Note: Descriptions are shown in the official language in which they were submitted.
~ 3
AN OVERLOAD PROTECTOR FOR TELECOMMUNICATIONS SYSTEMS
This invention relates to overload protectors for
telecommunications systems.
In the telecommunications industry, it is conven-
tional prac~ice to provide overload protectors at central
offices for incoming lines. Such protectors protect tele-
phone equipment within cu~tomer's premises from damage suchas could be caused by overvoltage or overcurrent conditions.
These conditions may suddenly occur. For instance, an
overvoltage condition may be as a result of a lightning
strike to an outside line. Brief overcurrent conditions may
cause no damage, especially if an overcurrent condition only
slightly exceeds that for which a telecommunications circuit
is designed. However, more prolonged overcurrent conditions
which are only slightly in excess o~ the desired r~;
value may result in elevated overheating of circuitry to
cause gradual burning of insulation and other heat sponsored
damage to telephone equipment.
In order to protect a customer's circuitry and
equipment ~rom damage caused by overvoltage or overcurrent
conditions, an overload protector is provided in each line at
the central office. Each protector normally includes two
overvoltage protection unit devices and two overcurrent
protection devices, the protection devices providing in-
dividual overvoltage and overcurrent protection for each tip
line and each ring line.
In the main, an overload protector has previously
comprised a pair of spaced carbon electrodes or a gaseous
discharge arrangement. In overvoltage conditions, the
voltage passes across the space between the electrodes or
through a gas filled space to ground. on the other hand, an
overcurrent protection device normally comprises a coil of
; wire mounted around a sleeve, the coil being connected in
'
2 ~ i7J i3~r~ ~s
seriPs between a respective outside circuitry te~minal and a
corresponding central office terminal of the protector. Heat
generated in the coil by overcurrent causes solder to melt
thereby releasing a pin within the sleeve, the pin then being
spring urged into contact with the ground line thereby
shorting out the circuit.
In an overload protector of different structure, as
describ~d in U.S. Patent 4,434,449, a sleeve holding a coil
is mounted upon a line pin and held in a position spaced from
a ground line by solder. An overvoltage protection device is
mounted upon an end of the sleeve and is spring urged
downwardly towards the ground. Melting of the solder causes
the sleeve to move towards the ground, thereby shorting out
the circuit.
While conventional overload protectors operate in a
generally sa~isfactory manner, they axe of complex and
expen~ive structure with the overcurrent and overvoltage
devices and ground lines extending substantial distances
within housings away ~rom bases of the protectors, with the
terminals ~xtending from the bases away from the housings.
The present invention seeks to provide an overload
protector for telecommunications systems of a structure which
may be mor~ economical to produce than existing structures.
Accordingly, the present invention pxovides an
overload protector for a telecommunications system compris-
ing:- a dielectric ~ase and a dielectric housing extending
from one side of the base to define a chamber with the base;
a ground means comprising a ground terminal pin extending
from the other side of the base exteriorly of the chamber;
two pairs of signal terminal pins mounted within and
extending from the other side of the base, each pair for a
respective signal line; an overcurrent protection device
provided for each pair of signal pins, each overcurrent
protection device connected in a signal line in series
~ r~
be~ween the two pins of its respective pair and operable to
connect the signal line to the ground terminal pin upon
attainment of overcurrent conditions; and an overvoltage
protection device provided for each pair of signal pins, each
overvoltage protection device connected to a respective
signal line at one side of the overvoltage protection device
and to the ground ~erminal pin at the other side of the
over~oltage protection device, the overvoltag~ protection
device being operable upon attainment of overvoltage
conditions in the respectiva line to permit current to pass
from the signal line to the ground pin, characterized in that
the ground means is located entirely in the ; ~~iate
vicinity of the base, and each overvoltage protection device
is located entirely in the immediate vicinity of the base and
is electrically connected in that i s~;ate vicinity to its
respective signal line and to the ground terminal pin.
With overload protectors according to the inven-
tion, the overvoltage protection device of each conducting
means is in the immediate vicinity of the base so that the
connection from each signal lina to the ground terminal pin
is rendered as small as possible and is not required to
extend away from the base and into the housing. Hence, the
amount of conducting material employed in the overload
protector is minimized. In one construction of conducting
means, a ground conductor extends from the ground pin and is
electrically separated from its respective signal line by its
associated overvoltage protection device. Alternatively, the
overvoltage protection device i5 mounted with one side
directly in electrical contact with the ground terminal pin
and without the interposition of the ground conductor.
In practical constructions, each overvoltage
protection device comprises a solid state overvoltage
protection unit. A solid state overvoltage protection unit
may be of extremely small size commensurate with enabling a
conducting means to be disposed completely at the base
thereby eliminating the need for a ground line to extend away
from the base and into the housing.
In a preferred arrangement using a solid state
overvoltage protector unit for each overvoltage conducting
means, the conducting means also includes a ground conductor
extending laterally from the ground terminal pin with the
solid state overvoltage protection unit mounted between the
ground conductor and the respective signal line. The ground
conductor may extend across and engage one side of the base
and registration means are preferably provided for locating
the ground conductor in a required desirad fixed position
upon t~e base. Alternatively, the ground conductor is
embedded within the base and in further arrangements, the
whole of each conducting means including the solid state
overvoltage protection unit is totally embedded within the
base .
