JPH0351271B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electric heating furnace apparatus equipped with a mechanism for guarding against leakage current.

[Conventional technology and problems]

The leakage current of an electric heating furnace is mainly influenced by the electrical insulation properties of the thermal insulator that constitutes the furnace wall, and generally exhibits a large value compared to other electrical equipment. For this reason, electric heating furnaces require a mechanism to protect the human body from leakage current for safety reasons.

Currently, a leakage circuit breaker is used as this guard mechanism, and the circuit is cut off when an excessive leakage current flows, but in reality, this circuit breaker is not necessarily designed rationally for use. It's hard to say that it is.

For example, during initial drying after manufacturing an electric heating furnace, the furnace wall thermal insulation still contains moisture, which often causes excessive leakage current and trips the earth leakage circuit breaker, which hinders work execution. I often come.

In addition, the insulation properties of this furnace wall thermal insulation material generally tend to decrease as the temperature rises, and the degree of this decreases as the period of use passes, so the leakage current increases as the furnace temperature rises, and the most There is always the inherent, and often actual, fear that the leakage current will exceed the set current value of the earth leakage breaker and the circuit will be interrupted during heating near the required rated operating temperature.

Of course, one way to eliminate these drawbacks is to increase the thickness of the furnace wall thermal insulation, but
This is undesirable because it increases the size and weight of the furnace body. Another method is to set the current value of the earth leakage breaker to a large value in advance, but this weakens the significance of using the earth leakage breaker from a safety standpoint and should be avoided as much as possible.

The present invention has been made in view of the above problems, and even if the set current value of the earth leakage breaker is set to a low value based on normal safety standards, the above problems will not occur. The aim is to provide an electric heating furnace with excellent safety and usability characteristics.

[Means for solving problems]

As a result of intensive research into the above-mentioned problem, the present inventor has created the following mechanism.

That is, an electric heating furnace has an electric heat load and a furnace wall consisting of an inner thermal insulation layer, an outer thermal insulation layer, a conductive layer interposed between the two thermal insulation layers, and a furnace shell that covers the outside of the thermal insulation outer layer, and an electric heat load. The furnace shell is grounded, the electrical wire for the electric heating load is passed through the earth leakage breaker and the zero-phase current transformer, and the conductive layer and The electric heating furnace apparatus is characterized in that the connected electric wire is connected to the electric heating load grounded electric wire through the zero-phase current transformer.

[Examples and effects]

In FIG. 1, the apparatus of the present invention includes an electric heating furnace 1;
It has an earth leakage breaker 2 and a zero-phase current transformer 3. The electric heating furnace 1 has a furnace wall having a thermally insulating inner layer 4, a conductive layer 5, a thermally insulating outer layer 6 and a grounded furnace shell 7, and an electric heating load 8.
and has. The conductive layer 5 may be a box made of a copper plate, and its thickness is preferably about 0.5 to 1 mm. The thermally insulating inner layer 4 and outer layer 6 can be formed of the same material, but it is more effective to form the inner layer 4 with a material that has particularly high heat resistance and to form the outer layer 5 with a material that has particularly high electrical insulation. It is.
From the viewpoint of thermal insulation and electrical insulation, it is desirable that the inner layer 4 is sufficiently thicker than the outer layer 6. For example, if the total thickness of the insulating layer is 250 mm in an electric furnace with a maximum heating temperature of 1100°C, the inner layer 4 is 200 mm,
The outer layer 6 may be 50 mm thick, the inner layer 4 may be made of ceramic material, and the outer layer 6 may be made of asbestos material.

In this embodiment, three three-phase alternating current wires 10, 11, and 12 are connected to the earth leakage breaker 2 from a power source 9, and one of the wires 11 is grounded. The three electric wires that have passed through the earth leakage breaker 2 then pass through a zero-phase current transformer 3 and are connected to an electric heat load 8. Further, an electric wire 13 tangential to the conductive layer 5 passes through the zero-phase current transformer 3 and is connected to a ground electric wire 11 . Furthermore, the output line 14 of the zero-phase current transformer 3 is input to the earth leakage breaker 2. The furnace shell 7 of the electric furnace 1 is grounded.

The zero-phase current transformer 3 can be constructed of an iron core with a secondary winding. Wire 1 inside the iron core
0, 11, and 12 are passed through. When current flows through each wire, magnetic flux is generated in the iron core, and a secondary current flows through the secondary winding. However, since all the wires pass through each wire, the sum of the induced secondary currents is 0 under normal conditions. , no current flows through the output line 14 after all. here,
If there is an earth leakage, a zero-sequence current flows, and a corresponding secondary current is input to the earth leakage breaker 2 via the output line 14. If the input level exceeds the rated sensitivity current value, the earth leakage breaker 2 is shut off.

In this embodiment, since the electric wire 13 connected to the conductive layer 5 passes through the zero-phase current transformer 3 and is grounded, even if a leakage current occurs, it is collected by the conductive layer 5 and the zero-phase current transformer 3 is grounded. No secondary current passes through the current transformer 3 and is generated from the zero-phase current transformer 3. Therefore, the leakage current can be effectively grounded without operating the earth leakage breaker 2. If the leakage current crosses the conductive layer 5 and reaches the furnace shell 7, since the furnace shell is grounded, a zero-sequence current will flow, and when a certain reference level is reached, the earth leakage circuit breaker will cut off the input current. Operate.

〔Effect of the invention〕

In the device of the present invention, a conductive layer is provided within the furnace wall and is grounded through the zero-layer current transformer, so even if there is a leakage current, the surface of the electric furnace Not only can it be effectively grounded without reaching the ground, but the input current will not be cut off by an earth leakage breaker. Further, since the earth leakage breaker is activated only when a leakage current exceeding a certain level reaches the furnace shell and flows to the ground, the set current value of the earth leakage breaker can be set low from the viewpoint of safety. Furthermore, even in such a case, the circuit breaker will not operate frequently and prevent electric heating. Further, by forming the inner layer of the furnace wall of the electric heating furnace thickly with a heat-resistant material and forming the outer layer thinly with an electrically insulating material, heat insulation and leakage prevention can be effectively achieved.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the electric heating furnace apparatus of the present invention. DESCRIPTION OF SYMBOLS 1... Electric heating furnace, 2... Earth leakage breaker, 3... Zero-phase current transformer, 4... Thermal insulation inner layer, 5... Conductive layer, 6
...Thermal insulation outer layer, 7... Furnace shell, 8... Electric heat load,
9...Power supply, 10-14...Electric wire.

Claims (1)

[Scope of Claims] 1. An electric heating device having a furnace wall comprising an inner thermal insulation layer, an outer thermal insulation layer, a conductive layer interposed between the two thermal insulation layers, and a furnace shell covering the outside of the thermal insulation outer layer, and an electric heating load. It consists of a furnace, an earth leakage breaker, and a zero-phase current transformer, the furnace shell is grounded, and the electric wire for the electric heat load is passed through the earth leakage breaker and the zero-phase current transformer, An electric heating furnace apparatus characterized in that an electric wire connected to the conductive layer is connected to the electric heating load grounded electric wire through the zero-phase current transformer. 2. In the device according to claim 1,
An electric heating furnace apparatus characterized in that the thermally insulating inner layer has high heat resistance, and the thermally insulating outer layer has high electrical insulation. 3. In the device according to claim 1 or 2, the electric heating load electric wire is composed of three three-phase alternating current electric wires, one of which is grounded, and the electrically conductive layer An electric heating furnace apparatus characterized in that an electric wire connected to the grounded electric wire is connected to the grounded electric wire through the zero-phase current transformer.