Apparatus Of Medical And Other Types

Abstract

Medical, dental or other apparatus contacting living beings are connected to their sources of electrical energy by safety devices which, upon connection to the sources, prevent energization of the apparatuses if, when the connections are made, faults such as transposed connections, and/or discontinuity in the connections, exist.

Description

BACKGROUND, FIELD OF THE INVENTION

The present invention relates to medical, dental and other apparatus which may contact a living being unsafely due to fault in the connection of the apparatus to the electrical source for energizing the apparatus. For example, such connection may be made by polarized plugs which through wear can be inserted wrongly, so that what is nominally the "hot" conductor connecting the hot side of the electrical source, is actually the "neutral" conductor, and vice versa, that is to say, the connections are transposed. Again, there may be discontinuity in a "ground" conductor, for example.

Under various circumstances, these types of connection faults, namely, transposition and discontinuity, may create leakage currents through a living being contacting the apparatus, and at levels dangerous to that being. In some cases, too, such faults may be harmful to the apparatus.

BACKGROUND, DESCRIPTION OF THE PRIOR ART

There are various well-known expedients for providing safety in using apparatus of the class described. As noted above, for example, polarized plugs are intended to assure proper connections, and a ground conductor is provided.

In addition, the apparatus may be provided with fault sensing means, such as a ground detector, fuse, or the like, which responds to the existence of a fault by disconnecting the apparatus; an isolating transformer may be interposed between electrical source and apparatus, and so forth.

The foregoing measures, however, are of a before the fact, or after the fact character, so are not necessarily effective at the time connection is made. For instance, deenergizing the apparatus in response to fault current, means that this safety measure cannot take effect until after fault current flows. If the living being is a patient, ill, injured or debilitated, often no leakage current at all through the patient should be tolerated.

SUMMARY OF THE PRESENT INVENTION

According to the present invention, a safety device senses the electrical nature of the connections upon the connection being made, and if this electrical nature is incorrect, prevents the source from energizing the apparatus. More particularly, the device senses voltages, and if these indicate a transposition and/or discontinuity fault, the device operates relays, latches or the like, which in effect prevents the apparatus from being energized at all. Accordingly, if measures such as polarization, grounding, etc., fail, the present invention detects such failure and prevents energization. Further, since the present invention acts in response to voltage, it acts before fault current can flow in a living being in contact with the apparatus.

The utility of the present invention extends from use with hand tools used by individuals in good health, to use with electroencephalography apparatus, electrical surgical apparatus, and the like, wherein the tissues of a living being, very commonly in ill health, have low electrical resistance contact with apparatus electrodes, as well as electrical contact with other structures of the apparatus and the immediate environment.

In the preferred embodiment of the invention, the connection of the apparatus to the energy source is by means of a hot conductor and a neutral conductor, and there is also a ground conductor, and the safety device includes a differential transformer having a pair of primary windings, one of which is to be energized by the voltage between hot and neutral, and the other of which is to be energized by the voltage between hot and ground. The transformer also has a secondary winding, and this is so related to the primary windings that unless there is voltage across both primaries, and the voltages are properly related there is a voltage across the secondary. The safety device includes a switch for allowing the apparatus to be energized, i.e., for turning it on, but if there is a voltage across the secondary, the switch is prevented from turning the apparatus on.

In another embodiment, the hot to ground and hot to neutral voltages fire a pair of silicon controlled rectifiers, or similar devices. The rectifiers are in series with each other and with a solenoid for which when energized causes a relay to permit the apparatus to be turned on.

In still another embodiment, a first solenoid is energized by hot to ground voltage to unlatch the switch for turning the apparatus on. A second solenoid is energized by hot to neutral voltage to allow the switch to be latched on.

