US3573551A

US3573551A - Protection against electrical shock

Info

Publication number: US3573551A
Application number: US833463A
Authority: US
Inventors: Richard C Sircom
Original assignee: EASTECH Ltd
Current assignee: EASTECH Ltd
Priority date: 1969-06-16
Filing date: 1969-06-16
Publication date: 1971-04-06
Anticipated expiration: 1988-04-06
Also published as: CA925155A

Abstract

To reduce the danger of electrical shock in two-wire nongrounded electrical systems, the leakage current from line L1 to ground is balanced by an injected current from ground to line L1, and similarly the leakage current from line L2 to ground is balanced by an injected current from ground to line L2. In this way, the leakage current from line L1 to ground cannot pass through a human body from ground to L2, since the leakage current form a closed current loop.

Description

United States Patent [72) lnventor Richard C. Sircom Windsor, Nova Scotia, Canada [21 Appl. No. 833,463 [22] Filed June 16, 1969 [45] Patented Apr. 6, 1971 [73] Assignee Eastech Limited Windsor, Nova Scotia, Canada [54] PROTECTION AGAINST ELECTRICAL SHOCK [56] References Cited UNITED STATES PATENTS 2,786,148 3/1957 Hunt 317/17X 3,2 30,424 l/l966 Gagniere 3 17/10 Pn'm'ary Examiner-J. D. Miller Assistant Examiner-Harvey F endelman Attorney-Stevens, Davis, Miller and Mosher ABSTRACT: To reduce the danger of electrical shock in twowire nongrounded electrical systems, the leakage current from line L] to ground is balanced by an injected current from 13 8 Dawn! ground to line L1, and similarly the leakage current from line [52] US. Cl. 317/17, L2 to ground is balanced by an injected current from ground 307/92 to line L2. [5 1] Int. Cl 02h 9/00 In this way, the leakage current from line L1 to ground can- [50] FieldotSearch 317/ l7, 18, not pass through a human body from ground to L2, since the 26', 307/92; 328/7; 174/5 leakage current form a closed current loop.

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Patented" April 6, 1971 FIG. 6C

PROTECTION AGAINST ELECTRICAL SHOCK This invention relates to protective means adapted to reduce the danger of electrical shock in two-wire nongrounded electrical systems.

Systems of this general type are commonly used in situations where the danger caused by an electrical shock is greater than would normally be the case. Thus in wet conditions, such as in association with swimming pools, where a very good ground connection can exist from the body of a human, and in conditions such as during surgical operations where electrical apparatus is used on particularly sensitive parts of the human body, a current can flow through part of a human body which is sufiicient to cause death.

Existing systems are effective to reduce considerably the danger of the passage of relatively large ground currents, but it has been found that often a relatively small ground current can also be lethal. The dangerous level of current is so low that even the relatively small capacitative line-to-ground leakage current in a normal system is large enough to be fatal.

An object of the present invention is the provision of protective means adapted to' reduce the danger of electrical shock in two-wire nongrounded electrical systems, and capable of providing protection even against the relatively small capacitative line-to-ground leakage currents.

According to the present invention, in protective means adapted to reduce the danger of electrical shock in a two-wire nongrounded electrical system, current generating means cause a first current to be injected between ground and a first of two lines of the system, this current being substantially equal to and in phase with a first leakage current between that first line and ground, and the current generating means cause a second current to be injected between ground and the second of the two lines of the system, this current being substantially equal to and in phase with a second leakage current between that second line and ground, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of a two-wire nongrounded electrical power supply system, and of protective apparatus applied thereto;

FIGS. 2 to 5 are explanatory diagrams relating to leakage currents and the present invention; and

FIGS. 6A and 68, when arranged side-by-side as indicated in FIG. 6C, together show the circuit diagram of a capacitative-leakage-current suppressor shown in FIG. 1.

Referring first to FIG. I, an isolating transformer 1 having a primary winding Pl connected across an alternating current supply 3 and having a grounded electrostatic screen 5, has a secondary winding S1 which is used to energize an isolated power system through a contact breaker panel 6.

The contact breaker panel 6 is orthodox in that it includes two insulated bus bars 7 and 9 connected respectively to the two ends of the secondary winding SI, and a plurality of automatic overload contact breakers CB1, CB2, and CB3. A main manual circuit breaker MB is arranged to permit breaking of both connections to the bus bars 7 and 9, and each circuit breaker CB1, CB2, and CB3 also is effective to break both poles of the supply.

