Electrical safety receptacle assembly

A contact assembly for alternating current electrical receptacles is disclosed in which electricity is not applied to the pins of a mating electrical plug until the plug is inserted substantially entirely into the receptacle assembly. The wiper contacts which engage the pin when it is pushed into the receptacle are initially not connected to the electrical source. When the tip of the pins engage the bottom of the receptacle housing, it engages a separate contact assembly which makes electrical connection to the electrical source only after the pins are inserted almost entirely into the receptacle. The exposed portion of the pins of the electrical plug are therefore not energized until the plug is almost fully inserted, thus preventing inadvertent touching of the exposed pins and the resultant electrical shock.

TECHNICAL FIELD 
This invention relates to alternating current electrical receptacles and, 
more particularly, to receptacles having reduced risk of accidental shock. 
BACKGROUND OF THE INVENTION 
In order to provide adequate electrical contact between common household 
electrical plugs and the corresponding electrical receptacles, it has been 
common to provide a sliding contact between the pins of the electrical 
plug and a substantial length of the mating contacts in the receptacle. 
This sliding contact takes place at an extended length of a central 
portion of the receptacle contacts and the pins of the plug. In order to 
assure an adequate, low-resistance contact, the receptacle contacts are 
generally bifurcated so as to grasp and hold the pin between two contact 
elements arranged to exert a closing force on the pin. While such prior 
art receptacles have normally provided adequate electrical contact with 
the pins of the mating plug, such contact deteriorates under environmental 
conditions where the wiping surface of the bifurcated contacts may become 
coated with nonconductive material. More importantly, such prior art 
electrical receptacles initiate electrical contact while the pins are 
inserted into the receptacle only a small portion of their entire length. 
It is therefore possible to inadvertently touch an exposed portion of the 
plug pins and receive an electrical shock. This is a particular danger for 
small children or elderly persons who have difficulty in removing the plug 
and tend to insert their fingers under the plug to assist in removal. 
Small children also tend to play with electrical plugs, thereby increasing 
the danger. 
It would be an improvement from a safety viewpoint to permit electrical 
contact inside of an electrical receptacle only after the pins have been 
inserted substantially all of the way into the receptacle, leaving very 
little of the pin length exposed. Unfortunately, the tips of the pins tend 
to vary in size and position to such an extent that a solid electrical 
contact cannot be guaranteed if only the tips of the pins are used to make 
contact. Worse, such tip-only contacts tend to be intermittent, damaging 
the electrical appliances connected to the plug. 
SUMMARY OF THE INVENTION 
In accordance with the illustrative embodiment of the present invention, 
the danger of exposed electrical pins is reduced or eliminated by using 
the tips of an electrical plug to operate entirely separate electrical 
contacts which only then allow electricity to flow through the bifurcated 
contacts and into the pins of the electrical plug. Such separate contacts 
are arranged to close only when the pin is essentially all of the way into 
the receptacle and hence to energize the the wiper contacts and the pins 
only when the plug is almost fully inserted. More particularly, the 
bifurcated contacts of the receptacle are not connected directly to the 
electrical conductors carrying electricity. Instead, the contacts include 
at the innermost end at least one normally open contact which is closed by 
movement of the tip of the pin into contact with this innermost end of the 
contacts. 
The major advantage of the late-connecting safety receptacle of the present 
invention is the reduction or elimination of exposed, live contact pins 
during the insertion and removal operations when the users hand are in 
contact with the electrical plug. In addition, the bifurcated wiper 
contacts provide a natural way to engage and operate a pair of oppositely 
disposed positive closure contacts which do not rely on a wiping action to 
provide contact. Such positive closure contacts tend to produce less 
sparking at initial contact engagement and final contact disengagement, 
thereby reducing pitting and wear on the contact surfaces. 
The late-operating safety contacts of the present invention can be used in 
portable electrical extension cords as well as in fixed wall receptacles. 
Indeed, the danger of exposed electrical pins is greater for extension 
cords where two hands are typically used to insert and remove the plug 
from the receptacle. 
In a preferred embodiment, the positive closure contacts are formed with a 
cup-shaped stationary contact permitting dual, oppositely disposed 
contacts to engage the inner surface of the same cup, thereby to provide 
two separate and distinct electrical contact paths through the same cup 
contact. Moreover, the cup-shaped contacts allow easy manufacture of the 
safety receptacle by forming natural holders for dummy contact assemblies 
to be inserted during injection molding, later to be removed and replaced 
with operating contact assemblies.

