CA1152558A - Fail safe surge arrester systems - Google Patents

Abstract

A B S T R A C T

An air gap surge arrestor has two overlapping layers of electrically conductive layers and an inter-mediate insulating layer interposed between the conduc-tive layers. The intermediate layer has predetermined aperture in the area of overlap between the conductive layers to establish an air gap between the conductive layers. All of the layers are flexible to permit a bending thereof to conform to the shape of an electrode of a device to be protected by the surge arrester.

Description

1~5;~558 CROSS-REFERENCE TO RELA~ED APPLICArrION
Thi~ application is related to commonly assign-ed Canadian Application Serial No. 298,463, filed March 8th, 1978, and is a division of commonly assigned Canadian application Serial No. 313,~79, filed October 16, 1978.
sACKGROUND
Gas tube overvoltage protectors are widly used for the protection of equipment from overvoltage conditions which may ~e caused by lightning, hîgh voltage line contact, and the like.
It is also a widely practiced technique to associate various fail-safe arrangements with such tu~es and with other types of protectors, e.g., air gap arresters, to meet various contingencies. For example, the presence of a sustained overload, as where a power line has come in continued contact with a protected telephone llne, produces a concomitant sustained ionization of the gas tube and the resultant passage of heavy currents through the tube. Such currents will in many cases destroy the overvoltage pro-tector and may also constitute a fire hazard.
One common approach to this problem is to employ fusible elements which fuse in the presence of such over-loads and provide either a permanent short circuiting of the arrestor directly, or function to release anot~er mechanism, e.g. a spring loaded shorting bar, which provides the short circuit connection (commonly, the arrester electrodes are both shorted and grounaed). The presence of the permanent short and ground condition serves to flag attention to that condition thus signalling the need for its inspection or replacement. Examples of this type of .

~5;25~3 failsafe protection are found in U.S. Patents 3,254,179;
3,281,625; 3,340,431, 3,396,343; and 3,522,570. Several of these patents also incorporate with the fail-safe feature, a backup air gap arrangement so that there is both failsafe fusible (short~ type protection as well as backup air gap protection.
Still another approach, disclosed in commonly assigned Canadian application serial No. 298,463, is based on the discoveries that an effective fail-s~afe function can be achieved by employing a non-metallic fusible mater-ial and that important advantages are consequently realized.
The fusible material is an electrical insulator which in the exemplary embodiments is interposed between one or more of the electrodes and the shorting mechanism. Surprisingly, the response of the non-metallic material to thermal con-ditions is precise and, moreover, does not leave an insul-ative ~ilm in the course of fusing which might otherwise interfere ~ith the short circuit contact.
The need exists, nonetheless, to develop fail-safe arrangements which provide both surge and failure pro-tection for gas tube arresters.
SUMMARY
The present invention is directed to fail-safe surge arrester assemblies in which both back-up surge and air gap back-up protection is provided with economically pro-ducible systems.
The invention according to the parent application No. 313,479 overcomes the problems of the prior art by pro-viding a combination fail safe and air gap device for use with a gas filled surge arrester, comprising first and second metallic electrically conductive layers, t~e layers

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being in overlapping relationship, and an intermediate insulating layer of non~metallic ~usihle material inter-posed between and in contact with the first and second layers to prevent short circuiting therebetween ex6ept .
in the presence of a sustained overload causing the fusible material to fuse and yield to permit establish-ment.of a short circuit between the first and second layers, the intermediate layer hav;ng at least one pre-determined aperture therein in the area of oYerlap be-tween the first and second layers to define an air gap electrode therebetween.
On the other hand the invention of thi.s d;vi-sional application may be considered as an air gap surge arrester comprising a sealed laminar assembly having first and second layers of electrically conductive metallic material, a port;on of the conductive layers-being in over-lapping relationship; and an intermediate insulating layer interposed between the first and second layers, the in-sulating layer having a predeterm;ned aperture in the area of overlap between the conductive layers, whereby an air gap is established between the first and second layers, each of the layers being flexible to permit a bending thereof to conform to the shape of an electrode of a device to be protected by the arrester.
BRIEF DESCRIPTION OF THE DRAWINGS
- Figure 1 is an elevation view, par-tly in sche-matic, of a gas filled arrester with a first embodlment of th;s invention;
Figures 2 and 3 are cross-sectional views taken along lines 2-2 and 3-3, respectively, in Figure l;

