GB1603031A - Field effect devices for protection against over voltages - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO FIELD EFFECT DEVICES FOR PROTECTION AGAINST OVER VOLTAGES (71) We, SOCIETA ITALIANA TELECOMUNICAZIONI SIEMENS S.p.A., of Piazzale Zavattari 12, 20149 Milan, Italy, an Italian Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to devices for protection against over voltages. Such devices may be used, for instance, in radar systems, telecommunication lines, telesignalling lines, and circuits connected to them, and may comprise two electrodes arranged in a vacuum-sealed envelope devoid of radioactive elements and containing a gas.

Overvoltages occur along lines and across apparatus connected thereto owing to atmospheric conditions (lightning, static charges, etc.), accidental contact with, or induction due to, power transmission lines, or to other causes. Generally speaking, the most dangerous overvoltages occur along a telecommunication line and reach the terminals of the line which are generally formed by exchange apparatus, repeaters, user's sets, etc. These overvoltages result in an electric discharge taking place, which causes damage to or malfunction of the line terminal.

To avoid such a drawback, gas-discharge protection devices are generally provided at the input of line terminals and connected to the line wires and to the earth. Such protection devices are designed to produce an electric discharge inside them when a voltage exceeding a predetermined threshold is present across the said wires.

These protection devices thus prevent the said overvoltages from reaching the line terminals, or at least ensure that overvoltages are attenuated to such a level that they are no longer dangerous for persons or for the apparatus. The above-mentioned deviced generally comprise at least two electrodes located in a vacuum-sealed envelope containing a low-pressure gas.

In the protection devices of known type, use is generally made of emission radioactive isotope designed to generate ioinic pairs in the gas contained in the envelope, such ion pairs facilitating discharge priming when overvoltages occur.

These protection devices have the disadvantage that, should the envelope break, the radioactive isotope escapes and thus contaminates the environment. This could constitute a source of danger for persons working near the protected apparatus.

Other solutions, also known, include the use of radioactive material to form at least part of the envelope (or to make radioactive at least part of the envelope). Protection devices in accordance with these solutions, although being in solid state, may become a source of risk for persons.

According to the invention there is provided a field-effect device for protecting against overvoltages, comprising at least one section disposed in an envelope containing a gas, the or each section comprising two primary electrodes and a pair of secondary electrodes arranged to generate an electric field between the secondary electrodes stronger than that existing between the primary electrodes, the secondary electrodes being provided in the same envelope in parallel via at least one capacitor with the primary electrodes, the secondary electrodes extending parallel to each other.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a known type of protection device and its respective circuit diagram for the purpose of explaining the invention; Figure 2 shows a Paschen curve for better illustrating the operation of the protection device of Figure 1; and Figure 3 shows a circuit diagrams of a preferred embodiment of the invention.

Figure 1 shows a known type protection device comprising an envelope or vessel 1 through which pass two terminals 2 insulated from each other. The said terminals 2 are connected to a pair of primary electrodes 3 which are connected m parallel with a pair of secondary electrodes 4.

The secondary electrodes 4 are arranged between the said primary electrodes 3, thereby priming the ionization process in the gas present between the primary electrodes 3.

The operation of the above described protection device will be described with reference to the Paschen curve shown in Figure 2.

The Paschen curve is characteristic of the type of gas used and of the material of which the electrodes are made. Such a curve is obtained by indicating, against a double logarithmical scale, in the abscissa the product of the distance d between two electrodes contained in a gaseous environment at a pressure p by the pressure itself, and in the ordinate the corresponding priming or breakdown voltage V across the two electrodes.

It will be noted that the priming or breakdown voltage V gradually decreases as the product p.d. decreases until it reaches a minimum value at the point A.

A further decrease in the product p.d.

results in the priming or breakdown voltage V increasing again.

Since the said electrodes 3 and 4 are contained in the same envelope or vessel 1, the pressure p is the same for both of them, and thus it is possible to vary the priming voltage V by modifying only the distance d between each pair of electrodes.

Once the pressure p0 inside the envelope is known, the distance d2 between the primary electrodes 3 is chosen so that the said electrodes operate in the conditions indicated at A in the characteristic curve.

For the secondary electrodes, the working point is B which is determined by the product po.dl and the voltage V'.

The operating point A is characterized in that it has a negligible field-effect and a low priming voltage V. The operating point B is instead characterised in that it has a high field-effect and a priming voltage greater than the voltage at the point A.

Should an overvoltage with slow increase occur, the protection device operates under the conditions existing at the point A, and thus priming occurs between the said primary electrodes 3.

In the case of a rapidly increasing overvoltage, the primary electrodes, in the absence of ion pairs would prime with a delay such that part of the overvoltage impulse would pass through, which could result in damage to or malfunction of the line terminal downstream.

