WO2007081090A1

WO2007081090A1 - Safety device for magnetic field-free heating cable

Info

Publication number: WO2007081090A1
Application number: PCT/KR2006/005232
Authority: WO
Inventors: Jong-Jin Kil
Original assignee: Jong-Jin Kil
Priority date: 2006-01-13
Filing date: 2006-12-06
Publication date: 2007-07-19
Also published as: KR100709095B1

Abstract

Disclosed herein is a safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a Negative Temperature Coefficient (NTC) thermistor layer wound around the first heating wire, a second heating wire wound around the NTC thermistor layer, a constant melting temperature resin or synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the constant melting temperature resin or synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a temperature detection operation during a half cycle of a single-phase two-wire AC power source connected between the first and second heating wires and performing a heating operation during a remaining half cycle. The safety device includes a short circuit detection unit connected in series with the metal shield and connected to a ground side wire, which is one of two wires of the single-phase two-wire AC power source.

Description

SAFETY DEVICE FOR MAGNETIC FIELD-FREE HEATING

CABLE

Technical Field

[1] The present invention relates to a safety device using a metal shield that surrounds the outer circumferential surface of a magnetic field-free heating cable to block an electrical field that leaks out of the heating cable used in a heating apparatus for bedding, such as an electric floor, an electric mat or an electric pad. Background Art

[2] A heating cable, which includes two heating wires and in which current flows through the two heating wires in opposite directions, thus canceling an induced magnetic field, has been proposed. Furthermore, another heating cable, in which an insulating resin or a Negative Temperature Coefficient (NTC) thermistor is inserted between a first heating wire and a second heating wire, the first and second heating wires are disposed in opposite directions, and various types of blocking means are provided to the outside of the heating cable such that the temperature of the heating cable can be automatically detected and controlled with reference to a temperature set by a user, and such that electromagnetic waves and an induced magnetic field can be blocked, has been proposed. Furthermore, another heating cable, which blocks an electrical field to prevent no electrical field from leaking out using a metal shield wound around a second heating wire, has recently been developed.

[3] As an example, one of the above-described heating cables is described below with reference to the accompanying drawing. FIG. 1 is a diagram showing the construction of a typical magnetic field-free heating cable. Referring to FIG. 1, the magnetic field- free heating cable includes a core thread 1, a first heating wire 2 connected to one side of a power source and wound around the core thread 1, a Negative Temperature Coefficient (NTC) thermistor 3 wound around the first heating wire 2 and configured to have a resistance value that decreases in proportion to an increase in temperature, a second heating wire 4 wound around the outer circumferential surface of the NTC thermistor 3, and a metal shield 6 wound around the second heating wire 4. The magnetic field-free heating cable is covered with an insulating sheath layer 7. The first heating wire 2 and the second heating wire 4 are disposed coaxially or parallel to each other.

[4] When the Alternating Current (AC) input power source is turned on, a temperature detection signal current, which depends on variation in the temperature resistance value of the NTC thermistor 3 disposed between the first heating wire 2 and the second heating wire 4, is output to a voltage detection rectifier in the first half period of an AC cycle. The temperature detection signal current passes through the first heating wire 2, and flows through the second heating wire 4 after the direction thereof is reversed at the NTC thermistor 3.

[5] In this case, the directions of the respective temperature detection signal currents flowing through the first heating wire 2 and the second heating wire 4 are opposite each other, so that magnetic fields are offset, therefore the temperature detection signal currents flows therethrough in a magnetic field-free state. Furthermore, the electrical field leaking out of the second heating wire 4 is primarily blocked, and an electrical field leaking out due to electrical charges present between the intervals of the second heating wire 4 is blocked by the metal shield 6. The metal shield 6 must be separately grounded.

[6] Conventionally, in order to block the electrical fields and, at the same time, minimize the thickness of the heating cable, a scheme in which a process of disposing the insulating layer between the second heating wire 4 and the metal shield 6 is omitted is used. In this case, the internal resistance of the metal shield increases as the length of the heating cable increases, and the voltage across the two ends of the metal shield increases in proportion to an increase in resistance, so that there is a disadvantage in that the electrical field blocking attenuation ratio is reduced. Accordingly, in the above-described construction, in order to solve the problem, a constant melting temperature resin insulating layer 5 is provided therein.

