Note: Descriptions are shown in the official language in which they were submitted.
&ilberts Case 285
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SURGE VOLTAGE ARRESTER WITH VENTSAFE FEATURE
Background of the Invention
This invention relates to improvements surge voltage
arresters for line protectors of the type used for pro
tecting telephone lines and like communication lines
from over-voltage and over-current conditions.
20 Surge voltage arrester of the cold cathode gas discharge
tube type serve as the primary arrester and source of
protection in various line protectors of the station or
central office type. Such line protectors may also include
a carbon or other type of air gap back-up protector in the
~5 event of a failure of the primary surge arrester as a
result of leakage of gas from the tube due to a broken
seal or similar damage. A gas tube arrester which has
failed in this manner will be difficult to detect because
the line to which it is connected continues to operate
30 properly. Thus, it is desirable to provide some type of
air gap or secondary surge arrester as a "back-up" or
ventsafe feature in the event of failure of the gas tube
arrester. Line protectors embodying these surge voltage
arresters are frequently installed under conditions
35 wherein dust, moisture and other contaminants can enter
the secondary air gap. This can alter the breakdown vol-
tage characteristics of the air gap.
3L~S~S57
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In providing secondary or back-up protection of the air
gap type the ideal situation is to construct the air gap
with a breakdown surge voltage that is slightly above
the breakdown surge voltage of the gas tube arrester.
In this way the secondary air gap is not utilized so
long as the gas tube is functioning properly. However9
the idealized situation is not attainable on a mass
production basis Thus, there is always a range over
which the gas tube breaks down, and this will depend
upon many factors, including production tolerances as
well as the number of times the gas tube has fired.
Likewise, as far as the air gap is concerned, production
tolerances, electrode surface conditions, and other
factors will result in a variation of breakdown voltages
from unit to unit.
Where the air gap is made quite small in order to provide
a low breakdown voltage, there is the possibility that
the breakdown voltage of the air gap may in some cases
be below that of the gas tube, in which event the air
gap would break do~n while the gas tube is still func-
tioning properly. Moreover, in a typical arrangement inwhich the electrodes of the air gap are of metal, the air
gap will short out after one or a few discharges 9 leaving
a surge arrester unit that has short-circuited the line
but which nevertheless has a properly functioning gas
tube. On the other hand, if the air gap is madelarge in an
attempt to prevent short circuitry, its breakdown voltage
may be so high that it exceeds the specifications or
requirement of the user. Therefore, in surge arresters
having facing metal surfaces that define the secondary
arc gap a compromise has been attempted so as to provide
an arc gap which is small enough to break down at a
low enough voltage for useful purposes, but which break-
down voltage is nevertheless above the breakdown voltage
of the gas tube.
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Summary o'f'the 'Inven'tion
An object of this invention is to provide an improved
surge voltage arrester assembly that utilizes a gas tube
as a primary surge arrester and an air gap or secondary
surge arrester in the event of failure of the gas tube
arrester due to leakage or from other causes. The assem-
bly may be of the type having ei~her a two electrode or
a three electrode gas tube and in each case the air gap
has facing metal electrode surfaces, one of which is
roughened and coa~ed with graphite or the like.
The surge voltage arrester is characterized as having a
primary surge arrester of the cold cathode gas tube type
and a secondary surge arrester of the air gap type, the
breakdown voltage of the secondary arrester being great-
er than the breakdown voltage of the primary arrester but
having a breakdown voltage less than that of the primary
arrester upon loss of its gaseous medium, said arresters
being housed together and being adapted to be connected
to form parallel electric circuits from a line to be
protected to ground, said secondary arrester having its
air gap defined by facing electrode surfaces, at least
one of said facing electrode surfaces having a coat-ing
of a material that enhances surge voltage breakdown
across said facing surfaces at the coating so that for
a given surge breakdown voltage of the air gap, the air
gap is wider than would be the case in the absence of
such coating, said facing surfaces being respectively
on an annular portion of an electrically conductive cup
that contains said gas tube and a rim of an electrode
that forms part of said gas tube, and in which said
air gap is annular in configuration.
