EP2415068B1 - Dimmable amalgam lamp and method for operating the amalgam lamp while dimmed - Google Patents

Description

The invention relates to a dimmable amalgam lamp, with a quartz glass tube which encloses a discharge space containing a filling gas and which is closed at its two ends with bruises, is passed through at least one current feedthrough to a respective helical electrode in the discharge space, wherein at least one of the bruises a cavity having an opening to the discharge space for receiving an amalgam supply, which is temperature controlled by means of the helical electrode, and with a dimming device, by means of a nominal lamp current I_nominal to a lower actual lamp current I_ist can be reduced.

Furthermore, the invention relates to a method for operating an amalgam lamp when dimming.

State of the art

Such amalgam lamps are low pressure mercury lamps in which amalgam is used to increase performance. Amalgam lamps are used for technologically demanding applications that require high UV radiation densities and high reliability, such as UV sterilization and oxidation.

In this case, a supply of solid amalgam is introduced into the discharge space in addition to the filling gas. The effect of the amalgam is to control the mercury vapor pressure within the discharge space enclosed by the lamp body.

From the US 2006/267495 A1 Such an amalgam lamp is known. This consists of a quartz glass tube made of quartz glass, which is closed on both sides with bruises through which a current feedthrough is laid in the discharge space to a helical electrode. For introducing an amalgam supply is proposed solid To include amalgam in an auxiliary container open to the discharge space. The additional container is positioned behind one of the electrodes. Since the container is open to the filling gas, the solid amalgam is in thermodynamic equilibrium with the filling gas of the lamp. The additional container protrudes either through the cylinder jacket surface of the quartz glass tube or through one of the bruises into the discharge space. In this case, an additional fixation of the amalgam supply is provided in the additional container by means of a fused hook-shaped holder.

The melting of the additional container for receiving the amalgam supply requires an additional process step and is associated with the risk of loss of the quartz glass tube.

This disadvantage avoids a dimmable amalgam lamp according to the type mentioned, as they are known from WO 2007/091187 A1 is known. It is proposed to provide one of the bruises with a cavity open to the discharge space, into which the amalgam reservoir is introduced. In an alternative embodiment it is provided that the amalgam reservoir is introduced into a spherical container which has an opening at the top and which is held by a metal strip which is embedded in the pinch. The metal strip protrudes into the spherical container at the same time and anchors the amalgam reservoir in it.

In the vicinity of the amalgam supply a heating coil is provided, which has its own circuit and a temperature control. As a result, the amalgam supply can be kept at a certain temperature, thus ensuring the highest possible efficiency of the amalgam lamp.

A particular problem arises when dimming the amalgam lamp. In amalgam lamps, where the amalgam reservoir is located on the inside wall of the discharge space, the amalgam has an optimum temperature at the nominal lamp power, thereby ensuring an optimal mercury vapor pressure. When dimming, however, the heat flow from the discharge area between the electrodes to the amalgam reservoir decreases so that it becomes colder and the mercury vapor pressure and the efficiency of the amalgam lamp decrease.
According to the WO 2007/091187 A1 the heating current is a function of the degree of dimming of the lamp.

However, the dimming level of the lamp does not correspond to the lamp current I ist. Because the lamp current I_ist is also affected by external influences, such as the outside temperature.

The US 2004/195954 A1 describes a sterilization lamp in the form of a low-pressure mercury vapor discharge lamp, in which a container is attached to an amalgam deposit on the outer wall of the lamp bulb and is fluidly connected to the interior of the lamp envelope. In another embodiment, the amalgam deposit is held in a sleeve by means of a hook. This is passed through a frontal compression of the lamp bulb and positioned in the area outside the discharge arc behind an electrode. It is open to the interior of the lamp bulb.

At the time of the WO 2007/091187 A1 known amalgam lamp is counteracted the cooling of the amalgam supply by the temperature in the region of the amalgam and the Dimming level can be determined and a corresponding adjustment of the temperature of the separate heater is carried out.

However, the separate heater and the temperature measuring and control device require a considerable amount of additional design effort.

Technical task

The invention is therefore based on the object to provide a structurally simple amalgam lamp, which maintains a high efficiency of UV-C emission even when operating at lower power (dimming).