[7] Referring to a circuit diagram shown in the lower portion of FIG. 1, heating current is input to the first heating wire 2, and flows through the second heating wire 4 after the direction thereof is reversed. The first and second heating wires 2 and 4 are entirely surrounded by the metal shield 6 and the metal shield 6 is separately grounded, so that all of the electrical charges are discharged.

[8] Temperature detection and heating can be performed without using a separate temperature sensor temperature due to the operation of the above-described heating cable NTC thermistor, so that, when the heating cable is overheated to a high temperature, a heating operation is automatically interrupted and, at the same time, the temperature can be controlled. In a heating apparatus for bedding, such as an electric floor, an electric mat or an electric pad, which comes into close contact with the human body and carries a risk of a fire, a function of guaranteeing the safety thereof is important, so that it is preferred that a safety device for interrupting the flow of current at the time of the occurrence of a short circuit in the heating cable or the circuitry, other than a function of automatically controlling temperature, be additionally provided.

[9] In greater detail, when the constant melting temperature resin insulating layer 5 disposed between the second heating wire 4 and the metal shield 6 is melted and damaged and, thus, the second heating wire 4 and the metal shield 6 are short-circuited with each other, a possibility of causing a fire or an electric shock increases due to the abrupt increase in resistance. Accordingly, a safety device, which has a simple construction and operates rapidly and which monitors an insulation state between the second heating wire 4 and the metal shield 6, and prevents current from flowing in a high resistance state by quickly interrupting the supply of power in an emergency, is required.

Disclosure of Invention Technical Problem

[10] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a safety device for a magnetic field-free heating cable for bedding, which has a simple circuit structure and rapidly interrupts the flow of overheating current generated due to short-circuiting between a second heating wire and a metal shield.

[11] Another object of the present invention is to provide a safety device, which, in a heating cable for bedding, which is thin and is required to be flexible, detects the insulating performance and any abnormality in an insulating layer wound around the second heating wire such that the increase of the thickness of the heating cable is minimized by using a surrounding metal shield that is grounded to block an electrical field, thus allowing the insulating layer to be implemented to have a minimal thickness so as not to be excessively thick.

[12] A further object of the present invention is to provide a safety device, which allows the thickness of the insulating layer to be reduced, thus enabling the amount of raw materials to be reduced and minimizing manufacturing costs. Technical Solution

[13] In order to accomplish the above object s, the present invention provides a safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a first synthetic-resin insulating layer wound around the first heating wire, a second heating wire wound around the first synthetic-resin insulating layer, a second synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the second synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a heating operation using a single-phase two-wire AC power source connected between the first heating wire and the second heating wire, the safety device including a short circuit detection unit connected in series with the metal shield and connected to a ground side wire, which is one of two wires of the single-phase two-wire AC power source. [14] In addition, the present invention provides a safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, an NTC thermistor layer wound around the first heating wire, a second heating wire wound around the NTC thermistor layer, a constant melting temperature resin or synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the constant melting temperature resin or synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a temperature detection operation during a half cycle of a single-phase two-wire AC power source connected between the first and second heating wires and performing a heating oper ation during a remaining half cycle, the safety device including a short circuit detection unit connected in series with the metal shield and connected to a ground side wire, which is one of two wires of the single-phase two- wire AC power source.

[15] The short circuit detection unit includes at least one short circuit detection resistor, which is connected between a ground and the metal shield, a temperature fuse, which is connected between a ground and the metal shield, or a photocoupler, which is connected in parallel with the short circuit detection resistor connected between a ground and the metal shield.

[16] Furthermore, the short circuit detection unit includes at least one short circuit detection and short circuit excessive current blocking condenser, which is connected between a ground and the metal shield, or a photocoupler, which is connected in parallel with the short circuit detection and short circuit excessive current blocking condenser.

[17] Furthermore, the short circuit detection unit includes a solenoid coil, which is connected between a ground and the metal shield, a magnetic proximity switch, which is disposed at a location close to the solenoid coil, or a magnetic resistance device, which is disposed at a location close to the solenoid coil.