In each case, the safety device is preferably a more or less integral part of the apparatus with which it is used. This is because the faults with which the invention deals arise externally of the apparatus, and in its connections to the electrical source which energizes. Typically, there is a three-conductor power cord terminating at one end in the apparatus, and at the other end in a plug having male electrodes. A corresponding plug, having female connectors, provides, with the male connectors, the ground, neutral, and hot terminals of the system. The safety device of the invention preferably provides the termination of the power cord at the apparatus, so faults occuring between the apparatus and the energy source will prevent the apparatus from being turned on, if there is a discontinuity or conductor transposition in the connections at any point between source and safety device. Thus either plug may be incorrectly wired, they may be joined together in wrong polarity, the ground connection may be broken, and so on, and the safety device will detect this and prevent the apparatus from being manually switched on by the operator of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a schematic diagram of a safety device according to the invention in connection with a source of electrical energy and apparatus connected to said source;

FIG. 2 shows a safety device having a differential transformer and so much of FIG. 1 as is necessary to illustrate the operation of the safety device with the transformer;

FIGS. 3 and 4 show other varieties of differential transformers usable in place of that of FIG. 2;

FIG. 5 shows a safety device having silicon controlled rectifiers (SCR's) and so much of FIG. 1 as is necessary to illustrate the operation of the safety device with the SCR's;

FIG. 6 shows a safety device having solenoids and so much of FIG. 1 as is necessary to illustrate the operation of the safety device with the solenoids; and

FIG. 7 shows a safety device like that of FIG. 5 as used in a transformer-isolated system.

In FIG. 1, reference numerals 1, 2 and 3 designate the respective hot, neutral and ground terminals at the plug-in or other connection of the respective hot, neutral and ground conductors 7, 8 and 9, by means of which electrically energizeable apparatus A is to be energized by a source AC of electrical energy.

Voltage sensing means V is connected at points 7, 8 and 9 to conductors 4, 5 and 6, respectively, for controlling a switch S in accordance with the relationship among the voltages at the said points. Switch S has movable contactors 10, 11 and 12 for conductively bridging the electrical discontinuity of the respective fixed-contact pairs 13 and 14, 15 and 16, and 17 and 18. The dashed-line ganging 19 represents mechanism of switch S for moving contactors 10, 11 and 12.

It is to be supposed that the contactors 10, 11 and 12 move in unison, and are normally open. It is to be further supposed that points 7, 8 and 9 and the system of contacts 13 through 18 are structurally integrated as intimately as practical with the portions of apparatus A with which they connect electrically. The first of these portions is represented by impedance Z, representing the electrical load to be driven by source AC and the second of these portions is represented by impedance Z.sub.2 , representing that part of the apparatus which is not supposed to be energized. Normally, Z.sub.2 will include the leakage impedance, to housing or chassis structure, of electrodes, tools, or the like, for contacting a human being.

The function of the means V is to "look" at the voltages at terminals 1, 2 and 3, in order to see if they are appropriate for electrically connecting Z.sub.1 and Z.sub.2 to them. If they are, then means V "tells" switch S to close the contact pairs 13 and 14, 15 and 16, and 17 and 18. If not, means V remains silent, so to speak, so the contacts 10, 11 and 12 remain in the condition illustrated, and apparatus A remains electrically isolated from source AC.

For example, suppose that the terminals 1, 2 and 3 represent a plug-in connection. Then the ends of conductors 4, 5 and 6 at the terminals are incorporated in a plug, normally having male electrodes, and the connections of ground and source AC at the terminals are incorporated in a plug, normally having female electrodes.

For one reason or another, it is sometimes possible to incorrectly mate the plugs with, say, hot and neutral electrodes transposed, hot to neutral and neutral to hot. Again, such transpositions may exist in the wiring of, say, the female electrodes to source AC and ground, so that mechanically proper plug mating is actually electrically incorrect. Further, there can be breaks in the wiring to the left of terminals 7, 8 and 9.

In general, it is undesirable to energize the apparatus A whenever any transposition of connections exists to the left of points 7, 8 and 9, or whenever any electrical discontinuity exists to the left of points 7, 8 and 9.

The basic action of means V is to operate switch S only if it senses source voltage between terminals 7 and 8, and between terminals 7 and 9, simultaneously. Thus, if conductor 4 is connected to terminal 2, and conductor 5 is connected to terminal 1, then if Z.sub.3 is sufficiently small, then there is substantially zero voltage between points 7 and 8. Or, if instead there is electrical discontinuity between point 9 and ground G, then there will be zero voltage between terminals 7 and 9.

