Note: Descriptions are shown in the official language in which they were submitted.
H. A. Bergersen et al 4-1
. (Revision)
1~7~3319
ELECTRICAL RESI5TANCE ~EATING ELEMENT
- Background of the Invention
~ he present invention relates to electrical resistance
he~ting elements and in partic.ular to elements having a flat
S configuration and consisting of a number of metal strips
- arranged in a meander like pattern.embedded in or laminated
withi~.an-insulation ~ody.
- ---Such heating elements are well known from U.S. Patents
Nos. 3,263,307 and 3,336,557 ~both to O.G. Lund et al) as
wèll as from U.S. Patents Nos. 4,~25,893 and ~,092,626 (both
to H.A. Bergersen), a main feature of these inventions being
that-the resistance metal strips are mads of a material having
a-melt-ing point lower than 200C. When such elements are
used às electrical heating of apartments and houses and the
install~tion necessitates close contact with combustible~
mat-erial such as wood and wallpaper, i~ is essential that
the temperature rise of the heating element surroundings
at no place exceeds 150~C. This requirement can be achieved
by using as a resistance strip a metal alloy consisting of
61.5~ tin, 37.7~ lead and 0.8% antimony, which has a melt-
ing point at 183C.
A heating element, where low melting resistance strips
are welded between two layers of insulating material, acts
as one large thermal fuse if it is operated under abnormal
conditions, for example, when it is unduly covered by heat
insulating materials. When the temperature in such cases
approaches the melting point of the alloy (170C+), the
mechanical properties of the foil strips become very poor,
and in this state, the foil may fracture at any time before
the melting point is reached. At the moment the strip
H.A. Bergersen et al 4-1
(Revision)
~7833~9
fracture starts, the cross-section of the strip is reduced~
and it melts instantly, causing the current-path to be broken.
The heating element must thereafter be replaced with a new one.
There are, however, also known resistance heating
elements having only one fuse. Such elements are for instance
described in U.S. Patent No. 3,417,229 (G.J. Shomphe et al),
in which there is inserted a thermal fuse ~or each individual
heater unit. Such units will only be guarded against over-
heating if the fuse itself is overheated. Such overheating
will occur either-if the fuse itself is'excessively thermally
insulated or if ~he heater unit draws excessive current. The
fuse-will melt at a desired temperature when overheated and
~ cut the current so that the heater unit itself probably
need~not be replaced. If, however, a hea-ter unit area some-
what-displaced from the fuse is overheated, the wallpaper
and: surroundings may catch fire before the fuse cuts the
current-path.
- The-safest type of resistance heating el~ments is
' therefore considered to be that previously described. Experi-
i~ 20 ments have shown, however, that it is not necessary for each
and every square centimeter of the resistance element to be
capable of fusing at 'the desired low temperature.
- - ' Summary o~ the Invention
~he'main objec~ of the present invention is the'provision
25- of a new'and improved heating element which maintains the
excellent hea~Lng and installation properties of existing
ele~ents.' `
'` A feature of the invention is the provision of an
electrical resistance heating element in which at least
one fuse portion is incorporated in a resistance metal strip.
By using a heating element in accordance with the
present inv~ntion thermal overheating will cause breakdown
of the current path at defined places, improving the safety
and reducing the risk of overheating
The thermal fuses must be distributed in such a way that
if the resistance element, in addition to the usual installa-
tion covering, i5 partly covered by various items, such as
- boarding, furniture, carpets, etc., the surface temperature
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H.A. Bergersen et al 4-1
~L7~3~L~3 ( Revi s ion )
on the covered area must a~ no place exceed a critical value.
The degree of unauthorized covering may vary with type and
size of material or article, and the worst case will occur if
;heat transfer from a certain area is effectively blocked.
