US 3771098 A
A means for monitoring the integrity of a ground circuit in the connection of an electrical apparatus to an electric utility outlet through a power cord and plug. The means comprises a wall receptacle having two contacts, each of which is connected to the utility ground conductor, and a plug having a ground prong comprising two members which are adapted, respectively, for engaging the two ground contacts of the wall receptacle.
Description (OCR text may contain errors)
United States Patent 1 Dempsey [451 Nov. 6, 1973 1 GROUND WIRE MONITORING SYSTEM  Inventor: Robert H. Dempsey, Napa, Calif.
 Assignee: Neurodyne-Dempsey, Inc., Napa,
 Filed: May 16, 1972  Appl. No.1 253,892
 US. Cl. 339/14 R, 339/32 R, 339/183,
' 339/176 M, 324/51  Int. Cl. H01r 3/06, H011 17/18  Field of Search 339/14 R, 14 L, 14 P,
339/14 RP, 14 T, 182 R, 182 L, 182 T, 183, 31R,31B,31L,31M,31T,32RM,33,176 R, 176 M, 177 R; 324/51  References Cited UNITED STATES PATENTS 3,289,149 11/1960 Pawloski 339/183 3,546,657 12/1970 Cook 339/60 3,335,388 8/1967 Karol 339/14 R 2,170,131 8/1939 Doremus 339/14 L X 3,537,061 10/1970 Haag et a1 339/176 R X 1 Primary Examiner--Marvin A. Champion Assistant Examiner Robert A. Hafer Att0rney.las. M. Naylor et a1.
9 57 ABSTRACT A means for monitoring the integrity of a ground circuit in the connection of an electrical apparatus to an electric utility outlet through a power cord and plug. The means comprises a wall receptacle having two contacts, each of which is connected to the utility ground conductor, and a plug having a ground prong comprising two members which are adapted, respectively, for engaging the two ground contacts of the wall receptacle.
One of the ground prong members is connected in a conventional manner through the power cord of the apparatus to the apparatus case.. The second ground prong member is connected to the apparatus case through a continuity-sensing device whereby any failure of the ground conductor and the power cord or the connection of the ground prong to the ground contact and the wall receptacle will be immediately detected.
In an alternative embodiment, the second power cord ground conductor is connected directly to the apparatus case and through the plug and wall receptacles to a remote continuity-sensing device to monitor the integrity of the ground circuit not only in the apparatus power cord and plug-to-wall receptacle connection, but also in the utility wiring of the structure in which the apparatus is located.
3 Claims, 5 Drawing Figures GROUND WIRE MONITORING SYSTEM BACKGROUND OF THE INVENTION The medical arts are making increasing use of many types of electrical appliances and apparatus in the diagnosis, treatment and care of hospital patients. In many applications the patient is required to be in personal physical contact with the case of such apparatus. Since many types of such apparatus require high voltages in their operation, there is a risk of electrical shock to such patients resulting from the use of equipment that is improperly grounded.
Hospital utility wiring and hospital equipment commonly employ a three-conductor system with one of the conductors serving as a ground. Electrical connection of such equipment to the utility wiring of the hospital is made by means of a conventional three-prong plug and mating three-socket wall receptacle.
It has been found that repeated plugging and unplugging of such equipment from wall receptacles can wear the plug and receptacle contacts, and fray and break portions of the apparatus power cord, resulting in a loss of electrical continuity of the ground conductor. This loss of continuity in the ground conductors is not readily apparent, however, since it does not affect the hot and neutral lines and, accordingly, does not interfere with the operation of equipment plugged into the receptacle.
Accordingly, it is an object of the present invention to provide a means of monitoring the continuity of a ground circuit of an electrical apparatus while it is plugged into an electrical power outlet.
Another object of this invention is to provide a systent for monitoring the continuity of such a ground circuit that is simple, inexpensive and requires a minimum of special equipment and wiring. I
Still another object of the present invention is to provide such. a ground continuity monitoring system by employing electrical power outlets that are compatible with conventional three-prong plugs and electrical plugs that are compatible with conventional threesocket receptacles.
These and other objects are achieved by means of a three-socket wall receptacle for connection to electric utility wires, which includes a ground socket having two electrical contacts disposed therein in a jack-like configuration and independently connected to the ground conductor of the electric utility wires. A power cord plug is provided having a ground prong which includes two concentrically spaced conductor members adapted for engaging the two contacts in the ground socket of the wall receptacle. One of the ground prong members is connected directly to the case of the electrical apparatus through the apparatus power cord in the conventional manner.
The other ground prong member is connected to a continuity sensing device which in turn is connected to the case of the apparatus, whereby the continuity of the ground circuit through the apparatus power cord and the plug and receptacle connection is continually monitored.
In an alternative embodiment of the present invention, both members of the ground prong are independently connected to the apparatus case through the power cord. One contact of the ground socket is connected to the ground conductor of the utility wiring. The other contact of the ground socket is connected to a remote continuity sensing device which in turn is connected to utility wiring. In this embodiment the continuity monitor will protect any loss of ground in the utility wiring as well as in the power cord and plug connection of the apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a perspective view of the power cord, plug and wall receptacle of the present invention with the apparatus associated with the power cord shown schematically,
FIG. 2 is a sectional view of the power cord plug and the wall receptacle of the present invention,
FIG. 3 is a schematic diagram of the electrical connection of a conventional power cord plug and wall receptacle,
FIG. 4 is a schematic diagram of the electrical connections of the power cord plug and wall receptacle of the present invention, and
FIG. 5 is a schematic diagram of the electrical con nec tions of a power cord plug and wall receptacle of an alternative embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an electrical apparatus, shown generally at 10, includes a case 11 having an electrical device such as a motor 12 mounted thereon. A power cord 13 includes wires 14 and 15 which are connected to motor 12, and wires 16 and 17 which are connected to case 11. The connection of the wire 17 to the case may be direct, as shown in this figure, or through a sen-. sor, as will become apparent from the subsequent discussion.