DETAILED DESCRIPTION 
Referring more specifically to FIG. 1, there is shown an exploded 
perspective view of a portion of an electrical alternating current 
receptacle utilizing the late-closing safety contacts of the present 
invention. More particularly, the receptacle of FIG. 1 comprises a body 
portion 10 and a cover portion 11 designed to mate together and registered 
by use of a guide pin (not shown) on the underside of cover 11 which fits 
into guide hole 12 in body portion 10. Body portion 10 defines a 
symmetrical pair of cavities 13 and 14 for receiving a fixed contact 
assembly 15 made of conductive material and including a four-sided open 
box 16 and a conductive tab 17. When assembled, box 16 fits over raised 
land 31 to insure proper positioning of fixed contact assembly 15 when the 
receptacle is fully assembled. Tab 17 includes a slot 18 which engages a 
interior wall 19 in body portion 10 and a screw 20 for connecting an 
electrical wire to tab 17. Tab 17 is shown only partially; in fact, a 
mirror image contact assembly extends in a forward direction into a mirror 
image cavity in body 10 which forms the fixed contact for another 
electrical receptacle in the common household duplex receptacle. 
A double-ended, movable bifurcated contact element 25 is arranged to fit 
into a recess 26 on the underside of cover 11 which recess communicates 
with a pin opening 27 in the face of cover 11. Contact element 25 
comprises a box-shaped central portion 28 with two pairs of oppositely 
disposed bifurcated contact elements 29 and 30. Contact elements 29 are 
arranged to engage an electrical pin or blade on a plug inserted into 
receptacle cover 11. Contact elements 30 are arranged to electrically 
contact the inner surfaces of box 16 on fixed contact assembly 15 when the 
two bifurcated contact elements 30 are separated by the insertion of an 
electrical pin or blade into the pin slot 27. Since bifurcated contacts 30 
are located on the remote end of contact element 25 from slot 27, the 
electrical pin or blade does not engage contact elements 30 until the pin 
or blade is inserted almost all of the way into slot 27. This operation 
can be better seen in FIGS. 2 and 3, to be described below. 
It is to be noted that a fixed contact assembly similar to contact assembly 
15 is located in each of four cavities defined within receptacle body 
portion 10. Similarly, a movable contact assembly similar to contact 
assembly 25 is likewise located in each of four recesses similar to recess 
26 on the underside of cover portion 11. Each set of fixed and movable 
contacts is paired with another set of fixed and movable contacts to form 
a dual pin receptacle for receiving the two pins or blades of a standard 
alternating current plug. As is standard for electrical wall outlets, two 
such receptacles are fabricated together into a single duplex receptacle. 
Only one set of contacts are shown in FIG. 1 for simplicity. 
Turning then to FIG. 2, there is shown a partial cross-sectional view of 
the receptacle of FIG. 1 showing the assembled contacts. A partial view of 
a standard electrical plug 32 is also shown having a pin 33 which is 
partially inserted into slot 27 of receptacle cover portion 11. As can be 
seen in FIG. 2, the tip of pin 33 is just initiating engagement with 
bifurcated contacts 30, and the ends of contacts 30 have not yet engaged 
the fixed contacts 16. Since the fixed contacts 16 are the electrically 
energized contacts (due to the electrical connection by way of screw 20), 
no electricity has yet been applied to contacts 30, contacts 29 and hence 
to pin 33. Note that the pin 33 is therefore not energized at this time 
and hence cannot inadvertently apply a shock to the user who inserts a 
finger or other conductive tool under the plug 32 while inserting or 
removing plug 32. 
In FIG. 3 there is shown a partial cross-section view of the receptacle of 
FIG. 1 showing the assembled contacts and the plug 32 fully inserted into 
slot 27. It will be noted that the bifurcated contacts 30 are fully 
separated by the insertion of pin 33 therebetween, and have engaged the 
inner surfaces of box 16 to provide electrical contact and thereby 
energize the pin 33. The bifurcated contacts 29 serve as further contact 
areas for electrical connection with pin 33 while, at the same time, 
providing frictional resistance to the withdrawal of plug 32 from the 
electrical receptacle. This frictional resistance insures that the plug 32 
will not inadvertently be removed from the receptacle 10-11 by minor 
forces exerted on the plug 32 or the electrical wire connected thereto. 
FIGS. 1, 2 and 3 show the application of the late-connecting safety 
contacts to standard electrical duplex wall outlets. Such safety contacts 
can be applied to any other form of electrical plug and socket connection 
by modifications obvious to those skilled in the art. As an example, FIGS. 
4, 5, 6 and 7 show the application of the present invention to the 
receptacle end of a standard extension cord. 
