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Патент USA US2410370

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Oct- 29, 1946,
" E. M. THIELERS ET AL
2,410,370
THERMAL CIRCUIT INTERRUPTER
'
Filed Sept. 20, 1943
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Patented Oct. 29, 1946
2,410,370
"UNITED STATES PATENT OFFICE
THERMAL CIRCUIT INTERRUPTER
vErnst Martin Thielers, Stockholm, and Walter
Otto Wilhelm Broberg, Nynashamn, Sweden,
assignors to Telefonaktiebolaget L. M. Ericsson,
Stockholm, Sweden, a company of Sweden
Application September 20, 1943, Serial No. 503,118
In Sweden October 20, 1942
8 Claims.
1
(Cl. 200—123)
.
2
Within telephone technic a fuse heat-coil has
been used comprising a heat-producing winding
acting upon an interruption point which consists
of two metal parts, soldered together by an easily
fusible alloy. As a rule,.the heat-producing coil
shown in Fig. 2. In this ?gure, J and K are two
springs, K being provided with a slot in which
the narrow part of the heat-coil cover A may be
inserted. This causes the springs J and K to be
pushed apart and good contactis thus obtained.
is inserted inv ametalcover and wound on a nar
L is a spring which'in its normal rel-axed posi
tion rests against a strip M and at its upper end
has a slot in which the pin F is to be inserted.
The contact at the strip M is then broken and
row metal spindle bored lengthways, from one
side so as to receive a pin which, by a fusible
alloy, is soldered to the wall of the boring (in
terruption poin
.
i
,
-'
' vWhen current ?owsthrough the heat-produc
10 the spring L presses against the pin head, this
pressure generally amounting from 250 to 300 g.
The described heat-coil operates as follows.
The soldering metal which ?xes the pin F to the
spindle C softens at a pre-determined rise of tem
ing winding, heating of the easily-fusible solder
follows which, at a pre-determined current, causes
the solder to soften or tO-fllse-So that the pin,
actuated by an elastic force, can be pulled out of 15
perature, caused by excess current, whereupon
the boring thereby causing interruption of the
the spring L is released thereby pulling out the
pin. Current interruption follows and spring L
is applied against the strip M, thus closing an
alarm circuit.
current. At the same time, the spring adapted to
pull out the'pin may be brought against a con
tact and close‘ an alarm circuit.
» ~-
.
An arrangement of this kind is shown in Figs.'1
and 2 on the drawing, Fig. 1 showing the heat
coil and Fig. 2 the heat-coil mounted in springs
on a ?tting.
~
~
.
.
The heat-coils are most frequently used in tele
phone exchanges where they are placed in con
nection with main distributing frames.
'
goils combined with lightning protectors and
ning protectors and fuses according to Fig. 3,
25 where the incoming line is marked N, the fuses
Fig. 4 is a diagram representing the character
istic curve of the heat-coils.
1
Fig. 5 is a sectional view of an embodiment of
the invention.
_
_
In Fig. 1 which shows a heat-coil ‘in, cross-sec 30
tion view, A isa metal cover, B an insulating
sleeve in the‘lshapev of a: truncated cone, generally
made of ebonite or-?ber, the middle of which is
The
' heat-coils are frequently combined with light
Fig. 3 is a diagram of a circuit with the heat
O,» lightning protectors A, heat-coils P and the
line to internal exchange devices Q. In this com
bination the fuse generally has a limiting cur
rent of 3 9... Whilst the heat-coil is adapted to
interrupt at lower values.
.
-
Due to its structure, the heat-coil has char
acteristic curves differing from the characteristic
curves of a fuse consisting of a fuse~wire. Fig.
4 gives the characteristic curves of one of the
most frequently used kinds of heat-coils. It has
threaded so that the spindle can be screwed in‘ to
the insulating sleeve; E is insulating washers, in
a limiting current of about 0.16 a. and, at a. load
of 0.25 a., an voperating time of about 20 seconds
and at 0.7 a. an operating time of 2 seconds. The
heat-coil is thus considerably slower than a fuse
- sulating the spindle from the cover A, F a .pin
40 containing a fuse-wire. The resistance of that
with a, head; w-hichpin is inserted in C and
?xed by an easily-fusible solder (fusing point be
coil is about 20 ohms.
» ‘
is
Owing to said characteristic curves the heat
ing normally about 70° 0.). Gjis a heat-produc
coil is adapted for use in combination with a fuse
ing winding whose one end is in contact with the
spindle C and whoseiother end is soldered to the 45 (fuse-wire) of about 3 a., the fuse-wire operating
at high excess currents which generally are of
‘short duration, while the heat-coil is adapted
The outer terminal clamp of the heat-coil is
to interrupt at lower non-allowable currents
formed partly by the cover A, partly by the pin F.
which sometimes may be of longer duration.
As a rule,\the heat-coils are mounted in so
The heat-coil thus described has however sev
called strips and fastened by means of springs, as 50
eral drawbacks.‘ Primarily, the resistance is high
coveratI-I.