Power cord 13 terminates in a plug 18 which has conventional power prongs l9 and 20, connected to wires 14 and 15, extending therefrom in the usual manner. Plug 18 also has a ground prong 22 extending there from which includes coaxially spaced conductive members 23 and 24, connected to wires 16 and 17, respectively, and separated by a non-conductive sleeve 25 which terminates in a collar 26. In the preferred embodiment the prongs are arranged in conventional volt grounded plug pattern and adapted to be'plugged into a conventional 120 volt grounded receptacle.
A wall receptacle 27, adapted for mounting on a wall in the conventional manner, has means forming sockets 28, 29 and 30 therein for receiving prongs 19, 20 and 22 of plug 18, respectively. The sockets are preferably arranged in conventional 120 volt grounded receptacle pattern and adapted to receive a conventional 120 volt grounded plug.
Referring to FIG. 2, sockets 28 and 29 are of conventional construction and include contacts 32 disposed along the inner surface thereof and engaged by prongs prong 22 are the same as those of conventional plug ground prongs. Accordingly, the system of the present invention is compatible with conventional systems.
Also disposed in socket 30 is a spring clip 40, attached to receptacle 27 at a terminal 42 on the outer surface thereof. Spring clip contact 40 is adapted to engage the end of element 24 when prong 22 is inserted in socket 30. Ground conductor 43 of the utility wiring is connected to terminal 39. A wire 44 connects terminal 42 to ground conductor 43 or a remote continuity sensing device, depending upon the embodiment within which the wall receptacle is used.
The advantage of employing the plug and receptacle construction of the present invention will be more fully understood if reference is made to the schematic diagrams shown in FIGS. 3 and 4.
In FIG. 3, a diagram of the prior art arrangement is shown wherein a three-wire power cord, plug and a receptacle is employed. Wires 14 and 15 supply electric energy from conductors 34 and 35 to motor 12. Wire 16 connects case 11- to ground conductor 43. However, if wire 16 breaks or if the juncture of wire 16 and conductor 43 becomes defective, case 11 will no longer be grounded and could be subject to dangerously high potential levels due to leakage from motor 12.
Referring to FIG. 4, in the present invention a second ground wire 17 is connected through plug 18 and receptacle 27 to a conductor 44 which communicates with the utility ground conductor 43. Wire 17 is also connected to a continuity sensing device 21 which in turn is connected to frame 11. Thus, if wire 16 is broken, or the juncture of wire 16 and conductor 43 becomes defective, sensor 21 will detect a loss of continuity and give a suitable alarm indication. Accordingly, any loss of ground in the power cord or plug and receptacle junction of the apparatus will be immediately evident to personnel operating the apparatus.
While the conductor 44 is shown as a conventional wire, it might take the form of a strap connecting the terminals 39 and 42. Such a strap might be formed as a removable separate part or a removable knock out formed between the terminals.
An alternative embodiment of the present invention, illustrated in FIG. 5, provides a direct connection of a second power cord ground wire 17 to case 11, and through plug 18 and receptacle 27 to a conductor 44', which is connected to a remote continuity sensing device 21. Continuity sensing device 21 is in turn connected to the utility wiring through which the apparatus is connected at a point remote therefrom. Thus, in this embodiment, sensor 21 monitors the continuity of the utility ground wiring, as well as the power cord and plug and wall receptacle junction. Also, in this embodiment a single scanning continuity sensing device may be used to monitor several different electrical apparatus (not shown) through other ground conductors such as 44a, 44b and 440.
While preferred embodiments of the invention have been illustrated and described, it should be understood that the invention is not intended to be limited to these embodiments, but rather is defined by the accompanying claims.
1. A receptacle for use in a ground conductor, said receptacle comprising:
a. a base member having first, second and third sockets therein arranged in conventional volt grounded receptacle pattern with the first socket having a contact therein adapted to be connected to a hot conductor, the second socket having a contact therein adapted to be connected to a neutral conductor and the third socket having a contact therein adapted to be connected to a ground conductor; and,
b. a second contact in the third socket electrically insulated from the first contact therein, said second contact being adapted to be connected to a second conductor and being disposed so as to not interfere with the insertion of a conventional 120 volt grounded plug into the receptacle.
2. A plug for use in a ground conductor, said plug comprising:
a. a base member having first, second and third prongs extending therefrom and arranged in a conventional 120 volt grounded plug pattern with the first prong having a conductor portion adapted to be connected to a hot conductor, the second prong having a conductor portion adapted to be connected to a neutral conductor and the third prong having a conductor portion adapted to be connected to a ground conductor;
b. a second conductor portion on the third prong electrically insulated from the first conductor portion thereof, said second conductor portion being adapted to be connected to a second conductor and being disposed so as to not interfere with insertion of the plug into a conventional 120 volt grounded receptacle.
3. A power connector comprising:
a. a receptacle having first, second and third sockets therein, said sockets having contacts therein adapted to be connected to hot, neutral and ground conductors, respectively;
b. a plug adapted to be plugged into said receptacle and having first, second and third prongs extending therefrom for insertion into the respective first, second and third sockets of the receptacle upon plugging of the plug into the receptacle, each prong having a conductor portion adapted to engage the contact in the socket into which it is inserted; and,
c. a second contact in the third receptacle socket and a second conductor portion on the third prong adapted to engage the second contact of the third socket upon plugging of the plug into the receptacle.