[18] In addition, the present invention provides a safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a first synthetic-resin insulating layer wound around the first heating wire, a second heating wire wound around the first synthetic-resin insulating layer, a second synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the second synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a heating operation using an AC power source connected between the first heating wire and the second heating wire, the AC power source being a single-phase two- wire AC power source or a single-phase three- wire AC power source, the safety device including an AC power input selection switch connected to the heating cable; and a short circuit detection unit connected to a side of a ground wire, which is one of two single phase wires, when the AC power source is the single-phase two-wire AC power source, and connected to a side of a hot wire, which is one of three single phase wires, when the AC power source is the single-phase three- wire AC power source, according to operation of the AC power input selection switch.

[19] In addition, the present invention provides a safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, an NTC thermistor wound layer around the first heating wire, a second heating wire wound around the NTC thermistor layer, a constant melting temperature resin or synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the constant melting temperature resin or synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a temperature detection operation during a half cycle of a single-phase two- wire AC power source connected between the first and second heating wires and performing a heating operation during a remaining half cycle, the AC power source being a single- phase two-wire AC power source or a single-phase three-wire AC power source, the safety device including an AC power input selection switch connected to the heating cable; and a short circuit detection unit connected to a side of a ground wire, which is one of two single phase wires, when the AC power source is the single-phase two-wire AC power source, and connected to a side of a hot wire, which is one of three single phase wires, when the AC power source is the single-phase three- wire AC power source, according to operation of the AC power input selection switch.

Advantageous Effects

[20] In a heating cable that can automatically detect and control temperature, when insulating resin disposed between the second heating wire and the metal shield is damaged or melted, the present invention automatically cuts the ground wire out of the circuit, or interrupts the flow of current, so that it can improve the safety of the heating cable and the temperature controller, which are used for electrical heat bedding.

[21] Furthermore, the present invention guarantees the safety of the device even though the insulating resin wound around the second heating wire is constructed to be thin, so that the overall thickness of the heating cable is small, and the amount of raw materials can be reduced. Brief Description of the Drawings

[22] The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[23] FIG. 1 is a diagram showing the construction of a typical magnetic field-free heating cable; [24] FlG. 2 is a diagram showing an electrical connection between a safety device, a temperature controller and a heating cable, according to an embodiment of the present invention;

[25] FlG. 3 is a diagram showing an electrical connection between a safety device, a temperature controller and a heating cable according to another embodiment of the present invention;

[26] FlG. 4 is a circuit diagram showing examples of short circuit detection units implemented in various ways, according to the present invention;

[27] FlG. 5 is a block diagram showing a single-phase three- wire power source supply circuit and a single-phase two-wire power source supply circuit;

[28] FlG. 6 is a circuit diagram showing an electrical connection between a safety apparatus, a temperature controller and a heating cable according to another embodiment of the present invention; and

[29] FlG. 7 is a circuit diagram showing an electrical connection between a safety apparatus, a temperature controller and a heating cable according to another embodiment of the present invention. Best Mode for Carrying Out the Invention

[30] The constructions of the present invention are described in detail with reference to embodiments of the present invention, shown in the accompanying drawings below.

[31] FlG. 2 is a diagram showing an electrical connection between a safety device, a temperature controller and a heating cable. Referring to FlG. 2, a short circuit detection unit 30 is connected to a line for grounding a metal shield 6. Current controlled by a power control unit 20 flows through a first heating wire 2, and flows through a second heating wire 4 after the direction thereof is reversed at the temperature detection unit 40. In this case, an electrical field, which leaks out due to electrical charges present between the intervals of the second heating wire 4, is radiated to the outside via a constant melting temperature resin insulating layer 5, and is then blocked by an external wound metal shield 6. The blocked charges are discharged to ground along the metal shield 6.

[32] The constant melting temperature resin insulating layer 5 wound around the second heating wire 4 is made of synthetic insulating resin, such as Polyvinyl Chloride (PVC), nylon, Teflon, silicon, PolyPropylene (PP) or PolyEthylene (PE), and the term 'constant melting temperature resin' refers to resin for which a melting point and a physical property variation point are set so that melting occurs at a constant temperature.

[33] Accordingly, the physical property of the constant melting temperature resin insulating layer 5 varies at a constant temperature. When the second heating wire 4 enters an overheated state and, thus, the second heating wire 4 and the metal shield 6 are short-circuited to each other due to the melting and damage of the constant melting temperature resin insulating layer 5, a voltage of 20 to 40 V, caused by current flowing through the metal shield 6, is applied across the two ends of the metal shield 6, and thus increase in resistance occurs. A short circuit detection unit 30 is formed at this location. When overvoltage is detected, the short circuit detection unit 30 functions to interrupt current so that the current does not flow. Accordingly, when the internal heating wire is overheated, the short circuit detection unit 30 functions to interrupt current so that overheating no longer occurs due to the melting of the constant melting temperature resin insulating layer 5.

