Assembly for the discharge of electric overvoltages

As a known "back-up device" for a gas-filled over-voltage arrester, a voltage-dependent resistor (6) is arranged centrally in a housing with the over-voltage arrester (1). The varistor (6) can, in this connection, be sealed with a moisture-repelling substance and it is arranged concentrically to a connecting pin (5) of the over-voltage arrester. By means of an auxiliary electrode (8) of annular disk shape placed on the varistor (6), an air gap (13) from the connecting pin (5) can be formed. The varistor, after its response, can be short-circuited via this air gap.

BACKGROUND OF THE INVENTION 
The present invention falls within the field of electric components and is 
intended for use in the construction of a gas-filled over-voltage arrester 
which is combined, in a metal housing, with a parallel-connected 
voltage-limiting device, so as to form a special assembly. Such assemblies 
are used, in particular, wherever it is necessary to provide additional 
assurance against the possible failure of a gas filled over-voltage 
arrester. 
In one known assembly of this type, the parallel-connected voltage-limiting 
device is developed as an air discharge gap. For example, for this purpose 
a thin insulating foil provided with holes is placed on one electrode of 
the over-voltage arrester which has a cylindrical connecting pin, the 
foil, in its turn, bearing an auxiliary electrode in the form of an 
annular disk. The air discharge gap thus formed and the over-voltage 
arrester are combined by means of a cylindrical metal housing to form an 
assembly. The housing electrically connects the auxiliary electrode to the 
corresponding electrode of the over-voltage arrester (U.S. Pat. No. 
4,736,269). In this case, the axial clamping of the air discharge gap and 
the over-voltage arrester is effected by a flanged edge of the housing, 
which at various places on its circumference is pressed down onto the 
auxiliary electrode. In this known embodiment, the centering of the 
over-voltage arrester in the metal housing is effected by a tubular 
ceramic insulator of the over-voltage arrester, the outside diameter of 
which is greater than the outside diameter of the two electrodes of the 
over-voltage arrestor. 
In another, similar embodiment of such an assembly, the electrodes and the 
insulator of the over-voltage arrester can have the same outside diameter. 
In that case, the centering of the over-voltage arrester in the housing, 
the inside diameter of which is greater than the outside diameter of the 
over-voltage arrester, is effected by indentations distributed uniformly 
on the periphery of the metal envelope of the container, the indentations 
being present in the region of the ceramic insulator (U.S. Pat. No. 
5,142,434). 
With these known assemblies, there is the danger that due to moisture the 
breakdown voltage of the parallel connected air discharge gap will be 
reduced to below the breakdown voltage of the over-voltage arrester and 
that thereupon the entire element will fail by a single response of the 
air discharge gap. 
It has therefore already been proposed to encapsulate this 
parallel-connected air discharge gap. An arrangement for this is known in 
which the air discharge gap consists of a flat, stepped ceramic cylinder 
bearing metal layers applied to it by vapor deposition. This discharge gap 
is placed on the one electrode of the over-voltage arrester and soldered 
to it. Furthermore, a metal cap is placed over the end of the over-voltage 
arrester provided with the applied discharge gap, the cap being soldered 
on one side to a metal coating of the discharge gap and sealed on the 
other side with respect to the cylindrical insulator of the over-voltage 
arrester by means of a silver-containing epoxide resin (U.S. Pat. No. 
4,707,762). Such an encapsulation of the air discharge gap is relatively 
expensive to produce. 
SUMMARY OF THE INVENTION 
A gas filled over-voltage arrester may have a tubular ceramic insulator 
with two electrodes arranged at the ends of the insulator, the second of 
the electrodes having an axially extending cylindrical connecting pin. The 
present invention provides an assembly consisting of over-voltage 
arrester, voltage limiting device, and metal housing so that the voltage 
limiting device is protected against the influence of moisture and the 
construction of the entire element is simple from a manufacturing 
standpoint. 
It is proposed, in accordance with the present invention, that the voltage 
limiting device consist of a metal-oxide varistor in the form of a hollow 
cylinder, having end sides that are metallized; that the ceramic insulator 
and the electrodes of the over-voltage arrester as well as the 
hollow-cylindrical varistor have the same or approximately the same 
outside diameter; and that the inside diameter of the metal envelope of 
the housing be greater than the outside diameter of the over-voltage 
arrester and of the varistor, at least one spacer being provided for the 
centering of the over-voltage arrester and the varistor. 
The parallel electric connection of an over-voltage arrester and a voltage 
limiting device of the form of a metal-oxide varistor, is, in itself, 
known (German AS 23 55 421, claim 1, as well as column 2, lines 46 to 65). 
In this known parallel circuit, the unavoidable delay in breakdown of the 
gas-filled over-voltage arrester is eliminated by the use of a varistor 
having a response time that is only slightly above that of the 
over-voltage arrester where the response voltage of the varistor is, in 
general, the voltage at which the varistor conducts a current of 1 mA. 
