up to a specified earth leakage current threshold to be detected. The present invention also relates to an earth leakage detection circuit for power equipment having a source conductor, a sink conductor and earthed components forming an earthed conductor. The present invention further relates to a positive earth leakage detection circuit and a negative earth leakage detection circuit for power equipment, the circuit having a source conductor, a sink conductor and earthed components forming an earthed conductor. The present invention further relates to an earth leakage protection circuit which is based on a current source from a sourcing conductor and a current sink to a sinking conductor which can source and sink current up to a specified earth leakage current threshold to be detected. PRIOR ART [0002] Industry standards require that electrical machines and machine components especially in heavy electrical engineering used in various industries have earth leakage protection. Electrical power equipment often requires a safety mechanism that shuts the power off when an unexpected electrical current is detected above a specified threshold. Many electrical power items have a mechanically strong metal enclosure which, by nature, is conductive. When this enclosure can be touched by people it needs to be kept safely isolated from any high voltage conductors within the piece of equipment. Connecting the enclosure to a grounded conductor, called 'earthing' the equipment, provides a degree of safety as any accidental short circuits from high voltage conductors to the earthed enclosure would cause a current surge that would trip a circuit breaker or blow a fuse and as a result keep the enclosure safe to touch. [0003] Earth leakages may occur in electrical equipment which can lead to electrocution if not detected or if there are no means to interrupt the circuit to prevent that occurrence. Earth leakage may present as small stray voltages on a metal enclosure of electrical equipment. Earth leakage protection devices are intended to interrupt the circuit if a dangerous voltage is detected. 1 [0004] One known electric earth leakage protection device comprises a detection circuit, a control circuit, a switching power supply and a control switch. The detection circuit is used for detecting abnormalities of a live wire, a neutral wire and a ground wire in a system circuit and outputting a detection result to the control circuit. The control circuit is arranged between a power input end and a power output end of the system circuit and is used for controlling on/off of the live wire, the neutral wire and the ground wire in the system circuit according to the circuit detection result sent by the detection circuit. The switching power supply is connected between the live wire and the neutral wire of the circuit and is used for providing power for the control circuit. One end of the control switch is connected, between the power input end and the control circuit, to the neutral wire, and the other end of the control switch is connected, between the power output end and the control circuit, to the neutral wire. A portion, connected with the live wire, of the switching power supply is located between the control circuit and the power input end, and a portion, connected with the neutral wire, of the switching power supply is located between the control circuit and the power output end. With the electric leakage protection device, comprehensive protection on earth leakage of the live wire and the neutral wire and on ground wire electrification can be carried out. [0005] Also known is an Earth Leakage Circuit Breaker which is used as a safety device in electrical installations with high earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if a dangerous voltage is detected. Also used have been residual current circuit breakers which detect earth leakage current directly. A residual-current circuit breaker is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. A residual-current circuit breaker is designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or all short-circuit conditions. [0006] A residual-current device is an appliance leakage current interrupter sometimes known as a ground fault circuit interrupter. A ground fault condition is defined as: an unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment or earth. A residual-current device is designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 5-30 milliamperes) and disconnecting quickly enough (i.e. in about <300 ms), they 2 may prevent electrocution. They are an essential part of the automatic disconnection of supply and operate to switch off when a fault develops, rather than relying on human intervention, which would normally be too slow and too late. Residual-current devices operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between current flowing through a live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to another circuit), and the device will open its contacts. Its operation does not require a fault current to return via the earth wire in the installation, as the trip will operate just as well if the return path is via plumbing, the ground or any other current path. