MXPA96005467A - Detector to verify the integrity of a grounding to an electrical appliance - Google Patents

Abstract

The present invention relates to a detector for verifying the integrity of a ground connection to an electrical device, having terminals of electric and neutral current to feed current to the apparatus from the respective electric and neutral current feeders, of a supply electric that has a point to earth GMD, to connect to a ground terminal of the apparatus, such detector characterized in that it comprises: means of differential comparator circuit to compare a fractional voltage in the terminal to ground with a fractional voltage in the neutral terminal of the apparatus, and to produce a fault signal if a difference between them the fractional voltages exceed a predetermined threshold, wherein the differential comparator circuit means. comprise: means of energy supply c.d. Low voltage rail having a high voltage and low voltage rail, the low voltage rail being coupled to the ground terminal of the apparatus first voltage dividing means including a pair of resistors connected in series through the high voltage rails and low voltage and having a common junction, coupled to a first input of a comparator, second voltage separating means including a pair of resistors, connected in series between the high voltage rail and the neutral terminals of the apparatus and having a junction common, coupled to a second input compared

Description

DETECTOR TO VERIFY THE INTEGRITY OF AN EARTH CONNECTION TO AN ELECTRICAL APPLIANCE DESCRIPTION This invention relates to electrical protection systems associated with ground loss meshes. Over the last 15 to 20 years, earth leakage circuit breakers have been used (ELCB) often increased to protect against ground fault failures. The principle of the ELCB, as is well known, is that in the case of a ground fault failure, part of the current that normally flows through the live feeder and returns through the neutral, presents loss to ground. Consequently, there is an imbalance between the respective electrical (or phase) and neutral currents. The unbalance is detected and, if a predetermined threshold level is exceeded, a relay is activated that interrupts the power supply voltage supply to the electric and neutral power supplies. The value of the ELCBs manifests itself particularly when it is protected against ground loss in an apparatus having an electrically conductive housing, which is connected to ground. In such a case, if a failure occurs in the apparatus where the cover becomes electric, then the ground leakage current flows to the ground and immediately operates the ELCB. However, if the ground connection fails in such a way that there is no leakage path for the current to flow in case the cover becomes electric, then the ELCB will not activate until someone touches the electrically conductive housing and This form provides a leakage path to earth. In such a case, the leakage current passes through the person giving rise to the required imbalance between the electric and neutral aliquot currents, which causes the ELCB to operate. Under these circumstances, as long as the ELCB continues to operate, there is an inevitable loss to ground through the person who touches the device. In this way, the ELCB must be so adjusted that the loss current, which occurs in such circumstances, is not fatal. However, what constitutes a "fatal" ground leakage current varies from one person to another. In addition, the ground leakage current, which will actually flow through a person consistent with the cover of an electrical appliance becomes "electrical", is a function of the person's body resistance. People whose skin is suitable for being wet (mothers and children, for example) have a skin resistance much lower than the staff of the developing site, for example, whose skin tends to be dry and callused. An earth leakage current, which may present a slight shock, however, may prove fatal to the trainer. Furthermore, the magnitude of the earth leakage current, which can be fatal for someone younger, is so low that it adjusts the ELCB in order to be effective against the small leakage current to ground which, in practice, would result in false alarms and the unnecessary disconnection of the ELCB. In fact, there is no perfect solution to this problem. The best thing to do is to adjust the ELCB to operate at an "average" earth leakage current typically of the order of 30 mA, mindful of the fact that a current may actually be fatal to a segment of the population. When it is also kept in mind that, if a device's ground connection is absent or fails, then an earth leakage current will generally find a return path to the ground connection through a person who inadvertently touches a ground cover. It is clear that, in some cases at least, ELCBs give an apparent security impression, but not justified. It is an object of the present invention to provide a protection device, which either replaces or can be used in conjunction with an ELCB and which eliminates or significantly reduces the disadvantages described above. According to a broad aspect of the invention, a detector is provided to verify the integrity of a ground connection to an electrical appliance that has electrical and neutral terminals to supply power to the appliance from respective power and neutral power supplies of an electrical supply having a power point. ground connection for connecting to a ground terminal of the apparatus, such detector comprises: a differential