Note: Descriptions are shown in the official language in which they were submitted.
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HEAT COIL ASSEMBLY FOR A PROTECTOR UNIT AND AN OVERLOAD
PROTECTOR INCORPORATING SUCH AN ASSEMBLY
This invention relates to overload protectors for
cornmunications systems, and in particular is concerned with a heat
coil and line pin assembly oF a protector.
Overload protectors usually combine an overvoltage
protection device comprising two opposed electrodes spaced to define
a gap. One electrode is connected to a telephone line terminal and
the o-ther is connected to a ground terminal. On occurrence of a
voltage above a predetermined value on the telephone line, there is
a spark breakdown across the gap to ground. One such overvoltage
device is provided for each line of a telephone or other
communication system, that is Tip and Ring. There is also provided
an overcurrent device normally comprising a coil connected between
the central office and outside line terminals. On occurrence of a
current above a predetermined value, the coil heats up and causes a
fusible metal joint to melt, permitting movement of one mernber
relative to another and connecting the line to ground. One such
device is again provided for each line and the device will also
operate if there is a continuous spark breakdown due to a constant
overvoltage condition. Heat is conducted from the spark gap device
to the fusible metal joint and melts the fusible metal.
It will be appreciated that large numbers oF
protectors are required and it is desirable -that the cost be as low
as possible. The assembly of protectors can be labour intensive and
thus any reduction in labour con-tent will assist in reducing costs.
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One particular -feature which requires careful assembly is the heat
coil assembly.
A heat coil assembly generally comprises a coil of
wire wound onto a spool. The spool has a central bore into which is
5 placed a pin. The spool and pin are joined by a Fusible alloy, wi-th
the pin extending from one end of the spool. One end of the wire is
connected to the central of-fice terminal in the protector and -the
other end is connected to the line terminal. It is uc.ually
necessary to wind the coil after the spool has been joined to the
pin. The joined spool and pin are then assembled to the rest of -the
pro-tector assembly and the ends of the wires connected. It would
assist in assembly if the line terminal, spool and pin could be
preassembled, then the wire wound onto the spool. Normally, this
cannot be done as there are two line terminals in the base of the
protector and access to wind is not possible.
In the present invention, the pin and line terminal
are formed as a unitary member, either by forming a single part or
by rivetting or otherwise joining individual sections together. The
spool can then be mounted on the pin and -the wire wound onto the
spool. One end of the wire is attached to the spool, or pin, prior
to winding. The terminal, pin, spool and coil unit is then
assembled to a protector base, and the o-ther end of the wire
connected to the central office terminal.
Thus, in the broades-t aspec-t of the invention, a heat
coil assembly for a protector unit comprises a line terminal having
a terminal portion and a pin portion forming an integral unit, a
spool mounted on the pin portion and joined thereto by a fusible
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alloy, the spool capable of sliding axially on the pin portion on
fusing of the fusihle alloy, and a coil wound on the spool, one end
of the coil connected to the spool, pin portion or line terminal
portion. The invention also provides a pro-tector having an
overvoltage pro-tec-tion device having one electrode connected to a
ground terminal and another electrode connected to the heat coil
assembly. A pro-tec-tor may have two heat coil assemblies and two
overvoltage protec-tion devices, within a casing, and a back-up
protection device may be positioned between each hea-t coil assembly
and the ground terminal, in parallel arrangement with the
overvoltage pro-tection devices.
The invention will be readily understood by the
following description of certain embodiments, by way of example, in
conjunction with the accompanying drawings, in which:-
Figure 1 is a front view of one form of protector
wi-th -the front wall of the housing removed, as on the line I-I of
Figure 2;
Figure 2 is a cross-section on the line II-II of
Figure 1;
Figure 3 is a perspective view of a line terminal and
coil assembly;
Figure ~ is a cross-section, on the line IV-IV of
Figure 1, of a pin and coil;
Figure 5 is a perspective view of the ground bar as
used in the arrangement of Figures 1 and 2;
Figure 6 is a perspective view of a line bar as used
in the arrangement of Figures 1 and 2;
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Figures 7 and 8 are front and side views oF an
alternative form of line terminal and pin; and
Figure 9 is a cross-section similar to that of Figure
2, illustrating an alternative form o-f protector.