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete. [0007] There are also in existence residual-current devices which have intentionally slower responses and lower sensitivities, designed to protect equipment or avoid starting electrical fires, but not disconnect unnecessarily for equipment which has greater leakage currents in normal operation. To prevent electrocution, A residual-current device should operate within 25-40 milliseconds with any leakage currents (through a person) of greater than 30 milliamperes, before life threatening electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current that a residual current device responds to. A small leakage current, such as through a person, can be a very serious fault, but would probably not increase the total current enough for a fuse or circuit breaker to break the circuit, and certainly not to do so fast enough to save a life. [0008] Normally the resistance between any grounded component and any power conductor is very high. Many faults can develop slowly where the resistance between the earthed component and a power conductor drops as, for instance, when partially conductive damp dirt touching both conductors builds up over time. A further safety method is to pre-empt an unwanted low resistance short by watching for small currents between the earthed component and the power conductors. These are called earth leakage currents and they occur over unwanted earth leakage circuits. [0009] Equipment compliance standards specify the thresholds for earth leakage current. Present earth leakage detection circuits use resistor bridges to provide small voltages corresponding to the amount of earth leakage current. The accuracy of these detection circuits is often affected by the power conductor voltages, the temperature of the circuitry and electrical noise produces signals that need to be filtered out to provide a correct signal for an earth 3 leakage fault. SUMMARY OF THE INVENTION [0010] The present invention provides an earth leakage detection circuit for power equipment having a source conductor, a sink conductor and earthed components forming an earthed conductor. The circuit is based on a current source from the sourcing conductor and a current sink to the sinking conductor which can source and sink current up to the specified earth leakage current threshold to be detected. Both the current source and current sink are connected to the earthed conductor in a way that biases the earthed conductor to a certain voltage. Within a range of this voltage the current source and current sink circuits are unable to source or sink their threshold current as the voltage is outside their current regulating operational range. Using a current source and current sink results in a low dynamic impedance current measuring system compared with the common resistive bridge circuits. This design is also far less susceptible to the voltage noise caused by high power switching circuits commonly found in power equipment that can cause false earth detect signals to be generated. [0011] When an earth leakage circuit forms, the earthed conductor's voltage will move away from the biased voltage. Once the earth leakage current exceeds the specified threshold the earthed conductor's voltage will exceed a limit of the voltage range and cause the source or sink circuit to begin regulating their respective specified threshold current. Circuitry detects when the current source or current sink begin to regulate their current which then signals an earth leakage fault condition. Typically a processor reads these signals and then takes appropriate action such as shutting down the equipment. One reason for the accuracy of the earth leakage circuit is due to the current source and current sink being insensitive to any variation in the sourcing and sinking conductor's voltage difference. [0012] The accuracy of the earth leakage is also due to a temperature compensated design where components with complementary temperature characteristics are selected. The response is accurate across a wide temperature range and control circuitry of power equipment often experiences a wide operating temperature range. Typically a processor reads these signals and then takes appropriate action such as shutting down the equipment. One reason for the present invention's accuracy is due to the current source and current sink being insensitive to any variation in the sourcing and sinking conductor's voltage difference. [0013] In its broadest form the present invention comprises: 4 an earth leakage detection circuit comprising; a current source, capable of drawing current from a sourcing power conductor, a current sink, sinking current to a sinking power conductor; said current source and current sink each connected to earthed equipment which requires earth leakage detection protection; the current source and current sink enabling the earthed equipment having a low impedance to bias the earthed equipment to a voltage that is above an operational threshold for the current source and below an operational threshold for the current sink. [0014] According to a preferred embodiment when there is no earth leakage, a small current flows from the source conductor through the current source, to the sink conductor. When there is no earth leakage, the small current which flows from the source conductor through the current source to the sink conductor passes via at least one diode between the source conductor and the current source. The small current passes through at least one diode between the current sink and sink conductor. Preferably the small current biases the earthed equipment to a predetermined voltage. When an unacceptable earth leakage occurs between the earthed equipment and the source conductor, the earthed equipment has its voltage biased towards the source conductor and beyond a first voltage threshold that causes the current source to limit the amount of current. [0015] When an unacceptable earth leakage occurs between the earthed equipment and the sink conductor, the earthed equipment has its voltage biased towards the sink conductor and beyond a voltage threshold that causes the current sink to being limiting the amount of current. When a current limit is reached, the corresponding current source raises a limiter feedback signal, to turn on a transistor. The transistor limits the current through diodes which draw the signal to a microcontroller through diodes. According to one embodiment the diodes prevent current limiter feedback from the current sink from affecting the current source. According to an alternative embodiment, the diodes prevent current limiter feedback from the current source from affecting the current sink. [0016] When a current limit is reached, the corresponding current source raises a limiter feedback signal, to turn on a transistor during which current through diodes is limited and pulls a signal to the microcontroller down through diodes. [0017] When an earth leakage occurs between the earthed equipment and the power conductor, the voltage is diverted away from the biased voltage to pass one of the thresholds and the corresponding current source or sink begins regulating the current. Within these 5 thresholds the current source and current sink source and sink less than their specified regulated current. [0018] The present invention provides an alternative to the known prior art and the shortcomings identified. The foregoing and other objects and advantages will appear from the description to follow. In the description reference is made to the accompanying representations, which forms a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. In the accompanying illustrations, like reference characters designate the same or similar parts throughout the several views. