Electrical plug

An electrical plug for connecting the first and second conductors of an electrical lead to an electrical power outlet. The plug is designed to contain a fuse. When the plug is connected to the power outlet and the fuse makes a proper electrical connection, a signal member provides a signal.

BACKGROUND OF THE INVENTION 
1. Cross-Reference to Related Applications 
This application is related to copending, commonly assigned Application No. 
07/415,820 (Tucker), filed contemporaneously with this application, the 
disclosure of which is incorporated herein by reference. 
2. Field of the Invention 
This invention relates to electrical plugs, particularly electrical plugs 
which are useful for making an electrical connection between an electrical 
lead and an electrical power outlet. 
2. Introduction to the Invention 
It is known to insert a light, or other signal, into an electrical circuit 
so that it is easy to determine whether current is flowing through a 
heater or other operational electrical device which forms part of the 
circuit. The light can, for example, be placed in a plug through which the 
device is connected to a wall outlet in a building or to another power 
source. It is also known to insert a fuse, circuit breaker, ground fault 
circuit interrupter (GFCI), or other circuit protection device into an 
electrical circuit so that if the current exceeds a safe level or 
otherwise behaves in an abnormal fashion, the protection device is 
converted from its normal state into an abnormal state in which it 
disconnects the circuit from the power supply or otherwise prevents 
maintenance of the abnormal current. The protection device can, for 
example, be placed in a plug through which a heater or other operational 
device is connected to a wall outlet in a building or to another power 
source. When an electrical circuit contains both a signal light and a 
protection device, absence of the signal light may signify failure of the 
operational device itself, or failure of the electrical connections to the 
operational device, or conversion of the protective device into its 
abnormal state, e.g. failure ("blowing") of the fuse. 
Self-regulating strip heaters which comprise elongate electrodes connected 
by a conductive polymer resistive element are well-known. In operation, 
the thermal output of these strip heaters varies in response to changes in 
the thermal environment and thus serves to limit the maximum temperature 
which the heater achieves. Such heaters are often used to provide freeze 
protection of pipes, such as domestic or commercial water lines, or to 
maintain a constant temperature for pipes in process industries. A number 
of measures have been proposed for assisting the safe operation of such 
heaters. Reference may be made, for example, to U.S. Pat. No. 4,436,986 
(Carlson) and U.S. Pat. No. 4,822,983 (Bremner et al) and to copending 
commonly assigned application Ser. No. 404,730, filed Sept. 8, 1989 
(Emmett). The disclosure of each of these patents and applications is 
incorporated herein by reference. 
SUMMARY OF THE INVENTION 
I have now realized that when a conductive polymer strip heater is 
connected to a power supply through an electrical plug which is inserted 
into a conventional wall outlet and which contains a fuse or other 
protective device, it is very useful to include, as a part of the plug, a 
signal device, e.g. a light-emitting diode, which is connected between the 
two legs of the heater circuit so that it will provide a signal (e.g. will 
be lit up) only when the power is on and the protective device is in its 
normal state. 
In a first aspect, this invention provides an electrical plug for 
connecting an electrical lead comprising a first conductor and a second 
conductor to an electrical power outlet, which plug comprises 
(1) a first contact member which comprises (a) a first prong suitable for 
insertion into one socket of the power outlet, and (b) a first 
fuse-contact section; 
(2) a second contact member which comprises (a) a first conductor-contact 
section to which the first conductor can be physically and electrically 
connected, and (b) a second fuse-contact section; 
(3) a third contact member which comprises (a) a second conductor-contact 
section to which the second conductor can be physically and electrically 
connected, and (b) a second prong suitable for insertion into the other 
socket of the power outlet; and 
(4) a signal member which is connected between the second and third contact 
members and which provides a signal when current is passing therethrough, 
whereby, when the first and second prongs are inserted into the power 
outlet, the signal member provides a signal if a fuse connects the first 
and second fuse contact sections and does not provide a signal if there is 
no electrical connection between the first and second fuse contact 
sections. 