Gilberts Case 285
5S 7
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Brief Description of the Drawin
FI~. 1 is a sectional view of a surge voltage arrester
assembly of the present invention and shown embodied
in a known type of line protector;
FIG. 2 is a fragmentary sectional view on an enlarged
scale taken along line 2-2 of FIG. l;
FIG. 3 is an enlarged fragmentary portion of FIG. l;
FIG. 4 illustrates a three element gas tube form of the
invention and with the section line taken along the
longitudinal axis of the tube; and
FIG. 5 is an enlarged fragmentary ~ortion of FIG, 3.
Detailed Description
Referring now to the drawing there is shown a station
protector 10 embodying a surge voltage arrester assembly
of the invention. The protector comprises a sheet metal
housing or cap 12 having an annular sidewall portion 14
containing an annular flange or stop-shoulder 16. Below
the shoulder 16, the sidewall 14 is formed with a screw
thread 18 for threading into the well 61 of a protector
block 62, as will be presently more fully described.
The cap 12 also includes an end wall 20 which i~ opposite
to the open end of the cap 12.
Mounted within the cap 12 are several coaxial parts which
provide the primary and secondary surge arrester assembly
of the invention. More specifically9 there is a gas tube
22 having opposed electrodes 24, 26 that define an arc
gap 28 therebetween. The electrodes 24, 26 are separated
by a tubular insulator 30 of ceramic or the like to which
~ilberts Case 28~
~5;~5S7
the electrodes 24, 26 are brazed or soldered in the
usual manner, Thus, the electrodes respecti~ely have
annular electr~de flanges 3~, 34 at which the electrodes
24, 26 are silver soldered to the ends of the insulator
30 by rings 27.
10 The gas tube 22 is coaxially housed within a tubular
structure that is in the form of a metallic cup 36
- h~ving a cylindrical sidewall 38. The gas tube 22 fits
closely within the confines of the cup 36 although the
gas tube and parts assembled therewith may slide relative
15 to the cup so as to facilitate assembly of those parts.
Near the open end of the cup 36 the sidewall 38 has
diametrally enlarged annular cylindrical skirt or end
portion 40 which surrounds the peripheral edge surface
20 45 Of the electrode flange 34. This end portion 40 de-
fines the open end of the cup 36 and is radially spaced
from the surface 45 of the electrode flange 34, thereby
defining a secondary air gap 42 of annular configuration.
This arrangement provides a secondary or back-up surge
25 arrester-
Prior to assembly of the parts of the protector 10, theend portion 40 is roughened on its inner cylindrical sur-
face at least in the region that will be presented to the
30 air gap 42 when the parts are assembled. This roughening
may be done by sandblasting. A coating of carbonaceous
material such as graphite 43 is then applied to the
roughened area which makes adherence of the graphite
~ to the metal possible. The graphite may be applied
as an annular band by rubbing a pencil or other graphite-
containing tool against the roughened surface and then
blowing off the excess with air. The graphite band 43
is thus opposite to the peripheral surface 45 of the
electrode flange 34 and generally coextensive therewith.
The surface 45 may or may not be sandblasted and coated
Gilberts Case 285
~5;~557
-- 6 -
with graphite.
It is within the scope of this invention to apply other
voltage breakdown enhancing materials as the band 43.
Also, the graphite might possibLy be applied as a sus-
pension that is painted on the end portion 40. In any
event the effect of the graphite 43 is to permit a wider
gap 42 for the same breakdown voltage than would be
possible in the absence of the graphite.
The end of the electrode flange 34 has a metal contact 48
thereagainst with an annular groove 52 for receiving an
annular 0-ring 53. The O-ring 53 is of pliable material,
preferably an elastomer, for example silicone rubber,
although other elastomers might also be suitable. The
0-ring is of a diameter such that it seals against the
inside surface of the end portion 40 near its lower end.
The metallic cup 36 is coaxially housed within a metallic
grounding cage 50 having an end wall:52 and a plurality
of circumferentially spaced, spring-like fingers 54.
The spring fingers are compressed radially inwardly when
the cup 36, together with the arrester assembly, are
inserted as a unit within the open end of the cap sidewall
14. In this regard a solder pellet 56 is inserted into the
cage 50 prior to insertion of the assembled cup and gas
tube so that the solder pellet lies between the end wall
o~ the cup 36 and the end wall 52 of the cage 50. A coil
compression spring 58 bears at one end on the end wall
20 and at its opposite end against the flat end wall 52
of the grounding cage. During assembly of the protector,
the sealing ring 53 and the contact 48 prevent the gas
tube 22 from coming out of the cup 36.
