Note: Descriptions are shown in the official language in which they were submitted.
~137S~
CROSS-REFERENCE TO RELATE~ APPLICATION
This application is related to commonly assigned
Canadian Appli~ation Serial No. 298,463, filed March 8th,
197g .
BACRGROUND
Gas tube overvoltage protectors are widly used for
the protection of equipment from overvoltage conditions which
may be caused by lightning, high voltage line contact, and the `!
like. -
It is also a widely practiced technique to associate ~`
various fail-safe arrangements with such tubes and with other
types of protectors, e.g., air gap arresters, to meet various
contingencies. For example, the presence of a sustained over-
load, as where a power line has come in continued contact with
a protected telephone line, 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 protector 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 overloads
and provide either a permanent short circuiting of the arrestor
directly, or function to release another mechanism, e.g., a
spring loaded shorting bar, which provides the short circuit
connection (commonly, the arrester electrodes are both shorted
and grounded). The presents of the permanent short and ground
condition serves to flag attention to that condition thus
signalling the need for
~,~
Pg/
1~7~
its inspection or replacement. Examples of this type of
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 bo~h fail-
safe 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-safe function can
be achieved by employing a non-metallic fusible material
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
t 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 fîlm in the course of fusing which might otherwise
interfere with the short circuit contact.
The need exists, nonetheless, to develop ail-safe
arrangements which provide both surge and failure protection
~or gas tube arresters.
SUMMAR~
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 producible
systems.
Accordingly, the present invention may be summarized
as follows:
n~ f~ ~ r 3 _
1~375g~i
~ .
A combination fail sa:Ee and air gap device for use
with a gas filled surge arrester, comprising first and
second metallic electrically conductive layers, the
layers being in overlapping relationship, and an inter-
mediate insulating layer of non-metallic fusible material
interposed between and in contact with the first and second .
layers to prevent short circUitlng therebetween except in
the presence of a sustained overload causing the fusible
material to fuse and yield to permit establish~ent of a
short circuit between the first and second layers, the
intermediate layer having at least one predetermined aper-
ture therein in the area of overlap between the first and
second layers to define an air gap electrode therebetween.
sRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an elevation view, partly in schematic,
of a gas filled arrester with a first embodiment of this
invention;
Figures 2 and 3 are cross-sectional views taken along-
lines 2-2 and 3-3, respectively, in Figure l;
Figure 4 is an enlarged plan vi.ew of the air gap
device shown in Figure l;
Pg/~ d 4 -
75~i
Fi~ure 5 is a cross-sectional view taken along line
5-5 in Figure 4;
E'igure 6 is a plan view, partly in schematic and
partly in cross-sect:ion, of a gas fil].ed arrester of the
second embodiment;
.~ ,
sd/i~`6 -4A-
.
1~75~5
,, .
1 1I Figure 7 is a cross-sectional view tsken along line 7-7
2 ¦1 in Figure 6 and illustrating sn air gap device and clip with
- 3 1l fusible material on the clip legs;
4 1¦ Figure 8 is a cross-sectional view taken along line 8-8
! in Figure 6;
- 6 il Figure 9 is a cross-sect~onal view taken along line 9-9
7 ~¦ in Figure 6 and il~ustrating an air gap device and fusible member
8 ' in the form of a cylindrical sleeve about the gas tube;
9 Figure 10 is an enlarged plan view of an air gap device ¦
l used in Figure 6;
11 , Fig-~re 11 is a longitudinal cross-sectional view ~aken
12 along l~ne 11-11 in Figure 10;
13 , Figure 12 is a top plan view of another air gap device
- 14 l similar to that illustrated in Figure 10;
'I Figure 13 is a longitudinal cross-~ectional view taken
: 16 ll along line 13-13 in Figure 12;
17 ll Figure 14 is a bottom plan view of the embodiment in
18 Il Flgure 12; and
19 ll Figure 15 i6 a cross-sectional view taken along line
'i 15-15 in Figure 12 and enlarged for clarity of illustration.
21 1
22 DE~ILED DESCRIPTI~N
,-
23 ¦ While this invention is susceptible of embodiment in
~any dlfferent ~orms, there is shown in the drswings and will
1 herelnafter be described in detail a preferred embodiment of the
26 ' invention, and modifications thereto, with the understand~ng that
27 ~I the preser.t disclosure is to be considered as an exemplification
-
_ 5 _
,~ . . : .
