FI84868C - pipe resistance - Google Patents

Description

1 84868

This invention relates to a tubular resistor comprising a metallic tubular sheath, a resistor wire helically substantially interposed therein, an insulating material filling a space between the resistance wire coil and the pipe sheath, power supply pins connected to the ends of the resistance wire coil, and sheaths Power supply pins extending inside the 10 stone sheaths have been introduced.

In conventional pipe resistors as described above, a small leakage current is always generated through the insulation of the resistor to the grounded metal pipe sheath. This leakage current increases as the temperature of the tube resistor rises, being normally well below 1 mA in the resistors used in the 15 electric heaters. But even the wrong rock size and stacking (small stones tightly stacked) can double the leakage current by two to three times. This is due to an increase in the temperature of the resistor as the cooling deteriorates. However, the use of the heater is not affected by even a larger leakage current, because the heater is earthed, ie there is no voltage difference between the outer surface of the heater and ground.

However, leakage flows are increasingly being monitored. There are devices that detect leakage current, i.e. the so-called RCDs that are tuned to operate if a leakage current exceeds a certain limit. This limit depends on the object to be protected, but is usually less than 30 mA in households.

It has been found that pipe resistors such as those described above, when present in large groups, as is often the case with electric heaters, can cause RCDs to trip. In practice, it has been found that in the first minutes of the heater heating, the leakage current initially increases quite sharply and then decreases to a value of less than 1 mA per 35 resistors. The older the resistance, the higher the peak value of the leakage current 2 84868 at the beginning of the heating. With a one-year unused resistor, this peak leakage current can exceed up to 20 mA. The leakage current changes of such an older resistor as a function of time when the current is switched on are illustrated in the curve of Figure 3 of the accompanying drawings.

The reason for the occurrence of said leakage current peak is the moisture inside the resistance tube. Namely, the magnesium oxide used as the insulating material filling the space between the resistance wire coil and the put-stone sheath is hygroscopic, so that when the resistor is cold, it absorbs moisture from the air. This migration of moisture into the resistor cannot be prevented either by hermetically sealing the ends of the resistor tube, because, for example, the surface powers present in electric heater resistors are so high that such a hermetic seal is not possible due to the pressure inside the tube jacket.

Figure 1 of the accompanying drawings schematically shows the structure of a conventional tube resistor corresponding to the description given in the introduction above. The resistor thus comprises a metallic tubular sheath 2, a substantially centrally arranged resistance wire spiral 1, a space between the resistance wire spiral 1 and the pipe sheath 2 filling the magnesium oxide insulating material 3, power supply pins 4 connected 4 have been imported. Power supply pins 4, which are more conventionally made of steel, are necessary so that the temperature of the power supply conductor 30 to be resisted at its connection point, which is provided with a connector 6 in the structure of Fig. 1, does not become too high. The temperature of the resistance wire coil itself is above 1000 ° C, and in order to reduce this temperature to a value suitable for the supply conductors, the length 35 of the steel pin 4 must usually be at least 5 cm. Thus, in practice, the steel pin 3 84868 pi 4 becomes, at least immediately after the current is connected to the resistor, substantially colder than the resistance wire coil 1. Thus, it is conceivable that the moisture in the magnesium oxide 3 which evaporates when the resistance wire coil 1 suddenly heats up and protruding in the direction of the end of the pipe jacket 2 partially condenses at the outer end of the pipe jacket 2 of this steel pin 4, causing an increase in leakage current. In the case of the resistance wire spiral 1 itself, however, there is little leakage current, because its temperature 10 rises very rapidly so high that the water evaporates into an electrically non-conductive gas. Even after a while, the steel pin heats up and the water vapor finally pushes out from inside the pipe jacket 2, leading to a significant decrease in the leakage current. Such a mechanism of action would explain quite well the leakage current curve observed in practice, as shown, for example, in Figure 3 of the accompanying appendix.

According to the invention, it has now been found that the leakage current peak occurring immediately after switching on can be substantially eliminated from the resistors when the power supply pins 20 are insulated with respect to the insulating material filling the space between the resistor spiral 1 and the pipe jacket 2 at least from the pipe jacket ends.

Preferably, the current supply pins can be provided with an insulating coating at least on the inner parts of the pipe jacket closest to the ends of the pipe jacket to insulate the power supply pins with respect to the insulating material at least on the parts closest to the ends of the pipe jacket. Preferably, the insulating coating extends over the entire portions of the tube of the power supply pins.

The tube resistor according to the invention will now be described in more detail with reference to the accompanying drawing, in which Figure 1 shows the end of a prior art tube resistor, Fig. 2 shows the end of a tube resistor 448488 4 shows the behavior of the leakage current as a function of time 5 in the pipe resistor according to the invention.

The prior art pipe resistor according to Figure 1 and its leakage current behavior have already been described above with reference to Figures 1 and 4. Figure 2 shows an exemplary embodiment of a pipe resistor according to the invention and uses the same reference numerals as in Figure 1 for the components of Figure 1. The pipe resistor shown in Fig. 2 differs from the tube resistor in Fig. 1 only in that an insulating coating 7 is formed on the outer surface of the power supply pin 4 along its entire length. This coating can be, for example, enamelling. In Fig. 2 it is shown that the insulating coating extends over the entire length of the power supply pin 4, but as stated above, this is not necessary and the insulating coating could be located only for part of the length of the pin 20, this part being in the area inside the pipe jacket 2 and closest to its end. when the pipe resistor starts to heat up it is initially coldest. In practice, however, it is probably simplest to coat the entire connecting pin 4, with the possible exception of the portion 8 of the smaller diameter shown in Figures 1 and 2, to which the resistance spiral 1 is attached.

Figure 4 shows the leakage current behavior of a resistor according to the invention as a function of time immediately after switching on the current. It can be seen from the curve in Figure 4 that no peak value occurs immediately after switching on, but that the leakage current increases relatively evenly during the first six minutes and then remains at a constant value of 0.32 mA in the curve of Figure 4. Thus, there is a significant improvement over previous 5,84868 silicon tube resistors, where the maximum peak leakage current a few minutes after power up can be as high as 20 mA.

The tube resistor according to the invention has been described above by means of only one exemplary embodiment and it will be appreciated that several modifications may be made to the exemplary embodiment disclosed herein in accordance with the above principles without departing from the scope of the appended claims. The most important thing in the invention is that direct current-carrying contact between the power supply pin 4 and the insulating material 3 is avoided, at least in those areas of the power supply pin 4 which are closest to the ends of the resistor.

Claims (2)

A pipe resistor comprising a metallic, tubular sheath (2), a resistor tree coil (1) located in this substantially centrally located one, an insulating material (3) which fills the space between the resistance tree coil (1) and the tube sheath (2) of the resistance tree coil. (1) ends connected power supply pins (4) and insulating material sleeves (5) arranged to close the ends of the tube sheath (2) through which insulating material sleeves the power supply pins (4) extending past these sleeves (5) into the tube sheath (2). ), characterized in that the power supply pins (5) have an insulating coating (7) at least on the inside of the tube sheath (2) near the ends of the tube sheath (2), to isolate the power supply pins (5). with respect to the insulating material (3) in the foodstuff, the parts which are closest to the ends of the tube sheath (2). Pipe resistor according to claim 1, characterized in that the insulation coating (7) extends to all the parts of the power supply pins (4) which are on the inside of the pipe casing (2).