[34] FlG. 3 is a diagram showing an electrical connection between a safety device, a temperature controller and a heating cable according to another embodiment of the present invention. Referring to FlG. 3, the short circuit detection unit of the present invention may be used even for a heating cable in which a thermistor is not used, only a resin insulating layer 3a is disposed between the first heating wire 2 and the second heating wire 4, and respective ends of the first heating wire 2 and the second heating wire 4 are directly connected to each other, like the construction of claim 1. In this case, temperature detection is not performed, but the temperature of the heating cable may be generally measured using a separately provided temperature.

[35] FlG. 4 is circuit diagrams showing examples of short circuit detection units implemented in various ways, according to the present invention. FlG. 4 shows embodiments for performing a voltage and short circuit detection operation using various types of construction.

[36] FlG. 4 (a) shows a scheme in which the short circuit detection unit 30 between two ends A-B is implemented using a short circuit detection resistor 10. When current flows through the short circuit detection resistor 10, the short circuit detection resistor 10 is cut out of the circuit due to overheating. That is, when high power is applied to a low power resistor, the resistor is cut out of the circuit. This scheme causes only the ground line of the metal shield 6 to be cut out of the circuit, so that excessive current caused by the metal shield 6 is interrupted but the power source itself is not cut out of the circuit.

[37] FlG. 4 (b) shows a scheme in which a temperature fuse 11 is used. In this scheme, the temperature fuse 11 is cut by a heating phenomenon depending on current flowing through the short circuit detection resistor 10. The temperature fuse 11 is an element that is connected to cause current from the power source to flow or to be interrupted, and the current from the power source is completely interrupted when the temperature fuse 11 is cut.

[38] FlG. 4 (c) shows a scheme in which a photocoupler 13 is used. When a pho- tocoupler 13, such as a Silicon Controlled Rectifier (SCR), a TRIAC, a transistor, a CDS or a LOGIG, is used, a signal is transmitted between the input light-emitting side and output light-receiving side of the photocoupler 13 via light. Accordingly, the scheme facilitates the control of the circuit because the input light-emitting side and the output light-receiving side are insulated from each other. FlG. 4 (d) shows a scheme in which the light-emitting diode is connected in parallel with an additional current-limiting positive voltage diode 14, and thus the breakage of the photocoupler 13 due to the application of overvoltage is prevented.

[39] FlG. 4 (e) shows a scheme in which a short circuit detection and short circuit excessive current blocking condenser 15 is used. Although the metal shield is short- circuited when a short circuit detection and short circuit excessive current blocking condenser having a small capacity is used, short-circuit current does not flow because the short circuit detection and short circuit excessive current blocking condenser 15 has a high electrical resistance value, so that excessive current can be prevented. Particularly, in a half wave current heating scheme, direct current is obtained in a half wave, and the direct current does not pass through the short circuit detection or through the short circuit current interruption condenser 15. This scheme also causes the ground line of the metal shield 6 to be cut out of the circuit, so that excessive current caused by the metal shield 6 is interrupted but the power source itself is not cut out of the circuit. The reason why the schemes shown in FlG. 4 (a) and (e) are used is because a desired result can be achieved even though only a single element is used. Since FlG. 4 (f) and (g) are schemes in which photocouplers 13 are respectively used, the respective operations of which are the same as those of FlG. 4 (c) and (d), detailed descriptions thereof are omitted.

[40] In the case where the photocoupler 13 and the short circuit detection resistor 10 or the photocoupler 13 and the short circuit detection and short circuit excessive current blocking condenser 15 are connected in parallel with each other, the breakage of the photocoupler 13 due to the application of overvoltage can be prevented.

[41] FlG. 4 (h), (i) and Q) are schemes in which solenoid coils 16 are respectively used.

A magnetic proximity switch 17, as shown in FlG. 4 (i), is provided along with the solenoid coil 16, or a magnetic resistance device 18, as shown in FlG. 4 Q), is provided along with the solenoid coil 16. The magnetic proximity switch 17 is a device configured such that a switch is opened and closed by a magnetic field induced by the solenoid coil 16.