rg/i~ - 3 -5~i8 Figure 4 is an enlarge~ plan yie~ of the air gap device shown in Figure l;
Figure 5 is a cross-sectional view taken along line 5-5 ih Figure 4;
Figure 6 is a plan view, partly in schematic and partly in cross-section, of a gas filled arrester of the second embodiment;

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jj Figure 7 is a cross-sectional view taken along l~ne 7-7 2 ,l in Figure 6 and illustrating an air gap device and clip with

3 ! fusible material on the clip legs; .

4 1! Figure 8 is a cross-sectional view taken along line 8-8 S ¦, in Figure 6;
6 i¦ Figure 9 is a cross-sectional view taken along line 9-9 7 ¦ in Figure 6 and illustrating an air gap device and fusible member 8 Ij in the form of a cylindrical sleeva about the gas tube;
9 ,I Figure 10 is an enlarged plan view of an air gap device ...
ll used in Figure 6;
11 , Figure 11 is a longitudinal cross-sectional view taken 12 , along line 11-11 in Figure 10;
13 ,I Figure 12 is a top plan view of another air gap device 14 ,¦ similar to that illustrated in Figure 10;
ll Figure 13 is a longitudinal cross-sectional view taken 16 11 along line 13-13 in Figure 12;
17 !~ Figure 14 is a bottom plan view of the embodiment in 18 I Figure 12; and , 19 1! Figure 15 is a cross-sectional view taken along line l! 15-15 in Figure 12 and enlarged for clarity of illustration.
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23 ~ While this invention is susceptible of embodiment in 24 many different forms9 there is shown in the drawings and will i hereinafter be described in detail a prsferred embodiment of the 26 ,, invention9 and modifications thereto, with the understanding that 27 I the present disclosure is to be considered as an exemplification ,.
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- 1 ! f the principles o~ the inven~ion and is not intended to limit 2 ,! the invention t~ the elbodiments illustrat¢,d.
3 ¦1 In the embodimPnt ~llustrated in Figures 1-3 snd 6-9, 4 11 a gas tube 20 is provided, the tube includlng a center body 20A
1, and electrode end caps 20B each separated from the center body 6 ! 20A by a respective insulated sleeve section 20C.
7 I The arrester 20, which is of kn~wn construction and may 8 I comprise for example mII Model 31, has its end electrodes (not 9 1i sh~wn) extending inwardly from the end caps 20B toward the center .
'` of the tube interior to define a gap between the electrodea.
11 ' Spacing and dimensions are such ~hat each electrode also forms 12 'l a gap with the center body conductive casing section 20A.
13 I The tube is filled with a gas and the electrode end 14 jl caps 20B are each provided as by welding with a lead 21B and jl terminal 22B, e.g., a spade lug, for connection to the circu~t 16 1l to be protected. Center body 20A is likewise provided with a 17 ~! lead 2LA welded thereto and the associated connection 22A
18 ~ for connection to ground. .
19 !! In the presence of overvoltage conditions the gas ln ¦~ tube 20 ~onizes thereby creating in known manner, conductive 21 , shunting paths bet~een each line of the protected circuit and 22 l ground (via the respective terminal lead 21B and ground lead 23 1~ 21A).
24 ¦ A short circuiting means 25,illustrated as a clip, i5 ¦ disposed between each line electrode 20B and the ground electrode 26 ~l 20A. Clip 25 is illustrative, since it will be understood by 27 l, those skilled in the art that other clip arrangements are readily 28 , adaptable to this function.