The invention also includes a method o~ making an
overvoltage protector for a telecommunications sy~tem
comprising:- providing a dielectric base and a dielectric
housing ~or mountiny upon one s.ide of the base: providing a
ground means comprising a ground terminal pin; mounting the
ground terminal pin and two pairs of signal terminal pins
within the base with the pins extending ~rom the other side
of the base; providing an overcurrent protection device for
each pair of signal pins on the one side o~ the base by
connecting the overcurrent protection device in a signal line
in series between the two pins of its respective pair so as
to be operable to connect the signal line to the ground
terminal pin upon attainment of overcurrent conditions;
providing an overvoltage protection device for each pair of
signal pins and electrically connecting it between a
respective signal line and the ground terminal pin, each
overvoltage protection device operable upon attainment of
overvoltage conditions in its respective line to permit the
current to pass from the signal line to the ground pin; and
mounting the housing upon one side of the base, the
overcurrent protection devices and the overvoltage protection
devices sealed within the confines of the base and the
housing assembly; characterized in locating the ground means
entirely in the immediate vicinity of the base, and locating
each overvoltage protection device entirely in the ; -~;ate
vicinity of the base and connecting each overvoltage
protection device in that immediate vicinity to its
respective signal line and to the ground terminal pin.
One embodiment of the invention will now be
described, by way of example, with referencP to the
accompanying drawings, in which:-
Figure 1 is an exploded isometric view of an
overload protector according to the embodiment;
Figure 2 is an exploded isometric view of the
overload protector of the e~bodiment taken in the opposite
: direction from Figure 1;
Figure 3 is a side elevational view, partly in
cross-section, of the assembled protector of the embodiment
and taken in the direction of arrow III in Figure 2;
Figures 4 and 5 are composite vertical cross-
sectional views of the assembled overload protector taken
through Fiyure 3 and Wit~l a housing and other parts omitted
for clarity;
Figure 6 is a cross-sectional view in side eleva-
tion, and to a greatly enlarged scale, of a semiconductor
voltage surge protection device included in the protector of
the first embodiment;
Figure 7 is a plan view, in the direction of arrow
VII in Figure 2, of part of the assembly of the protector of
the first embodiment;
Figure 8 is a cross-sectional view through the base
after assembly of the component parts and taken along line
VIII-VIII in Figure 2; and
Figure 9 is an isome~ric view of part of the
assembly taken in the same direction as the exploded view of
; Figure 2.
~ '3
As shown by the embodiment in Figures 1, 2 and 3,
an overload protector 10 for a telecommunications system
comprises a dielectric base 12 and a dielectric housing 14
which, as shown in Figure 3, extends from one side of the
5 base to define a chamber 1~ when the housing and base are
assembled together.
The base supports a ground terminal pin 18 o~ a
ground means and two pairs of signal terminal pins. Each
pair of signal terminal pins comprises an outside plant pin
20 and a central office pin 22. All of the pins are received
through and are carried by the base 50 as to extend
exteriorly of the assembled protector in the manner shown in
Figures 3, 4 and 5. As can be seen, as is conventional for
signal terminal pins in overload protectors, the outside
plant pins 20 extend further outwardly from the base than the
central office pins 22 for test purposes.
An overvoltage protection device is provided for
each pair of signal pins. Each overvoltage protection device
is a packaged solid state protection unit, i.e. a semi-
conductor voltage surye protection device which generally
comprises two flat metal electrodes and a semiconductor
voltage surge protection element sandwiched between the
electrodes. The structure may be as shown in Figure 6 which
compri~es semiconductor voltage surge protection element 28
positioned between and electrically connected to two
electrodes 30 and 32. The electrodas are connected to the
voltage surge protection element 28 by layers of solder 34.
Annular synthetic resin sealing member 36 extends around and
is spacPd from the element 28 and is sealingly adhered to
both of the electrodes. The resin is a dielectric, flexible
environmentally stable ma~erial which is non-disruptive under
; heat conditions created by a voltage surge through the
device. A suitable material for this purpose i5 an RTV
thermosetting material or is considered to be one of a family
of olefin acrylic copolymers. Included in thi~ family are
ethylene acrylic acids, ethylene methacrylic acids, propylene
acrylic acids, propylene methacrylic acids and metal salts
and esters thereof. These resins readily adhere to the metal
electrode when hot, are flexible, dielectric and stable. A
particularly suitable resin is an ethylene acrylic acid
copolymer.
Each of the semiconductor voltage surge protection
devices 26 is disposed in the immediate vicinity of th2 base.