S As illustrated in Figures 1 and 2, a protector has a
base 10 and a housing 11 which is generally removably attached to
the base, but may also be permanently attached. There are five
terminals or pins extending through the base 10, two line terminals
12 and 13 for connecting to outside lines, two line terminals 14 and
15 for connecting to central office lines, and a ground terminal 16
for connecting to a ground. Mounted wi-thin the housing are two
protector devices 17, in the present example gas tube protector
devices. The arrangement as illustrated in Figure 1 is symrnetrical
about a vertical center line, with one protector device 17 for one
line, for example Tip, and another protector device 17 For the other
line, that is Ring. For convenience, only one protector device and
associated items will be described in detail but the description
applies to the other device unless otherwise stated.
Considering Figure 2, the line terminal 13 extends up
through the base 10 and at its upper end is rive-tted, or otherwise
attached, to a short, laterally extending plate 18. The terminal 13
is attached at one end of -the plate 18, and at the other end is
a-ttached a pin 19 extending upward away from the base 10. At -the
upper end of the pin 19 is positioned a coil assembly 20. The coil
assembly comprises a spool 21 and a coil oF wire 22 wound on the
spool. The spool is attached to the pin by a fusible alloy. One
end oF-the wire 22 is connected to -the spool while the other end is
connected to an extension 23 of the central office terminal 15.
Thus a connec-tion exists between the two line
terminals, 13 and 15, via plate 18, pin 19, spool 21 and wire 22.
Also, as the spool 21 contacts the bottom electrode of the protector
device 17 there is a connection From the line to the bottom
electrode.
The terminal 13, plate 18, pin 19, spool 21 and wire
22 are seen in perspec-tive in Figure 3, together wi-th the terminal
15 and extension 23. The plate 18, pin 19, spool 21 and wire 22 are
seen in cross-section in Figure 4. The thin layer of Fusible alloy
between the spool and pin is indicated at 25.
The protector device 17, as an example, comprises two
electrodes in opposition, spaced apart to define an arc gap. The
electrodes are sealed in a ceramic tube with the gap at a
sub-atmospheric pressure. The electrodes each have a flange, 26,
which extends beyond the ceramic tube. At the lower end the spool
21 is in contact wi-th the flange 26 of the lower electrode. The
flange at the upper end is contacted by a compression spring 27.
The upper end of the spring 27 rests in and is in contact with a
shallow cup or recess 28 at the upper end of a ground member 29.
The ground member ex-tends down to the base 10 and is attached to the
ground terminal 16 (Figure 1). The lower end o-F the spring is
located by a central pin 30 extending up from the flange 26 of the
upper electrode.
As stated, the arc gap in the device 17 is at
sub-atmospheric pressure. If -the seal between an electrode and the
ceramic tube breaks, the device becomes vented to atmosphere and the
breakdown voltage is too high. ThereFore, a back-up gap device is
usually provided for each line. These are indicated at 32. Each
back-up device comprises two electrodes 33 spaced by and bonded -to a
thin disc 34 of dielectric ma-terial, such as Mylar (trademark). The
disc has a small hole a-t its center and ac-ts to deFine a gap between
the electrodes 33.
As seen in Figure 1, the ground member 29 has two
cups or recesses 28 at its -top end, one for each line. The back-up
devices 32 are positioned on the top, outer surfaces of the cups 28.
A line member 35 has its top end 36 extending over 'che top of a
back-up device and the line member extends down inside the housing
11 and is in contact at its lower end 37 with the plate 18. A line
member 35 is provided For each line and the associated protector
device 17 and back-up device 32. The line members are pressed into
contact with -the back-up devices 32 at the upper ends and into
contact with plates 18 at their lower ends on assembly of the
housing 11 to the base 10.
