Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND ~F THE INVENTION
This invention relates to electrically heated glass
panel refrigerator doors, and more partlcularly, to a control
system for substantially preventing electrical shock when the
glass panel of a ~efrigerator door is accidentally broken.
Glass panel refrigerator doors, such as the type shown
in U.S. Patents 2,987,7B2; 3,131,421; 3,499,245; and 3,629,972,
have a glass panel mounted in a movable metal door frame. In
order to keep the glass panel substantially fogless and frost free
and to substantially prevent the frame from "sweating" and collect-
ing moisture, the glass panel and frame are equipped with an
electric panel heater and door heater, respectively. Typically,
the electric panel heater includes a heating element such as a
transparent electrically conductive coating on one of the inacces-
sible surfaces of the panel.
Normally, the electrically heated glass panel and
frame do not present any shock hazard because the panel and door
heaters are inaccessible, however, if the glass panel is acci-
dentally broken, the electrically conductive or "live" panel
heater and/or door heater may become accessible, creating a
hazardous condition which can cause electrical shock to persons
who accidentally touch or contact the broken panel.
It is therefore desirable to provide an electrically
heated glass panel refrigerator door system which substantially
prevents electrical shock if the glass panel of the refrigerator
door is accidentally broken. While various protective devices
and circuits, such a those found in U.S. Patents 3,4~9,551;
3,475,594; and 3,52~,753, have been previously employed with
v~rying degress of success in an attempt to solve this problem,
30 there still exists the need for a new and improved system which
is reliable, quick ~nd fail-safe.
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SUMMARY OF THE INVENTION
A new and improved electrically heated glass panel
refrigerator door system is provided to prevent electrical
shock if the glass panel of the refrigerator door is accidentally
broken.
According to the invention there is provided an
electrically heated glass panel refrigerator door system, com-
prising: a glass-like panel assembly for permitting viewing
of products in a refrigerator and a movable door frame securing
said panel assembly for access into said refrigerator, said
panel assembly including at least one transparent glass-like
pane having a normally inaccessible surface; panel heating means
for heating said panel assembly to substantially prevent the
condensation of moisture and the formation of frost on said
panel assembly so that said panel assembly is substantially
fogless and frost free, said panel heating means including bus
bar means and an electrically conductive coating connected to
said bus bar means on said normally inaccessible surface of
said transparent glass-like pane, said bus bar means including
a first bus bar and a second bus bar spaced from said first
bus bar and connected to said electrically conductive coating;
door heating means for heating said door frame to substantially
prevent the condensation of moisture and the formation of frost
on said door frame; power supply means for energizing said door
heating means and said panel heating means; and interlock switch .
means connected to and between said power supply means and said
panel and door heating means for effectively disconnecting and
deactivating said door heating means and said panel heating
means from said power supply means in response to an unsafe
condition when said transparent glass-like pane is accidentally
broken causing the electrical resistance of said panel heating
means to increase with said panel heating means being dangerously
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accessible, said interlock switch means including fuse heating
means for emitting heat in response to said unsafe condition,
said fuse heating means connected in parallel with said panel
heating means and in series with said door heating means to
pass effective heating current when said panel heating means
resistance increases, and thermal responsive means operatively
associated with said fuse heating means for opening said circuit
connection between said power supply means and said door and
panel heating means in response to said heat from said fuse
heating means to substantially prevent electrical shock from
both said door heating means and said accessible panel heating
means on said broken pane.
In the illustrative embodiment the fuse heating means
includes a tubular ceramic heater which circumscribes the thermal
responsive means.
The interlock switch means can include a plurality of
thermal responsive devices and a plurality of fuse heaters to
increase the safety and reliability of the control system.