The contact breakers CB1, CB2 and CB3 are arranged to control the supply of alternating current electrical power to separate subcircuits SCI, SC2, and 8C3 and typically each subcircuit would supply a number of two-pin or three-pin power outlets 10 into which portable or movable electrical equipment could be plugged as and when required. v

Systems such as have been described so far are well known in the art and are used in environments, such as hospitals, where it is desired to eliminate the risk of ground fault currents. Since both poles of the supply are isolated from ground, no large ground current can flow unless two faults to ground exist, one from each pole of the isolated supply. However,

such systems are inherently defective in that there is a capacitive coupling between each live part of the system and ground, and although the capacitive currents which flow per foot run may be small, they total to a considerable current in large system.

In the system shown in FIG. I, contact breaker CB1 is used solely as a power supply for a capacitive-leakage-current suppressor 11 and a ground leakage current indicator 13. The suppressor 11 includes a first set of input terminals Al, A2, and A3 for connection respectively to a first pole of each of the subcircuits it is desired to monitor, and a second set of input terminals B1, B2 and B3 for connection respectively to the second pole of those subcircuits. The connections to terminals Al and B1 are actually internal leads of the suppressor I]. As indicated in FIG. 1, the suppressor 11 includes a series of variable potentiometers RIAl, RlA2, RlA3, R181, R182 and RIBS and these are associated respectively with the input terminals Al to B3. Associated with the group of terminals connected to first pole of the supply, i.e.: the terminals A1,A2 and A3, is a variable potentiometer R24A and associated with the group of tenninals connected to the other pole of the supply is a variable potentiometer R248.

FIG. 2 is a diagram showing how a capacitive leakage current can flow through leakage capacitance CX and a resistive leakage current can flow through leakage resistance RX from line L1 to ground and thence through any available impedance Z to the other line L2. FIG 3 illustrates how the similar leakage capacitance CY and leakage resistance RY between line L2 and ground together usually form the impedance Z. The direction of current flow is, in view of the alternating potential involved, also alternating. Referring back to FIG. 2, it will be seen that if a human body HB bridges the insulation between line L2 and ground, it shunts the impedance Z, and the leakage current between line L1 and ground now passes partly through that human body HB. If the impedance of the body is low compared with impedance Z which will often be the case, the leakage current will flow for the most part through the human body I-IB.

Referring now to FIG. 4, it will be seen that a first human body HBI connected between L2 and ground would shunt the impedance Z 2 presented by leakage capacitance C 2 and leakage resistance R 2 and would pass the leakage current from line L1 to ground on to line L2. On the other hand, a second human body HB2 connected between L] and ground would shunt the impedance Z 1 presented by leakage capacitance R l and leakage resistance R1, and would pass the leakage current from line L2 to ground on to line L1. The operation of .the suppressor I1 is indicated schematically in FIG. 5. Ideally, the suppressor produces a current 11 from ground to line LI which is exactly equal to leakage current 11 from line L1 to ground. Since there is no net current flow from line L1 to ground, a human body HBl connected between ground and line L2 will pass no current. Similarly, ideally the suppressor produces a current 12 from ground to line L2 which is exactly equal to leakage current 12 from line L2 to ground, a human body HB2 connected between ground and line Ll will pass no current. It will be understood that the presence of body HBI must not occur at the same time as the presence of body HB2, since then both bodies would carry currents.

The suppressor 11 (see FIGS. 6A and 68) includes a power supply 51 and two

subcircuits

53 and 55 which are associated respectively with line L1 and line L2. The power supply 51 provides separate DC supplies at I-ZOO v., Ov., and 200 v. to subcircuit 53 and to subcircuit 55. It also supplies a common DC supply at +15 v., Ov., and -l5 v., as indicated. In these two FIGS. leads A to G on each FIG. are connected to the corresponding leads on the other FIG. The components used in suppressor 11 include the following:

Considering first sub-circuit 53, terminals A1, A2 and A3 are indicated, and it will be noted that a lead 57 connects the input line L1 to the terminal Al while a lead 59 connects the input line L2 to terminal BI. Thus the suppressor uses one of its inputs to monitor the power supply to the suppressor.-

Transistors Resistors QlAl- RIAL 10,000 ohms. Q2 2N3711 BIA D0. Q3 R1A3 D0.

