WO2002019475A1

WO2002019475A1 - Plug and electric outlet for absorbing electromagnetic interference

Info

Publication number: WO2002019475A1
Application number: PCT/KR2001/001486
Authority: WO
Inventors: Ki-Kwon Jang
Original assignee: Jang Ki Kwon
Priority date: 2000-08-31
Filing date: 2001-08-31
Publication date: 2002-03-07
Also published as: CN1245784C; AU2001282665A1; CN1461505A

Abstract

Disclosed is a plug that comprises: a short current cutoff circuit installed in an electric device and including a cutoff capacitor having a grounded first terminal for restricting the short current; a first plug terminal for receiving AC electricity and supplying electrical energy to the electric device via a wire; a second plug terminal connected to a second terminal of the cutoff capacitor; and a ground check circuit for checking connection states of a ground terminal of an electric outlet with the second plug terminal and including a luminescent element haivng a first terminal connected to the second plug terminal; and a contact terminal connected to a second terminal of to the second plug terminal; and a contact terminal connected to a second terminal of the luminescent element and to be connectable with a human body. Therefore, the plug can remove the EMI via the ground, prevent short states, and checke the connection states of an electric outlet with the plug.

Description

Plug and Electric Outlet for Absorbing Electromagnetic Interference

BACKGROUND OF THE INVENTION (a) Field of the Invention

The present invention relates to a plug and an electric outlet that are

interfaces for supplying power to electric devices. More specifically, the

present invention relates to a plug and an electric outlet that provide a

ground-check circuit for breaking electromagnetic interference (EMI) and a short-preventing circuit.

(b) Description of the Related Art

General plugs have two to three terminals for supplying power to

electric devices that use voltages of 100 to 250 volts (V) and 50 to 60 Hz, and one of the terminals is grounded to absorb EMI and electromagnetic

fields.

In the case of a plug with two terminals, one terminal is grounded

and the other terminal flows AC (alternating current) electricity, and in the

case of a plug with three terminals, one terminal is grounded and other two

terminals flow AC electricity.

However, terminals of a conventional electric outlet into which a plug

is inserted do not display a ground terminal mark and an AC electricity

terminal mark, and accordingly, it may occur that a ground terminal of an

electric outlet and an AC electricity terminal of a plug are connected and become short circuited so that electric devices may become unstable and

users may be damaged.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a plug for absorbing

EMI, checking whether terminals of an electric outlet are matched with those

of the plug, and preventing a short current

It is another object of the present invention to provide an electric outlet for absorbing EMI, checking whether terminals of an electric outlet are

matched with those of the plug, and preventing a short current.

In one aspect of the present invention, a plug comprises: a short

current-cutoff circuit installed in an electric device, including a cutoff capacitor having a grounded terminal, for restricting a short current; a first

plug terminal receiving AC electricity and supplying electric energy to the

electric device; and a second plug terminal connected to another terminal of the cutoff capacitor of the short current-cutoff circuit.

In another aspect of the present invention, a plug comprises: a short

current-cutoff circuit including a cutoff capacitor, installed in the plug, for

restricting a short current; a first plug terminal for receiving AC electricity and

supplying electric energy to an electric device through a cable; a second plug

terminal connected to one terminal of the cutoff capacitor of the short

current-cutoff circuit; and a shield line wrapping the cable connected to the first and second plug terminals to remove electromagnetic interference (EMI), having one terminal connected to another terminal of the second plug terminal, and another terminal being grounded.

In still another aspect of the present invention, a plug comprises: first and second plug terminals respectively connected to terminals of an electric outlet, for supplying electric energy to an AC electricity-using device; a third plug terminal connected to a ground terminal of the electric outlet, and being grounded, for removing electromagnetic interference (EMI); and a short current-cutoff circuit including a cutoff capacitor having one terminal connected to the second plug terminal and another terminal connected to the third plug terminal.