A more detailed explanation of the invention is pro-
vided in the following description and appended claims taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic circuit diagram of an elec-
trically heated glass panel refrigerator door system in accord-
ance with principles of the present invention; and
FIGURE 2 is a fragmentary schematic circuit diagram
illustrating a portion of another electrically heated glass panel
refrigerator door system in accordance with principles of the
present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT
Referring to Figure 1 of the drawings, an electrically
heated glass panel refrigerator door system 10 has a glass panel
1~ 3~347
refrigerator door 12. Such refrigerator doors are desirably
used in supermarkets, convenient food stores, beverage stores
and the like in refrigerated display cases and cabinets from
which purchasers may select refrigerated beverages, foods and
other products. The doors may be employed on self-contained
refrigerator cabinets or i.n connection with walk-in coolers or
other built-in refrigerated display facilities and commercial
refrigerators which have shelves or bins to hold and display
products for sale. Such doors are used for both normal temper-
ature
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.~
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and low temperature refrigeration. Normal temperature appli-
cations involve the display of dairy products and the like at a
temperature slightly above the freezing temperature. Low tem-
perature applications involve the display of frozen foods, ice
cream and the like, at a temperature well below freezing. Desir-
ably, the customer selects the products or merchandise by look-
ing through the glass-like panel assembly 14 of door 12, opening
the door 12 and removing the merchandise from the refrigerator.
Preferably, the refrigerator door 12 is of a swinging
or pivotally hinged type of construction, such as the type shown
in U.S. Patents 2,987,782; 3,131,421 and 3,499,245, although othe~
types of refrigerator doors can be used with this system such as
the sliding ~ype construction shown in U.S. Patent 3,508,361.
The glass-like panel assembly 14 of refrigerator door
12 generally takes the form of a glass panel mounted in a movable
rectangular metal door frame 16. Panel assembly 14 desirably is
of the insulated type and includes a plurality of parallel light-
transmissive and preferably transparent glass-like panes 14a made
of glass or impact-resistant plastic to clearly display and per-
mit viewing of the merchandise or products in the refrigerator.Preferably, the panel assembly 14 has three glass panes, although
in some circumstances it may be desirable to use one, two, four
or even more glass panes.
Spaces are hermetically sealed between the edges of the
pane to provide dead air spaces therebetween, so that moisture
will be excluded from the dead air spaces between the panes. The
dry air between the panes generally has good insulating value.
While such spaces are typically filled with dry air, they can be
filled with an inert gas, such as nitrogen.
~L13~)8~7
In order to prevent the glass panel assembly 14 from
"fogging up" and accumulating frost, which would interfere with
full visibility of the products in the refrigerator, the door 12
is equipped with panel heating means such as an electrical panel
heater 18 having an electrical heating element, which preferably
takes the form of a transparent electrically conduc~ive coating
18a or layer on one of the normally inaccessible surfaces of the
glass panes 14a. Desirably, the electrically conductive coating
18a is intimately bonded ox fused to the rear or inner surface of
the front glass pane such as shown in U.S. Patent 3,475,594 and
prevents the condensation of moisture as well as formation of frost
on the panel assembly 14 so that the panel assembly is substan-
tially fogless and frost free. The ends of the electrically con-
ductive coating are connected to bus bar means, such as first and
second opposed bus bars or leads 18b and 18c, respectively. The
bus bars are connected to wires or lines 22b and 22c which pass
through the door and are suitably connected to power supply means,
such as an electrical power source 24 or line.
In order to prevent the condensation of moisture and
the formation of frost on the metal door frame 16, the door frame
is provided with door heating means, such as one or more internal
door heaters 20, which can take the form of heating cables mounted
in the hollow core of the door frame, such as shown in U.S. Patent
3,499,245. The door heater 20 is connected to bus bars 18b and
18c by lines 22b and 22~, respectively, and is connected to an
outlet of the power source 24 via line wires 26b, 26c and 26d.
The power source or line is adapted to supply an alternating
current at 110 volts and 60 cycles or any other desirable voltage
and frequency.
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In accordance with principles of the present inven-
tion a novel interlock switch means, such as interlock switch
28 is provided to effectively disconnect and deactivate the panel
heater 18 and door heater ~0 if the glass panel assembly 14 is
accidentally shattered or otherwise broken. The interlock switch
generally defines a control system or circuit which is generally
reliable, quick and substantially fail-safe.