Diodes Btu". 10 megohms. D1... IN5212 R5 100,000 ohms. D IN5212 R6 82,000 ohms. D3"... IN5212 R7... 8,200 Ohms. D4..." 1N5212 R8 4,700 ohms.

R9 15,000 ohms. CR1 IN914 R101". 4,700 ohms. CR2 1N914 R11 470 ohms. CR3. IN914 R12 Do. CR4-" VR180 1113.. 4,700 ohms. CR5- V3180 1114",. 470,000 ohms. CR6 3842 3.15.... Do. CR7- 384Z 1116..-. Do. CR8..- 3842 R17 Do. CB9.-- 3842 Rl8 4,700 ohms. R19- 330,000 ohms.

Capacitors 320..-. 4,700 ohms R21 220 ohms. C1 0.0033mid. R22-.. Do. C2 250mfd B23 100ohms. C3 0.1 mid. R24A" 2 megohms. C4. 20mm. R24B. Do. C5-.- 20mtd.

C6 50mid C7 50mfd Through the series combination of capacitor C1 and potentiometer RlAl, the line L1 is connected to ground when switch SlAl is closed. SlAl is one set of contacts of a multiple switch, other sets of contacts being indicated at S1A2, S1A3, SlBl, S1B2, and S1B3. Transistor Q1A1 has its base connected to the slider of potentiometer RlAl, and its emitter is at very close to ground potential, since its base is connected to ground through resistor RlAl. This emitter is connected to a differential amplifier formed by transistors Q2 and Q3, and the output of transistor QlAl is compared with the average of the amplifier output voltage, the averaging being effected by a low-pass filter formed by resistor R19 and capacitor C2. Differences between these two voltages are amplified by differential amplifier in phase-opposition to stabilize the output voltage operating point at ground voltage. The input voltage to transistor QlAl is proportional to the current flowing through capacitor C1, and since the reactance of the capacitor C1 is much higher than the resistance of resistor RlAl, this current will be in phase with the currents flowing in the leakage capacitance between line and ground in the subcircuit SCI and the apparatus connected to it, and also will be proportional to that current. By adjustment of the resistor RlAl, this proportionality can be adjusted so that the output current flowing from ground-to-line is equal to the leakage current flowing from line-to-ground, to result as discussed above in a closed current loop between line L1 and ground.

It will be seen that each of the inputs Al, A2 and A3 has associated with it an input circuit 61 which, in the case of input A1 contains capacitor C1, resistor RlAl, transistor QlAl, and an emitter resistor R2Al. The other input circuits 61 are similar to that one, and each (when the associated circuit breaker is closed) will supply an AC current through an associated adding resistor R3A1, R3A2 or R3A3 to the input of the differential amplifier, so that the subcircuit 53 operates to balance the leakage current from line L1 to ground for the cir' cuits selected by closure of the circuit breakers CB2 and CB3.

Dealing now in more detail with the subcircuit 53, the output stage is a push-pull complementary-symmetry circuit including transistors Q8, Q9, Q and Q11, in which the voltage between the output on the collectors of transistors Q9 and Q10 and the positive and negative supply rails (i.e., the leads at 4:200 V. and at --200 V.) is shared equally by transistors Q8-Q9, and Q10-Q11 respectively. This is effected by a voltage dividing chain of resistors R14, R15, R16 and R17, the

values of which resistors are equal, and the midpoint of which resistive chain is connected to the output, i.e. to line L1 through DC isolating capacitor C3. Emitter-follower transistors Q6 and Q7 hold the emitters of transistors Q9 and Q10 at voltages midway between the output voltage and the positive and negative supply rail voltages, respectively. Output transistors Q8 and Q11 are driven in phase opposition by transistors 04 and Q5 through Zener coupling diodes CR4 and CR5 respectively. These reduce the collector-to-emitter voltage applied to transistors Q4 and Q5, allowing the use of low voltage transistors in this stage.

The idling current in the output stage is determined by the voltage difference between the bases of transistors Q4 and Q5, which is held at a constant value by the current flowing through forward-biased diodes CR1, CR2 and CR3. This current flows through the resistive coupling chain formed by resistors R8 and R9, from the collector of transistor Q3. Negative current feedback is applied by sensing the output current as a voltage drop across resistor R23 in the return line from ground to the common of the dual power supply. This signal is fed back through a summing resistor R5 to the junction of resistors R3A1 to R3A3 and the base of transistor 02, and is in phase-opposition to the input.