comparator circuit for comparing a voltage at the neutral connection with a voltage at the ground terminal of the apparatus and producing a fault signal if a difference between such voltages exceeds a predetermined threshold. Such a detector can be used merely to give an indication that the ground connection to a device is failing. Preferably, the detector further includes interrupting means responsible for the fault signal to open a switch contact in at least one of the electric current and neutral feeders. BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it can be put into practice, some preferred embodiments will now be described, by way of non-limiting example, with reference to the accompanying drawings in which: FIGS. Ib and show schematic representations of an apparatus incorporating a detector according to the invention; Figure 2 is a circuit diagram schematically showing the detector according to the invention: Figure 3 shows schematically a detail of an auxiliary detector for use with the invention; and Figure 4 shows schematically how the detector, according to the invention, can be used with a conventional ELCB or contactor. The Figure shows an apparatus 10 representing an electrical load 11, which is connected to electrical and neutral current terminals 12 and 13 respectively. The current is fed to the apparatus 10 via the respective electric and neutral power supplies 14 and 15 of an electrical supply represented schematically by a transformer winding 16. In practice, the transformer winding 16 is a single phase of a power supply. 3 phases that have a secondary transformer winding connected to star whose star point, constituting the neutral feeder 15, is connected to ground, GND. Associated with the apparatus 10 is a ground terminal 17, which itself is connected to GND. It should be understood that the ground terminal denoted GND is, in fact, provided by the power supply company and is usually anchored to a local bus on the customer's distribution board (not shown). Connected between the neutral terminal 13 and the ground terminal 17 of the apparatus 10 is a detector 18 for verifying the integrity of the ground connection between the ground terminal 17 and GND. For the sake of integrity, it should be noted that the potentials associated with the ground terminal 17 and the far ground connection of the power supply at the distribution transformer site, also denoted by GND, are nominally the same. However, in reality they are not physically connected to the same point and therefore it is legitimate that there is a slight discrepancy between the real potentials associated with distant and local ground connections. The maximum allowable discrepancy is set by the utility company and is typically less than 12 V. The detector 18 detects whether the difference between the voltages at the neutral terminal 13 and the ground terminal 17 exceeds the maximum voltage fluctuation allowed by the power supply company and, if so, produces a fault signal indicative of a fault ground connection to the apparatus 10. Fig. Ib schematically shows a fault condition, wherein the electrical power terminal 12 is connected to the terminal to ground 17. Providing that the ground terminal 17 is connected to GND, there will be a leakage current through GND, so that there will be a disparity between the electric current and the neutral feeder current of sufficient magnitude to cause the operation of a conventional ELCB (shown in Fig. 4). Fig. Shows still another fault condition causing an effective short circuit between the electrical power terminal 12 and the ground terminal 17, but where, belonging to a fault ground or absent connection, the ground terminal 17 it is not connected to GND. Alternatively, pertaining to the corrosion, for example, of the direct ground connection of the customer distribution board to the ground terminal 17, the path between the ground terminal 17 and GND may have an increased resistance, so that the terminal to ground 17 is a potential slightly larger than GND. This situation is dangerous since a person who touches the device and makes a connection to its electric power terminal 12, offers an alternative route for the leakage current to flow. An increase in the impedance of the direct connection to GND causes an increase in the leakage current, which branches through the person. As explained above, this leakage current can, in some cases, be fatal. Referring now to Fig. 2, a circuit diagram of the detector 18 is shown comprising a low voltage rail 20 connected to the ground terminal 17 of the apparatus 10 and a high voltage rail 21 through which a regulated voltage of 6 V c.d exists. The voltage c.d. regulated is derived, in conventional manner, via a low-pass transformer 22, whose primary winding 23 is connected through the electric and neutral power supplies 14 and 15, and whose secondary winding is connected to a normal bridge rectifier 25 connected in its negative output to the low voltage rail 20 and connected at its positive output, via a rectifying diode 26, to an integrated circuit regulator 6 V 27. A first voltage separator comprising resistors 28 and 29, is connected through the voltage rails 20 and 21 and a common junction 30 thereof are connected to the reversing input of a comparator 31, in order to provide a reference voltage signal, which is diverted from the ground potential by a quantity fixed. A second voltage divider, comprising resistors 32 and 33, is connected between the high voltage rail 21 and the