U.S. patent number 3,753,261 [Application Number 05/206,288] was granted by the patent office on 1973-08-14 for continuous ground and polarity monitor.
Invention is credited to Keith R. Thaxton.
United States Patent |
3,753,261 |
Thaxton |
August 14, 1973 |
CONTINUOUS GROUND AND POLARITY MONITOR
Abstract
A disk has three apertures therein for receiving the hot,
neutral and ground prongs of an a.c. line plug. In use the disk is
slipped onto the prongs and positioned between the body of the line
plug and a mating electrical outlet socket. An indicator lamp is
fixedly attached to the disk and electrically coupled across the
hot and ground prongs by friction tight contact at the
corresponding apertures in the disk. The indicator lamp glows as
long as a ground connection and proper polarity are maintained in
the electrical outlet.
Inventors: |
Thaxton; Keith R. (Mountain
View, CA) |
Family
ID: |
22765730 |
Appl.
No.: |
05/206,288 |
Filed: |
December 9, 1971 |
Current U.S.
Class: |
340/649; 340/687;
340/656; 439/490 |
Current CPC
Class: |
H01R
13/717 (20130101); H01R 13/7177 (20130101) |
Current International
Class: |
H01R
13/717 (20060101); H01R 13/66 (20060101); G08b
021/00 () |
Field of
Search: |
;340/252,255,256,253R
;324/122 ;174/66 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Claims
I claim:
1. A ground-absence detector device for use with a three prong plug
engagable in a mating electrical receptacle having hot and neutral
line terminals and a ground terminal, the detector device
comprising:
a wafer-like member having three apertures therein, said apertures
being spatially arranged and configured to receive the three prongs
of said three-prong plug, said wafer-like member having a
predetermined thickness which is small enough to permit the
installation of said wafer-like member on said prongs between the
body of said plug and said mating receptacle;
an electrical indicating means affixed to said wafer-like member;
and
means for electrically couping said indicating means across said
hot line terminal and said ground terminal, said
coupling means including:
an electrical contact disposed at the aperture of said wafer-like
member which receives the prong of said plug corresponding to said
hot line terminal, said contact being configured to frictionally
engage said last named prong;
a length of cord extending from said wafer-like member and having
an end terminal connectable to a ground point on an electrical
appliance associated with said plug; and
conductor means for coupling said indicating means to said contact
and said cord.
2. The device of claim 1, wherein said indicating means includes a
light-emitting element having a visible portion, and means for
mounting said element to extend said visible portion away from a
plug on which said detector device is installed.
3. The device of claim 2 wherein said mounting means includes an
elongated member extending from said wafer-like member beyond said
plug body and having a longitudinal axis substantially parallel to
the longitudinal axis of said plug.
4. The device of claim 2, wherein said light-emitting element is a
neon lamp and said last named electrically coupling means includes
a voltage dropping resistor.
5. The device of claim 1, wherein the predetermined thickness of
said wafer-like member is less than three-sixteenths inch.
6. The device of claim 1, wherein said conductor means is embedded
in said wafer-like member.
7. The device of claim 6, wherein said wafer-like member is coated
with an electrically insulating potting material.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to electrical safety
devices, and more specifically to a device for indicating the
absence of a ground connection and proper polarity in an electrical
outlet or appliance.
A three-wire system used to power an electrically operated
appliance or instrument has two main current carrying conductors,
commonly referred to as the hot and neutral conductors, and a third
conductor which serves to safely ground the electrical equipment.
The power or line plug from the electrical equipment has three
prongs corresponding to the hot, neutral and ground conductors, and
these prongs mate with corresponding conductors in a supply
receptacle such as a wall socket or the like.