By experiments it will be possible to de~exmine the maximum
size of a randomly placed area of the heating element which
may be effec~ively thermally insulated without causing rise
of the ~urface temperature to ~he critical value at any place
within the covered area. This area is called the critical
area. The thermal fuses will there~ore have to be distributed
- in ~such a way that at least one such ~use will operate if an
area-larger than the critical area is covered sufficiently
to ~lock the heat transfer.
In many instances, ~hermal insulation of a certain area
of a hea~ing element is not effective. In experiments done
in=order to de~ine--the critical area, a lO0 mm thick mineral
wool mat was used às excess covering on the outside of the
regu~ax sur~ace material.
~ In order to simulate a heating element ceiling installa-
tion, the following arrangement was used. A heating element,
500 x 1200-mm in size~ constructed from meander formed lead
~antimony strips laminated between plastic sheets, was installed
between a~200 mm thick mineral wool mat in horizontal position
and a 12 mm thick chip board, the board facing downward. At -
stabiIized condi~ions, the heating element was operatingat 210 W/m with a maximum temperature on the chip board
o~ 78~C.
In order to simulate harmful covering of the ceiling,
pieces of 100 mm thick mineral wool mats were placed against
the lower side of the chip board. The critical temperature
was chosen to be 175C, and the critical area turned out to
be in the order of 400 cm2. The size of the critical area
-will decrease with decreased chip board thickness. It will
also decrease if the chip board is exchanged with materials
having lower sideways heat conduction. The preferred shape
for the critical area is a circle, but a square will give a
close approximation.
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7~3~9
The well-known process of making heating elements of
the described type is to start from a block of a desired alloy
and roll this block into a metal sheet of 5-25 ,u thickness.
The sheet is thereafter cut to produce a des;red resistance
strip web, e.g. a meander pattern, while laminating the resis-
tance strip with insulation material on one or both sides.
When making a suitable resistance element of the pre-
sent invention, it is essential that the main metal component
is inexpensive, and lead is still considered to be the most
suitable material. However, in order to make the lead sheet
less brittle, it should be alloyed with about 1% antimony.
The distributed discrete thermal fuses are obtained
by locally introducing a layer of metal that alloys to lead
giving a composition having a melting point lower than 200C.
A suitable material is a metal selected from the group consis-
ting of tin, lead/tin/antimony alloy and lead/bisrnuth alloy.
In summary, according to a first broad aspect of the
present invention, there is provided a method for making an
electrical heating element comprising: forming a sheet of
electrical resistance material; arranging a layer of fuse
material in a predetermined pattern on the resistance material;
pressing the fuse material and the resistance material together
to form a layer of uniform thickness, and cutting the pressed
layer in a meander pattern.
According to a second broad aspect of the present
invention, there is provided an electrical heating element
comprising: a layer of an electrical resistance material having
a predetermined melting temperature; and a fuse material having
a melting temperature below said predetermined melting temper-
ature arranged in a predetermined pattern incorporated in saidresistance material, saidfuse material having been pressed
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into said resistance material to form a layer havin~ a uniform
thickness.
Brief Description of the Drawing
The above mentioned and other objects and features of
the present invention will become more apparent by reference
to the following description taken in conjunction with the
accompanying drawings, in which:
Figure 1 illustrates various ways of thermal fuse
arrangements,
Figure 2 schematically shows fuse material placed on
a base material sheet,
Figure 3 illustrates alternative fuse material arrange-
ments on a heating element, and
Figure 4 shows examples of discrete thermal fuses on
the resistance strips.
Description of the Preferred Embodiment
... ... _ .
There are, as illustrated in Figure 1, several ways
of introducing thermal fuses in the resistance strip. In
Figure la the base material 1 is provided with an insert 2 of
fuse material. In Figure lb the fuse material 2 is embedded
between two base material layers. In Figure lc the base
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- H.A. Bergersen et al 4-1
(Revision)
material sheet or strip is broken by a full cross-section of
fuse material 2. While a cross-section as illustrated in
Fig. la and lb can be obtained by a rolling process, the
Fig. lc version will also include a soldering process before
rolling. Fig. 2 schematically illustrates various ways of
arranging fuse material strips 2 on an uncut base material
sheet 1.