In a further aspect of the present invention, an electric outlet comprises: an electric outlet ground terminal connected to a ground terminal of a plug, for removing EMI flowing to an electric device; first and second electric outlet terminals for supplying AC electricity supplied by an external power source to the electric device through the plug, and having one terminal connected to either of a neutral line or a ground line of the external power source; and a ground-check circuit for checking which one of the first and the second electric outlet terminals is connected to the ground line or the neutral line of the external power source, comprising: a light emitting device having one terminal connected to the first electric outlet terminal; and a contact terminal connected to another terminal of the light emitting device and a human's body.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and

constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of

the invention:

FIG. 1 shows a plug that includes a ground-check circuit and a short current-cutoff circuit according to a first preferred embodiment of the present

invention;

FIG. 2 shows a plug that includes a ground-check circuit and a short

current-cutoff circuit according to a second preferred embodiment of the present invention;

FIG. 3 shows a plug that includes a ground-check circuit and a short

current-cutoff circuit according to a third preferred embodiment of the present

invention;

FIG. 4 shows an electric outlet that includes a ground-check circuit

and a short current-cutoff circuit according to a preferred embodiment of the

present invention;

FIG. 5(a) shows an electric outlet switch turned off according to a

preferred embodiment of the present invention;

FIG. 5(b) shows an electric outlet switch turned on according to a preferred embodiment of the present invention;

FIG. 5(c) shows a relationship between the electric outlet switch and

an electric outlet ground terminal according to a preferred embodiment of the

present invention; and

FIG. 6(a) and 6(b) show exemplified multi-taps using an electric outlet according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, only the preferred embodiment

of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in

various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in

nature, and not restrictive.

FIG. 1 shows a configuration of a plug that comprises a ground-

check circuit and a short current-cutoff circuit according to a first preferred

embodiment of the present invention. As shown, the plug comprises a short

current-cutoff circuit 10; a first plug terminal 1 1 ; a second plug terminal 12;

and a ground-check circuit 13.

In this instance, the first plug terminal 1 1 flows the AC electricity, and

the second plug terminal 12 is connected to a neutral line. In the short current-cutoff circuit 10, a cutoff capacitor 10-1 and a

charge-removal resistor 10-3 are connected in parallel, and one terminal of

the cutoff capacitor 10-1 and one terminal of the charge-removal resistor 10-

3 are grounded. When a short occurs, the short current flows through the

other terminals of the cutoff capacitor 10-1 and the charge-removal resistor

10-3, and the cutoff capacitor 10-1 reduces the magnitude of the short current to cut off the shortcurrent. The short current flowing to the capacitor

is proportional to a variation of voltage during a predetermined time, and

capacitance of the capacitor as expressed in Equation 1 . Therefore, when a capacitor of a small capacitance is selected, the magnitude of the short

current can be reduced. The charge-removal resistor 10-3 removes a charge remaining in the capacitor. Equation 1

ι = C — dt where "/^'" represents the short current, "C" indicates the capacitance

of the capacitor, "dt" expresses time variation, and "dv" shows voltage

variation. In this instance, the short current-cutoff circuit 10 is installed in an

electric device.

The first plug terminal 1 1 receives AC electricity and supplies electric

energy to the electric device through a wire, and the second plug terminal 12

is connected to the other terminals of the cutoff capacitor 10-1 and the

charge-removal resistor 10-3. Therefore, when the ground terminal of the electric outlet and the

second plug terminal 12 are connected, no current flows to the short current-

cutoff circuit 10, but when the AC electricity-flowing terminal of the electric

outlet and the second plug terminal 12 are connected, the short occurs, the

short current is generated to the short current-cutoff circuit 10, and as described above, the magnitude of the short current is reduced because of

the capacitance of the cutoff capacitor 10-1 .

The ground-check circuit 13 checks whether the ground terminal of

the electric outlet and the second plug terminal 12 are correctly connected, and it comprises a protection resistor 13-1 ; a neon lamp 13-3; and a contact terminal 13-5. One end of the protection resistor 13-1 is connected to the

second plug terminal 12, one end of the neon lamp 13-3 is connected to another end of the protection resistor 13-1 , and another end of the neon lamp 13-3 is connected to the contact terminal 13-5.

In the above-noted ground-check circuit 13, when the ground

terminal of the electric outlet is connected to the second plug terminal 12 and

the contact terminal 13-5 is connected to a human's body such as a hand,

the current does not flow, and hence, the neon lamp 13-3 is not turned on,

thereby indicating that the electric outlet and the plug are appropriately

connected.

However, when the AC electricity-flowing terminal of the electric

outlet and the second plug terminal 12 are connected and the contact terminal 13-5 is connected to the human's body, the AC electricity flows

through the neon lamp 13-3, and hence, the neon lamp is turned on thereby representing that the electric outlet and the plug are not appropriately connected. Referring to FIG. 1 , an operation of the plug according to the first preferred embodiment of the present invention will now be described.