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS [0019] The present invention will now be described in broad detail according to preferred but non limiting embodiments wherein; Figure 1 shows a schematic view of a circuit according to a preferred embodiment of the present invention; and Figure 2 shows the circuitry external to the current source and current sink for an implementation with a single earth leakage signal to a control system's microcontroller. DETAILED DESCRIPTION [0020] Referring to figure 1 there is shown a schematic view of a circuit 1 according to a preferred embodiment. Circuit 1 includes a source conductor 10 and a sink conductor 16 that provide electrical power to equipment. Circuit 1 further comprises one or more earthed components or equipment 14 that are safe when they cannot source or sink a specified current from or to the corresponding source conductor 10 or sink conductor 16. [0021] A current source 12 and a current sink 18 are provided that can source or sink a specified amount of current to or from the earthed equipment or components 14 6 when the voltage of the earthed equipment 14 is below a given sourcing voltage or above a given sinking voltage. The current source 12 and current sink 18 provide an output signal when the sourced or sunk current reaches the threshold where the current source 12 or current sink 18 begin to limit the amount of current. The current source 12 provides current from the source conductor 10 to the earthed equipment 14. When the earthed equipment 14 do not sink sufficient current from the current source 12 it biases the earthed equipment voltage in a positive direction. Likewise the current sink 18 provides current from the earthed equipment 14 to the sink conductor 16. When the earthed equipment 14 does not source sufficient current for the current sink 18 it biases the voltage at the earthed equipment in a negative direction. When there is no earth leakage the small below-threshold current from the current source 12 matches the small below-threshold current to the current sink 18 and, as a result, the earthed equipment 14 are biased to a certain voltage. When an earth leakage current to either the source conductor 10 or the sink conductor 16 begins to develop then the earthed components' 14 voltage is biased towards the corresponding conductor 10 or 16 having the earth leakage current path. When the earth leakage current reaches the specified current threshold the corresponding current source 12 or current sink 18 begin to regulate their current to this limit and effectively change their response from a low impedance response to a high impedance response. When the current source 12 or current sink 18 begin to regulate they generate a corresponding signal that indicates the earth leakage current has exceeded its specified limit. [0022] Referring to figure 2 there is shown a schematic view of circuitry 2 external to the current source 12 and current sink 18 for an implementation with a single earth leakage signal to a control system's microcontroller. When there is no earth leakage, a small current flows from the source conductor 10 through the current source 12, through diodes 20, 22 and 24, the current sink 18 and diode 26 to the sink conductor 16. This small current biases the earthed equipment 14 to a certain voltage. When an unacceptable earth leakage occurs between the earthed components 14 and either the source conductor 10 or sink conductor 16 then the earthed components 14 have their voltage biased towards the corresponding source conductor 10 or sink conductor 16 and beyond the corresponding voltage threshold that causes the current source or current sink to begin limiting the 7 amount of current. When a current limit is reached the corresponding current source 12 or current sink 18 raise a limiter feedback signal, 28 or 30, to turn on transistor 32 or 34 which in turn limits the current through diode 36 or 38 which sends the signal to the microcontroller down through diode 40 or 42. Diode 40 and diode 42 prevent the current limiter feedback from the current sink 18 from affecting the current source 12 and vice versa. Detection circuits detect when the current source or current sink begin to regulate the current and provide a corresponding signal to the control system. [0023] The general purpose of the present invention, which is described herein in detail, is to provide an earth leakage circuit arrangement which while possessing some of the attributes of the known earth leakage devices has the additional advantages along with novel features that result in a new, advantageous and more efficient means for controlling earth leakage which is not anticipated, rendered obvious, suggested, or even implied by any of the prior art devices, either alone or in any combination thereof. [0024] Although the invention will be described according to preferred embodiments it will be appreciated that other variants are contemplated. The design and construction may be variously modified, without departing from the spirit and scope of the invention, as defined in the claims. [0025] It will be recognized by persons skilled in the art that numerous variations and modifications may be made to the invention as broadly described herein without departing from the overall spirit and scope of the invention. 8