In a second aspect, this invention provides an electrical plug for 
connecting an electrical lead comprising a first conductor and a second 
conductor to an electrical power outlet, which plug comprises 
(A) a conductor-receiving member which comprises 
(1) a first channel in which the first conductor can be placed, and 
(2) a second channel in which the second conductor can be placed; and 
(B) a conductor-connecting member which comprises 
(1) a first contact member which comprises (a) a first prong suitable for 
insertion into one socket of the power outlet and (b) a first fuse-contact 
section, 
(2) a second contact member which comprises (a) a first conductor-contact 
section and (b) a second fuse-contact section, 
(3) a third contact member which comprises (a) a second conductor-contact 
section and (b) a second prong suitable for insertion into the other 
socket of the power outlet, and 
(4) a signal member which is connected between the second and third contact 
members and which provides a signal when current is passing therethrough; 
the conductor-receiving member and the conductor-connecting member being 
(i) movable relative to each other between a unique mated configuration and 
a plurality of demated configurations; 
(ii) such that in at least some of the demated configurations, the first 
conductor can be placed in the first channel and the second conductor can 
be placed in the second channel; 
(iii) such that if the first conductor has been placed in the first channel 
and the second conductor has been placed in the second channel, the 
conductor-receiving member with the conductors placed therein and the 
conductor-connecting member can then be brought into the mated 
configuration, in which mated configuration the first conductor makes 
physical and electrical contact with the first conductor-contact section 
and the second conductor makes electrical contact with the second 
conductor-contact section; and 
(iv) such that they cannot be brought into the mated configuration if the 
first conductor or the second conductor has been placed in direct physical 
contact with the respective conductor-contact section instead of being 
placed in the respective channel; and 
(C) closure means for maintaining the conductor-receiving member and the 
conductor-connecting member in the mated configuration, 
whereby when the first and second prongs are inserted into the power 
outlet, the signal member provides a signal if a fuse connects the first 
and second fuse-contact sections and does not provide a signal if there is 
not electrical connection between the first and second fuse-contact 
sections. 
In a third aspect, this invention provides an electrical plug assembly 
which comprises 
(A) a plug comprising 
(1) a first contact member which comprises (a) a first prong suitable for 
insertion into one socket of a power outlet, and (b) a first fuse-contact 
section; 
(2) a second contact member which comprises (a) a first conductor-contact 
section to which a first conductor can be physically and electrically 
connected, and (b) a second fuse-contact section; 
(3) a third contact member which comprises (a) a second conductor-contact 
section to which a second conductor can be physically and electrically 
connected, and (b) a second prong suitable for insertion into the other 
socket of a power outlet; 
(4) a signal member which is connected between the second and third contact 
members and which provides a signal when current is passing therethrough, 
and 
(B) a fuse which is positioned between the first and second fuse contact 
sections, 
whereby, when the first and second prongs are inserted into the power 
outlet, the signal member provides a signal if the fuse electrically 
connects the first and second fuse contact sections and does not provide a 
signal if there is no electrical connection between the first and second 
fuse contact sections.

DETAILED DESCRIPTION OF THE INVENTION 
The electrical plug of the invention is used to connect an electrical lead 
comprising a first conductor and a second conductor to an electrical power 
outlet. The power outlet may be a wall outlet either inside or outside of 
a building, or it may be part of a power supply or other suitable source 
of power. The outlet may be two-hole for connection to a phase and a 
neutral wire, or three-hole for connection to a phase, a neutral, and a 
ground (earth) wire. The electrical lead may be any electrical component 
with a first conductor and a second conductor, e.g. an electrical power 
cord. The plug is particularly useful in making an electrical connection 
to a strip heater, i.e. an elongate heating element which comprises at 
least two electrodes. For a connection made with this type of heater, the 
first conductor is one electrode of the heater and the second conductor is 
the other electrode of the heater. The strip heater may comprise a 
conductive polymer, i.e. a composition in which a particulate conductive 
filler is dispersed or otherwise distributed in a polymeric component. 
Particularly preferred are self-regulating conductive polymer heaters, 
i.e. those which exhibit PTC (positive temperature coefficient) behavior. 