Zinc oxide in particular is used as varistor material (U.S. Pat. No. 
3,905,006). 
In an assembly developed in accordance with the present invention, a 
negative influence of moisture on the response voltage of the voltage 
limiting device is substantially excluded by the use of a varistor. If the 
assembly is particularly endangered by moisture during operation, the 
varistor can be additionally sealed by a moisture-repelling substance in 
the nonmetallized region of its surface. A hardenable silicone oil or a 
silicone resin could be used as the moisture repelling substance, the 
sealing being effected by impregnation or immersion followed by heat 
treatment. However, the varistor can also, possibly in addition, be 
provided with an insulating coating on its inner and outer surfaces in the 
form, for instance, of a glaze. It is also possible to seal the entire 
assembly at the open end of the metal housing, for instance by means of a 
cast-resin sealing compound. 
Since no special shaping with closely defined dimensions is necessary for 
the manufacture of the varistor and no special measures of adjustment are 
required for the arranging thereof, the individual parts of the assembly 
can be easily handled and are thus accessible to automated production. 
The centering of the arrester and the varistor in the metal can be 
effected, for instance, by several spacers, in the form of indentations, 
provided on the metal housing and distributed uniformly on its 
circumference, in the region of the outer surface of the ceramic insulator 
as well as in the region of the outer surface of the varistor. For 
example, three indentations each can be provided, and can be developed 
either as points or in the form of axially extending jags. In such a case, 
it is necessary to provide the outer surface of the varistor with an 
insulating coating, for instance a layer of glass or glaze. 
Another technique for centering employs a tubular insulating part as a 
spacer, the part being arranged between the metal housing on the one hand 
and the over-voltage arrester and varistor on the other hand. Such an 
insulating part may consist of a tapelike winding and, in particular, of a 
shrinkdown plastic tubing which surrounds the over-voltage arrester and 
the varistor. Such an insulating part furthermore provides assurance that 
no electric discharge can take place between the second electrode of the 
over-voltage arrester and the metal housing. 
From a manufacturing standpoint there is particularly suitable a centering 
in which the metal housing is provided with indentations only for the 
centering of the over-voltage arrester and in which an annular spacer of 
insulating material, for instance a rubber ring or a piece of tubing, is 
provided for centering the varistor. 
Alternatively, to center the varistor, a spacer may be used which consists 
of a cuplike auxiliary electrode which rests on the second electrode and 
the wall of the auxiliary electrode consists of several fingerlike tabs, 
the free ends of these fingerlike tabs resting resiliently against the 
inner wall of the metal housing with the interpositioning of an insulating 
layer consisting of a fusible plastic. Such a spacer also assures 
so-called fail-safe behavior of the assembly (see U.S. Pat. No. 4,912,592, 
part 8, and U.S. Pat. No. 4,984,125, part 14). In this connection, the 
insulating layer can be applied to the ends of the fingerlike tabs or to 
the inner wall of the metal can, or an insulating foil can be arranged 
between the metal can and the cap. 
The arrangement with respect to each other of over-voltage arrester and 
varistor on the one hand and metal housing on the other hand can be 
facilitated, particular from a manufacturing standpoint, if an auxiliary 
electrode in the form of an annular disk, the outside diameter of which 
corresponds to the inside diameter of the metal envelope of the housing, 
is placed on the second electrode of the varistor. In this case, upon the 
manufacture of the component, the edge at the open end of the metal 
housing is pressed against the auxiliary electrodes at various places 
distributed along the periphery, with the auxiliary electrode forming a 
good abutment. In this connection, the auxiliary electrode itself can be 
developed as a cup spring, or be additionally provided with a cup spring, 
to assure dependable axial clamping of the parts. 
The use of an auxiliary electrode in the shape of an annular disk can 
furthermore serve to create an additional discharge path in the region of 
the component. This is present when the inside diameter of the auxiliary 
electrode of annular-disk shape is 0.2 to 2 mm larger than the outside 
diameter of the cylindrical connection pin of the second electrode of the 
over-voltage arrester. By this dimensioning, a discharge gap by which the 
varistor can be short circuited is formed between the auxiliary electrode 
and the cylindrical connection pin. It enters into action when the 
over-voltage arrester has failed and the varistor has responded and if, 
due to the current load and the heating of the varistor resulting 
therefrom, the annular gap between the auxiliary electrode and the 
connecting pin is ionized by gases emerging from the varistor. The arc 
then produced leads either to a short circuit between auxiliary electrode 
and metal pin or to the melting of a solder pellet arranged below or above 
the component, such as known, for instance, from Fig. 1 of U.S. Pat. No. 
4,502,087. 