In addition, the invention has for its object to provide a procedure for operating the amalgam lamp at dimming, which ensures a high efficiency of UV-C radiation.

With regard to the amalgam lamp, this object is achieved on the basis of an amalgam lamp with the features of the type mentioned in the introduction that the current feedthrough to helical electrode comprises a forward line and a return line for a heating current I_add, wherein a control device is provided by means of the heating current I_add in Depending on the height of the actual lamp current I_ist is adjustable, and wherein the control device is designed to set the heating current "I_add" in dependence on the height of the actual lamp current in accordance with the following design rule: $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{i\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

The tempering of the amalgam reservoir serves to generate a mercury vapor pressure in the discharge space, which is independent of the current performance of the amalgam lamp and ensures optimal efficiency for the UV radiation. There is a range of optimal temperature of the amalgam, which is independent of the nominal power of the amalgam lamp.

For tempering the amalgam supply, the helix electrode adjacent to the amalgam supply is used. This thus serves both to generate an arc and to maintain a predetermined temperature of the amalgam supply.

In operation, the arc attacks the surface of the electrode so that it is heated by the arc. This heating depends on the power of the arc and is transferred to the amalgam supply by thermal radiation. In comparison, the contribution of the arc to heating the amalgam supply is low.

In the case of the amalgam lamp according to the invention, a tempering of the amalgam supply enclosed in the pinch is thus provided over the heated electrode, without the need for an expensive additional heating device or the like.

When dimming the amalgam lamp, however, the lamp current decreases from the nominal value I_nominal (100% power) to a lower value and accordingly reduces the heat flow from the coil to the amalgam reservoir, which thereby does not reach the predetermined temperature. As a result, the vapor pressure of mercury within the discharge space and thus also the UV-C emission sink to a value below the optimum.

To compensate for this heat loss, an additional current is sent through the helical electrode according to the invention. The additional current heats the helical electrode, which is close to the amalgam reservoir, beyond the temperature that would otherwise set with dimmed lamp power.

The amount of the additional current depends on the difference between the nominal power and the requested power when dimming.

The control device is used to adjust the additional current so that even in dimmed operation an optimal temperature of the amalgam supply is maintained and so a high efficiency of UV-C emission can be achieved. If the sum of the additional current and the actual lamp current in dimmed operation is greater than 2 times the nominal lamp current, the amalgam will overheat. If the sum of the additional current and the actual lamp current in dimmed operation is less than 0.5 times the nominal lamp current, the amalgam supply, on the other hand, becomes too cold. In both cases, the efficiency of UV-C emission decreases. It has been shown that a complex temperature control for the temperature of the electrode or the amalgam supply can be dispensed with under these boundary conditions.

Ideally, the additional current is adjusted by means of the control device in response to the dimmed actual lamp current so that the sum of additional current and actual lamp current corresponds exactly to the nominal lamp current. The deviations from this ideal value are in the range of +/- 10% (relative to the nominal lamp current). Therefore an embodiment of the amalgam lamp is proposed, in which the control device is designed to set the additional current "I_add" in accordance with the following design rule: $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{i\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

An additional current in this range ensures maximum UV-C emission efficiency both when operating with nominal lamp power and when operating with a dimmed amalgam lamp.

wobei I_nominal (in A) der nominale Lampenstrom der Amalgamlampe und die Konstante k = 0,25 x 10^-3 (in m²/A) ist.In a particularly preferred embodiment of the amalgam lamp according to the invention it is provided that the Amalgamvorrat of the helical electrode has a distance "L" (in m), which is set in dependence on the nominal lamp current on the basis of the following equation: $$\sqrt{2\mathrm{kI\_nominal}}\ge \mathrm{L}\ge \sqrt{1/2\mathrm{kI\_nominal}.}$$

where I_nominal (in A) is the nominal lamp current of the amalgam lamp and the constant k = 0.25 x 10 ^-3 (in m ² / A).