[42] The magnetic resistance device 18 is a semiconductor device having three terminals, like a transistor. Accordingly, when voltage is applied to the magnetic resistance device 18, the value of current, which flows through the magnetic resistance device 18, varies according to a magnetic signal and a corresponding voltage is output, so that the gate of SCR/TRIAC is driven by the signal voltage, therefore the fuse can be cut.

[43] The magnetic resistance device 18 described above is insulated from a detection unit in the same manner as the photocoupler, so that it is optionally applied to the circuit, therefore heating power can be controlled by adding an auxiliary circuit, or the fuse can be cut. The construction insulated as described above has an advantage in that it can be coupled to any circuit part, but an uninsulated construction has a limitation in constructing the circuit.

[44] FlG. 5 is a block diagram showing a single-phase three- wire power source supply circuit and a single-phase two- wire power source supply circuit, and FIGS. 6 and 7 are circuit diagrams. Referring to FlG. 5, a power source for transmitting power to homes is generally classified as a single-phase three- wire 220 V power source, shown in FlG. 5 (a), or a single-phase two- wire 220 V power source, shown in FlG. 5 (b). A power of 220 V from both terminals of a single-phase three- wire power source unit, as shown in FlG. 5 (a), is supplied to the heating cable through a temperature controller. In this case, the neutral wire of the power source unit is grounded, power controlled by the temperature controller is applied to the heating cable through the temperature controller, and ground is achieved via a virtual ground between the temperature controller and an electric mat. In the case of the single-phase two- wire power source, one of two power supply lines is grounded, and a power of 220 V from the two terminals of the power source unit is supplied to an temperature controller, as shown in FIG. 5 (b).

[45] Referring to FIGS. 6 and 7, the present invention allows a single-phase two- wire power source or a three-phase two- wire power source to be selected according to the operation of a selection switch SW. That is, the single-phase two- wire power source is selected when the selection switch SW is connected to a selection terminal E, and the three-phase two- wire power source is selected when the selection switch SW is connected to a selection terminal F.

[46] In the case of the single-phase three- wire power source, a virtual ground is formed using condensers 21 and 22. The metal shield 6 is connected to the virtual ground, and thus the electrical field is blocked. In this case, when the voltage of the condenser 21, which functions as a short circuit detection unit, is increased from 110 V to 220 V, or when the voltage of the condenser 22 connected between the second heating wire 4 and the metal shield 6 is decreased from 110 V to 0 V, a short circuit can be detected.

[47] In the case where a phenomenon, such as the cutting of a wire, a short circuit, or overheating, occurs, short circuit operation short circuit current I is applied to a fuse connected with a TRIAC 60a, and thus the fuse is cut out of the circuit. In this case, the short circuit detection unit includes a resistor 10 or a magnetic resistance device 18 for causing signal variation using a solenoid 16, and is connected with the TRIAC 60. Reference numeral 62 is a protection resistor.

[48] The condenser 21 is an internal condenser having a capacitance between the second heating wire 4 and the metal shield 6, and the condenser 22 has the same capacitance as the condenser 21. When the capacitances of the two condensers are the same, a voltage of 220 V is divided into two voltages, each of which is 110 V, and thus the voltage difference between a ground point and a virtual ground point becomes 0 V. Since the voltage difference obtained through the division of the capacitance is 0 V, and the voltage difference depending on phase angles is also 0 V because the AC phase angles are the same, the potential between the ground point and the virtual ground point voltage is a 0 V in terms of potential voltage and phase. If the voltage were changed to 110 V using a resistor, instead of the condenser 21, the potential between the ground point and the virtual ground point could not be 0 V, due to the AC phase difference. The AC phase difference due to the resistor is 90 degrees, so that the potential of the metal shield becomes 0 V due to the virtual ground, therefore an electrical field is blocked.

[49] The operation of the short circuit detection unit constructed as described above is described below. When the second heating wire is overheated or the second heating wire and the metal shield are short-circuited to each other due to the defect of the constant melting temperature resin insulating layer, the short circuit detection unit, located between the metal shield and the ground wire, detects variation in voltage. In this case, the ground wire of the metal shield is cut out of the circuit due to applied resistance, or the heating wire itself is cut out of the circuit, so that a fire due to overheating or an electric shock on a human body can be prevented.