The dimensioning of the varistor, i.e., the determination of height and 
wall thickness, as well as the selection of the material (in particular 
having a base of zinc oxide), can be such that the response voltage of the 
varistor lies in a defined manner, for example, at least 40% above the 
response voltage of the over-voltage arrester. With a response voltage of 
the over-voltage arrester of about 350 volts, the varistor can therefore 
be so dimensioned that it conducts, with an applied voltage of 
100 volts a current of .ltoreq.0.5 .mu.A, 
200 volts a current of .ltoreq.5 .mu.A, 
650 volts a current of about 1 MA, 
750 volts a current of about 1 A and 
1000 volts a current of about 30 A.

DETAILED DESCRIPTION 
The assembly of FIG. 1 consists of an over-voltage arrester 1, a 
voltage-dependent resistor 6, and a metal container or housing 9 with an 
auxiliary electrode 8. The over-voltage arrester 1 has in this connection 
a first electrode 2, a ceramic insulator 4, and a second electrode 3. The 
second electrode 3 has a cylindrical axially arranged connection pin 5. 
The voltage-dependent resistor 6 in the form of a zinc-oxide varistor has a 
hollow cylindrical shape, the two end surfaces being metallized--in a 
manner not shown in detail--and both the inner and the outer surfaces are 
provided with a glaze 7. This glaze has a layer thickness of about 0.1 to 
0.25 mm. The height of the hollow cylinder is, for instance, 3.1 mm with 
an outside diameter of about 7.3 mm and a wall thickness of about 2.1 mm. 
Furthermore, the varistor is sealed with a moisture repelling substance. 
The varistor 6 is placed on the second electrode 2 of the over-voltage 
arrester and arranged concentric to the connecting pin 5. Again, an 
auxiliary electrode 8 in the shape of an annular disk is placed on the 
varistor, the inside diameter of its ring being somewhat smaller than the 
inside diameter of the varistor 6 and its outside diameter being greater 
than the outside diameter of the varistor 6. Varistor 6 and over-voltage 
arrester 1 have approximately the same diameter. 
Over-voltage arrester 1, varistor 6, and auxiliary electrode 8 are arranged 
within a cylindrical metal housing 9. The over-voltage arrester 1 lies on 
the bottom of the housing. A plurality of indentations 11 are distributed 
on the periphery on the edge 10 of the housing 9 for fixing the 
over-voltage arrester, the varistor and the auxiliary electrode in place 
within the metal housing. Furthermore, uniformly distributed indentations 
12 on the periphery of the metal container serve to center the 
over-voltage arrester and the varistor within the housing and are provided 
both in the region of the ceramic insulator 4 and in the region of the 
varistor 6. 
The inside diameter of the auxiliary electrode 8 which has the shape of an 
annular disk is furthermore so dimensioned that an air gap 13 is formed 
between the auxiliary electrode and the connecting pin 5, the width of the 
air gap being between 0.1 and 1 mm. 
In the embodiment shown in FIG. 2, a tubular insulating part in the form of 
shrinkdown tubing 14 is provided for centering of the over-voltage 
arrester 1 and the varistor 6 within the metal housing 91. The tubing 
surrounds the over-voltage arrester and the varistor. The shrinkdown 
tubing eliminates the need for centering indentations in the metal housing 
91 such as provided in the embodiment of FIG. 1. Furthermore, the 
shrinkdown tubing 14 prevents possible short-circuiting between the 
electrode 3 and the metal container 91 in the event of the failure of the 
over-voltage arrester. 
In another embodiment illustrated in FIG. 3, the embodiment of FIG. 1 is 
modified to include a cup spring 17 which is placed on the auxiliary 
electrode 8. The indentations 11 in the metal housing 92 thus surround the 
cup spring 17 by means of which the arrester 1, the varistor 6, and the 
auxiliary electrode 8 are clamped axially to each other. Furthermore, only 
the over-voltage arrester 1 is centered by means of indentations 12. A 
piece of tubing 15, the outside diameter of which in attached condition is 
somewhat smaller than the inside diameter of the metal housing 92, is 
placed on the varistor 6 to center it. A rubber ring 16 can also be used 
instead of the piece of tubing 15. 
In another embodiment shown in FIG. 4, the over-voltage arrester 1 is 
centered in the metal housing 92 by means of several indentations 12, as 
in the embodiment shown in FIG. 3. The end sides of the varistor 6 are 
provided with a metallization 61 and the rest of the surface is sealed in 
moisture-tight manner by a silicone wax. A cup-shaped electrode 20 is 
provided to center the varistor. The bottom 21 of varistor 6 is seated on 
the electrode 3. The wall of the varistor 6 consists of fingerlike tabs 
22, similar to those tabs disclosed in U.S. Pat. No. 4,132,915, Fig. 4, 
part 32. The ends 23 of the fingerlike tabs lie resiliently against the 
inner wall of the metal housing 92, the ends 23 being individually 
surrounded by an insulating layer 24 consisting of a fusible plastic.