During the temperature control of the amalgam supply enclosed in the pinch, the distance between the amalgam supply and the helical electrode plays an essential role. The larger the nominal lamp current, the higher the temperature of the electrode during operation of the amalgam lamp, and the greater must be the distance between the electrode and amalgam reservoir to adjust the temperature in the amalgam reservoir to the desired level and overheat the amalgam prevent. Overheating can lead to a deviation of the optimal mercury vapor pressure and concomitantly to a reduction in the efficiency of the emitted UV-C radiation.

wobei I_nominal (in A) der nominale Lampenstrom und die Konstante
k = 0,25 x 10^-3 (in m²/A) ist.According to the invention, it is therefore provided that the amalgam reservoir has a spacing from the helical electrode which is set as a function of the nominal lamp current using the following equation: $$\sqrt{2\mathrm{kI\_nominal}}\ge \mathrm{L}\ge \sqrt{1/2\mathrm{kI\_nominal}.}$$

where I_nominal (in A) is the nominal lamp current and the constant
k = 0.25 x 10 ^-3 (in m ² / A).

The distance between the amalgam reservoir and the helical electrode is understood to be the distance between the longitudinal axis of the radiator coil and the amalgam reservoir namely, the longitudinal axis position of the arc facing outside of the coil and the longitudinal axis position of the electrode facing the outside of the amalgam supply, as shown in FIG. 1 is shown schematically. The distance is determined by the length of the helical electrode power supply lines and the diameter of the helix. With the same coil diameters, only the length of the power supply lines to the electrode is decisive, which are also referred to below as "legs".

At a smaller distance, it may happen that the amalgam reservoir is overheated; and at a distance above said range there is a risk that the amalgam supply remains too cold. In both cases, the efficiency of the UV emission decreases.

In a particularly preferred embodiment of the amalgam lamp according to the invention, provision is made for the distance "L" (in m) between amalgam reservoir and helical electrode to be set using the equation: $$\mathrm{L}=\sqrt{\mathrm{kI\_nominal}}\pm \mathrm{0}.\mathrm{2}\sqrt{\mathrm{kI\_nominal}}$$

According to the invention, the pinch is provided with a cavity within which the amalgam reservoir is received. The cavity is formed in the simplest case in the production of the pinch using a special mold. In this cavity, the amalgam reservoir is reliably fixed, so that it can not escape even in tilted positions of the amalgam lamp.

For the fixation of the amalgam reservoir within the cavity, a measure is preferred in which the cavity opening has an opening width which is impassable for the amalgam reservoir.

The amalgam reservoir is present as a solid solid and has a shape and size that prevents leakage from the cavity opening in the discharge space as a solid. The placement of the amalgam reservoir in the cavity here requires the introduction of amalgam in a flowable state and subsequent solidification to the solid amalgam, which fills the cavity completely or partially. A suitable measure for this will be explained in more detail below with reference to an embodiment.

Alternatively or additionally, it has also proven to be advantageous if in the cavity a holding element protrudes, which is anchored to the amalgam reservoir.

The retaining element contributes to the fixation or additional anchoring of the amalgam reservoir within the cavity and is preferably at least partially embedded in the respective pinch.

In this case, a first embodiment of the amalgam lamp has proven to be in which the holding element consists of quartz glass and is guided from the outside through the pinch in the cavity.

The holding element in this case has an elongated cylindrical part which extends through the pinch and allows handling and alignment of the holding element before making the pinch. The holding element further has a part projecting into the cavity, which can be provided with a hook and which serves to anchor the amalgam supply. The holding element is made of quartz glass, so that differences between the thermal expansion coefficients of the holding element and the material of the pinch, which is also the quartz glass, are avoided.

In an alternative and equally preferred embodiment of the amalgam lamp according to the invention it is provided that the holding element consists of metal and is connected to a power supply line of the electrode.

The metallic holding element is in this case welded to a supply line for the power supply of the electrode. This results in a predetermined position of the holding element with respect to the cavity to be produced. Therefore, it must be ensured in the production of the pinch that the free end of the retaining element comes to rest in the cavity to be produced. On the other hand, it is unnecessary to introduce and align a holding element in an additional process step.