The electrodes of a self-regulating heater are generally elongate metal 
wires or braid which are parallel and spaced apart. They are attached to 
or embedded in a resistive element which comprises the conductive polymer 
and is often in the form of a continuous strip. In order to provide 
environmental protection and electrical insulation, it is common for the 
resistive element and the electrodes to be covered by a dielectric layer, 
e.g. a polymeric jacket. A metallic grounding braid is often present over 
the dielectric layer in order to provide physical reinforcement and a 
means of electrically grounding the strip heater. 
The term "PTC behavior" is used in this specification to denote a 
composition or an electrical device which has an R.sub.14 value of at 
least 2.5 or an R.sub.100 value of at least 10, and preferably both, and 
particularly one which has an R.sub.30 value of at least 6, where R.sub.14 
is the ratio of the resistivities at the end and the beginning of a 
14.degree. C. range, R.sub.100 is the ratio of the resistivities at the 
end and the beginning of a 100.degree. C. range, and R.sub.30 is the ratio 
of the resistivities at the end and the beginning of a 30.degree. C. 
range. Self-regulating conductive polymer heaters which exhibit PTC 
behavior, and appropriate conductive polymer compositions, are disclosed 
in U.S. Pat. Nos. 3,858,144 (Bedard et al), 3,861,029 (Smith-Johannsen et 
al), 4,188,276 (Lyons et al), 4,388,607 (Toy et al), and 4,426,339 (Kamath 
et al), and copending, commonly assigned application Ser. No. 404,730 
(Emmett), filed Sept. 8, 1989. The disclosure of each of these patents and 
applications is incorporated herein by reference. 
The invention is illustrated by the drawings. FIG. 1 shows a schematic 
circuit diagram for the plug which is designed for insertion into an 
electrical power outlet. A first contact member 2 comprises a first prong 
4 which can be inserted into one socket of the outlet and a first 
fuse-contact section 6. A second contact member 8 comprises a second 
fuse-contact section 10 and a first conductor-contact section 12 to which 
a first conductor of an electrical lead can be physically and electrically 
connected. A third contact member 14 comprises a second conductor-contact 
section 16 to which the second conductor of an electrical lead can be 
physically and electrically connected and a second prong 18 which can be 
inserted into the other socket of the outlet. A signal member 20 is 
electrically connected between the second and third contact members. In 
this embodiment, the signal member 20 is electrically in series with a 
first resistor 22 and a second resistor 24. When the first and second 
prongs 4, 18 are inserted into the power outlet, the signal member 20 
provides a signal if a fuse 26 connects the first and second fuse-contact 
sections 6, 10. No signal is provided if there is no electrical connection 
between the first and second fuse-contact sections 6, 10. The signal 
member 20 may be a light, e.g. a light emitting diode (LED), which 
provides a visual signal, a bell or other apparatus which provides an 
audio signal, or an electrical switch which can generate an alarm signal. 
Other types of signal members may be appropriate in different 
circumstances. 
The first and second fuse-contact sections 6, 10 are positioned to receive 
a fuse 26. The selection of a specific fuse is dependent on the normal 
operating conditions and the anticipated fault conditions. Particularly 
preferred when the connection is to be made to a strip heater is a very 
fast acting fuse, i.e. a fuse which has little, if any intentional delay 
in the overload region and which "trips" (opens) very rapidly when the 
current in the circuit comprising the fuse exceeds the rated value of the 
fuse. Appropriate fuses are very fast-acting ceramic ferrule fuses with a 
current rating of 10 amperes and a voltage rating of 125/250 volts. Such 
fuses are available, for example, from the Bussman Division of Cooper 
Industries under the name Buss GBB.TM.-10. While it is possible to use one 
of the measures disclosed in U.S. Pat. No. 4,822,983 (Bremner et al) in 
conjunction with this invention, I have found that, when a fuse as just 
described is employed, excellent results can be obtained without using a 
sensor conductor as disclosed in that patent. 