When the user holds the plug body by hand, the user is naturally connected to the contact terminal 13-5, and the user inserts the plug into the electric outlet. If the ground terminal of the electric outlet and the second plug terminal 12 of the plug are connected, the ground terminal of the electric outlet, the neon lamp 13-3, the contact terminal 13-5, and the user's body are connected to ground, and accordingly, it becomes an equipotential such as a ground state, and the neon lamp 13-3 is not turned on.

Therefore, the AC electricity flows to the electric devices through the first plug terminal 11 of the plug, and in this instance, no current flows to the cutoff capacitor 10-1 of the short current-cutoff circuit 10 connected to the second plug terminal 12.

Accordingly, the short current-cutoff circuit 10 is installed in the electric device and is connected to a ground terminal of the electric device to be used.

If the second plug terminal 12 of the plug is connected with the AC electricity-flowing terminal of the electric outlet while the user's hand is connected to the contact terminal 13-5, the second plug terminal 12, the

neon lamp 13-3, the contact terminal 13-5, the user's hand, and ground are

connected, and accordingly, the AC electricity flows to the neon lamp 13-3,

and the neon lamp is turned on so that the user finds the wrong connection.

Also, the AC electricity supplied through the electric outlet flows to

the second plug terminal 12 and the short current-cutoff capacitor 10-1 and

reduces the magnitude of the short current, thereby preventing problems.

As described above, the EMI generated in the electric devices can be removed through grounding, dangers of short accidents caused by

grounding can be previously warned, and the short current is restricted, so the user can safely use the plug.

FIG. 2 shows a configuration of a plug that includes a ground-check circuit 13 and a short current-cutoff circuit 10 according to a second

preferred embodiment of the present invention. As shown, the plug comprises a short current-cutoff circuit 10; a first plug terminal 11 ; a second

plug terminal 12; a ground-check circuit 13; and a shield line 14.

Since the first plug terminal 1 1 , the second plug terminal 12, and the

ground-check circuit 13 are matched with those of the first preferred

embodiment of the present invention, no further description will be provided.

The shield line 14 of aluminum or copper wraps cables and is

grounded to remove the EMI.

The configuration of the short current-cutoff circuit 10 is matched with that of the first preferred embodiment excepting that one terminal of the

cutoff capacitor 10-1 and the charge-removal resistor 10-3 are connected to

the second plug terminal 12, and their other terminals are connected to the

shield line 14. In this instance, the short current-cutoff circuit 10 is installed in

the plug, differing from the first preferred embodiment. It is also possible for

the second plug terminal 12 and the shield line 14 to be connected, and for

the short current-cutoff circuit 10 to be installed in the electric device in

connection with the shield line 14, identical with the first preferred embodiment.

Next, an operation of the second preferred embodiment of the present invention will be described.

The operation of the second preferred embodiment is basically

matched with that of the first preferred embodiment, excepting that in the second preferred embodiment the ground is through the shield line 14, and

in this process, when the AC electricity-flowing terminal of the electric outlet

and the second plug terminal 12 are connected, the AC electricity passes

through the neon lamp 13-3, the contact terminal 13-5 and the user's body,

and is grounded to turn on the neon lamp, and when it is shorted, the short

current flows to the cutoff capacitor 10-1 , is greatly reduced, and is

exhausted through the shield line 14.

FIG. 3 shows a configuration of a plug that comprises a ground-

check circuit 13 and a short current-cutoff circuit 10 according to a third preferred embodiment of the present invention. As shown, the plug comprises a first plug terminal 11 ; a second plug terminal 12; a third plug terminal 15; a short current-cutoff circuit 10; and a ground-check circuit 13.

The first and second plug terminals 11 and 12 are connected to a terminal of the electric outlet to supply the AC electricity to the electric devices.

The third plug terminal 15 is connected to the ground terminal of the electric outlet to remove the EMI.

In the short current-cutoff circuit 10, the cutoff capacitor 10-1 and the charge-removal resistor 10-3 are connected in parallel and are installed in the plug identically with the first preferred embodiment, and one terminal of each of the cutoff capacitor 10-1 and the charge-removal resistor 10-3 are connected to the second plug terminal 12, and their other terminals are connected to the third plug terminal 15. Their operation is matched with that of the first preferred embodiment, so it will not be described.

The configuration and operation of the ground-check circuit 13 is matched with that of the first preferred embodiment, which will not be described.

The charge-removal resistor 10-3 of the first to third preferred embodiments removes a remaining charge from the cutoff capacitor 10-1.