When the materials 1 and 2 illustrated in Figs 1 and 2
are lead and tin respectively (preferably the fuse material
2 should be a lead/tin alloy), it is assumed that the heat
resulting from normal operation of the heating element will
cause the materials in the contacting area to alloy so that
the small portion of the alloyed material will have a
desired melting point of about 180~C. It may, however, be
'more desirable to ensure during the manu~acturing process
and_before placing the heating element into normal operation
tha~'the base~f~se'contact area is alloyed to have the
desired melting point.
~hen a heating element is manufactured according to the
above or other methods, an element is obtained wherein any
one of--the distributed discrete thermal fuses will alloy in
a ratio of-approxLmately 60% tin and 40% lead, melt and break
the cùrrent path if a randomly placed area of a certain size
tcritical areaj o the heating element including the thermal
fuse is covered in an unauthorized manner.
~ While thè material of the resistance metal strips has
bee~'indicated as lead and tin as well as lead and bismuth,
othe~ materials may be used, for examplet steel as base
material and silver/copper as fuse material, or chromium/
nickel as base material and brass as fuse material, etc.
Fig. 3 illustrates three alternative ways of arranging
fuse material strips 7 on a heating element 5. The electrical
resistance strips 6 are for convenience indicated to form a
meander pattern, but it should be obvious that the resistance
strip or strips can be arranged in any convenient manner.
While the fuse material strips are shown as full drawn lines,
it will be understood that the fuse strips will not form a
current path along these lines. This statement is at least
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H.A. Bergersen et al 4-1
(Revision)
true for the illustrations of Figs. 3a and 3c, where the
final product will look somewhat like the partial view of
the heating element shown in Fig. 4a. In Fig. 3b, however,
where the fuse strips are placed so as to be parallel with
the longitudinal parts of the meander resistance strip
pattern, the fuse strip will carry current along a sub-
stantial part of its length.
A discrete thermal fuse'is obtained at all crossings or
over~appings between a fuse strip 7 and a resistance strip 6
in Figs. 3a, 3b and 3c. In accordance with the present
invention the discrete thermal fuses should be distributed
so tha~ at least one fuse will be covered wholly or partly
by a critical area 8, which. is randomly placed on the heating
. element 5. As described, the relevant fuse will operate,
i.e. melt, when it is covered so that as its temperature
reaches-the cri~ical value, a break will occur in the
current'path.'~
Figs.~4a and 4b schematically illustrate an expanded
.view` of a part.of the heating element 5 of Figs. 3a and 3b
incLuding resistance strips 6 which'are provided with a
number of discrete thermal fuses 7' and 7" respectively.
~ The fuse material strips should preferably be placed
on the resistance material base foil at some stage prior to
cutting.of the base foil into the desired pattern of
resistance strips (Fig. 2). In this way it is ensured that
- the fuse material strips are also cut to provide the desired
number of discrete thermal fuses on the resistance strips.
:- Another possible method is to'arrange the discrete
fuses on the resistance strips after the base sheet has
been cut into the desired pattern, but such a method will
be rather complicated,.and difficulty will also be
'experienced in obtaining the proper contact between the
two materials-.
In all embodiments of th.e present invention proper
contact or alloyed connec~ion between the two materials
wilL be facilitated if the material surfaces are clèan and
free from corrosion and oxide films before the contacting
operation.
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H.A. Bergersen et al 4-1
(Revision~
.
- While we~have described above the principles o~ our
invention in connection with specific apparatus, it is to
be clearly understood that this description is made only
by way of example and not as a limitation to the scope of
our invention as set forth in the objects thereof and in
the accompanying-claims.
TNT:MM
August 24, 1981
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