Electrical switching device for thermal and overvoltage protection

A normally open temperature sensitive electrical switching device includes a heat fusible electrically conductive pellet which melts and provides an electrically conductive path through the switching device at a predetermined temperature level and, further, includes arc contacts defining an air gap across which arcing will occur when a sufficient potential is supplied to the switching device. A first electrically conductive electrode and second electrically conductive electrode are electrically connected to first and second electrical leads provided for an electrical connection to the device. A nonconductive mounting arrangement holds the first and second electrodes in spaced relation. The heat fusible conductive pellet is positioned in contact with the second electrode and defines the air gap of predetermined dimension with the first electrode. Current will therefore flow between the first and second electrical leads when the voltage across the air gap is sufficient to cause arcing or when the fusible pellet is heated to its predetermined fusion temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to temperature sensitive electrical switching 
devices and, more particularly, to a temperature sensitive device which is 
normally open but which closes an electrical circuit when the temperature 
of the device is raised to a predetermined temperature level and which 
device provides an air gap path through the device to permit arcing across 
the gap when a voltage of sufficient magnitude is applied to the device. 
Thermally actuatable switch constructions are presently known in which an 
electric circuit is opened or closed in response to the switch being 
heated to a predetermined temperature level. In U.S. Pat. No. 3,875,546, 
issued Apr. 1, 1975, to Merrill, and U.S. Pat. No. 3,519,972, issued July 
7, 1970, to Merrill, both assigned to the assignee of the present 
invention, temperature responsive electrical switches are disclosed in 
which a sliding electrical contact is held against a second electrical 
contact by a relatively stiff spring, which spring bears upon a normally 
solid, heat fusible, nonconductive pellet. When the pellet fusion 
temperature is reached, the pellet will melt and the stiff spring will no 
longer oppose the force of a somewhat weaker spring which then moves the 
sliding contact away from the second contact. 
A normally open electrical switching device is disclosed in U.S. Pat. No. 
3,189,508, issued Apr. 27, 1965, to Merrill and assigned to the assignee 
of the present invention. In the device disclosed therein, a sliding 
contact is moved into electrical contact with a stationary contact after 
fusion of a nonconductive heat fusible material. 
It is also known to provide momentary overvoltage protection for various 
types of electrical machines by placing arc contacts in parallel 
electrically with the power terminals of the machine to be protected. When 
there is a momentary surge of line voltage, the excessive potential across 
the contacts will result in arcing and the machinery will therefore be 
protected. If the overvoltage condition should exist for a substantial 
period of time, the high current will blow the line fuses in the power 
supply line, permanently disconnecting the protected machine from the 
source of line power. If, on the other hand, the overvoltage condition 
exists only for a relatively short period of time, the arcing across the 
protective arc gap contacts may cease before sufficient current has passed 
through the line fuse to cause the fuse to blow. The protected machine 
will, therefore, resume its normal operations. 
Heat sensitive, normally open electrical switches have in the past been 
placed in physical proximity to a protected electrical machine and 
connected electrically in parallel with the machine. If the machine should 
overheat during operation, the switch will close, thus causing a large 
current to flow through the power lines and resulting in the line fuse 
being blown. Previously, such thermal protectors have been separate from 
overvoltage protection devices, and added labor required for their 
connection into the circuits. Additionally the use of two discrete devices 
has increased the cost of the protection circuitry. 
Thus, it is seen that there is a need for a combined normally open heat 
sensitive switching device and an overvoltage arc protector in which the 
air gap is accurately controlled and in which the construction of the 
device is simple and its operation extremely reliable. 
SUMMARY OF THE INVENTION 
A normally open, temperature sensitive electrical switching device having 
first and second electrical leads comprises a first electrically 
conductive electrode means which is electrically connected to the first 
electrical lead and a second electrically conductive electrode means which 
is electrically connected to the second electrical lead. A non-conductive 
means for mounting the first and second electrode means in spaced relation 
is provided. A heat fusible pellet means is positioned in contact with the 
second electrode means and defines an air gap of predetermined dimension 
with the first electrode means. The pellet means is made of a material 
which will melt at a predetermined temperature and form an electrically 
conductive path between the first and second electrode means. The air gap 
is sufficient to permit arcing thereacross when an excessive voltage is 
applied to the first and second electrical leads. 
An insulator means may be positioned between the conductive pellet and the 
first electrode means such that the predetermined dimension of the air gap 
therebetween is maintained. The first electrically conductive electrode 
means may include an electrically conductive case which is connected to 
the first lead, with the case having a first end defining an opening into 
a central cavity and a second end which is closed. The nonconductive means 
for mounting the first and second electrode means in such an arrangement 
may include a means for positioning the second electrode in the central 
cavity defined by the case. An electrically conductive spacer means in the 
cavity and a spring means may be provided for urging the spacer means into 
direct abutting contact with the insulator means. The insulator means may 
be an annular piece of mica such that the air gap is defined through the 
center opening in the insulator means between the pellet means and the 
electrically conductive spacer means. 
Accordingly, it is an object of the present invention to provide a heat 
sensitive electrical switching device which also includes an overvoltage 
protective air gap; to provide such a device in which an electrically 
conductive path will be provided between the power leads when the device 
is heated to a predetermined temperature level; to provide such a device 
in which the device operation is extremely reliable and in which the 
device assembly is simple; to provide such a device in which the 
overvoltage air gap is dimensioned precisely; and, to provide such a 
device in which the device actuates at substantially the desired 
predetermined temperature level. 
Other objects and advantages of the present invention will be apparent from 
the following description, the accompanying drawings and the appended 
claims.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to a unique electrical switching device which 
provides overvoltage protection and thermal protection for electric 
apparatus. 