As described, the plug prevents short problems that may be generated while performing a grounding process to remove the EMI. FIG. 4 shows an electric outlet that comprises a ground-check circuit

and a short current-cutoff circuit. As shown, the electric outlet comprises first

and second electric outlet ground terminals 1 10a and 1 10b; first and second

electric outlet terminals 1 11 and 112; a short current-cutoff circuit 1 13; a

ground-check circuit 1 14; a switch 1 15; and a noise canceller 116.

The first and second electric outlet ground terminals 110a and 1 10b

are connected to a ground terminal of the plug, so as to remove the EMI that

flows to electric devices.

The first and second electric outlet terminals 1 1 1 and 1 12 supply the

AC electricity supplied by an external power source to electric devices or electric machines through the plug, and either of the first and second electric

outlet terminals is connected to either of a neutral line or a ground line of the external power source.

In the short current-cutoff circuit 1 13, a cutoff capacitor 1 13-1 and a

charge-removal resistor 1 13-3 are connected in parallel, and one of the

terminals of the cutoff capacitor 1 13-1 and the charge-removal resistor 1 13-3

are connected to the second electric outlet ground terminal 1 10b, and the

other terminals are connected to the first electric outlet terminal 1 1 1 .

When a short occurs, the short current flows to the electric outlet

ground terminal 1 10 through the cutoff capacitor 1 13-1 and the charge-

removal resistor 1 13-3 in the short current-cutoff circuit 1 13, and the cutoff

capacitor 1 13-1 reduces the magnitude of the short current to cut off the short current.

If the first electric outlet terminal 111 of the electric outlet according to the present invention is connected to either of a neutral line or a ground line of the external power source, and is combined with a grounded terminal of the plug of the electric devices or the electric machines, the short current does not flow since the first electric outlet terminal 111 and the terminal of the plug are equipotential.

However, if the first electric outlet terminal 111 is connected to a terminal to which the AC electricity is supplied from the external power source, and is connected to a grounded terminal of the plug of the electric devices or the electric machines, the first electric outlet terminal 111 and the terminal of the plug become short circuited.

In this instance, the short current flows through the cutoff capacitor 113-1 , and the short current flowing to the cutoff capacitor 113-1 is proportional to the variation of the voltage and the capacitance of the capacitor during a predetermined time as expressed in the above equation. Therefore, when a small capacity of the capacitor is selected, the short current can also be reduced.

The charge-removal resistor 113-3 removes the charge remaining in the capacitor.

The ground-check circuit 114 checks which terminal of the first or second electric outlet terminals 111 or 112 is connected to the ground line or the neutral line of the external power source, and it comprises a protection

resistor 1 14-1 ; a neon lamp 1 14-3; and a contact terminal 1 14-5.

In this instance, one end of the protection resistor 1 14-1 is

connected to the first electric outlet terminal 1 1 1 , one end of the neon lamp

1 14-3 is connected to another end of the protection resistor 1 14-1 , and

another end of the neon lamp 1 14-3 is connected to the contact terminal 1 14-5.

In the above-configured ground-check circuit 1 14, when the first

electric outlet terminal 1 1 1 is connected to a neutral line or a ground line of the external power source, and the contact terminal is connected to a

human's body such as a hand, the neutral line or the ground line, the first

electric outlet terminal 1 1 1 , the contact terminal 1 14-5, the human's body,

and ground become equipotential, and the AC current does not flow and the neon lamp is not turned on. Therefore, the user finds that the first electric

outlet terminal 1 1 1 is connected to the neutral line or the ground line of the

external power source.

When the first electric outlet terminal 1 1 1 is connected to a terminal

that supplies the AC electricity of the external power source, and the contact

terminal is connected to the human's body such as a hand, the AC electricity

flows to the terminal that supplies the AC electricity of the external power

source, the first electric outlet terminal 1 1 1 , the contact terminal, the

human's body, and ground, and accordingly, the neon lamp is turned on. Hence, the user finds that the first electπc outlet terminal 1 1 1 is connected to

the AC electricity-supplying terminal, and the user can correctly connect the

plug of the electric devices or the electric machines.

In this instance, the protection resistor 1 14-1 uses a resistor of great

resistance so as to reduce the voltage supplied to the human's body and the

neon lamp.

The switch 1 15 disconnects the first and second electric outlet

ground terminals 1 10a and 1 10b when the plug is not inserted, and connects the first and second electric outlet ground terminals 1 10a and 110b when the

plug is inserted.