'
'
‘H
m
"
2,410,370
3
which is particularly perceptible when a tele
phone communication is established over several
exchanges, 40 to 45 ohms for every fuse-point
thereby being connected to the line. The con
struction described involves a reduction of the
resistance and at the same time increase of the
operating current which is seen from the table
below, taken from “Katalog over Telegrafverkets
materiel” (Catalogue from the Swedish Telegraph
Department) :
Fusing current,
ampcres at a
Resistance
fusing time of
20 seconds
0 .27
0.45
O .52
0.85
2 .00
Still more inconvenient is however the following
observation made by the inventors. It has been
found that the heat-coil has fairly narrow limits
which with the heat-coil having a resistance of
20 ohms, amounts to about 0.16 and about 1 a.
When the current exceeds 1 a. it may occur that
(l) The insulation of the winding wire carbonizes
and short-circuits occur without the pin
4
desirable to coat the winding with some ?xing
agent (for instance thin “cellone” solution). The
heat capacity of the winding must not be too
high. Consequently, the wire diameter should be
selected rather small as, for instance, with a coil
having a resistance of 10 ohms it may be about
0.10 mm.
It should also be observed that, at the points
where the heat-coil rests against the springs,
the surfaces of contact should be made as small
as possible. For that purpose the pin head in
its inner portion has a conical form. Further,
the circular slot at the lower part of the cover
may be conically shaped. When the heat-coil
is fastened in its springs the above described
springs I and K will rest against small surfaces
on both sides of the lower ?ange of the cover.
Easily-fusible alloys with various fusing tem
peratures may serve as soldering metal. Nor
mally, an alloy with a fusing point of 65-70" C.
is used.
The described heat-coil is adapted for use pref
erably in telephone technics but may be used
also in other ?elds, where its working properties
may be found useful.
We claim:
1. In a device of the character described, an
electrically conductive cover, an electrically eon
ductive tube arranged within the cover, insulat
being pulled out and causing alarm. The
ing means supporting said tube in said cover, a
30 pin soldered in said tube, a heating coil wound on
heat-coil then offers no protection.
(2) The winding wire is burnt Without the pin
said tube for fusing the pin solder, the pin and
cover being engageable with fastening springs and
being pulled out and giving alarm.
the heat capacity of the heating coil and parts
When these heat-coils are used in combination
with tube fuses of 3 a. capacities a ?eld of the 3 heated by said coil and heat losses of said insulat
ing means and fastening springs being so reduced
intensity of current of between 1-3 a. exists with
that the interruption point for all excess currents
in which no protectors, operating reliably, are
over the limiting current attains its fusing tem
available.
perature at least as early as the leading coil at
After a close research the inventors have suc
tains a temperature injurious thereto.
ceeded in constructing a heat-coil, whose resist
ance amounts to less than half of that of the
above described heat-coil type and which for all
intensities of current above the limiting intensity
breaks the circuit and causes alarm and which
may be inserted in the same strips as those used
for the present heat-coils. The invention is char
acterized in that, by excess current, the point of
interruption (i. e. the solder) attains the fusing
or softening temperature earlier or quite as early
as the heat-producing winding attains a tem
perature injurious to the wire or its insulation.
This is obtained by reducing the heat capacity
as much as possible and by counteracting the
heat-conduction from the pin to the bottom of
the cover.
Heat losses caused by radiation are
left out of consideration by this invention.
Fig. 5 shows an embodiment of the invention.
R is a cover of metal, S a thin tube of metal (pref
2. In a device of the character described, an
electrically conductive cover, an electrically con
ductive tube arranged within the cover, relatively
thin disc like insulating members supporting the
tube within the cover, a .pinsoldered in said tube
and a heating coil wound on said tube for fusing
the pin solder.
3. An arrangement as claimed-in claim ,2 where
in the thickness of the wall of said tube does not
exceed .5 mm.
4. An arrangement as claimed in claim 2
wherein the internal diameter of said tube is ap
proximately 1 mm.
,
5. An arrangement as claimed in claim 2
wherein the heating coil is coated with a thin
?xing agent.
6. An ‘arrangement as claimed in claim 2
wherein the ends of the tube are ?anged and
erably with good heat conductivity). T indicates
insulation washers, for example of ?ber, which
engaged with the disc like insulating members.
are secured at the ends of the tube, the tube
ends for this purpose being stamped to form a
small ?ange. U is the pin arranged in the tube
gaged with the disc like insulating members, said
cover having depressions therein for retaining the
insulating members in position.
and being soldered with easily-fusible alloy.
Finally, V is the heat-producing winding, whose
ends are soldered to the tube or the cover, re
spectively. The parts maintain their position by
means of several annular depressions X formed
in the cover.
The winding is arranged so as to be wound in
as few layers as possible (preferably one layer)
and as close to the tube as possible. It is also
'7. An arrangement as claimed in claim 2
wherein the ends of the tube are ?anged and en
8. An arrangement as claimed in claim 2
wherein the pin and cover are provided with por
tions engageable with fastening springs, said por
tions being shaped to minimize the area of con
tact with said springs.
ERNST MARTIN THIELERS.
WALTER OTTO WILI‘IEL'M BROBERG.
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