neutral terminal 13 of the apparatus via a rectifying diode 34. A common junction 35 of the second voltage divider is connected to the input without inversion of the comparator 31 and also connected to the low voltage rail 20 via a 6 V zener diode 36 connected in parallel with a filter capacitor 37. An output 38 of the comparator 31 is connected to the base of a bipolar junction transistor NPN 39 , whose emitter is connected to the low voltage rail 20 and whose collector is connected, via a relay 40, to the unregulated voltage high rail. A resistor 41 is connected between the unregulated high voltage rail and the base of the bipolar junction transistor 39, which is connected via a capacitor 42 to the low voltage rail 20. In practice, a rectifier diode (not shown) is connected through the relay coil 40 in order to derive any upper return EMF generated by the coil 40 and thereby prevent damage to the bipolar junction transistor 39. In order to increase the safety of the detector 18, it is provides a second comparator 43, whose output is commonly connected to the output 38 of the first comparator 31, and whose inverting and non-inverting inputs are connected to GND and to the neutral terminal 13 of the apparatus via respective first and second voltage separators, and peripheral components, which are shown without marks in the figure since they are, in all aspects, identical to the circuit system described above in relation to the first compares 31. The first and second comparators 31 and 43 are constituted by an integrated double comparator circuit such as National Semiconductors LM193 series. The rectifying diode 34 avoids the negative half cycle of the supply voltage c.a. which is compared to the ground voltage on the low voltage rail 20, so that the respective comparators 31 and 43 are affective only for the half positive cycle of the supply of c.a. Also, any fluctuation of c.a. associated with the ground voltage is derived by means of a filter capacitor 44 connected between GND and the high voltage rail 21, while the rectifying diode 26 has been shown to provide even greater stability. The operation of the detector 18 is as follows. Normally, the output voltage of the comparator 31 is low, so that the base voltage of the bipolar junction transistor 39 is low and the bipolar junction transistor 39 is cut off. When a difference between the voltage levels appears in the reversal inputs and if reversal of any of the comparators 31 and 43, a predetermined threshold is exceeded, which is determined by the values of the circuit components, then the output 38 of the comparators fed to the base of the bipolar junction transistor 39 becomes high and the bipolar junction transistor 39 is switched and the relay 40 is activated. Relay 40 can be connected to produce an alarm indicative of a fault ground connection, or can be connected to an interruption device such as an ELCB or contactor, to automatically disconnect the electricity supply from the apparatus 10, as described in more detail below with reference to Figs. 3 and 4 of the drawings. In a particular embodiment of the detector 18 reduced to practice, the values of the resistors 28 and 29 in the first voltage separator are both 240 KO, while the values of the resistor 32 are 5.5 KO and that the resistor 33 is 3.3 KO. The capacitor values 37, 42 and 44 are respectively 22μF, 100 / xF and 470μF. The value of the resistor 41 is 1 KO. The detector 18 described above with reference to Fig. 2 of the drawings is effective in the circuit configuration shown in Fig. Ib to indicate that the potential to ground and that the neutral terminal 13 does not differ by more than a threshold allowed . However, in the case of a fault, which effectively has short circuits of the electric current terminal 12 with the ground terminal 17, as shown in Fig. 1, the ground terminal 17 is still connected to GND via power supply 14 and the secondary winding 16 of the power supply. If the length of the electric power supply 14 is not so great that the combined impedance of the secondary winding 16 of the electric power supply 14 does not produce too great a voltage drop, then it may happen that the voltage at the terminal to ground 17 and that the terminal to ground far from the power supply, and attached to the neutral feeder 13, are within the allowed tolerance. In this case, the detector 18 will register a "healthy" condition although, in reality, the ground terminal 17 is not directly linked to GMD. Fig. 3 shows an auxiliary detection circuit 50 for detecting a difference in voltage between the ground point GND of the electrical supply and the neutral terminal 13 of the apparatus and comprising a bridge rectifier 51 having rectifying diodes 52a, 52b and 53a , 53b. The diodes 52a and 52b as well as the diodes 53a and 53b are connected in a reciprocal manner through the neutral and GND terminals. With respect to the reciprocal connection of the two diode pairs, each branch of the bridge rectifier 51 has a very high impedance, in excess of 200 KO, the minimum impedance between the neutral and ground feeders allowed by the power supply company is electricity. The positive and negative outputs of the bridge rectifier 51 are connected to a relay coil 54 operatively connected in series with a latch normally a closed switch contact 55 and further operably connected to a latch normally a closed switch contact 56 and to a latch normally an open switch contact 57, whose function is explained below with reference to Fig. 4 of the drawings.