By adjustment of potentiometer R24A, an input voltage can be added at the base of transistor 02 which will be in phase with the line-to-ground voltage, and which will therefore produce an output current from ground-to-line equal to the resistive leakage current flowing from line-to-ground through permitted resistive leakage paths, such as through resistors R14, R15, R16 and R17, as well as in external circuits such as the leakage detecting indicator 13.

Subcircuit 55 is a duplicate of subcircuit 53 described above, but in this case line terminal L2 is connected to input terminal B1 and so this subcircuit monitors and compensates for leakage from line L2 to ground. It is necessary to use separate power supplies, fed from individual windings on the power transformer, since the midpoint of the power supply as regards sub-circuit 55 is not the same signal potential as that in subcircuit 53.

It will be appreciated that although three input terminals Al, A2 and A3 (for line L1) and three input terminals B1, B2 and B3 (for line L2) are provided, any number of such input terminals can be used, in two groups, one for each line L1 or L2, when the total number of subcircuits to be monitored for capacitative leakage currents exceeds three.

In the use of the apparatus as illustrated in FIG. 1, the various potentiometers RlAl to RIM, and the potentiometers R24A and R24B, are adjusted in sequence to obtain a minimum reading on the meter of the ground leakage monitor. Basically, each of the two potentiometers R24A and R24B requires only one adjustment to bring the indicated fault to a minimum value, and the other potentiometers may require repeated adjustment until all leakage currents have been balanced. This adjustment can be expedited by first leaving contact breakers CB2 and CB3 open, and adjusting the potentiometers to balance only the leakage currents for the subcircuit SCI. Next the contact breaker CB2 can be closed and adjustment made only to potentiometers R1A2 and R1B2 and possibly R24A and R24B, to compensate for leakage currents in subcircuit SC2. Finally, contact breaker CB3 can be closed and potentiometers R1A3 and R1B3 adjusted to compensate for leakage currents in subcircuit SC3, a final adjustment of potentiometers R24A and R24B possibly being necessary.

Since the apparatus described above does not automatically adjust itself to compensate for changes in the leakage currents in the apparatus in the system, it is necessary to repeat the adjustment whenever equipment is switched into or out of circuit.

It is to be noted that when the suppressor 11 is switched off, by the use of a common operating knob for all the switches shown in it, all parts of the suppressor are isolated from the electrical supply system.

The apparatus which has been described above enables the leakage current which can flow between a line and ground through a human body to be kept to very small values. The apparatus is used in conjunction with a ground leakage indicator such as the'indicator 13 shown, so that any major ground fault many of the electrical circuits will be noted by that indicator. The suppressor ll acts to compensate for line-to-ground currents which are inherent in the use of the electrical apparatus, and which therefor cannot be dealt with by cessation of the electrical supply. it will be seen that whereas known types of ground-current protection equipment deal with fault conditions, the suppressor of the present invention deals with normal leakage currents.

By provision of separate adjusting means for compensation of the capacitive leakage currents of various subcircuits, it is possible to deal with the situation where the ratio of capacitive leakage current to resistive leakage current varies from subcircuit to subcircuit.

lclaim:

1. Protective means adapted to reduce the danger of electrical shock in a two-wire nongrounded electrical system, in which current generating means cause a first current to be injected between ground and a first of two lines of the system, this current being substantially equal to and in phase with a first leakage current between the first line and ground, and in which the current generating means cause a second current to be injected between ground and the second of the two lines of the system, this current being substantially equal to and in phwe with a second leakage current between that second line and ground, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

2. Protective means as claimed in claim 1, wherein the system is an alternating current system and theleakage currents include capacitative components.

3. Protective means as claimed in claim 1, wherein the system is an alternating current system, and the protective means include first adjustable'means by which compensation for capacitive leakage currents is provided and second adjustable means by which compensation of resistive leakage currents is provided.

4. Protective means as claimed in claim 1, wherein the system includes a plurality of subcircuits and the protective means include separate adjusting means associated with each of those subcircuits for compensation of capacitive leakage currents in that circuit.

5. Protective means as claimed in claim 1, wherein two inherently similar but separate amplifier devices are arranged to compensate for line-to-ground leakage currents respectively from one line to ground and from the other line to ground, each amplifier device being arranged to respond to the leakage current and to apply a substantially equal compensating current between ground and its associated line.

6. Protective means as claimed in claim 5, wherein the system includes a plurality of subcircuits and the protective means include separate adjusting means associated with each of those subcircuits arranged for compensation of capacitive leakage currents in that circuit.