In the foregoing, it will be observed that the emphasis is on preventing initial energization of apparatus A under various fault conditions in the connections of apparatus A to source AC and ground. This is critical to electrical safety where the apparatus A is being used in connection with living beings in various enfeebled states due to illness, accident, general debility, and so on. This is because it has to be assumed that any fault-caused current flow at all through these beings may be dangerous to them. It is common to provide apparatus A with various protective means which operate in response to the effect of electrical faults on the operation of the apparatus. That means that if the apparatus is contacting a living being, and the fault is of the kind that would cause current to flow through the living being, then the latter must be subject to the fault current while the protective means is assessing the situation, so to speak, to see if it is necessary to shut-off the power, or whatever. The present invention, on the other hand, simply does not permit the apparatus to operate unless the electrical conditions and connections to the left of switch S are correct.

FIG. 2 shows a preferred embodiment of my invention, only so much of FIG. 1 being shown in FIG. 2 as is necessary to show how the added detail fits into FIG. 1.

In FIG. 2, the contacts 10, 11 and 12 of the switch S are normally closed now, instead normally open, as before. However, another set of movable contacts 20, 21 and 22, along with the contact pairs 23 and 24, 25 and 26, and 27 and 28, provide the normally open property of switch S, FIG. 1.

The switch S is driven by a solenoid 30 suitably mechanically connected, by means not shown, to the ganging 19. When the winding of the solenoid is energized, the contacts 10, 11 and 12 are driven to their open positions. Preferably, the switch is biased closed so that if the solenoid is deenergized, the contacts 10, 11 and 12 automatically resume their closed positions.

The contacts 20, 21 and 22, on the other hand, having suitable ganging 29, are manually operated as by toggle or rocker T which one pushes one way or the other to open or close the contacts 23 through 28, as desired.

Before connecting the conductors 3, 4 and 5 to the source, toggle T is pushed to bring contacts 20, 21 and 22 to the positions shown. The connections are then made, upon which voltage sensing means V, in the form of a differential transformer having primary windings 30 and 31, secondary winding 32, and cores 33 and 34, is enabled to see what voltages exist at points 7, 8 and 9.

Windings 30 and 31 are connected across points 7 and 8, and points 7 and 9, respectively, and are wound on cores 33 and 34, respectively, each winding inducing flux only in its respective core. Further the winding senses are such that when the voltages at 7, 8 and 9 are correct, the flux in core 34 is opposite in sense to the flux in core 33, insofar as inducing voltages in winding 32 is concerned.

Secondary winding 32 is wound over both cores and therefore when the core flux changes has voltage induced therein proportional to the algebraic sum of the fluxes in the cores.

It will therefore be evident that in only one condition will there be zero voltage induced in winding 33, and that is that voltage between points 7 and 9 is equal and opposite to the voltage between points 7 and 8. Under all other conditions (other than the trivial one when the conductors 4, 5 and 6 are not connected to any voltages), there will be a net voltage induced in secondary winding 32.

As winding 32 is connected to the winding 35 of a solenoid 130, the solenoid becomes energized whenever one of the fault conditions described supra exists. When energized, solenoid 130 of course actuates contacts 10, 11 and 12 to open position. Therefore, when one initially connects conductors 3, 4 and 5 to terminals 1, 2 and 3, solenoid 130 immediately breaks the electrical continuity at contact pairs 13 and 14, 15 and 16, and 17 and 18, if there is a fault at the time the connection is made. Hence, operating toggle T cannot turn apparatus A on unless there is not fault.

If desired, contact system 20-28 can be provided with a latch L, which will latch the contacts on, provided the solenoid 130 has not acted. In this case, the solenoid 130 can be caused to disable the latch through ganging 19, so that when the solenoid is energized, the contact system 20 through 28 will not latch closed, and thereby the operator will be apprised that there is a fault.

It will also be observed that where some fault current in apparatus A can be tolerated, the contact system 10 through 18 can be eliminated if latch L is used. In this case, the operator will operate toggle T expecting it to latch. However, when solenoid 30 energizes due to detection of a fault, the contact system 20 through 28 will not latch. This requires the contact system 20 through 28 to be normally biased open, unless the latch is operative.