1 vf the principles of the inventlon and is not intended ~o llmit
2 1I the invention to the e~h~diments illustrated.
3 ¦ In the embodi~en~ illustrated in Flgure!s 1-3 and 6-9,
4 1 a gas tube 20 i8 provided, the tube lncluding a c:enter body 20A
¦¦ and electrode end caps 20B each separated from the center body
6 1 20A by a respective insulated sleeve;section 20C.
7 I The arrester 20, which is of ~lown constructioa and may
8 l¦ comprise for e~ample TII Model 31, has its end electrodes ~not
9 ll shown) extending inwardly from the end caps 20B toward the center
, of the tube ~nterior to define a gap between the electrodes.
11 Spacing and di~ensions are such that each electrode also forms
12 l a gap with the center body conductlve casing section 20A.
13 , The ~ube is filled with a &as and the electrode end
14 '1 c~ps 2QB are each provided as by welding with a lead 21B and
,, terminal 22B, e.g., a spade lug, for connection to the circuit
16 ll to be protected. Center body 20A is likewise provided with a
17 ll lead 21A welded thereto and the associated connection 22A
18 , for connection to groundO
19 1 In the prese~ce of overvoltage conditions the gas i~ ¦
~I tube 20 ioni~es thereby creating in known manner, conducti~e
21 l¦ shunting paths between each line of the protected circuit and
22 ¦I ground (via the respective ter~inal lead 21B and ground lead .;
~3 11 2LA)~
24 ¦ A short circuiting ~eans 25,illustrated as a clip, is
Ij disposed between each line electrode 20B and the ground elec~rode
26 l 20A. Clip 25 is illustrative, since it will be understood by
27 Ij those skllled in the art that other clip arrang~ments are readily
~-~8 ,' adaptable to this function.
., ., l
.
1~375~S
Il .
ll .
1 ', Each ~l~p 25, which is illustratively of grain oriented
2 tin plated carbon s~eel, heat treated for s~ress relief fsom
3 'I hydrogen embrittlement after plating, includes a first set of
4 spring fingers 26 resiliently engaging, respectivelyl end cap
S I,' (line electrode) 20B and another set of spring fingers 27 dis-
6 ll posed about cen~er body (ground electrode) 20A. The spring fin-
7 I gers 26 and 27 are integrally connected by the bridge section 28
8 , 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 detail in Figures 2, 7 and 9, the fingers 27 of the
12 short^circuit clips are spaced from contact with center body 20A l
13 by reason of fusible elements 30, 31 and 32 described in greater ¦
14 ' detail below. Specifically, each of the fingers 27 includes a
,, contact portion 27A which is urged in the direction of contact ¦
16 lI with grounded center body 20A and which consequently presses j
resiliently on the fusible member interposed therebetween.
18 l Fusible elements 30, 31 and 32 are of non-metallic,
19 , electrically insulative composition. Suitable materials will
¦ have melt temperatures in the range corresponding to ther~al con-
21 ditions at srrester thermal overload and will have suitable
22 dielectric strength, dielectric constant, dissipation factor and ; .
23 ! volume and surf8ce resistivity to provide the requisitive insula-
24 I tive functioD. The preferred material should also be free of
1 embrittlement or plastic flow due to aging and high ambient tem-
26 1 perature efects, be non-in1ammable under the overload conditionsl,
27 , have good mechanical properties and be inert to corrosives and
~-, 1 , .
28 i weather.
,~, ,
,, . . . . : , ,
75~a~
1 I Exemplary of such a cla6s of m~te~ als are certain of ¦ I
2 li the fluoroplast~cs, sueh as fluorinated ethylene propylene polyme~
3 , (FEP), the pol~mer perfluoroalkoxy (PFA), the modified copolymer
4 ¦, of ethylene and tetrafluoroethylene (ETFE) (m~rketed under the
!; DuPont Company trademark ~efzel), and poly (ethylene-chlorotri-
6 fluoro-ethylenej(E-CTFE copolymer) marketed under the Allied
7 I Che~ical Corporation mark Halfar. (The fluoroplastic poly-
8 1I tetrafluoroethylene (TFE), on the other hand, does not have suit-
9 l, able melt properties for the illustrated application.) In the
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 and 25
12 respectively.