In the conventional electπc outlet, the first and second electric outlet

ground terminals 1 10a and 1 10b are always connected, and the conventional electric outlet is protruded to be connected to the ground terminal of the plug when the plug is not inserted, and accordingly, when the

conventional electric outlet ground terminal and the AC electricity-supplying

terminal are connected, they become short circuited and may provoke electric shocks. Therefore, the switch for the electric outlet according to the

present invention becomes open and prevents short problems through the

protruded second electric outlet ground terminal 1 10b when the plug is not

inserted, and performs the original ground function when the plug is inserted.

To perform this operation, the switch 1 15 for the electric outlet

according to the present invention comprises a safety rotary cover 1 15-1 , which is a nonconductor; a supporting spring 1 15-3; a ground spring 1 15-5; a

ground-terminal connection post 1 15-7, which is a conductor; and a spring

contact terminal 1 15-9, which is a conductor.

FIG. 5(a) shows the switch 1 15 turned off. As shown, when nothing

is inserted into a cavity connected with the first and second electric outlet

terminals 1 1 1 and 1 12, the safety rotary cover 1 15-1 covers the cavity, and

since the ground spring 1 15-5 and the ground-terminal connection post 1 15-

7 are not connected in this instance, the first and second electric outlet

ground terminals 1 10a and 1 10b are not connected. In FIG. 5(b), when two terminals of the plug are inserted, the safety rotary cover 1 15-1 is rotated, and the supporting spring 1 15-3 and the

ground spring 1 15-5 are compressed. In this instance, the supporting spring 1 15-3 is supported by the nonconductor, and the ground spring 1 15-5

electrically connects the ground-terminal connection post 1 15-7 and the spring contact terminal 1 15-9. As shown by FIG. 5(c), the ground-terminal

connection post 1 15-7 is thereby connected to the first electric outlet ground

terminal 1 10a, and the spring contact terminal 1 15-9 is also connected to

one end of the cutoff capacitor 1 13-1 .

Therefore, when the plug is not inserted, the switch 1 15 prevents the

problem of the AC electricity flowing to the protruded first electric outlet

ground terminal 1 10a to develop a short circuit.

The noise canceller 1 16 comprises a capacitor connected to the first and second electric outlet terminals 111 and 112, and it filters noise included in the AC electricity and removes the same.

FIG. 6(a) shows an exemplified 200 to 250 volts multi-tap using the electric outlet according to a preferred embodiment of the present invention, and FIG. 6(b) shows an exemplified 100 to 120 volts multiple plug power bar using the electric outlet according to a preferred embodiment of the present invention. As shown in FIGs. 6(a) and 6(b), a plurality of first electric outlet terminals 111 are electrically connected, a plurality of second electric outlet terminals 112 are electrically connected, and a plurality of electric outlet ground terminals 110 are electrically connected. Also, the ground terminal is connected to first terminals of the charge-removal resistor 113-3 and the cutoff capacitor 113-1 , their second terminals are connected to the first electric outlet terminal 111 , and one terminal of the ground-check circuit 114 is connected to the first electric outlet terminal 111. As described, the present invention can remove EMI, prevent short problems that may be generated during the EMI removal, and check connection states of the electric outlet to the plug.

While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A plug comprising: a short current-cutoff circuit installed in an electric device, including a cutoff capacitor having a grounded terminal, for restricting a short current; a first plug terminal receiving AC electricity and supplying electric energy to the electric device; and a second plug terminal connected to another terminal of the cutoff capacitor of the short current-cutoff circuit.

2. The plug of claim 1 , further comprising a charge-removal resistor connected to the cutoff capacitor in parallel, having a grounded terminal connected to the grounded terminal of the cutoff capacitor, and having another terminal connected to the second plug terminal, for removing a charge remaining in the cutoff capacitor.

3. The plug of claim 1 , further comprising a ground-check circuit for

checking whether a ground terminal of an electric outlet is correctly

connected to the second plug terminal, comprising: a light emitting device

having a terminal connected to the second plug terminal; and a contact

terminal connected to another terminal of the light emitting device and a

human's body.

4. The plug of claim 3, further comprising a protection resistor having

a terminal connected to the second plug terminal and another terminal

connected to one terminal of the light emitting device, for protecting the light emitting device and reducing a voltage supplied to the human's body.

5. A plug comprising: a short current-cutoff circuit including a cutoff capacitor, installed in the plug, for restricting a short current; a first plug terminal for receiving AC electricity and supplying electric energy to an electric device through a cable; a second plug terminal connected to one terminal of the cutoff capacitor of the short current-cutoff circuit; and a shield line wrapping the cable connected to the first and second plug terminals to remove electromagnetic interference (EMI), having one terminal connected to another terminal of the second plug terminal, and another terminal being grounded.