With regard to the method for operating the amalgam lamp according to the invention in dimming, the above-mentioned object is achieved in that the additional current I_add is set as a function of the height of the actual lamp current I_Ist in accordance with the following design rule. $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{i\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

By this mode of operation can be ensured that even when dimming the amalgam lamp, the amalgam reservoir is maintained at a temperature which ensures high efficiency of UV-C radiation. Because when dimming the nominal lamp current I_nominal (corresponds to 100% of the lamp power) is reduced to a lower value, so that the helical electrode assumes a lower temperature, and thus also the amalgam supply cools, which is tempered according to the invention of this electrode.

According to the invention, an additional current is passed through the helical electrode, which depends on the difference between the nominal power of the amalgam lamp and the requested power during dimming.

By adjusting the additional flow within the range mentioned in equation (1), an optimal temperature of the amalgam reservoir is maintained and thus a high efficiency of the UV-C emission. With an additional flow below the lower limit, there is a risk that the amalgam will be too cold, so that the mercury vapor pressure and as a result of which the UV-C emission decreases; on the other hand, with an additional flow above the stated upper limit, there is a risk that the mercury vapor pressure will become too high, which likewise leads to a reduction in the UV-C emission. It has been shown that even with dimming operation of the lamp can be dispensed with a complex temperature setting for the amalgam supply.

In this case, the additional current I_add is ideally set so that the sum of additional current and actual lamp current corresponds exactly to the nominal lamp current. Slight deviations from this ideal case are readily acceptable, for example, deviations in the range of +/- 10% of the nominal lamp current. Accordingly, a method for operating the amalgam lamp is proposed in which the additional current is set in accordance with the following design rule: $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{i\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

Operating the amalgam lamp in this range will allow for optimal UV-C emission efficiency, both at nominal lamp power and dimmed amalgam lamp operation.

embodiment

Figur 1

ein Detail einer Ausführungsform der erfindungsgemäßen Amalgamlampe in einer Vorderansicht,

Figur 2

das Detail gemäß Figur 1 in einer Seitenansicht bei einem Schnitt entlang der Linie A-A,

Figur 3

eine Seitenansicht einer weiteren Ausführungsform einer erfindungsgemäßen Amalgamlampe im Detail,

Figur 4

eine weitere Ausführungsform der erfindungsgemäßen Amalgamlampe im Detail in einer Vorderansicht, und

Figur 5

eine Ausführungsform der erfindungsgemäßen Amalgamlampe mit einem Schaltbild, das einen Teil der Stromversorgung zeigt.

The invention will be explained in more detail with reference to embodiments and drawings. It shows in a schematic representation in detail:

FIG. 1

a detail of an embodiment of the amalgam lamp according to the invention in a front view,

FIG. 2

the detail according to FIG. 1 in a side view in a section along the line AA,

FIG. 3

a side view of another embodiment of an amalgam lamp according to the invention in detail,

FIG. 4

a further embodiment of the amalgam lamp according to the invention in detail in a front view, and

FIG. 5

an embodiment of the amalgam lamp according to the invention with a circuit diagram showing a part of the power supply.

FIG. 1 shows schematically one of the two ends of an amalgam lamp 20, which is characterized by a nominal power of 800 W (at a nominal lamp current of 8 A), a radiator length of 150 cm and thus by a power density of slightly less than 5 W / cm. It consists of a quartz glass tube 1, which is closed at its ends with bruises 2, are embedded in the molybdenum foils 3 and the ends of metallic terminals 4 to a helical electrode 5. For this purpose, the electrode 5 has "legs" 15 which are connected to the molybdenum foil 3.

Between the electrode 5 and a second electrode (opposite in FIG FIG. 1 not shown), an arc 13 is generated in operation, the foot 14 ends on the surface of the electrode 5. The upper edge of the electrode, on which the base 14 of the arc 13 attacks, is marked with a dashed line 12.