Reference is made to FIG. 1 in which a prior art protective arrangement is 
illustrated schematically. An electrical device to be protected, such as 
motor 15, is positioned in a cabinet 17 and connected electrically to 
power lines 19 and 21. Fuse 23 is provided in series with the motor 15 in 
order to provide protection for excessive current conditions. A normally 
open thermally actuatable switching device 25 is positioned adjacent the 
motor 15 in cabinet or housing 17 and is connected electrically in 
parallel with the motor 15. Should motor 15 overheat for any reason such 
that the predetermined temperature level for actuation of the device 25 is 
exceeded, device 25 will become conductive and motor 15 will be shunted. 
Since a current path will be provided across lines 19 and 21, a large 
current will be drawn and fuse 23 will be blown quickly. Switching device 
25 has generally been of the type which becomes permanently conductive 
when the predetermined temperature level is exceeded. 
In a somewhat similar manner, arc contacts 27 define an arc gap 29 which 
provides protection for motor 15 against voltage surges on the power lines 
19 and 21. The spacing of the arc gap will be set such that arcing will 
begin when a predetermined voltage level is exceeded. When this level is 
exceeded, the arcing across gap 29 will effectively shunt motor 15 and 
provide protection against damage from overvoltage transients. Should the 
overvoltage condition exist for a substantial period of time, fuse 23 will 
be blown. 
Reference is now made to FIGS. 2, 3 and 4 in which is shown a temperature 
sensitive electrical switching device having provision for overvoltage 
protection, which device embodies the present invention. The switching 
device has first and second current carrying electrical leads 31 and 33, 
respectively, and provides a completed electrical circuit between these 
leads when the temperature of the device exceeds a predetermined 
temperature level. Additionally, an overvoltage arc gap is provided such 
that current will flow between leads 31 and 33 when the voltage across the 
device exceeds a set potential level. A first electrically conductive 
electrode means is connected to first electrical lead 31 and includes 
electrically conductive case 35 having a first end 37 defining an opening 
into a central cavity 39 and a second end 41 which is closed. The first 
electrically conductive electrode means also includes an electrically 
conductive spacer means 43 and a spring means 45. 
A second electrically conductive electrode means includes a conductive 
member 47 which is connected to the second lead 33. Nonconductive means 
for mounting the first and second electrode means in spaced relation 
includes a ceramic bushing 49 which is held in place by a groove 51 in 
case 35. The outer end 37 of case 35 is crimped inwardly against a 
shoulder of the ceramic bushing 49 to secure it in place. A sealing 
compound 53 covers the end of the bushing and seals the opening into 
cavity 39. As shown in the drawings, the second electrically conductive 
electrode means is positoned within cavity 39. 
A heat fusible conductive pellet 55 is positioned in central cavity 39 in 
electrical contact with the second electrode means. The pellet 55 defines 
an air gap of predetermined dimension with the first electrode means. The 
pellet means 55 may advantageously be formed of various alloys. One such 
alloy which has been found to be useful is composed of 55.5% bismuth and 
44.5% lead by weight, having a fusion temperature of approximately 
255.degree. F. 
Positioned between the pellet means 55 and the conductive spacer means 43 
is an insulator means 57 which maintains a predetermined gap between the 
spacer and pellet means. As seen in FIG. 4, insulator means 57 is annular 
in shape and may be formed of mica or other insulative material. 
When connected for protection of an electrical device, the switching device 
of the present invention will be positioned in close proximity to the 
protected device and connected electrically in parallel therewith. The 
voltage supplied to the protected device will also be provided across 
leads 31 and 33. During normal operation of the protected device, there 
will be no current flow between leads 31 and 33 since bushing 49, sealing 
compound 53 and insulator means 57 will provide electrical isolation 
between the first and second electrically conductive electrode means. 
Should, however, the temperature of the protected device rise such that 
pellet means 55 is heated to a temperature exceeding its fusion 
temperature, actuation of the switching device will occur, as shown in 
FIG. 3. The melted pellet material 59 will bridge the gap between 
conductive member 47 and conductive case 35 and thus provide an electrical 
connection between leads 31 and 33. As can be seen from FIG. 3, spring 
means 45 will have moved conductive spacer means 43 and insulator means 57 
to the right. The spring 45 is provided in the device to exert a force at 
all times on the conductive pellet means 55. This constant force is 
provided in order to insure actuation at the desired predetermined 
temperature level. 
It has been found that pellets of the type used in the present invention 
which are formed of a conductive metallic material may develop oxide 
coatings on their outer surfaces. Such coatings will not melt at the 
predetermined temperature level at which the pure alloy will fuse, but may 
have a substantially higher fusion temperature. Although not particularly 
strong, the oxide coating may maintain the structural integrity of the 
pellet even after the pellet is heated above its fusion temperature. By 
applying force to the pellet, uniform temperature actuation is obtained 
since the outer oxide coating will be crushed after the unoxidized 
interior pellet material has fused. The pellet 55 is generally annular in 
shape and is positioned in cavity 39 such that it contacts member 47 but 
is held out of contact with the conductive case 35 until fusion occurs. 
The air gap which is provided for overvoltage protection of the protected 
device is defined between the pellet means 55 and the spacer means 43. The 
annular insulator means 57, by its thickness, maintains precisely the 
desired gap dimension. The gap is defined through the interior opening in 
the insulator means 57. It will be appreciated that variation on gap 
dimension and, consequently, variation in the electrical potential 
required for arcing may be effectuated simply by providing insulator means 
57 of varying thicknesses. 
While the form of apparatus herein described constitutes a preferred 
embodiment of the present invention, it is to be understood that the 
invention is not limited to this precise form of apparatus and that 
changes may be made therein without departing from the scope of the 
invention.