13 With reference to the embodi~ent of Figures 1-5, fusible
14 ~~ element 30 is generally rectangular in shape and interposed be-
tween a first layer 35 and a second layer 36 of conductive mate-
~i . . '.
16 , risl, e.g., copper. Layers 35 and 36 are generally rectangular
17 in shape and in register but smaller in dimensions than layer 30
18 ~o that layer 30 extends beyond the periphery of the conductive
19 ,' layers. Insulative layer 30 includes two rectangular openings
il 30A which together with the thickness of the layer 30 provide a
21 il, pair of air gaps between the first and second layers, Preferably,
22 1I the air gap is about 3 mils and provides a strike voltage in the
23 l' range of 500 to 1000 volts.
24 1I The entire assembly 40 including the first and second
1l layers 35 and 36 and insulative layer is a safety device which is
26 positioned circumferentially about the center body 20A as shown
~7 in Figures 1 and 2. Fingers 27 engage copper lsyer 35 and bias I ;
Z8 it against fusible layer 30, layer 36, and center body 20A, I
,
-- 8 --
.
~ ~37S~ l
,, . , .
During normal operation of the arrest~r 20, transient
2 surges produce ionization in the normal ~anner to protect the
3 ~ subject equipment. If, however, a sustained surge condition
4 occurs as where a line is permanently oontacted by a higher voltage
line, the resultant ioniz8tion currents fl~wing through the ar-
6 rester produce excessive heat; the ~usible layer 30, placed in
7 I the arrester region to respond to this heating, thereby fuses.
8 ' As this occurs, spring fingers 27, and in particular the contact
9 sections 27A thereof, move layer 35 lnto contact with layer 36
and center body 20A as the fusible layer 30 yields and flows.
11 When electric~l contact is made a short circuit is established
12 between the respective end cap and the center body thus providingi
13 a fail-safe (short~ 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 does not re8ult in a lo~s of protection; the air gap provides
19 l, back-up protection prior to arre8ter replacement.
1ll Figures 6-15 illustrate still further modifications to
21 l~ the invention. In these embodiments the air gap devices 50 and
22 ,l 60, Figures 10 and 12 respectively, are arranged in longitudinal
23 ,¦ relation8hip on the gas tube 20 beneath the clips 25.
24 11 The fusible elements may take alternative forms. In
,I Figure 7, tubul~r sleeves 31 are arranged about the fingers 27 of
26 I clips 25 so that a layer of fusible material ls interposed between
27 contact sections 27A and center body 20A, In Figure 9, a tubular
,1 . I
g _ ,
. .
1~3~5~
.
,, . .
1 1! sleeve 32 is arr~nget circumferentially about center body 20A to
2 ~ m~inta~n contact section 27A of the clip in spaced relation~h~p
3 1l thereto. Additionally, sleeve 32 overlaps device 50 or 60 to re-
4 ~I tain the device on snd in contact with center body 20A.
¦ Air gap device 50, Figures 10 and 11, includes a first
6 ¦I conductive layer 51 in the form of a rectangular iayer of metallic
7 ~ I! conductor material, e.g. copper. Layer 51 is t~kn~e in electrical
8 ¦ contact with end cap 20B by clip 25. A second layer 52 of conduc-
9 1 tive material is in overlapping relationship with layer 51. Layer
¦ 52 is in direct contact with center body 20A. Interposed between
11 l' layers 51 and 52 is a non~-metallic layer 54 of insulating materiaL
12 '' Layer 54 may be of the type previously described or a high melting
13 point material, such as a polyimide, an exemplary example is the
14 polyimide sold under the designation Kapton and may be surface
li coated with adhesive to secure layers 51 and 52. Layer 54 includesl
16 1l an aperture 54A therein to define an air gap between the ~ rlapping,
17 ll 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 gaps 54A are provided. However, it will be understood that the
i device wor~s egually well when the gas tube has one line electrode
21 and one ground electrode, I
22 , In the air gap device 50, the insulating layer 54 e~- ¦
23 , tendg béyond the periphery of both layers 51 and 5~. Layers 51
24 1,l snd 52 may be fabricated by known methods, preferably by printed
¦ circuit techniques.
26 !¦ Figures 12-15 illustrate sn air gap device similar to
27 ldevice 50 which has been modified to improve its contact and
28 I 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.~.
-10- .