To accommodate each of the devices 26, the base extends
further into the chamber 16 at a thicker part 38, as shown in
Figures 4 and 5, so as to define a recess 40 at one side of
the base. The recess has a projection 42 ~rom the end
surface 44 of the recess (shown particularly in Figure 2) to
provide a shallow part of the recess flanked by two deeper
recess parts at opposite sides of the projection 42. As is
claar from the Figures 1 to 5, the outside plant pins 20
extend through holes 46 in the base so as to project from the
recess surface 44 into the chamber 16. A signal conductor
comprising conductor pl~te 48 is provided to electrically
connect each of the outside plant pins 20 to a respective
voltage surge protection device 26. Each conductor plate 48
has two part circular ends which merge to provide a waisted
region 50. The one end of each plate is formed with a
concentric hole 52 which tightly receives a respective pin 20
so as to make electrical engagement with it. Each plate 48
is retained by its pin 20 upon the end surface 44 of the
recess with the waisted section 50 of the plate passing
through a restricted part 54 of the recess (see Figure 7).
This locates the other semi circular end 55 of the plate upon
the end surface 44 in a section 56 of the recess which is of
part cylindrical shape and is formed by opposing concave
walls of the part 38 of the base and the projection 42 from
the end surface 44. Figure 7 shows the structure with a pin
20 and devio2 26 remo~ed for clarity.
The ground ~eans is located entirely in the
immediate vicinity of the base. In this respect, tha ground
terminal pin 18 extends through the part 38 of the base to
:'
terminate at the surface of the base defining the chamber 16.
At that end of the pin there is provided a ground conductor
58 of the ground means. This ground conductor is a spring
plate which comprises two U-shaped and in line legs 60 formed
from a main end part 62 of the plate. The main end part of
the plate is secured ~o the ground pin for electrical contact
with it. One portion 63 of each leg 60 extends from the main
end part 62 of the plate, and laterally of the ground
terminal pin, across and contacting the part 38 of the base
to project outwardly over the recess 40. Each leg 60 then
extends downwardly at the base of its U~shape into the recess
40 with the other portion 65 of the leg 62 resiliently
flexible relative to portion 63. The portions 65 o~ the legs
62 extend across the recess sections 56. Registration means
is provided to locate the ground conductor 58 in a desired
fixed position upon the base. The registration means
comprises an extension 67 of the projection 42, the extension
registering snugly between the two U-shaped legs 60 (Figure
3).
As can be seen, with the parts of the protector
assembled together, each voltage surge protection device 26
is housed within a respective rece~s section 56 and is
contained between the free end of an associated conductor
plate 48. In each recess section 56, a conductor plate 48
and a leg portion 65 engage the two electrodes disposed at
the two sides of a respective voltage surye protection device
26. This is particularly clear from Figure 8. Each leg 65
place~ a resilient down pressure upon its device 26 to ensure
that the device remains in place during overvoltage con-
ditions and does not disintegrate.
As can be seen from the above description, with the
overvoltage protection devices disposed entirely in thP
immediate vicinity of the base, then the ground means, which
includes the ground terminal pin 18 and the conductor 58, may
be of minimal size, i.e. they do not extend into the chamb~r
16 of the completed protector. Their positioning and size is
commensurate with grounding contact with the voltage surge
protection devices 26 so as to minimize the grounding path
required from each of the outside plant pins 20 through the
conductor plates 48, voltage surge protection devices 26 and
through the ground conductor 58 into the ground terminal pin
18. Minimization in the sizes o~ the grounding elements and
the overvoltage protection devices reduces the materials
required to provide an operative overload prot~ctor.
Reduction in the amount of materials for grounding purposes
is accompanied by a reduction in cost and simplification in
design.
As can be seen more particularly from Figures 3, 4
and 5, the chamber 16 is provided substantially solely, for
the purpose of accommodating spindles 69, which are formed as
coaxial extensions of the outside plant pins 20, and the
accompanying overcurrent protection devices provided on those
spindles. Each overcurrent protection device comprises a
coil 70 of electrical wire mounted upon a spool 72. The coil
70 and spool 72 may be of conventional co~structîon for
overcurrent protection devices. Each coil 70 i8 connected by
its ends, and in conventional manner, to the two pins of its
associated pair i.e. an outside plant pin 20 and a central
office pin 22 (see particularly, Figures 4 and 5). Each
spool 72 is mounted upon the spindle 69 of its pin 20 and is
held in position axially along the pin by being soldered
thereto in a position spaced from one side of respective leg
portion 63 of the ground plate 58. Each spool is urged in a
direction towards the ground plate and towards the base by a
compression spring 74 (Figures 2 and 3 only) which surrounds
the spindle 69 of the pin and is compressed between the spool
and an upper part of the housing 14 (Fiyure 3) when the
housing is located in a position upon the base. As can be
seen from Figure 1, and particularly from Figure 9, each side
of each leg 60 of the ground conductor 58 is provided with a
projection 76 which extends partly around each of the two
pins 20 and is spaced from each of the pins by a part
circular edye surface 78. A lower end ~lange 80 of each of
the spools ~2 overlaps, in plan view, the extension 76.
Thus, if either signal line is subjected to overcurrent
conditions sufficient to cause overheating of the coil and
melting of the solder between spool and pin 20, the
respective compression spring 7~ forces the spool downwards
so as electrically to engage the ground plate 58 thus
grounding out the signal line.