It is imperative for the safe operation of the electrical equipment
that the integrity of the ground connection be maintained. This
requires ground continuity from the line plug to the equipment, and
equally importantly, a proper ground connection in the supply
socket. Heretofore, it has been common practice to periodically
test the ground conductor in supply sockets to insure that the
ground is properly connected. In hospitals, for example, wall
outlets are generally inspected once each month for proper polarity
and electrical ground continuity, as well as for proper tension on
a ground prong when extracted from the ground receptacle, thereby
to insure the proper frictional contact between the ground prong
and receptacle. Such routine monthly inspections are obviously time
consuming and costly. In addition, such inspections are no
guarantee against the ground continuity being broken at some time
during the interim period between inspections. A ground connection
can easily be broken by the presence of dirt on the electrical
contacts or by the abusive jerking of a plug from its socket, for
example. Consequently, the hospital patient or the operator of the
electrical equipment may be unknowingly exposed to hazardous, and
possibly fatal, electrical shock.
SUMMARY OF THE INVENTION
The present invention is a device which continuously monitors the
continuity of the electrical ground to which the line plug of an
appliance or instrument is connected. One embodiment of the
invention, as illustrated, includes a wafer or disk like member
having three apertures which are configured and spaced to permit
the disk to be mounted on the three prongs of a line plug. Rigidly
attached to the disk is an indicator lamp. The lamp is electrically
coupled across the hot and ground conductors through a pair of
electrical contacts which are exposed at two of the apertures in
the disk to frictionally engage the corresponding prongs of the
plug on which the disk is mounted.
In use, the prongs of a plug are inserted into the apertures in the
disk, and the disk is pushed into place against the body of the
plug, thereby to permit the plug to be inserted into a wall
receptable or the like. The indicator lamp is disposed adjacent to
the plug body so as to be in clear view of an operator or user of
the associated electrical equipment. When a ground connection is
absent, the lamp goes out, thereby to warn that the electrical
equipment should not be used before the ground connection is
restored.
Another embodiment of the invention includes a disk or wafer-like
member and indicator lamp having one lead connected to the hot
conductor as described above. However, the other lamp lead is
connected to a length of cord, the end of which is connected to the
chassis or cabinet of the electrical appliance. This arrangement
serves to monitor the continuity of the ground wire in the line
cord that extends from the appliance to the earth ground.
The invention features a device which is small, inexpensive and
easily slipped onto the prongs of a line plug. Once the device is
mounted in place on the plug, it will continuously monitor the
appliance ground connection. The device obviates the need for
routine, time consuming, and costly inspections of wall receptacles
to detect broken ground wire conditions or poor frictional
engagement of the ground prong and mating receptacle because the
absence of a safe ground is indicated immediately and clearly by a
dark lamp.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of one embodiment of the present
invention along with an associated three-prong line plug.
FIG. 2 is a combined schematic and sectional view taken along line
2-2 in FIG. 1.
FIG. 3 is perspective view illustrating another embodiment of the
invention installed on a line plug which is inserted into a wall
socket.
FIG. 4 is a combined diagramatic, schematic and sectional
illustration of the embodiment of the invention shown in FIG.
3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a wafer-like member or disk
11 formed of an insulating material such as epoxy board or plastic.
The disk 11 has three apertures, 13, 15, 17 therein which are
spaced apart and shaped to receive the three prongs 19, 21, 23,
respectively, of a three-prong line plug 25. The plug 25 may be of
the conventional a.c. line type used wherever an equipment
grounding conductor is required along with the two line conductors.
The prongs 19 and 21 are connected respectively to the so-called
neutral and hot conductors in the line cord 27, whereas the prong
23 is connected to the grounding conductor in cord 27. According to
the common practice dictated by the National Electrical Code, prong
19, i.e., the left-hand prong as viewed from the cord end of plug
25 in FIG. 1, is always the neutral conductor, and prong 21, i.e.,
the right-hand prong, is always the hot conductor of the power
line.