In particular, the two systems of contacts may be replaced by a manually operable circuit breaker having, in effects, contacts 20, 23 and 24, controlling only the hot conductor, and having a solenoid structure corresponding to, and replacing solenoid 130, which when energized by voltage prevents the contactor 20 from making contacts 23 and 24, or if the contacts 23 and 24 are made, trips the contactor 20 off the contacts 23 and 24. Such circuit breakers are well known in the art.

It is to be noted, that since the windings 30 and 31 are voltage-sensing elements, it is possible to provide very fast disablement of the latch. That is, even though some fault current might flow, since it is voltage which is sensed, not fault current, the action of solenoid is not tied to sensing some predetermined level of fault current before it can disable the latch.

Other differential transformer arrangements can be used instead of the one illustrated in FIG. 2. Thus, in FIG. 3, the transformer is really two transformers having cores 36 and 37, and, in effect dividing winding 32 between them in the form of windings 32A and 32B. In this case, windings 32A and 32B are wound to aid each other for like-directed fluxes in the cores, if correct connections result in oppositely directed fluxes induced by windings 30 and 31, due to their winding sense. If the winding sense of windings 30 and 31 have the opposite result on flux sense, then the windings would be wound to oppose such other for like-directed fluxes. Flux direction here would be characterized by its clock-sense in the cores.

In FIG. 4, the transformer has what amounts to an E-I Type core interrupted by an air gap which prevents windings 30 and 31 from being inductively related to each other. The core structure is analogous to that of FIG. 2, in that winding 32 is a unitary winding wound around a leg of core part 36A and a leg of core part 37A. The core is also analogous to that of FIG. 3 in that core parts 36A and 36B define a closed flux path equivalent to that of core 36, FIG. 3, and that core parts 37A and 37B define a closed flux path equivalent to that of core 37, FIG. 3.

In the embodiment of FIG. 5, the normally open contact system 10 through 18 is intended to be actuated manually, as by means of pushbutton P. It may be supposed that one pushes the button down to move the contacts 10, 11 and 12, by means of suitable ganging 49, to close the corresponding fixed-contact pairs. However, the system is normally latched open by a latch 38 operated through suitably ganging 39, by solenoid 130. Thus, in order to close the contact system, it is necessary to energize the solenoid 130, which then pulls the latch out of the way of the pushbutton, allowing it to be pushed down. In this case, the switch could also be constructed to latch the contact system closed.

Solenoid 130 is energized only when an SCR 40 and an SCR 41 are firing at the same time. SCR 40 is fired by the voltage between points 7 and 8, and SCR 41 is fired by the voltage between points 7 and 9, so the two rectifiers correspond to windings 30 and 31, FIGS. 2, 3 and 4. The anode-cathode junctions of the rectifier perform the summing function of the transformer-type voltage-sensing means of FIGS. 2, 3 and 4, except that in FIG. 5, this function is utilized to unlatch the contact system 10 through 18 so that it can be manually actuated to closed state whereas, in FIGS. 2, 3 and 4 it is employed to latch the contact system into its normally open state.

Since the voltages between points 7 and 8, and between points 7 and 19 is normally rather higher than necessary to fire the sort of SCR that would be used in the circuit of FIG. 5, voltage divider resistances 42 and 43 provide the gates 44 and 45 of SCR's 40 and 41 with predetermined fractions of the voltages between points 7 and 8, and points 7 and 9.

Diodes 47 and 48 protect the gate cathode junctions of the SCR's from reverse voltages, and diode 54 prevents the solenoid 130 from chattering during initial buildup of current in the relay winding.

Since latch 38 has only to be pulled out of the way long enough to allow pushbutton P to move a relatively short distance, a normally closed switch 50 may be provided to open the connection of the SCR's to the neutral conductor at point 51. Thus, after the contact system 10 through 18 has been actuated to closed condition, the SCR's can be made to cease firing by opening switch 50.