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Each clip 25, ~hich is i~lus~ratively of grain oriented 2 i tin plated carbon steel, heat treated for stress relie~ from 3 'I hydrogen embrittlement after plating, includes a first set of 4 ! spring fingers 26 resiliently engaging~ respectively, end cap l,l (line electrode) 20B and another set of spring fingers 27 dis-6 I,j posed about senter body (ground electrode~ 20A. The spring fin~
7 ~11 gers 26 and 27 are integrally connected by the bridge section 28 8 I of each clip. The spring fingers 26, as best illustrated in 9 Figures 3 and 8, are in direct contact with the end caps 20B to ~ provide electrical contact therewith. Conversely, as shown in .
11 greater detai~ in Figures 2, 7 and 9, the fingers 27 o the 12 short-circuit,clips are spaced from contact with center body 20A
13 by reason of fusible elements 30, 31 and 32 described in greater 14 1! detail below. Specifically, each of the fingers 27 includes a I contact portion 27A which is urged in the direction of contact 16 I with grounded center body 20A and which consequently presses 17 , resiliently on the fusible member intexposed therebetween.
18 i Fusible elements 30, 31 and 32 are of non-metallic, !
19 ll electrically insulative composition Suitable materials will I have melt temperatures in the range corresponding to ~hermal con-21 ~ ditions at arrester thermal overload and will have suitable 22 ¦! dielestric strength, dielectric constant, dissipation factor and 23 ¦ vol~me and surfaoe resistivity to provide the requisitive insula-24 i tive function. The preferred material should also be free of j embrittlement or plastic flo~ due to aging and high ambient tem-26 I perature effects, be non-infla~mable under the overload conditionl, -27 j have good mechanical properties and be inert to corrosives and , 1. ~, .
28 weather.

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~ , I Exe=plary of s~ch a class of =ate~ als are cer~sin of ¦
2 1¦ the fluoroplastics 9 such as fluorinated ethylene propylene polyme~
3 I (FEP), the polymer perfluoroalkoxy (PFA~ the modified copolymer 4 , of ethylene and tetrafluoroethylene (ETFE) ~marketed under the DuPont Company trademark Tefzel), and poly (ethylene-chlor~tri-6 il, fluoro-ethylene)(E-CTFE copoly~er~ marketed under the Allied 7 I Chemical Corporation mark Halfar. (The fluoroplastic poly-8 ¦! tetrafluoroethylene (TFE~, on the other hand, does not have suit-9 1l able melt properties for the illustrated application.) In the .
i examples, element 30 is formed of FEP film, and 31 and 32 are 11 i formed of approximately 38" long FEP tubing, sizes AWG 6 snd 2, 12 l; respectivelyO
13 With reference to the embodiment of Figures 1-5, fusiblej 14 l element 30 is generally rectangulsr in shspe and interposed be-ll tween a first layer 35 and a second layer 36 of conductive mate-16 j, rial, e.g., copper. ~ayers 35 and 36 are generally rectangular 17 i~ in shape and in register but smaller in dimensions than lsyer 30 1~ ' so that layer 30 extends beyond the periphery of the conductive 19 ',1 isyers. Insulative layer 30 includes- ~wo rectangular openings ji 30A which together with the thickness of the layer 30 provide a 21 ii pair of air gaps between the first and second layers. Preferably, 22 i~ the air gap is about 3 mils and provides a strike vo7tage in the 23 i range of 500 to 1000 volt3. -24 ll The entire assembly 40 including the first and second ii layers 35 and 36 and insulative layer is a safety device which is 2~ positioned circumferentially about the center body 20A as shown 27 in Figures 1 and 2. Fingers 27 engage copper layer 35 ~nd bias I ¦
28 it against fusible layer 30, layer 36, and center body 20A.