FIG. 2A shows an exploded view of one specific embodiment of the plug which 
is designed to connect the conductors of an electrical lead 28 to an 
electrical power outlet. In this figure the electrical lead 28 is a strip 
heater which comprises a resistive element 30, and embedded therein, a 
first conductor 32 and a second conductor 34. The heater is surrounded by 
a metallic grounding braid 36. Both the first conductor 32 and the second 
conductor 34 have been stripped of the conductive polymer which comprises 
the resistive element 30 to allow easy insertion into the 
conductor-receiving member 38. For other types of leads, e.g. insulated 
wires, it may not be desirable to remove the polymeric insulation or 
jacketing material. The conductor-receiving member 38 comprises a first 
channel 40, a second channel 42, and a third channel 44, designed for 
insertion of the first conductor 32, the second conductor 34, and the 
grounding lead 36, respectively. Each channel is a tunnel which has a 
frusto-conical opening, both features sized so that the conductors 
inserted in them can be held by frictional forces. The walls of the 
channels serve to prevent contact between the conductors. A radial opening 
46, 48, 50 is cut through each tunnel to allow electrical connection of 
the inserted conductor to the conductor-contact sections. Once the 
conductors are inserted into the conductor-receiving member 38, it is 
mated to the conductor-contact connecting member 52. As a result of the 
design of the conductor-receiving member, which may include positioning 
pins or other design elements which are not shown, there is a unique mated 
configuration. When the two pieces are mated, the first conductor 32 is in 
physical and electrical contact with the first conductor-contact section 
12, the second conductor 34 is in physical and electrical contact with the 
second conductor-contact section 16, and the grounding lead 36 is in 
physical and electrical contact with the grounding contact section 54. The 
first and second conductor-contact sections 12, 16 and the grounding 
contact section 54 may comprise insulation-piercing means for use when the 
conductors are insulated with a polymeric jacket. The conductor-connecting 
member further comprises first and second fuse-contact sections 6, 10 
designed to receive a fuse. The first connection-making section 4 
corresponds to the first prong of FIG. 1 and the second connection-making 
section 18 corresponds to the second prong. A ground-connection-making 
section 56 is also present to connect to the ground in the outlet. The 
embodiment shown also comprises a base 58 for the conductor-connecting 
member 52 which can be welded, glued, or snapped into place. A signal port 
60 is positioned so that the signal member 20 will be visible. FIG. 2B 
shows the bottom of the conductor-connecting member 52 with the base 58 
removed. The signal member 20, an LED, and first and second resistors 22, 
24 are visible. It is apparent that the first fuse-contact section 6 and 
the first connection-making section 4 are made from a single piece of 
metal, e.g. brass. In this embodiment, the second conductor-contact 
section 16 and the second connection-making section 18 are also made from 
a single piece of metal, as are the second fuse-contact section 10 and the 
fuse conductor-contact section 12. 
The conductor-receiving member 38 is preferably made from an insulating 
material, e.g. a polymer. It is particularly preferred that it be made 
from a transparent polymer, e.g. polycarbonate, so that there is a visual 
indication that the conductors are properly positioned. The 
conductor-receiving member 38 and the conductor-connecting member 52 are 
maintained in position by a closure means 62, e.g. a screw or a clamp. 
When mated properly, the plug provides adequate strain relief to the 
electrical lead. A minimum pull force of 20 pounds on the electrical lead 
is achieved. (The "pull force" or "pullout force" measures the amount of 
force required to pull the electrical lead 0.125 inch (0.318 cm) out of 
the plug. This procedure is described in copending, commonly assigned 
application Ser. No. 281,580 (Bautista) filed Dec. 8, 1988, the disclosure 
of which is incorporated herein by reference.) Proper mating of the 
conductor-receiving member 38 and the conductor-connecting member 52 
cannot be achieved if either the first conductor 32 or the second 
conductor 34 has been directly connected to the respective 
conductor-contact section 12, 16. 
When the fuse 26 is in place but no conductors are inserted into the 
conductor-receiving member, the plug may be used to determine whether wall 
outlets are properly wired and/or live. If the fuse is electrically intact 
and there is no signal when the plug is placed into the outlet, that is an 
indication that the outlet is defective. Additional signal members can be 
incorporated into the plug to signal whether the phase and neutral wires 
are wired correctly. When there is a ground connection to the plug, 
additional signal members will indicate whether there is actually a 
ground.