The pinch 2 at the illustrated end is provided with a cavity 9 which serves as a receptacle for an amalgam reservoir 6. The cavity 9 has an opening 7 to the discharge space 8. The opening width of the opening 7 is significantly narrower than the maximum clear width of the cavity 9 and also narrower than the maximum diameter of the amalgam reservoir 6, so that the amalgam is trapped in the cavity 9 and in solid form not in the discharge space. 8 can get. In the exemplary embodiment, the maximum opening width of the opening 7 is 2 mm.

wobei k = 0,25 x 10^-3 (in m²/A) ist. Der Abstand L beträgt bei der Amalgamlampe 20 im Ausführungsbeispiel etwa 4,5 cm. Gemäß der Erfindung wird dieser Abstand in Abhängigkeit vom nominalen Lampenstrom eingestellt, was in der Praxis durch Anpassung der Länge der Beinchen 15 erfolgt. Der Abstand L wird gemessen zwischen der Oberkante 12 der Elektrodenwendel und der Oberkante 16 des Amalgamvorrats, wie vom Blockpfeil "L" angedeutet.As a result, the amalgam reservoir 6 is fixed in the vicinity of the electrode 5. The electrode 5 is heated by the arc 13 to a temperature that depends on the current performance of the amalgam lamp 20 (see also FIG FIG. 5 ) and which affects the amalgam reservoir 6 depending on the distance L. The distance L between the amalgam reservoir 6 and the length position 12 of the foot point 14 is determined by the following equation: $$\mathrm{L}=\sqrt{\mathrm{kI\_nominal}}$$

where k = 0.25 x 10 ^-3 (in m ² / A). The distance L is approximately 4.5 cm in the case of the amalgam lamp 20 in the exemplary embodiment. According to the invention, this distance is adjusted as a function of the nominal lamp current, which is done in practice by adjusting the length of the legs 15. The distance L is measured between the upper edge 12 of the electrode coil and the upper edge 16 of the amalgam supply, as indicated by the block arrow "L".

The amalgam lamp 20 is equipped with a dimming and regulating device (not shown in the figure), which is described below FIG. 5 is explained in more detail.

From the view of FIG. 2 It can be seen that the opening 7 of the cavity 9 is narrowed in this viewing direction, so that the amalgam reservoir can not pass through the opening 7.

In FIG. 3 schematically a supplementary fixation of the amalgam reservoir 6 is provided by means of a quartz glass fiber 10, which extends through the pinch 2 into the amalgam reservoir 6 and which forms after production of the pinch with this a uniform quartz glass mass.

FIG. 4 shows a further embodiment of the amalgam lamp according to the invention, in which the amalgam reservoir 6 is additionally fixed by means of a metallic hook 11 in the cavity 9. The hook 11 is welded to one of the legs 15 for the electrode 5, and its free end extends into the amalgam 6.

The production of the amalgam lamp is explained in more detail on the basis of an exemplary embodiment:

In a quartz glass tube electrodes 5 are introduced and closed the two ends with bruises 2. To produce a pinch seal 2, a molding tool is used which has a recess which generates the pinch seal 2 including the cavity 9.

At the same time the power connection 4, 3; 15 embedded for the electrode 5 vacuum-tight. By means of an opening in the side wall of the quartz glass tube 1 to be subsequently closed again, solid amalgam is placed on the opening 7 of the cavity 9 and subsequently softened. Here, the amalgam flows into the cavity 9 and solidifies to the massive amalgam reservoir 6. It is essential that the amalgam reservoir 6 after solidification has a size that prevents the amalgam mass can pass through the narrowed opening 7 in the discharge chamber 8.

In the following, the dimmable amalgam lamp 20 according to the invention and a method for dimmed operation will be described FIG. 5 explained in more detail. The FIG. 5 shows the ends of the (broken) discharge space 8 of the amalgam lamp 20 according to FIG. 1 , with which in the discharge space 8 opposite, helical electrodes 5a, 5b, whose electrical connections are guided by the bruises 2. Both bruises 2 are provided with a cavity 9, but only one cavity 9 is filled with an amalgam reservoir 6, which is adjacent to the coil (electrode) 5a.

The power supply of the amalgam lamp 20 includes a first circuit "A", which serves to heat the electrode 5a, and a second circuit "B", which serves to apply the lamp voltage of nominally 100 volts. The circuits "A" and "B" are part of a control device 21.