When a voltage appears through the neutral terminal 13 and GND exceeding the inverse deviation voltage of the rectifying diodes in the bridge rectifier 51, then the bridge rectifier 51 conducts current between the neutral terminal 13 and GND, in order to activate the relay coil 54, which makes the normally closed switch contact 56 to latch open and the normally open switch contact 57 to latch closed. The activation of the relay coil 54 also causes the normally closed switch contact 55 to be connected in series with the relay coil 54 to be locked in the open manner, thus deactivating the relay coil 54 and preventing it from being overheated. However, since the normally closed switch contacts 55 and 56, as well as the normally open switch contact 57 are all latch contacts, they remain in their switched (i.e. activated) conditions even when the current to the coil is interrupted. relay 54, until it is manually reset by a normally open button 58 connected in series with a relay coil 59 through the high and low voltage rails 21 and 20, described above with reference to Fig. 2 of Figs. drawings. Referring to Fig. 4 of the drawings, respective feeders 60, 61 and 62 of a 3-phase supply are shown, each connected in series with the normally closed switch contacts 63, 64 and 65, which are adapted for simultaneous operation by a contactor 66 connected in series with the normally closed switch contact 56 described above with reference to Fig. 3 of the drawings. The contactor 66 together with the normally closed switch contact connected in series 56 are connected through the electric and neutral power supplies 14 and 15, via respective normally closed switch contacts 67, 68, which are operatively coupled to a switch Ground loss automatic (ELCB) 69 having a sensing coil 70. A resistor 71 is connected through the electric and neutral current feeders via the normally open switch contact 57, the resistor 71 being surrounded by the sensing coil 70 of the ELCB 69. When, belonging to an imbalance between the neutral and GND voltages, the relay 54 is activated momentarily, the normally closed switch contact latches are opened thereby connecting the contactor 66 through the electric and neutral power supplies 14 and 15, respectively. The contactor 66 in this way is activated and the normally closed switch contacts 63, 64 and 65 are opened, thus interrupting the energy to each of the 3 phase feeders 60, 61 and 62.