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1 ¦ During normal operation of the: arrester 20, transient - 2 surges produce ion~zation in the normal manner to protect the 3 ; subject equipment. If, however, a sustained surge condition 4 occurs as where a line is permanently contacted by a higher voltag~ I
line, the resultant ionization currents flowing through the ar-6 rester produce excessive heat; the ~usible layer 30, placed in 7 , the arrester region to respond to this heating, thereby fuses.
- 8 As this occurs~ spring fingers ~7, and in particular the contact 9 sections 27A thereof, move layer 35 int~o contact with layer 36 ~ .
and center bsdy 20A as the fusible layer 30 yields and flows.
11 When electrical contact is made a short circuit is established 12 between the respective end cap and the center body thus providing 13 a fail-safe tshort) action.
14 '~ Additionally, the air gap 30A between layer 35 and layer 36 provides back-up protection in the event of gas tube _ -~!
16 , failure. With this additional provision a failure of the gas 17 ~I tube in the open mode, as for example by reason of a gas leak, 18 j does not result in a loss of protection; the air gap provides ~~~
19 i, back-up protection prior to arrester replacement.
jl Figures 6-15 illustrate still further modifica~ions to 21 i the invention. In these embodiments the air gap devices 50 and 22 `j 60, Figures 10 and 12 respectively, are arranged in longitudinal 23 ¦I relationship on the gas tube 20 beneath the clips 25.
24 ~¦ The fusible elements may ta~e alternative forms. In c ~ Figure 7, tubular sleeves 31 are arranged about the fingers 27 of 26 , clips 25 so that a layer of fusible material is interposed betwee~ I
27 contact sections 27A and-center body 2CA. In Figure 9, a tubular ., . _ , , , , ~2SSI~

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1 li sleeve 32 is arranged circumferentially about cen~er body 20A to 2 1I maintain contact section 27A of the clip in spaced relationship 3 Ij thereto. Additionally; sleeve 32 overlaps device 50 or 60 to re . .
4 j~ tain the devlce on and in contact with center body 20A.
l¦ Air gap device 50, Figures 10 and 11, includes a first 6 ¦¦ conductive layer 51 in the form of a rectangular ia~ r of metallic v~' 7 I conductor material, e.g. copper. LRyer 51 is placc in electrical 8 contact with end cap 20B by clip 25~ A second layer 52 o~ conduc-9 tive material is in overlapping relationship with layer 51. ~ayer ¦ 52 is in direct contact with center body 20A. Interposed between ¦
11 ¦ layers 51 and 52 is a non-metallic layer 54 of insulating materiaL¦
12 ¦I Layer 54 may be of the type previously described or a high melting 13 'I point material, such as a pol~imide, an exemplary example is the 14 11 ~olyimide sold under the designation Kapton and may be surface llcoated with adhesive to secure layers 51 and 52. Layer 54 include~si 16 ¦1 an aperture 54A therein to define an air gap between the ~ rlapping 17 Ij portion of layers 51 and 52. Since gas tube 20 has two line 18 , electrodes, a pair of conductive layers 51 and associated air 19 `I gaps 54A are pr~vided. However, it will be understood that the ~__ I! device works equally well when the gas tube has one line electrode 21 ' and one ground electrode. -22 ,¦ In the air gap device 50, the insulating layer 54 ex-23 ii! tends beyond the periphery of both layers 51 and 52. Layers 51 24 ,1 and 52 may be fabricated by known methods, preferably by printed ¦ circuit technique~
26 Ij Figures 12-15 illustrate sn air gap device similar to 27 `, device 50 which has been modified to improve its contact and 28 'conforming characteristic with the gas tube 20 as well as faci-29 litating and improving its fabrication and operation.
More specifically, each first conductive layer 61, e.g.
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¦ copper, includes an end cap and/or clip corltact p~rtion 6L~ o~
2 1 generally rectaDgular shape. ~ortions 6LA are placed in direct 3 ii contact with their associatèd end caps (electrodes) 20B. A neck I ;
4 1 portion 61B connects portion 61A to a generally circular shape 1' portion 61G overlying the air gap formed by aperture 64A describe~ I
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7 ¦ Correspondingly, second conductîve layer 62 includes 8 two circular shaped portions 62A concentric with air gap 64A and g 1 interconnected by rectangular shaped portion 62B.
Insulating layer 64 is interposed between layers 61 and 11 6Z and formed with cut-out portions 64B. These cut-out portions I
12 ~j facilitate in the wrapping and conformance of the air gap device i 13 ~ about the gas tube.
14 1l' Moreover, with particular reference to Figure 15, the 1~ insulating layer 64 includes a layer 65 of plastic material of I ,~
16 ~i the types described above and is faced on each surface with an 17 ¦, adhesive layer 66 which bonds the layer 64 to the associated con-18 ~ ductive layers 61 and 62. Preferably, the edges 66A of the adhe-19 ~ sive layer adjacent the~ hole 64A in the plastic layer 65 is set 1I back a short distance. By way of illustration, with a hole dia- ¦
21 I meter of .05 inch in the plastic layer 65, a set back of .005 22 ~¦ inch provides sufficient clearance. The set back clearance ame-23 1l liorates the possibility of the sdhesive flowi..g into the air gflp 24 ~¦ during assembly. Moreover, the air gap dimension, e.g. 3 mil~, 1l must take into account the ~hickness of the adhesive, for example 26 jl ~hen the adhesive layers 66 are 1 mil thick, a plastic layer 65 27 of 1 mil is used to achieve a 3 mil air gap. The set of the ad-28 hesive also functions to prevent bridging or short circuiting .