.
.3754
i . . .1 . 1
1 co~per, includes an end cap snd/or clip contact portion 6LA of
2 I geDerslly rectangular shape. Portions 61A are plsced in direct
3 contact with their associated end caps (electrodes) 20B. A neck
4 1 portion 61B connects portion 6LA t~ a generally ~ircular shape
i' portion 61C overlying the air gap formed by aperture 64A describe~
6 1 below.
Correspondingly, second conductive layer 62 includes
8 ~ two circular shaped portions 62A concentric with sir gap 64A and ¦
g interconnected by rectang~llar shaped portion 62B.
Insulacing layer 64 is interposed between layers 61 and I -~
11 62 and formed with cut-out portions 64B. These cut-out portions
12 facilitate in the wrapping and conformance of the air gap device
13 ; about the gas tube.
14 Moreover, with particular reference to Figure 15, the
!~ insulating layer 64 includes a layer 65 of plastic material of
16 i the types de~cribed 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 snd 62. Preferably, the edges 66~ of the adhe-l
l~ !
19 , sive layer ad~acent the hole 64A in the plastic layer 65 is set
24 ~11 back a short distance. By wsy of illustration, with a hole dia- ¦
2~ meter of .OS inch in the plastic layer 65, a set back of .OOS
22 ¦1 inch provides sufficient clearance. The set back clearance ame-
23 I liorates the possibility of the adhesive flowi..g into the air gap
24 ¦ during assembly. Moreover, the air gap dimension, e.g. 3 mils,
¦ must take into account the thickness of the adh~sive, for example
26 l, when the adhesive layers 66 are 1 mil thick, a plastic layer 65
27 , o 1 mil is used to achieve a 3 mil air gap. The set of the ad-
, I
~8 ; hesive also functions to prevent bridging or short circuiting
,
~ '7S~S
' . ,
1 ll of the air g8p which might occur as a result of electrical dis-
2 charges if the adhesive entered the alr g8p.
3 The operation of the arrester assembly of Figures 6-15
4 ,l ls similar to that prev~ously described. During normal operation
of the arrester 20, transient surges produce ionization in the
6 ,I normal manner to protect the sub3ect equipment. If a sustained
7 I surge condition occurs, the resultant ionization currents flowlng
ô through the arrester produce excessive heat; the sleeves 31 or
9 I 32, placed in the arrester region to respond to this heating,
thereby fuse. As this occurs, spring fingers 27 move into con- j
11 tact with center body 20A as the fusible sleeve material beneath
~ those contacts yields and flows. When electrical contact is nade
13 a short circuit is established between the respective end cap and l
14 the center body th us providing a fail-safe (short circuiting~ action.
~ Non-mPtallic materials other than the foregoing may be
16 used as the fusible members provided they have appropriate elec-
17 ' trical insulation properties and undsrgo a predictable change of
18 ~echanical properties under the specified overload condition to
19 'I permit the short circuiting action to occur.
ll Moreover, the air gaps 54A or 64A provide back-up pro-
21 l¦ tection in the event of failure of the gas tube.
22 To facilitate use in a wide variety of applications,
23 ll the arrester assembly of Figures 1 & 6 may be potted in a modular
24 l¦ shell, the potting material therein being an epoxy compound.
ll Prior to the potting the arrester assembly may be wrapped and
26 l voids filled with PTFE or equivalent material (not shown).
27 iAlternatively, the arrester assembly may be used in d station
.- .
- 12 -
.
.
11~37~L5 ~' ,
Il .. ,
1 ¦ protector configuration well known in the art. Obviously, the
2 I present invention is useful with gas tube arrester having more or
3 11 less number of electrodes than the three electrode tube a,r~,es,ter
4 1 shown. The ability to provide an sir gap which i9 sealed from
env~..ns
5 ~J the enn~k~t~ by the laminar construction deccribed provides a
¦ significant advance.
7 ~ These modifications and others may be made by those ¦
8 1i skilled in the art without departing from the scope and spirit
9 ¦, of the present invention as pointed out in the appended claims.
i~1 ' !
'11 '''
12 ll
13 ii
14 ~ ;
17 1
18 1 l -
I
221 ~ l . , I
23 ~ ' ,.'
224 ~ . .
26
27
,
- 13 -