6. The plug of claim 5, further comprising a charge-removal resistor connected to the cutoff capacitor in parallel, having one terminal connected to the second plug terminal, and having another terminal connected to the shield line for removing a charge remaining in the cutoff capacitor, with one terminal of the cutoff capacitor being connected to the second plug terminal, and another terminal of the cutoff capacitor being connected to the shield line.

7. The plug of claim 5, wherein the second plug terminal and one terminal of the shield line are connected, one terminal of the cutoff capacitor is connected to another terminal of the shield line, and another terminal of the cutoff capacitor being grounded.

8. The plug of one of claims 5 to 7, further comprising a ground- check circuit for checking whether a ground terminal of an electric outlet is correctly connected to the second plug terminal, comprising: a light emitting device having one terminal connected to the second plug terminal; and a contact terminal connected to another terminal of the light emitting device and a human's body.

9. A plug comprising: first and second plug terminals respectively connected to terminals of an electric outlet, for supplying electric energy to an AC electricity-using device; a third plug terminal connected to a ground terminal of the electric outlet, and being grounded, for removing electromagnetic interference (EMI); and a short current-cutoff circuit including a cutoff capacitor having one terminal connected to the second plug terminal and another terminal connected to the third plug terminal.

10. The plug of claim 9, further comprising a ground-check circuit for

checking whether the ground terminal of the electric outlet is correctly

connected to the second plug terminal, comprising: a light emitting device

having one terminal connected to the second plug terminal; and a contact

terminal connected to another terminal of the light emitting device and a human's body.

11. The plug of claim 9, further comprising a charge-removal resistor connected to the cutoff capacitor in parallel, having one terminal connected to the second plug terminal, and another terminal connected to the third plug terminal for removing a charge remaining in the cutoff capacitor, with one terminal of the cutoff capacitor connected to the second plug terminal, and another terminal of the cutoff capacitor connected to the third plug terminal.

12. An electric outlet comprising: an electric outlet ground terminal connected to a ground terminal of a plug, for removing EMI flowing to an electric device; first and second electric outlet terminals for supplying AC electricity supplied by an external power source to the electric device through the plug, and having one terminal connected to either of a neutral line or a ground line of the external power source; and a ground-check circuit for checking which one of the first and the second electric outlet terminals is connected to the ground line or the neutral line of the external power source, comprising: a light emitting device having one terminal connected to the first electric outlet terminal; and a contact terminal connected to another terminal of the light emitting device and a human's body.

13. The electric outlet of claim 12, further comprising a short current- cutoff circuit for controlling a short current, comprising a cutoff capacitor having one terminal connected to the electric outlet ground terminal and another terminal connected to the first electric outlet terminal.

14. The electric outlet of claim 13, wherein the short current-cutoff circuit further comprises a charge-removal resistor connected to the cutoff capacitor in parallel, for removing a charge remaining in the cutoff capacitor.

15. The electric outlet of claim 12, wherein the ground-check circuit further comprises a protection resistor for controlling a voltage supplied to the human's body through the contact terminal.

16. The electric outlet of claim 13, wherein the electric outlet ground terminal comprises first and second electric outlet ground terminals, and the electric outlet further comprises a switch for disconnecting the first and second electric outlet ground terminals when the plug is not inserted, and connecting the first and second electric outlet ground terminals when the plug is inserted.

17. The electric outlet of claim 16, further comprising a switch including: a safety rotary cover for covering a cavity connected to the first and second electric outlet terminals; a supporting spring for restoring the safety rotary cover to its original location when the safety rotary cover is rotated and moved; a ground-terminal connection post connected to the electric outlet ground terminal when the safety rotary cover is rotated; a spring contact terminal for electrically connecting the ground- terminal connection post with one terminal of the cutoff capacitor when the safety rotary cover is rotated; and a ground spring for electrically connecting the ground-terminal connection post with the cutoff capacitor when the safety rotary cover is rotated.

18. The electric outlet of claim 12 or 13, wherein a plurality of first electric outlet terminals are electrically connected, a plurality of second electric outlet terminals are electrically connected, and a plurality of electric outlet ground terminals are electrically connected, and one terminal of the electric outlet ground terminal is connected to one terminal of the cutoff capacitor, another terminal is connected to the first electric outlet terminal, and one terminal of the ground-check circuit is connected to the first electric outlet terminal.