The ground member 29 and line member 35 are seen in
perspective in Figures 5 and 6 respectively.
Figures 7 and 8 illustrate an alternative form oF
line terminal, -For example terminal 13. The terminal, plate and pin
are Formed in one piece from wire. Other Forms can be used.
The operation is conven-tional. On occurrence oF an
overvoltage on a line, there is a spark or arc breakdown oF the gap
in the related device 17. In the event that the gap oF the device
17 becomes vented, or otherwise inoperative, then the gap in the
relatecl back-up device 32 becomes effective, breaking down at a
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voltage slightly higher than the gap in the device 17.
In the event of an overcurrent on a line, then the
coil assembly 20 associated with the line heats up and eventually
the fusible alloy layer 25 melts. This permits the spool 21 to be
pushed down on the pin 19, until the bottom of the spool contacts
the bottom end of the ground member 29, thus providing a ground
connection. This can also occur if there is a continous breakdown
due to a continuous overvoltage condition. Hea-t is transmi-tted from
the bo-ttom electrode of the device 17 to the spool 21 and thus melts
the fusible alloy.
Thus in normal breakdown on occurrence of an
overvoltage, the ground connection is via the gap in device 17 and
spring 27 to the ground member. In a back-up condition, the ground
connec-tion is via the line member 35, the back-up gap in device 32
to the ground member. In an overcurrent or continued overvoltage
condition, grounding occurs between the spool and the ground member.
I-t will be seen that the line pins 12 and 13, with
pla-tes 18, pins 19 and coil assemblies 20, each constitute a unitary
assembly provided prior to assembly in the protector. The spools 21
are joined to the pins 19 by the fusible alloy, and the pins 19 and
the terminals 12 and 13 attached to the plates. It is thus possible
to wind the wire 22 onto the spools prior to assembly to the
protector, but af-ter the spool, pin, plate and terminal have been
assembled into a unit. The pins 12 and 13, with plates and coil
assemblies, can be positioned in the base and the free end oP the
wire connected -to the other pins 1~ and 15. The pro-tectors 17 and
springs 27 are then assembled and positioned with the protector
resting on -the spool. The ground member 24 is attached to the
ground terminal 15 prior to assembly and the ground member is
assembled to the base by sliding the ground terminal into a bore
through -the base 10. The cup or recess 28 fits over the outer end
of the spring. The line mernber is then assemb'led with its lower end
resting on the plate 18 and its upper or outer end extending over
the back-up device 32. The whole is then pushed into the housing
11. The Free 'length of the line member 35 is slightly longer than
in its fully assembled condition, the lower end bending slightly on
Final assembly. The tapered spring 27 acts to center the protector
17. The assembly comprises a set of separate, removable and
replaceable parts, iF repair is desired for example.
If a back-up gap device is not required, then the
device 32 can be left out and a spacer inserted, although a back-up
device is genera'lly always used with gas tube protectors.
The invention can also be applied to non gas-tube
protectors, for example, carbon block protectors. Carbon block
protectors are not usually sealed into a housing, being a-t ambient
pressure. There-Fore venting does not occur. Also, carbon block
protec-tors usually fail by having a reduced breakdown voltage or by
developing a direct short between the electrodes. Thus, a back-up
device is no-t required.
Figure 9 illustrates a modiFica-tion to the pro-tector
of Figure 2, using a carbon block protector. The particular Form of
carbon block protector is not oF irnportance and in the example two
carbon blocks are positioned in a cerarnic housing 40, the carbon
blocks extending out the ends of the housing at 41 and ~2. The
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blocks are separated within the housing, -for example by a ring of
insulating material, and are not sealed to the housing 40. The
spring 27 rests on the outer end oF one carbon block, or electrode,
and the outer end of the other carbon block or electrode rests on
the spool 21. An aperture 43 in the top oF the housing 1l enables
testing to be carried out.