As shown in FIGS. 1 and 2, disk 11 narrows toward a portion 29 that
is formed to define a cylindrical cup 31 into which a hollow
elongated member or tube 33 is inserted and rigidly attached by a
tight friction fit or suitable bonding material. Mounted on the end
of tube 33 is a translucent plastic cap 35. Contained within the
tube 33 and cap 35 and disposed for view through cap 33 is an
indicator means which is preferably a neon lamp 37. However, other
types of light emitting elements may be used, such as conventional
incandescent lamps or electro-luminescent devices. Where it is
desired to have low power drain and low leakage current, on the
order of 100 microamperes or less, the light emitting element used
may be an electro-optical device such as a liquid crystal cell.
Neon lamp 37 is coupled by a pair of electrical conductors 39, 41
to electrical contacts 45, 43 at apertures 17, 15 respectively. As
described hereinabove, the apertures 17, 15 receive the prongs of
plug 25 which correspond to the hot line conductor and the ground
conductor; therefore neon lamp 37 is coupled in circuit across
these two conductors. A resistor 47 is connected in series with
conductor 41 to drop the line voltage to a value suitable for
operating neon lamp 37. The electrical contacts 43,. 45 project
slightly into the corresponding apertures 15, 17 to facilitate a
tight frictional engagement, and consequently a good electrical
connection with the prongs 21, 23 inserted into these
apertures.
The two electrical conductors 39, 41 and resistor extending from
neon lamp 37 are carried within tube 33 to disk 11 and the two
conductors are preferably embedded in the disk from the narrow
portion 29 thereof to the apertures 15, 17, as indicated by the
dashed outline of these conductors in FIG. 1. The portion of the
conductors disposed in disk 11 may be fine wires carried in
channels molded in the disk or they may be fabricated in thin
strips on the surface of the disk by suitable printed circuit
techniques, for example. In either case, the disk 11 is preferably
coated with potting material to insulate the electrical conductors
thereon from the external environment and to provide mechanical
strength.
The thickness, d, of disk 11 is preferably on the order 3/16 inch
or less, so as to permit the disk to be mounted on the prongs of a
line plug and still leave a substantial portion of the prongs
exposed for insertion into a wall receptacle.
In FIG. 1, the tube 33 for the indicator lamp is shown mounted in
vertical alignment with the aperture 17 in disk 11. However it is
to be understood that the indicator lamp may be mounted in a
different positional relationship with the three apertures 13, 15,
17. For example, the lamp mounting may be moved 90.degree.
clockwise, thereby to permit a pluraltiy of line plugs and
associated disks 11 to be used in a conventional multiple socket
wall receptacle without spatial interference from the indicator
lamps.
FIGS. 3 and 4 illustrate another embodiment of the invention. The
physical configuration of the disk 11 and indicator lamp is
substantially the same as that shown in FIG. 1, and the disk 11 is
shown in use when mounted on the prongs of plug 25 between the plug
and a wall receptacle 49. The embodiment of FIG. 3 includes a
length of cord 51 extending from cup 31. Attached to the end of
cord 51 is a clip or a terminal lug 53. Neon lamp 37 has one lead
connected to an electrical contact 43 at aperture 15 for the hot
line conductor in the same manner as described above with respect
to FIG. 2. However, as shown in FIG. 4, the other lead from lamp 37
is connected to cord 51 instead of a contact at the aperture 17 in
disk 11. Terminal lug 53 is attached directly to the chassis or
cabinet of an electrical appliance 55. With this arrangement, the
ground side of the lamp circuit is through cord 51, appliance 55
and back through the ground wire 57 in the line cord to the ground
conductor in the supply receptacle into which plug 25 is inserted.
Thus the lamp continuously monitors the continuity of the ground
connection from the earth ground to the appliance, thereby to
insure that ground wire 57 or the chassis ground is not broken.
In use, as long as the indicator lamp remains on, the operator is
insured that there is a ground connection. It is to be noted that
the device of the present invention also insures that the proper
line polarity exists in the wall receptacle. This is because the
lamp will not light if the neutral and hot conductors are
reversed.
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