Gilberts Case 285
:1~5~557
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The protector 10 is adapted to be mounted in the well
5 61 of the dielectric block or receptacle 62. This
block, which is of known construction, has a metallic
contact member 64 with an internal thread as shown for
receiving the cap thread 18. This contact member 64 is
usually connected to ground. At the bottom of the well
10 61 is a metallic contact 66 which is electrically
connected to the electrode 26 through the metal contact
48. Contact 66 is connected to the line to be protec~ed.
In threading the protector 10 into the ground contact
member 64 to the limit of the stop-shoulder 16, the
15 extreme end surface of the contact 48 will firmly engage
the line contact 66 by reason of the force of the spring
58.
The arc gaps 28 and 42 are electrically coupled in
20 parallel circuits from the line contact 66 to the ground
contact 64. The width of the arc gap 42 is such that
its breakdown voltage is greater than that of the break-
down voltage across the arc gap 28 of the gas tube 22.
Consequently, when the gas tube arrester is operating
25 properly as a primary surge arrester an over-voltage on
the line to be protected will result in a discharge
across the gas tube arc gap 28 to ground. The secondary
surge arrester will not discharge across the air gap 42.
However, if the gas tube should fail due to leakage,
30 some protection will be afforded by a discharge to ground
across the air gap 42 even though the breakdown voltage
thereacross is somewhat higher than the breakdown
voltage across the gas tube when the latter is functioning
normally. Because of~ the widened gap 42 with the graphite
35 surfaced electrode, there is less likelihood of the arc
gap 42 shorting out prematurely.
Gilberts Case 285
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In an overcurrent condition on the line due, for example 9
S to a prolonged voltage at the arcing voltage of the gas
tube, the heat within the protector 10 will cause the
solder pellet 56 to melt whereupon the force o~ the spring
58 will press the tips 60 of the grounding cage into
direct metallic contact with the line contact 66. This
10 results in a direct metallic connection cf the line to
be protected from the line contact 66 to the ground contact
member 64.
A three element gas tube version of the arreste~ assembly
15 is shown in Fig. 4. The primary or gas tube surge
arrester comprises opposed line electrodes 70, 70 and
a center or ground electrode 72. The several electrodes
are insulated from each other by ceramic insulators 74,
, 74 which are soldered by rings 76 to the respective
20 electrodes. The center or ground electrode 72 is hollow
to provide communicating coaxial cavities 77, 77 that
receive stem portions 78, 78 of the line electrodes 70,
70. The stem portions 78, 78 cooperate with the ground
electrode to provide primary arc gaps 79, 79 from each
25 line electrode to ground.
A secondary air gap 86 is also provided between each line
electrode 70 and the ground electrode 72. A metallic
cup 80, similar to cup 36, receives and contacts a line
30 electrode such that the open ends of the cups 80, 80
face each other. Each cup has a cylindrical sidewall
81 with a diametrally enlarged annular cylindrical end
portion 82 that is spaced from a rim 84 of the ground
electrode 72 to provide the annular secondary air gap
35 86. Each cup 80 is sandblasted and has a band of
graphite 83 applied thereto in the region of the gap 86,
Gilberts Case 285 ~S2~57
g
The sealing arrangement for eac:h air gap 86 also utilizes
a pliable elastomeric annular C)-ring 88 and may also
use a sealing compound 90. The 0-ring its into an
annular groove 92 in the grouncl electrode and is sized
to engage the end portion 82. The sealing compound 90
if used is disposed in a seconcl annular groove 94 in
the ground electrode 72 and seals against that electrode
as well as against the O-ring ~8 and the end portion 82.
The cups 80, 80 may be sized to fit into a clip type
receptacle for respective connections to the two sides
of the telephone line to be protected. The center elec-
trode may receive a clip or other connector in the
region between the two O-rings or the bands of sealing
compound 90, 90. Other conventional mountings for the
gas tube may be made as it is essentially cylindrical
in configuration and so lends itself to ready adaption
to known mountings.
As in Figs. 1 - 3, the primary arc gaps 79, 79 have break-
down voltages less than that of the secondary air gaps 86,
86 except when the gas tube becomes vented, in which case
the air gaps have the lower breakdown voltage. As a
result "ventsafe" protection is providPd for each side of
the protected line.