7. Protective means as claimed in claim 6, wherein the protective means also includes for each line a single adjusting means arranged for compensation of resistive leakage currents in the whole system.

8. A method of providing protection to reduce the danger of electrical shock in two-wire nongrounded electrical systems, in which a first current is generated in phase with the leakage current from a first of the two lines of the system to ground, this current being substantially equal to that leakage current, that current is injected between ground and the first line, a second current is generated in phase with the leakage current from the second of the two lines to ground, this current being substantially equal to that second leakage current, and that current is injected between round and the second line,

9. The method of claim and in which the system rs an alternating current system and the leakage currents include capacitive components.

10. The method of claim 8, and in which separate adjustments of the injected current are carried out to compensate respectively for capacitive and for resistive components of the leakage current.

11. A protective device suitable for connection to a twowire nongrounded electrical system and including connections respectively to ground, to a first of two lines of the system, and to a second of the two lines of the system, first current generating means adapted to sense the potential between the first line and ground and to produce an output current which is proportional to that potential and which can be adjusted to be in phase with leakage current from that line to ground, and to apply through its ground and first line connections a current adjustable to balance the leakage current from the first line to ground, and second current generating means adapted to sense the potential between the second line and ground and to produce an output current which is proportional to that potential and which can be adjusted to be in phase with leakage current from that second line to ground, and to apply through its ground and second line connections a current to balance the leakage current from the second line to ground, whereby the leakage from the first line to ground is precluded from passing on to the second line and the leakage current from the second line to ground is precluded from passing on the first line.

12. A protective device as claimed in claim 11, wherein first adjustable means are provided by which compensation for capacitive leakage currents is provided and second adjustable means are provided by which compensation of resistive leakage currents is provided.

13. A protective device as claimed in claim 11, wherein separate potential sensing means are provided arranged to be connectable separately to different subcircuits of the first line to be compensated, and the protective device is adapted to provide a total compensating current between ground and the first line which is the sum of the compensating currents required for the subcircuits of the first line, and similarly separate potential sensing means are provided arranged to be connectable separately to different subcircuits of the second line, to be compensated, and the protective device is adapted to provide a total compensating current between ground and the second line which is the sum of the compensating currents required for the subcircuits of the second line.

Claims

1. Protective means adapted to reduce the danger of electrical shock in a two-wire nongrounded electrical system, in which current generating means cause a first current to be injected between ground and a first of two lines of the system, this current being substantially equal to and in phase with a first leakage current between the first line and ground, and in which the current generating means cause a second current to be injected between ground and the second of the two lines of the system, this current being substantially equal to and in phase with a second leakage current between that second line and ground, whereby the part of the first leakage current which can flow from ground through a body to the second line, and the part of the second leakage current which can flow from ground through a body to the first line, are substantially reduced.

2. Protective means as claimed in claim 1, wherein the system is an alternating current system and the leakage currents include capacitative components.

3. Protective means as claimed in claim 1, wherein the system is an alternating current system, and the protective means include first adjustable means by which compensation for capacitive leakage currents is provided and second adjustable means by which compensation of resistive leakage currents is provided.

8. A method of providing protection to reduce the danger of electrical shock in two-wire nongrounded electrical systems, in which a first current is generated in phase with the leakage current from a first of the two lines of the system to ground, this current being substantially equal to that leakage current, that current is injected between ground and the first line, a second current is generated in phase with the leakage current from the second of the two lines to ground, this current being substantially equal to that second leakage current, and that current is injected between ground and the second line.

9. The method of claim 8, and in which the system is an alternating current system and the leakage currents include capacitive components.

11. A protective device suitable for connection to a two-wire nongrounded electrical system and including connections respectively to ground, to a first of two lines of the system, and to a second of the two lines of the system, first current generating means adapted to sense the potential between the first line and ground and to produce an output current which is proportional to that potential and which can be adjusted to be in phase with leakage current from that line to ground, and to apply through its ground and first line connections a current adjustable to balance the leakage current from the first line to ground, and second current generating means adapted to sense the potential between the second line and ground and to produce an output current which is proportional to that potential and which can be adjusted to be in phase with leakage current from that second line to ground, and to apply through its ground and second line connections a current to balance the leakage current from the second line to ground, whereby the leakage from the first line to ground is precluded from passing on to the second line and the leakage current from the second line to ground is precluded from passing on the first line.