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i ; , of e air p which ~ight oc~r as a result of electrical dis-2 charges if the adhesive entered the air gap.
3 The operation of the arrester assembly of Figures 6-lS
4 ~ is similar to that previously described. During normal operation j of the arrester 20, transient surges produce ionization in the 6 j! normal manner tD protect the subject equipment. If a sustained 7 I surge condition occurs, the resultant ionization currents flowing 8 through the arrester produce excessive heat, the sleeves 31 or 9 j 32, placed in the arrester region to respond to this heating, .
thereby fuse. As this occurs, spring fingers 27 move into con-11 tact with center body 20A as the fusible sleeve material beneath 12 those contacts yields and flows. ~n~en electrical contact is made ¦
13 a short circuit is established between the respective end cap and 14 the center body th us providing a fail-safe (short circuiting) action.
lS J Non-metallic materials other than the foregoing may be 16 t used as the fusible members provided they have appropriate elec-17 I trical insulation properties and undergo a predictable change of 18 1! mechanical properties under the specified overload condition to 19 i permit the short circuiting action to occur. -jl Moreover, the air gaps 54A or 64A provide back-up pro-21 li tection in the event of failure of the gas tube.
22 ¦I To facilitate use in a wide variety of applications9 23 1 the arrester assembly of Figures 1 ~ 6 may be potted in a modular 24 ,! shell, the potting ma~erial therein being an epoxy compound.
1l Prior to the potting the arrester assembly may be wrapped and 26 ~ voids filled with PTFE or equivalent material (not shown).
Alternatively, the arrester assem~ly may be used in a station ~' ! . , - .

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protector configuration well known in the art. Obviously, the present invention is useful with gas tube arrester having more or less number of electrodes than the three electrode tube arrester shown. The ability to provide an air gap which is sealed from the environs by the laminar construction described provides a significan-t advance.
These modifications and others may be made by those skilled in the art without departing from the scope and spirit of the present invention as pointed out in the appended claims.

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Claims (29)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An air gap surge arrester comprising a sealed laminar assembly having first and second layers of electrically conductive metallic material, a portion of said conductive layers being in overlapping relationship; and an intermediate insulating layer interposed between said first and second layers, said insulating layer having a predetermined aperture in the area of overlap between said conductive layers, whereby an air gap is established between said first and second layers, each of said layers being flexible to permit a bending thereof to conform to the shape of an electrode of a device to be protected by said arrester

2. An air gap surge arrester of claim 1, wherein said first and second laminar layers are coextensive in dimensions and in register at their peripheries.

3. An air gap surge arrester of claim 2, wherein said insulating layer extends beyond the periphery of said first and second layers.

4. An air gap surge arrester of claim 1, wherein said first and second laminar layers are copper.

5. An air gap surge arrester of claim 1, wherein said intermediate layer is a heat shrinkable plastic material.

6. An air gap surge arrester of claim 1, wherein said insulating layer aperture is circular in cross-section and the portion of said conductive layers overlapping said aperture have a generally circular shape concentric with said aperture.

7. An air gap surge arrester of claim 1, wherein said intermediate layer includes an adhesive on each surface in the area surrounding said aperture, said adhesive being set back a predetermined distance from the edge of the aperture.