Latch 38 may be provided with a biasing spring 52 urging it into the position shown, until the solenoid acts, pulling latch 38 to the left and compressing the spring between structure 53 and the latch, structure 53 representing stationary structure for supporting the latch, and other elements of the contact system. Accordingly, whenever the contact system is actuated to open condition, latch 38 will return to the position shown, and prevent actuating pushbutton P again, unless the solenoid is energized. Naturally, switch 50 must be closed before pushbutton P can be actuated again.

In FIG. 6, the structure corresponding to ganging 49 of FIG. 5 is shown as including a rigid stem 56 having a spur 57 projecting rigidly therefrom. As before, solenoid 130, latch 38 and spring 52 is provided to prevent pushbutton P from actuating the contact system to closed position, unless solenoid 130 is energized to shift latch 38 from the path of spur 57.

In addition, a second solenoid Z30, latch 138 and spring 152 are provided. Fixed structure 58 is also provided, corresponding to fixed structure 53 of FIG. 5, except that the former also supports and guides latch 138. Normally, latch 138 is in the position shown, unless solenoid Z30 is energized, whereupon latch 138 is moved by solenoid Z30 into a position over spur 57, thereby tensioning spring 157.

The contact system 10 through 18 is modified by the inclusion of fixed contacts 9A and 9B, which are manually closed by movable contactor 12. Contact 9A is electrically connected to point 9, and one end of the winding of solenoid 30 is electrically connected to contact 9B. As the other end of the winding of solenoid 30 is connected to point 7, therefore when conductor 4, 5 and 6 are connected to terminals 1, 2 and 3, voltage between terminals 1 and 2 will energize solenoid 130, which then moves the latch 38 out of the way of spur 51, allowing the pushbutton P to be used to actuate the contact system to the closed state by means of stem 56 and other elements of the mechanism represented by ganging 49.

When the button P is pushed down, the aforesaid mechanism compresses a spring 59 against a suitable portion of fixed structure 58. Accordingly, if the pushbutton is then released the spring 59 will force the same mechanism to actuate the contact system to the state shown.

However, if, when the button P is pushed, and the contact system goes to its closed state, there is voltage between points 7 and 8, then solenoid 230 is energized because its winding is connected between points 7 and 8. The mechanism including spur 57, latches 38 and 138, and so on, is so proportioned that in closed condition the spur 57 has moved far enough down to allow the latch 138 to shift to the right far enough to be in the way of spur 57 to sufficient extent to prevent the spring 59 from forcing the contact system out of its closed state, when the pushbutton P is released. Further, the proportioning is such that the latch 38 is held out of the way because the spur now occupies the space the latch 38 occupied before the solenoid 130 pulled it out of the way of the spur. Accordingly, if latch 138 does not shift to the right when the button P is actuated, then there is nothing to prevent spring 59 from pushing the button back up except the effect of spring 52 urging latch 38 against the end of the spur (note that when contacts 17 and 18 are closed by contactor 12, the connection of the winding of solenoid 130 to point 9 is open.) The spring 52 and the adjacent portions of latch 38 and spur 57 are designed so that friction or equivalent between latch and spur is not along sufficient to prevent the spring from opening the contact system, after the pushbutton P is released.

From the foregoing, it will be evident that unless there is voltage between points 7 and 9, the contact system cannot be closed. Further, even if it can be closed, it will stay closed unless there is also voltage between points 7 and 8.

In FIG. 7 are shown modifications which severally or individually may be applied to one and another of the previously described embodiments of the invention.

Thus, the contact system controlling the energization of apparatus A may be simplified to the movable contactor 11 and fixed contacts 15 and 16. Since the SCR circuitry of FIG. 5 is used here, the contact system 11, 15 and 16 is normally open, and closes only if the conditions at the connector terminals 1', 2' and 3' are correct. In passing, it is to be noted that terminals 1', 2' and 3' are shown in plug-in symbolism, but are of course equivalent to terminals 1, 2 and 3.

Another variation, commonly found in operating rooms, is isolation of the apparatus A from source AC by an isolating transformer 60 having primary winding 61 connected across source AC and center-tapped secondary winding 62, the respective ends of which are connected to terminals 1' and 2', and the center top of which is connected to terminal 3' via a ground fault detector D.

