Apparatus for monitoring the vacuum of a vacuum switch

An apparatus for monitoring the vacuum of a vacuum switch having at least one vacuum switch tube with a switch chamber and switch contacts is provided. The apparatus has a remotely interrogatable pressure measurement sensor in the form of a piezocrystal or surface acoustic (SAW) device arranged in the interior of the vacuum switch tube and a remote interrogation device placed external to the vacuum switch for monitoring the vacuum via the sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to vacuum switches and more 
particularly to an apparatus for monitoring the vacuum of a vacuum switch. 
2. Description of the Related Art 
Vacuum switches can perform their function, the interruption of currents, 
especially short-circuit currents, only if a certain minimum vacuum is 
present in the vacuum switch tube. If this is no longer the case due to a 
leak, in the extreme case this can lead during interruption to the 
destruction of the tube, with possible further harmful consequences. 
Previously it has been assumed that no vacuum loss arises in the switch 
tube, even after a long period of time. Due to the harmful consequences 
connected with such a vacuum loss, it is nonetheless desirable to be able 
to check the inner pressure of a vacuum switch built into the switching 
apparatus, without having to disassemble the switch tube from its mount 
for this purpose. From European patents 0 056 722 and 0 150 389, apparatus 
are known for the monitoring of the vacuum of vacuum switch tubes in the 
installed state. These references exploit what is called the Penning 
effect. Electrical and magnetic fields standing perpendicular to one 
another are produced, leading to a cold cathode discharge, whereby an ion 
current is produced whose value is proportional to the inner pressure of 
the switch tube. A disadvantage of these apparatus is that a measurement 
is not possible during operation, i.e. if the contact is open or closed. 
For measurement, in a testing mode the contact must be closed and the 
voltage must be applied between the contacts and the metallic coat. For 
measurement purposes, the switch must thus be completely isolated from the 
connections required during operation. For carrying out the measurement in 
standard vacuum switch tubes, it is also necessary to remove the tube from 
the switching apparatus so that the magnet required for the measurement 
can be attached. 
Furthermore, from European letters patent 0 309 852 a method for the 
verification of a vacuum in vacuum tubes is known in which, given a 
contact travel lower than the rated travel of the vacuum switch, the X-ray 
radiation resulting from the application of high voltage is acquired and 
evaluated as a confirmation of the presence of an operating vacuum. 
However, this method also has a series of disadvantages. Thus, a drive 
mechanism must be used that enables a mechanical intermediate position 
with a small rated travel. Measurement is not possible when the contact is 
closed or fully open. It can thus be measured only in a testing mode. 
Finally, it is necessary to separate the switch from the network, since 
the application of the field emission is not predictable, and thus the 
dielectric strength is not guaranteed during the testing so that arcing is 
possible. Also, in the testing mode the switch is not usable. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus to monitor 
the vacuum of a vacuum switch that avoids the disadvantages named above, 
i.e. so that a measurement is possible both in the open and in the closed 
state during operation. 
The object is solved by an apparatus for monitoring the vacuum of a vacuum 
switch having art least one vacuum switch tube with a switch chamber and 
switch contacts, wherein the apparatus has a remotely interrogatable 
pressure measurement sensor arranged in the interior of the vacuum switch 
tube; and a remote interrogation means for monitoring the vacuum via the 
sensor. 
Through the use of a pressure monitoring in the vacuum switch tube of the 
present invention, the disconnection of a higher-level switch can be 
performed. Harmful consequences can thus be avoided. 
It is particularly advantageous that the inventive apparatus includes a 
surface acoustic wave filter that can be remotely interrogated. The filter 
represents a purely passive component and requires no current supply. 
The following specification provides a further explanation of the invention 
on the basis of the figures.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The vacuum switch tube 1 shown in FIG. 1 includes in its switching chamber 
a fixedly arranged first switch contact 2 and a second switch contact 3, 
arranged movably opposite the first switch contact 2. The actual switching 
chamber is essentially formed from a ceramic tube 4 in which the two 
switch contacts 2 and 3 are concentrically arranged. At the upper end, the 
vacuum switch tube 1 is closed by a flexible metal bellows 5. A remotely 
interrogatable pressure measurement box 6 is arranged on the insulating 
ceramic tube 4. The measurement 6 box can be interrogated by a remote 
interrogation apparatus 7. 
A pressure measurement box shown is in FIG. 2 and includes a base 8, a ring 
9 and, as a cover the substrate of, a surface wave filter 10. This 
pressure measurement box can be square in shape, but is preferably 
circular. Suitable dimensions for the box are 2 mm in height and 10 mm 
diameter of the pressure measurement box. The interior of the pressure 
measurement box can be evacuated or filled with a gas. In each case, the 
pressure measurement box is hermetically sealed. If the interior of the 
pressure measurement box is evacuated, the covers 8 and 10 of the pressure 
measurement box do not experience any bending as long as the pressure 
measurement box is located in the intact vacuum of the vacuum switch tube 
1. If the vacuum is disturbed, the pressure in the vacuum switch tube 1 
thus increases, and the covers 8 and 10 of the pressure measurement box 
are bent inward. 
The effect of this bending on the surface wave filter, and the possibility 
of remote interrogation arising therefrom, is explained on the basis of 
FIG. 3, which shows a surface wave filter of this type. Such a surface 
wave filter includes as an essential component a substrate body 11. In 
most cases, the substrate body 11 is made of a piezoelectric, preferably 
monocrystalline, material. Suitable materials for the substrate body 11 
include quartz, lithium niobate, lithium tantalate, and the like. An 
interdigital converter 12, an antenna 13 connected therewith, and 
reflectors 14 are provided on the surface of the substrate body 11. If a 
high-frequency interrogation impulse 15 is now sent out from the remote 
interrogation apparatus 7. The antenna 13 receives this interrogation 
impulse and supplies it to the interdigital converter 12, which generates 
from the impulse an acoustic wave in the substrate body 11. The acoustic 
wave is again reflected into the interdigital converter 12 by the 
reflector 14, and from there is radiated via the antenna 13 as a response 
signal 16. The reflector 14 can be so coded, in a known way, that a 
correspondingly coded response signal is achieved. This response signal 16 
is in turn received by the remote interrogation apparatus 7. 
The ability of the surface wave apparatus shown in FIG. 3 to check the 
vacuum in a vacuum switch tube results from the property of the substrate 
body 11 to be sensitive to pressure and tension. In particular, the 
spacing of the response pulses, or their phase position, is dependent on 
the pressure-tension state of the crystal. A surface acoustic wave 
apparatus, as shown in FIG. 3, requires no current supply, since it is 
fully passive. 
A surface wave apparatus according to FIG. 3 can also be used as a 
measurement sensor without being arranged in a pressure measurement box 
according to FIG. 2, since the propagation of the acoustic waves is also 
dependent on the surrounding atmosphere. The wave propagation on the 
surface wave apparatus is modified upon penetration of air into the vacuum 
switch tube. 
On the other hand, the surface wave apparatus, somewhat like the pressure 
measurement sensor 6 in FIG. 1, is arranged in such a way that no 
shielding of the electromagnetic waves from and to the remote 
interrogation device 7 occurs. This is ensured since the ceramic tube 4 is 
transparent to electromagnetic waves. 
If the invention is used in a three-phase switching field, all three vacuum 
switches can be interrogated in parallel and at the same time with one 
remote interrogation device 7. Through the respective shielding of two 
tubes during maintenance after the determination of a tube defect, the 
defective vacuum switch tube can then be determined. 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications may be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. It is, therefore, intended that such 
changes and modifications be covered by the appended claims.