8. An air gap surge arrester of claim 1, wherein said air gap is about 3 mils.

9. An air gap surge arrester of claim 1, wherein the strike voltage is in the range of about 500 to 1000 volts.

10. An air gap surge arrester of claim 1, wherein said intermediate insulating layer is formed of fusible material.

11. An air gap surge arrester of claim 10, wherein said fusible material is a fluoropolymer.

12. An air gap surge arrester of claim 1, wherein said first and second layers and intermediate layer are generally rectangular in shape.

13. An air gap surge arrester of claim 1, and including a third layer of conductive material located in longitudinal spaced relationship from said first layer and overlapping at least a portion of said second layer, said intermediate layer having another predetermined aperture between said second and third layers, whereby an air gap is established between said second and third layers.

14. An air gap surge arrester of claim 13, wherein said insulating layer extends beyond the periphery of said conductive layers.

15. An air gap surge arrester of claim 13, wherein said first and third layers extend beyond the longitudinally remote ends of said insulating layer.

16. An air gap surge arrester of claim 15 wherein said insulating layer extends beyond the transverse peripheral por-tions of said conductive layers.

17. An air gap surge arrester of claim 15, wherein said insulating layer extends beyond the longitudinally remote ends of said second layer.

18. An air gap surge arrester of claim 13, wherein said insulating layer apertures are circular in cross-section and the portions of said conductive layers overlapping said aper-tures have a generally circular shape concentric with their associated aperture.

19. An air gap surge arrester of claim 13, wherein said intermediate layer includes an adhesive on each surface in the area surrounding said apertures, said adhesive being set back a predetermined distance from the edge of the associated apertures.

20. In a surge arrester assembly having a gas filled surge arrestor including at least two electrodes defining an ionization gap, and short circuit, clamp means biased towards a short circuit connection with said electrodes, the improvement comprising:
safety means interposed between at least one of said electrodes and said short circuit clamp means, said safety means including a sealed laminar assembly having first and second layers of electrically conductive metallic material in contact with said clamp means and electrode, respectively, a portion of said layers being in overlapping relationship, and an intermediate insulating layer inter-posed between said first and second layers, said insulating layer having a predetermined aperture in the area of over-lap between said conductive layers, whereby an air gap is established between said first and second layers, each of said layers being flexible and conforming to the shape of said electrode.

21. An assembly of claim 20, wherein said gas filled surge arrester is provided with two line electrodes and a ground electrode, said assembly including safety means for each line electrode located on said ground electrode.

22. An assembly of claim 20, wherein said intermediate layer is a meltable fluoropolymer.

23. In a total fail safe surge arrester assembly having a gas filled surge arrester including at least two electrodes defining an ionization gap, and short circuit clamp means biased towards a short circuit connection with said electrodes, the improvement comprising: air gap means interposed between a portion of said short circuit-ing clamp means and each of said electrodes, said air gap means including a sealed laminar assembly having first and second layers of electrically conductive material in contact with said clamp means and one of said electrodes, respectively, a portion of said layers being in overlapping relationship, and an intermediate insulating layer interposed between the overlapping portions of said conductive layers and defining an aper-ture therein to provide an air gap in the area of over-lap between said conductive layers, each of said layers being flexible and conforming to the shape of said electrode;
and non-metallic fusible means in thermal contact with said ionization gap and interposed between said clamp means and one of said electrodes to prevent short circuit connection therebetween except in the presence of sustained overload causing said fusible means to fuse and yield to permit establishment of said short circuit connections.

24. An assembly of claim 23 wherein said air gap means and said fusible means are located at the same electrode.

25. An assembly of claim 23 wherein said first layer is in direct contact with the other of said electrodes and said clamp means.

26. An assembly of claim 23 wherein said layer of in-sulating material is said fusible means.

27. An assembly of claim 23, wherein said fusible means is a sleeve positioned on said same electrode.

28. An assembly of claim 23, wherein said fusible means comprises a meltable fluoropolymer.

29. An air gap electrode device of claim 23, wherein said intermediate layer is polyimide.