Conductors 4 and 5 provide the full voltage of winding 62 across an inductance L, say the winding or windings of an electrical motor forming part of apparatus A. Conductor 6 connects to ground the housing or other apparatus structure which may contact the patient or the user of the apparatus, and which may more or less envelope the electrical load L and/or more or less segregate it mechanically from the patient or user.

In the present embodiment, while conductors 4 and 5 cannot be distinguished from each other as neutral and hot, nevertheless, it is as necessary here to prevent the contact system 11, 15 and 16 from closing under fault conditions, when the plug-in at terminals 1', 2', and 3', is made, as in the previous embodiments. Since points 7, 8 and 9 are electrically the same as before, as can be seen from considering their connections to the SCR circuitry and to the winding 62, it is evident that operation of FIG. 7, when the plug-in connection is made is as before, except of course, for the difference due to the simplification of the energization by means of solely contacts 11, 15 and 16.

The detector D is commonly found in transformer-isolated systems of the sort shown in FIG. 7, but is not essential to the operation of the SCR circuitry, etc., of FIG. 7. Usually the detector D merely signals the presence of a ground fault in apparatus A, namely, excessive leakage to ground via housing H. Such fault is not the sort of fault the present invention is intended to deal with, and is rather a possible property of the apparatus A, rather than of the connections of apparatus A to the winding 62. That is to say, the apparatus A is supposed to float off ground by reason of the isolating transformer 60, and if some part of the apparatus A has a ground fault this would not in general affect the interrelationship of the voltages on conductors 4, 5 and 6. What detector D does is determined on whether or not there is some voltage between ground and the conductor 6, when the connections to apparatus A are correct. If it detects no voltage, then housing H is grounded (which is safe, if not desirable), but if it detects a voltage greater than some predetermined magnitude, then that voltage exists between some part of the apparatus and ground, and is a potential hazard to the user or patient.

It will be evident from the foregoing that our novel safety device is susceptible of many modifications and may take a variety of forms. Many of these forms and modifications, though not shown, can be derived readily from the forms specifically disclosed. For instance, in FIG. 2, a latch, like 38 of FIG. 5, could be operated in place of contact system 10 through 18, for latching the contact system 20 through 28 against remaining closed. Also, signals, alarms or the like could be operated for the purpose of calling attention to connection fault. Again, "Triacs" and the like could be substituted for SCR's, and except in FIG. 7, the source of electrical energy AC may be either alternating or direct. We regard all such variations as falling within the scope of our invention.

Claims

Having described our invention in accordance with the requirements of the statute, we claim:

1. The combination with an electrically energizeable apparatus adapted normally to electrically contact a living being safely when said apparatus is electrically energized and free of fault, of

a safety device operatively associated with said apparatus for preventing fault from causing said apparatus to be in unsafe electrical contact with said living being;

said apparatus having a hot conductor, a neutral conductor and a ground conductor,

said hot conductor being connected to a first portion of said apparatus and being adapted to connect said first portion to the hot terminal of a source of electrically energy,

said neutral conductor being connected to said first portion and being adapted to connect said first portion to the neutral terminal of said source,

said ground conductor being adapted to connect a second portion of said apparatus to a ground terminal;

there being means, independent of said apparatus, electrically interconnecting said neutral terminal and said ground terminal, and said apparatus being energizeable in response to the electrical energy of said source available at said terminals;

said safety device including a switch normally interrupting electrical continuity of said hot conductor at a place between the said hot terminal and said first portion, said switch being normally independently operable of the rest of said faultproofing device, for establishing electrical continuity at said place;

said safety device also including voltage-sensing means connected in voltage-sensing relation to said conductors at points between said portions, on the one hand, and said terminals, on the other hand;

said voltage-sensing means being connected to said switch and being responsive to the several voltages at said points to substantially prevent said switch from being operative to establish electrical continuity at said place, under any one of the following conditions:

electrical discontinuity in said neutral conductor;

electrical discontinuity in said ground conductor;

connecting said hot conductor to said ground terminal and said ground conductor said hot terminal;

connecting said hot conductor to said neutral terminal and said neutral conductor to said hot terminal;

connecting said neutral conductor to said ground terminal and said ground conductor to said neutral terminal, provided said neutral and ground terminals are at different potentials.