The interlock switch 28 is connected between the power
source 24 and the door heater 20 and panel heater 18 via lines
26b and 26d, and desirably draws little power so as to not sub-
stantially interfere with the heating function of the door heater
20 and panel heater 18. In the preferred form, the interlock
switch 28 includes heating means, such as a fuse heater 28a,
connected to thermal responsive means, such as a thermal respon-
sive device or fuse 28b. The fuse heater 28a emits or generates
heat in response to an unsafe or hazardous condition when the
glass panel assembly is broken, causing the fuse to activate or
blow thereby effectively disconnecting the door heater 20 and
panel heater 18 from the power source 24. In the illustrative
embodiment, the fuse heater 28a takes the form of an elongated
tubular ceramic heater which longitudinally surrounds and circum-
scribes fuse 28b and desirably has leads connected in parallel
to or shunted across the opposed end leads of fuse 28b. The
tubular ceramic heater construction generally includes a ceramic
sleeve into which is embedded a pair of conductive termin~l rings
adjacent the ends of the sleeve, and a heating coil of finely
wrapped wire positioned between and connected to the conductivè
terminal rings. While the described ceramic heater is preferred,
it may be desirable in some circumstances to use a different
type of fuse heater construction such as a heater or thermal
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activating device which is operatively associated with but does
not necessarily surround the fuse 28b.
Normally, the fuse heater 28a is shunted out of the
circuit and is not activated and therefore passes little or no current.
If the gl~ss panel is accidentally broken, one or both of the
bus bars 18b or 18c, as well as the electrical conductive coating
18a may be severed, causing the panel heater 18 to become danger-
ously aceessible, however, in the present circuit such situation
causes the fuse heater 28 to energize in series with the door
heater 20 and panel heater 18. When energized, the fuse heater
28a generates a sufficient amount of heat to cause the fuse 28b
to melt thereby opening the circuit or line 26d
between the power source 24 and the door heater 20 and panel
heater 18 which effectively deactivates and disconnects the door
heater and panel heater from the power source 24. In this manner,
the hot leads 22b and 22c to the bus bars 18b and 18c and the
door heater 20 are quickly and safely neutralized to avoid pos-
sible electric shock or other haxm to patrons or other individuals
who may accidentally make physical contact with the panel heater,
broken pane or door heater.
When a 300 ohm ceramic resistor heater was used in
conjunction with a thermal fuse which opened at about 154F. in
a refrigerator door having a 300 ohm door heater and a 150 ohm
resistance in the electrically conductive coating, the fuse
opened in about 15 to 18 seconds after the glass panel and bus
bars were severed. Other values for the ceramic heater, fuse,
door heater and electrically conductive coating can be chosen
when desired.
It may be desirable in many circumstances to provide
an extra margin of safety to the circuitry of the present inven-
tion by providing a plurality of interlock switches, including at
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least one interlock switch connected to one of the conductive
lines of the power-source and at least one other interlock
switch connected to another conductive line of the power source.
One such circuit for accomplishing this result is illustrated
in Figure 2, in which a pair of interlock switches 128 and 130
are connected to lines 126d and 126c, respectively, of the power
source 124. Each of the interlock switches includes heating
means, such as a fuse heater 128a or 130a, operatively associated
with or connected to thermal responsive means, such as a thermal
responsive device or fuse 128b or 130b, with the fuse heaters
128a and 130a connected in series to each other along line 126b
and fuses 128b and 130b connected to power lines 126d and 126c,
respectively. The other end leads of fuse heaters 128a and 130a
are connected to door heater 120 and first bus bar 118bl respec-
tively, while the other end leads of fuses 128b and 130b are
connected to the second bus bar 118c and door heater 120, and the
first bus bar 118b, respectively, so that fuse heaters 128a and
130a are shunted across fuses 128b and 130b. Preferably, the fuse
heaters 128a and 130a and fuses 130a and 130b are of the same
type of construction as described in relationship to the fuse
heater 28a and fuse 28b of Figure 1, although in some circumstances
it may be desirable to use other types of fuse heaters or fuses.
The other components of the circuit 110 shown in Figure 2 are
substantially similar to the other components of the circuit 10
shown in Figure 1 and for ease of illustration and understanding,
the similar parts of the circuit 110 shown in Figure 2 have
been given numbers similar to the parts of the circuit 10 in
Figure 1, but increased by a hundred, such as door heater 120,
panel heater 118, glass panel assembly 114, etc. The operation
of circuit 110 is substantially the same as the operation of
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circuit 10 of Figure 1 with the additional safety and reliability
feature of two interlock switches 128 and 130 which effectively
disconnect both power lines 126c and 126d from the panel heater
118 and door heater 120 when the panel assembly is accidentally
broken.
Although embodiments of this invention have been shown
and described, it is to be understood that various modifications
and substitutions can be made by those skilled in the art with-
out departing from the novel spirit and scope of this invention.
What is desired to be protected and secured by Letters
P t t of tho Unitoa-~atoc is- :
a en
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