Industrial Applicability

[50] In a heating cable that can automatically detect and control temperature, when insulating resin disposed between the second heating wire and the metal shield is damaged or melted, the present invention automatically cuts the ground wire out of the circuit, or interrupts the flow of current, so that it can improve the safety of the heating cable and the temperature controller, which are used for electrical heat bedding.

[51] Furthermore, the present invention guarantees the safety of the device even though the insulating resin wound around the second heating wire is constructed to be thin, so that the overall thickness of the heating cable is small, and the amount of raw materials can be reduced.

[52] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] A safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a first synthetic-resin insulating layer wound around the first heating wire, a second heating wire wound around the first synthetic-resin insulating layer, a second synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the second synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a heating operation using a single- phase two-wire Alternating Current (AC) power source connected between the first heating wire and the second heating wire, the safety device comprising: a short circuit detection unit connected in series with the metal shield and connected to a ground side wire, which is one of two wires of the single-phase two-wire AC power source.

[2] A safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a Negative Temperature Coefficient (NTC) thermistor layer wound around the first heating wire, a second heating wire wound around the NTC thermistor layer, a constant melting temperature resin or synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the constant melting temperature resin or synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a temperature detection operation during a half cycle of a single-phase two- wire AC power source connected between the first and second heating wires and performing a heating operation during a remaining half cycle, the safety device comprising: a short circuit detection unit connected in series with the metal shield and connected to a ground side wire, which is one of two wires of the single-phase two-wire AC power source.

[3] The safety device to claim 1 or 2, wherein the short circuit detection unit comprises at least one short circuit detection resistor that is connected between a ground and the metal shield.

[4] The safety device to claim 1 or 2, wherein the short circuit detection unit comprises a temperature fuse that is connected between a ground and the metal shield.

[5] The safety device to claim 1 or 2, wherein the short circuit detection unit comprises a photocoupler that is connected in parallel with the short circuit detection resistor connected between a ground and the metal shield.

[6] The safety device to claim 1 or 2, wherein the short circuit detection unit comprises at least one short circuit detection and short circuit excessive current blocking condenser that is connected between a ground and the metal shield.

[7] The safety device to claim 6, wherein the short circuit detection unit comprises a photocoupler that is connected in parallel with the short circuit detection and short circuit excessive current blocking condenser.

[8] The safety device to claim 1 or 2, wherein the short circuit detection unit comprises a solenoid coil that is connected between a ground and the metal shield.

[9] The safety device to claim 8, wherein the short circuit detection unit comprises a magnetic proximity switch that is disposed at a location close to the solenoid coil.

[10] The safety device to claim 8, wherein the short circuit detection unit comprises a magnetic resistance device that is disposed at a location close to the solenoid coil.

[11] A safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, a first synthetic-resin insulating layer wound around the first heating wire, a second heating wire wound around the first synthetic-resin insulating layer, a second synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the second synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a heating operation using an AC power source connected between the first heating wire and the second heating wire, the AC power source being a single-phase two-wire AC power source or a single-phase three- wire AC power source, the safety device comprising: an AC power input selection switch connected to the heating cable; and a short circuit detection unit connected to a side of a ground wire, which is one of two single phase wires, when the AC power source is the single-phase two-wire AC power source, and connected to a side of a hot wire, which is one of three single phase wires, when the AC power source is the single-phase three- wire AC power source, according to operation of the AC power input selection switch.

[12] A safety device for a magnetic field-free heating cable, the heating cable including a first heating wire, an NTC thermistor wound layer around the first heating wire, a second heating wire wound around the NTC thermistor layer, a constant melting temperature resin or synthetic-resin insulating layer wound around the second heating wire, a metal shield wound around the constant melting temperature resin or synthetic-resin insulating layer, and an insulating sheath layer wound around the metal shield, the heating cable performing a temperature detection operation during a half cycle of a single-phase two- wire AC power source connected between the first and second heating wires and performing a heating operation during a remaining half cycle, the AC power source being a single-phase two- wire AC power source or a single-phase three- wire AC power source, the safety device comprising: an AC power input selection switch connected to the heating cable; and a short circuit detection unit connected to a side of a ground wire, which is one of two single phase wires, when the AC power source is the single-phase two-wire AC power source, and connected to a side of a hot wire, which is one of three single phase wires, when the AC power source is the single-phase three- wire AC power source, according to operation of the AC power input selection switch.