2. The invention of claim 1, wherein said safety device includes a solenoid, and

said switch has first and second contacts and a movable contactor movable from one said contact to the other and said contactor being effectively connected to said ground conductor at all times; said solenoid being connected between said first contact and said hot conductor;

said solenoid having a latch and said switch having a movable member movable to first and second positions, said first position being one in which said hot conductor has its said electrical continuity interrupted and said movable contactor contacts said first contact, and said second position being one in which said electrical continuity is established and said contactor contacts said second contact;

said latch being normally positioned to obstruct movement of said movable member from said first position to said second position, so long as said solenoid is not energized, but being responsive to energization of said solenoid to move out of the way of said movable member, said solenoid being said voltage sensing means.

3. The invention of claim 2, wherein said safety device includes means responsive to voltage between said hot conductor and one of the other said conductors, and to said switch being in said second position, for latching said switch in said second position.

4. The invention of claim 2, wherein said safety device has a second solenoid, and said second solenoid has a second latch;

said second solenoid being, with the first said solenoid, said voltage sensing means;

said second latch being normally positioned to allow movement of said movable member from said first position to said second position, but being responsive to energization of said second solenoid and to movement of said movable member to said second position to move to a position obstructing movement of said second member to said first position, so long as said second solenoid is energized.

5. The invention of claim 1, wherein said safety device includes a second switch and third switch, said second and third switches being normally open and being connected in series with one another and with a solenoid, between said neutral and hot conductors, said voltage-sensing means including means connected to said second and third switches for closing both said second and third switches in response to said voltage-sensing means sensing substantially the voltage of said source both between said hot conductor and said ground conductor, and between said conductor and said neutral conductor;

the first said switch being a movable member movable to first and second positions;

said first position being one in which each said conductor is electrically interrupted between said apparatus and said faultproofing device, said second position being one in which said conductor is electrically continuous between said faultproofing device and said apparatus;

said first switch having a latch normally positioned to obstruct movement of said member from said first position to said second position, but responsive to energization of said solenoid to move out of the way of said member.

6. The invention of claim 1, wherein said safety device and said voltage-sensing means includes first core means, second core means, primary windings on said core means, and a secondary winding on said core means;

one said primary winding being connected between said hot conductor and said neutral conductor for producing first flux in said first core means when there is voltage between said hot conductor and said neutral conductor, a second said primary winding being connected between said hot conductor and said ground conductor for producing second flux in said second core means when there is voltage between said hot conductor and said ground conductor; said first and second core means being arranged so that said first flux and said second flux are mutually independent of each other; said switch including means connected across said secondary winding to cause said switch to establish electrical continuity at said place only when said last two said voltages have the same sense and substantially the same magnitude.

7. The invention of claim 6, wherein said core means comprises two cores each having one and another of said primary windings thereon, and both having said secondary winding thereon.

8. The invention of claim 6, wherein said core means comprises two cores, each of said cores having a primary winding and a secondary winding thereon, the last said primary winding being the former said primary windings and each of the last said secondary windings being together the first said secondary winding.

9. A safety device for providing interconnection of an electrically energizable apparatus with a hot terminal, a neutral terminal and a ground terminal, said device having a first winding for connection between said hot terminal and said neutral terminal, a second winding for connection between said hot terminal and said ground terminal, a third winding, a first core means inductively relating said first winding to said third winding, and a second core means inductively relating said second winding to said third winding, said first winding being noninductively related to said second winding;

said safety device including switch means for manually interrupting electrical continuity of said interconnection, and being connected to said third winding for responding to voltage therein to restore said continuity when and only when said third winding has voltage therein corresponding to correct interconnection of said apparatus with said terminals, and correct voltages at said terminals.

10. The safety device of claim 9, wherein said third winding has one part wound solely on one said core means and another part wound solely on the other said core means.

11. The safety device of claim 9 wherein said third winding is wound around both said core means.