Claims (15)

1. An earth leakage detection circuit comprising; a current source, capable of drawing current from a sourcing power conductor, a current sink, sinking current to a sinking power conductor; said current source and current sink each connected to earthed equipment which requires earth leakage detection protection; the current source and current sink enabling the earthed equipment having a low impedance to bias the earthed equipment to a voltage that is above an operational threshold for the current source and below an operational threshold for the current sink.

2. An earth leakage detection circuit according to claim 1, wherein, when there is no earth leakage, a small current flows from the source conductor through the current source, to the sink conductor.

3. An earth leakage detection circuit according to claim 2, wherein when there is no earth leakage, the small current which flows from the source conductor through the current source to the sink conductor passes via at least one diode between the source conductor and the current source.

4. An earth leakage detection circuit according to claim 3 wherein the small current passes through at least one diode between the current sink and sink conductor.

5. An earth leakage detection circuit according to claim 4 wherein the small current biases the earthed equipment to a predetermined voltage.

6. An earth leakage detection circuit according to claim 5 wherein, when an unacceptable earth leakage occurs between the earthed equipment and the source conductor, the earthed equipment has its voltage biased towards the source conductor and beyond a first voltage threshold that causes the current source to limit the amount of current.

7. An earth leakage detection circuit according to claim 5 wherein, when an unacceptable earth leakage occurs between the earthed equipment and the sink conductor, 1 the earthed equipment has its voltage biased towards the sink conductor and beyond a voltage threshold that causes the current sink to being limiting the amount of current.

8. An earth leakage detection circuit according to claim 6 wherein, when a current limit is reached, the corresponding current source raises a limiter feedback signal, to turn on a transistor.

9. An earth leakage detection circuit according to claim 8 wherein, the transistor limits the current through diodes.

10. An earth leakage detection circuit according to claim 9 wherein the diodes draw the signal to a microcontroller through diodes.

11. An earth leakage detection circuit according to claim 10 wherein, diodes prevent current limiter feedback from the current sink from affecting the current source .

12. An earth leakage detection circuit according to claim 10 wherein, diodes prevent current limiter feedback from the current source from affecting the current sink.

13. An earth leakage detection circuit according to claims 1 or 12 wherein, when a current limit is reached, the corresponding current source raises a limiter feedback signal, to turn on a transistor.

14. An earth leakage detection circuit according to claim 13 wherein, when the transistor is turned on, current through diodes is limited and transmits the signal to the microcontroller through diodes.

15. An earth leakage detection circuit according to claim 14 wherein, when an earth leakage occurs between the earthed equipment and the power conductor, the voltage is diverted away from the biased voltage to pass one of said first and second thresholds. 2