The operation of the relay coil 54 also causes the normally open switch contact 57 to be closed so that the resistor 71 is connected through the electric and neutral current feeders 14 and 15, respectively. The value of the resistor 71 is thus chosen that the current, which flows in this way through it, exceeds the ground loss current of the ELCB 69, typically 30 mA. Since this current flows through the sensing coil 70 of the ELCB 69, this is interpreted by the ELCB 69 as an earth leakage current and the ELCB 69 operates so as to open the normally closed switch contacts 67 and 68 and from this form interrupts the individual phase supply through the electric and neutral power supplies 14 and 15. After diagnosing the cause of the fault and correcting it, the energy can now be restored and the button switch 58 (shown in FIG. 3) can be depressed in order to activate the relay coil 59. The relay coil 59 is operatively coupled to the latch switch contacts 55, 56 and 57 operated by the relay coil 54 so that, when the coil is activated of relay 59, the two normally closed switch contacts 55 and 56 are restored to their closed state, while the normally open switch contact 57 is restored to its open state.

It will be appreciated that one may wish to avoid the use of a contactor and an ELCB as shown in Fig. 4 and, if desired, one of these protection devices may be dispensed. However, both are shown and described in order to demonstrate that the auxiliary circuit 50 is equally well suited to act together with any of these devices. It will also be appreciated that the auxiliary detection circuit 50, being connected between the neutral feeder 15 and GND, does not require connection of the apparatus 10, since this is connected through the neutral input and ground connection points of the power supply. Accordingly, the auxiliary sensing circuit 50 is protected against any unavoidable voltage imbalance between the neutral and ground connections of the input power supply and causes the main contactor and / or the ELCB to be derived consequential to that condition. However, in the case where the ground terminal 17 of the apparatus 10 is connected to GND, then both the main detector 18 and the auxiliary detection circuit 50 serve to verify the integrity of the ground connection, even in the absence of a loss to earth. In the case of a ground loss, the auxiliary sensing circuit 50 is operating, even if the impedance resulting from the ground fault detected by the main detector 18 is too low to indicate a fault ground connection. Thus, according to the invention, a detector is provided that verifies the integrity of a ground connection of an input power supply, so that, in the event of a failure thereof, a voltage imbalance between the connections to ground and neutral, which allows the interruption of the electrical supply. By these means, the integrity of the ground connection can be ensured so that any ground leakage current will flow to GND, even if a metal cover of an electrical appliance becomes "electric current". This provides complete protection to a user of such an apparatus, since the ground leakage current flows through the user to the contrary to the proposed ELCBs used either in systems not connected to ground or in systems that are inadequately grounded.

Claims (8)

1. CLAIMS 1. A detector to verify the integrity of a ground connection to an electrical device, which has terminals of electric and neutral current to supply power to the appliance from respective electric and neutral power supplies, from an electrical supply that has a ground point GMD, to connect to a ground terminal of the apparatus, such detector is characterized in that it comprises: means of differential comparator circuit, to compare a fractional voltage in the terminal to ground with a fractional voltage in the neutral terminal of the apparatus, and to produce a fault signal if a difference between the fractional voltages exceeds a predetermined threshold; wherein the differential comparator circuit means, comprise: power supply means c.d. Low voltage rail having a high voltage and low voltage rail, the low voltage rail being coupled to the ground terminal of the apparatus, first voltage separating means including a pair of resistors connected in series across the voltage rails high and low voltage and having a common junction, coupled to a first input of a comparator, and second voltage separating means including a pair of resistors, connected in series between the high voltage rail and the neutral terminals of the apparatus and having a common joint, coupled to a second comparator input.
2. 2. The detector in accordance with the claim 1, characterized in that the differential comparator circuit comprises: interruption means, operatively coupled to an output of the comparator.
3. 3. The detector in accordance with the claim 2, characterized in that the interrupting means include a transistor switch, coupled to a normally closed interrupting device, connected in at least one of the terminals of electric and neutral current and being responsible for failure to open the interrupting device.
4. 4. The detector according to claim 2, characterized in that the differential comparator circuit includes at least two comparators, which have respective outputs, which are commonly connected to the interruption means.
5. 5. The detector according to claim 1, characterized in that the comparator is an integrated circuit.
6. 6. The detector according to claim 1, characterized in that it includes an auxiliary detection circuit, for detecting a difference in voltage between a point to ground and the electrical supply and the neutral terminal of the apparatus, such an auxiliary detection circuit comprising: very high impedance current connected between the neutral feeder and the ground point of the electrical supply to produce an unbalance signal, if the current flows in any direction between the neutral feeder and the ground point; and auxiliary interrupting means, operatively coupled to the normally closed interrupting device, connected and being responsible for the unbalance signal to open the switching device. The detector according to claim 6, characterized in that the current detector comprises a bridge rectifier having a pair of reciprocal rectifier diodes connected in series, connected, through the neutral feeder and the ground point of the electrical supply. The detector according to claim 6, characterized in that the auxiliary interruption means are connected in series with a elimination switch, so that in the operation of the interruption means, the elimination switch is opened, thus deactivating the means of interruption, and also means of refixation is provided to close the elimination contact.