The usefulness of the said secondary electrodes 4 resides in that, when an overvoltage impulse has a value V' such that V'/d1 exceeds the emission threshold of the electrode material, electron emission occurs. In this condition, owing to acceleration due to the electric field, a sufficient number of ion pairs are generated to immediately prime a discharge between the secondary electrodes, or to facilitate a discharge between the primary electrodes. The secondary electrodes thus perform the function that in the conventional protection devices is performed by a radioactive isotope.

In order for electron emission from the secondary electrodes to occur at lower voltages V, the distance between the electrodes being the same, the electrodes may be coated with, or made of, an electropositive material or a high-emission material (e.g. caesium or rubidium).

The same results i.e. an abundant emission of electrons, may be obtained by increasing the strength of the electric field, by giving the secondary electrodes 4 a suitable configuration (such as pointed configuration), and/or a suitable surface treatment (e.g. shot-blasting).

In the above described protection device, a discharge due to an overvoltage impulse prevailingly occurs between the secondary electrodes 4. When the impulse energy involved is rather large, particularly when the secondary electrodes are covered with, or made of, low-melting material, the secondary electrodes could be destroyed or damaged. To avoid this drawback, according to the invention, the secondary electrodes 4 are connected to the primary electrodes 3 by way of a capacitative junction diagrammatically illustrated in the form of capacitors 5 in Figure 3. Without departing from the scope of the invention, it is possible to limit the capacitative connection to one secondary electrode 4 only.

With reference to the diagram of Figure 3, the capacitors 5 limit the energy in the electric circuit in parallel with the primary electrodes 3.

In order not to negatively affect the discharge priming mechanism described above between the primary electrodes 3, it is necessary to effect a capacitative junction in such a way that the capacitance of the capacitors 5 is much higher than the capacitance between the secondary electrodes 4.

The voltage across the terminals 2 is distributed in a way inversely proportional to the capacitances of the members 4 and 5.

By way of example, if the said capacitors 5 have a hundred times the capacitance of the capacitor 4, the voltage across the secondary electrode 4 is about 98%, that applied across the terminals 2.

Protection devices have been described above, which comprise a pair of electrodes.

Without departing from the scope of the invention, it is possible to provide protection devices comprising a plurality of pairs of electrodes (e.g. multiple self-controlled discharges) by adopting the above described constructional and/or technological measures for each line electrode-earth electrode pair.

WHAT WE CLAIM IS: 1. A field-effect device for protecting against overvoltages, comprising at least one section disposed in an envelope containing a gas, the or each section comprising two primary electrodes and a pair of secondary electrodes arranged to generate an electric field between the secondary electrodes stronger than that existing between the primary electrodes, the secondary electrodes being provided in the same envelope in parallel via at least one capacitor with the primary electrodes, the secondary electrodes extending parallel to each other.

2. A device as claimed in claim 1, in which the secondary electrodes are separated from each other by a distance smaller than the distance between the primary electrodes.

3. A device as claimed in claim 1, in which the primary and secondary electrodes are coated with an electropositive material.

4. A device as claimed in claim 1, in which the primary and secondary electrodes are made of or coated with caesium or rubidium.

5. A device as claimed in claim 1, further comprising a plurality of line electrode-earth electrode pairs, a respective pair of secondary electrodes being connected via a respective at least one capacitor in parallel with each of the said pairs.

6. A device substantially as hereinbefore described, with reference to and as illustrated in Figure 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. By way of example, if the said capacitors 5 have a hundred times the capacitance of the capacitor 4, the voltage across the secondary electrode 4 is about 98%, that applied across the terminals 2. Protection devices have been described above, which comprise a pair of electrodes. Without departing from the scope of the invention, it is possible to provide protection devices comprising a plurality of pairs of electrodes (e.g. multiple self-controlled discharges) by adopting the above described constructional and/or technological measures for each line electrode-earth electrode pair. WHAT WE CLAIM IS:

1. A field-effect device for protecting against overvoltages, comprising at least one section disposed in an envelope containing a gas, the or each section comprising two primary electrodes and a pair of secondary electrodes arranged to generate an electric field between the secondary electrodes stronger than that existing between the primary electrodes, the secondary electrodes being provided in the same envelope in parallel via at least one capacitor with the primary electrodes, the secondary electrodes extending parallel to each other.

2. A device as claimed in claim 1, in which the secondary electrodes are separated from each other by a distance smaller than the distance between the primary electrodes.

3. A device as claimed in claim 1, in which the primary and secondary electrodes are coated with an electropositive material.

4. A device as claimed in claim 1, in which the primary and secondary electrodes are made of or coated with caesium or rubidium.

5. A device as claimed in claim 1, further comprising a plurality of line electrode-earth electrode pairs, a respective pair of secondary electrodes being connected via a respective at least one capacitor in parallel with each of the said pairs.

6. A device substantially as hereinbefore described, with reference to and as illustrated in Figure 3 of the accompanying drawings.