FIG. 4 shows a perspective exploded partial view of the receptacle end 40 
of a standard extension cord 41. A box-shaped fixed contact 42 is embedded 
in the receptacle end 40 of the extension cord 41. Fixed contact 42 has a 
closed bottom and an open top. An electrical wire 43 from cord 41 is 
connected to contact 42 by crimping or soldering or both. A movable 
contact assembly 44 is adapted to be inserted into a recessed opening 45 
in receptacle end 40 and held in place by spurs 46 on each side of the 
central box portion of contact assembly 44. Like contact assembly 25 in 
FIG. 1, contact assembly 44 in FIG. 4 has two pairs of oppositely disposed 
bifurcated contact elements 47 and 48. Bifurcated contact elements 47 are 
arranged to fit into fixed contact box 42 without touching the bottom of 
box 42. Bifurcated contact elements 48 have flanges 49 and 50 arranged to 
engage the edges of the recessed opening 45 in receptacle 40, as can be 
seen at the other opening 51 of receptacle 40. The fixed contact 42 can be 
better seen in FIG. 5 which is a perspective view of the contact 42 as 
seen from the rear with the wire 43 connected to the contact 42 by a crimp 
52. 
The late-closing safety receptacle of the present invention can be 
fabricated in an extension cord such as that shown in FIG. 4 by standard 
plastic injection overmolding techniques. The wires are first connected to 
the two fixed contacts such as contact 42 by crimping or soldering or 
both. These contacts are then inserted on the end of two posts defining 
the volume required for the movable contact assemblies such as assembly 
44. The contacts, with the posts inserted, are then place inside the 
injection mold and a nonconductive thermo-plastic material injected into 
the mold under pressure. Once the thermo-plastic material is semiset, the 
posts are retracted and the movable bifurcated contact assemblies 44 
inserted into the void left by the posts. The barbs 46 are locked into the 
setting thermoplastic material, thus preventing extraction, while the 
flanges 49 and 50 limit the inward positioning of the contacts 44. The 
late-closing safety contacts of the present invention therefore add very 
little to the cost of manufacturing electrical outlets, receptacles or 
extension cords. 
FIG. 6 shows a partial cross-section view of the receptacle end 40 of 
extension cord 41 showing a partial view of a plug 54 having an electrical 
pin or blade 55 connected thereto. In FIG. 6, pin or blade 55 is show as 
just starting to engage bifurcated contacts 47 which are not yet separated 
enough to engage the inside edges of box 42. It is to be noted that, at 
this position, where the plug 54 is almost entirely inserted into the 
receptacle 40, the contact assembly 44 is not yet electrically energized 
and hence pin 55 does not yet carry electricity. It is therefore 
impossible to get an electrical shock from the exposed portions of pin 55 
even if fingers or other tools are inserted under the plug 54. 
In FIG. there is shown a partial cross-section view of the receptacle end 
40 of extension cord 41 with the plug 54 fully inserted into the 
receptacle. It will be noted that, in this position, the bifurcated 
contacts 47 are fully separated and engage the inner surface of the fixed 
contact box 42 to complete the electrical connection through the extension 
cord 41 and receptacle 40 to plug 54. 
It will be noted that the safety receptacle in accordance with the present 
invention is entirely passive in that absolutely no action is required by 
the user in order to have full advantage of the safety features. It is 
therefore extremely effective for infants, small children and the elderly 
who are not apt to take the necessary actions required to activate active 
safety systems. It will also be noted that the improved receptacle of the 
present invention provides full wiping contact of the pin blades to the 
contact surface at the center of the blades as required by 
Underwriters'Laboratory standards. Finally, the near closure of the outer 
bifurcated contact elements presents a partially closed entrance to the 
receptacle, thus rendering it more difficult to insert foreign objects 
such as nails into the receptacle. These outer contacts are not, of 
course, live and hence present no danger of shock in and of themselves. 
One advantage of the box-shaped fixed contacts is the ability to provide 
two balanced electrical contact points. This eliminates intermittent 
connections which might otherwise result if the plug is forced from side 
to side. Both the wall receptacle and the extension cord receptacle can be 
manufactured in the same size and with the same features as existing 
receptacles, using the same materials and the same automated equipment, 
requiring only minor changes in the sub-assemblies and procedures to 
accommodate the late-closing contacts of the present invention. Note that 
the width of the bifurcated contact assemblies can be varied to suit the 
space available and the shape of the outer bifurcated contacts adjusted to 
increase or decrease the contact pressure on the inserted pins. 
It should also be clear to those skilled in the art that further 
embodiments of the present invention may be made by those skilled in the 
art without departing from the teachings of the present invention.