When dimming the amalgam lamp 20, the nominal current I_nominal (= 8 A) in circuit "B" is reduced. This reduces the temperature of the electrode 5 and thus also the temperature of the amalgam reservoir 6, so that the mercury concentration in the discharge space 8 decreases, thereby reducing the efficiency of the UV-C radiation. In order to compensate for this effect, an additional heating current I_add is conducted through the electrode 5a via the circuit "A", which leads to an increase in the temperature of the electrode 5a. This increase in temperature brings about an additional heating of the amalgam reservoir 6 arranged in the vicinity of the electrode 5a. The temperature increase of the electrode 5 is noticeable above all because an approximately 10 times higher thermal power density prevails in the vicinity of the base point 14 than the arc 13 (FIG. FIG. 1 ) between the electrodes 5a, 5b.

If the nominal current is reduced by 20% (I_act = 0.8 x I_nominal), the heating current I_add is increased such that: I_act + I_add = I_nominal (= 100%). The controller 21 is set to compensate for each decrease in the nominal current to 100% of the nominal current by a corresponding increase in the heating current.

This additional current ensures a maximum possible UV-C emission in dimmed operation.

Claims (9)

1. Dimmable amalgam lamp, comprising a quartz glass tube (1) that envelopes a discharge space (8) containing a filling gas, and is closed on its two ends by means of pinchings (2) through which at least one electrical current bushing (4) is guided to one coil-shaped electrode (5) each into the discharge space (8), whereby at least one of the pinchings (2) comprises a hollow space (9) that comprises an opening (7) towards the discharge space (8) and is designed to accommodate an amalgamate stock (6) and can be heated by means of the coil-shaped electrode (5), and comprising a dimming facility by means of which a nominal lamp current I_nominal can be reduced to a lower actual lamp current I_ist, whereby the electrical current bushing (4) to the coil-shaped electrode (5) comprises a supply line and a return line for a heating current I_add, and whereby an adjusting facility (21) is contained therein by means of which the heating current I_add can be adjusted as a function of the level of the actual lamp current I_ist, and whereby the adjusting facility is designed appropriately such as to adjust the heating current "I_add" as a function of the level of the actual lamp current in accordance with the following dimensioning rule: $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{I\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

   
2. Amalgam lamp according to any one of the preceding claims, characterised in that the amalgam stock is situated at a distance L (in m) from the coil-shaped electrode, whereby the distance L is adjusted as a function of the nominal lamp current in accordance with the following equation: $$\mathrm{\surd }\mathrm{2}\mathrm{kI\_nominal}\ge \mathrm{L}\ge \mathrm{\surd }\mathrm{1}/\mathrm{2}\mathrm{kI\_nominal},$$

   

   
   whereby I_nominal (in A) is the nominal lamp current of the amalgam lamp and the constant k = 0.25 x 10^-3 (in m²/A).
3. Amalgam lamp according to claim 2, characterised in that the distance L (in m) between amalgam stock and coil-shaped electrode is adjusted in accordance with the following equation: $$\mathrm{L}=\mathrm{\surd kI\_nominal}\pm \mathrm{0.2}\mathrm{\surd kI\_nominal}$$

   
4. Amalgam lamp according to any one of the preceding claims, characterised in that the opening width of the hollow space opening (7) is such that the amalgam stock (6) cannot pass through it.
5. Amalgam lamp according to any one of the preceding claims, characterised in that a holding element (10; 11) that is anchored to the amalgam stock (6) projects into the hollow space (9).
6. Amalgam lamp according to claim 5, characterised in that a part of the holding element (10; 11) is embedded in the pinching (2).
7. Amalgam lamp according to either one of the claims 5 or 6, characterised in that the holding element (10) consists of quartz glass and is guided from outside through the pinching (2) into the hollow space (9).
8. Amalgam lamp according to either one of the claims 5 or 6, characterised in that the holding element (11) consists of metal and is connected to an electrical current supply line (4) for the coil-shaped electrode (5).
9. Method for operating an amalgam lamp according to any one of the claims 1 to 8 upon dimming, characterised in that the additional current I_add is adjusted as a function of the level of the actual lamp current I_ist in accordance with the following dimensioning rule $$\mathrm{I\_nominal}-0.1\mathrm{I\_nominal}\le \mathrm{I\_add}+\mathrm{I\_ist}\le \mathrm{I\_nominal}+0.1\mathrm{I\_nominal}$$

   

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