COMMISSIONER'S DECISION
OBVIOUSNESS: Electric Fuse
A glass coating on a resistance wire reacts under overload to cause,
a rapid opening of the circuit. The claims were rejected under 45(4) as
this is shown in a prior patent.
Rejection: Affirmed.
This decision deals with a request for review by the Commissioner of Pat-
eats of a refusal of claims C1 to C18 inclusive of patent application
155,163. The refusal was made by an Office letter dated October 28, 1975
issued as a result of re-examination of the claims under Section 45(4) during
conflict proceedings.
The application was filed November 4, 1971 by Matthey & Mallory Limited and
is entitled "Fusing Resistor." Mr. G. Seaby represented the applicant at a
Hearing conducted by the Patent Appeal Board on March 3, 1976.
This application relates to an electrical safety resistor (i.e. a fuse)
having improved means for interrupting an electrical current flowing through
the fuse in the event of overloads. The conductive portion of the fuse is
coated with a glass which melts at a lower temperature than the conductor
itself, and reacts with the conductor to destroy it, and thus break the cir-
cuit. Temporary overloads will not melt the glass, but if they continue the
fuse interrupts the circuit at lower temperatures than would fuse the conductor
itself, and the risk of fire from hot fuses is reduced.
In the Office letter, claims C1 to C18 inclusive were rejected for failing to
patentably distinguish over the following prior art:
German Auslegeschrift 1,196,765, July 15, 1965, Kugelstadt
The Office letter said:
The applied publication teaches the use of a safety resistor
which is self destructing at electrical overloads by means of
a glass layer which adheres to the resistor at least in part.
The glass layer becomes ionic above its melting point and
destroys the resistor through electrolysis.
The Figure shows a coiled resistor inside a tubular base
which is sealed with resin around the electrodes.
Claims C1 to C18 are rejected for being anticipated by the
applied publication.
The applicant must reply to this letter to either cancel the
rejected claims C1 to C18 or show how their subject matter
is patentably different from the subject matter of the refer-
ence.
If an argument is presented that the subject matter of a
rejected conflicting claim is patentable, and the claim
continues to be a conflicting claim, the patentability of
such claim will be reviewed by the Patent Appeal Board before
a final decision is made by the Commissioner. In view of
this, the applicant should submit a full statement of the
reasons why the cited prior art is not pertinent and if an
oral hearing is desired before the Patent Appeal Board such
must be requested within the time limit of 3 months set
for response to this letter.
...
It appears that there is no patentable subject matter in the
present application over the German publication. However, this
will be dealt with at a later time.
In his response dated January 23, 1976 the applicant stated (in part):
The cited German reference does not teach the use of a protective
covering for the resistor, as claimed in conflicting claims C1 to
C7 and C10 to C18. In fact, there is no need for such a
protective covering, since the resistance film in the device of
the cited reference is on the inside of a tube. Strictly speaking,
the reference does teach the use of grooves formed in the resistance
material, as claimed in each of conflicting claims C6, C7 and C12
to C17. The resistance material of the cited reference is applied
as a strip in the form of a helix. The cited reference does not
teach the use of glass applied annularly around the resistor.
More importantly, the reference does not teach the use of a
metal oxide as the resistance film. The reference teaches the use of
noble metals, giving by way of example a gold-platinum metal layer. A metal
and a metal oxide are not equivalent, the metal oxide being an alloy
with entirely different characteristics from the metal. For example,
with regard to their electrical characteristics, the electrical
conductivity of a metal decreases as its temperature increases, while
the conductivity of an oxide increases as its temperature increases.
It will be noted that the use of a metal oxide as the resistance
material is claimed in each of conflicting claims C2 to C4, C8 and C11.
The advantage of using a metal oxide as the resistance material,
and in particular of tin oxide is set out in the paragraph
starting on page 3 of the disclosure.
The German patent describes an electrical safety resistor comprising a tubular
insulating base enclosing a spiralled coiled metal layer on the inner surface
of the base. Connecting wire leads extend through resin-sealed ends of the
resistor, and these are soldered to the coiled metal layer. A powdered glass
covers several coils of the metal so that when an overload occurs the glass
fuses to react with the metal layer to open the circuit. The applicant conductor
is wrapped around the outside of the non-conductive base, but we are satisfied
such structural alterations are of no material significance.
This application relates to resistors having a layer of material disposed over
a portion of the resistive film of the resistor. This material has a melting
point lower than the melting point of the resistive film and when melted it
reacts with the film to open the circuit. Claim C1 reads:
A resistor comprising: a non-conductive substrate, a resistive
film on said substrate, a layer of material disposed on at least a
portion of said resistive film having a melting point below the
melting point of said resistive film which when melted chemically
reacts with said film to render said portion thereof non-conductive,
metallic caps disposed at opposed ends of said substrate and overlying
said film, electrical leads coupled to said end caps, and an insulative
cover substantially surrounding said film and said caps.
At the hearing, the applicant and the Board agreed to confine the issue to
one of "obviousness," rather than "anticipation." Obviousness had been raised
on pages 1 and 2 of the Office letter where we find such statements as
"there is no patentable subject matter in the present application over the
German publication" and "or show how their subject matter is patentably
different from the subject matter of the references." It was clear to the
Board that a rejection for anticipation could not be sustained. There are
minor structural differences between the two fuses which preclude such a
rejection, but not necessarily one for obviousness.
The applicant contends that in the reference the melting temperature of
the glass is higher than the melting temperature of the film, whereas in
claims C1 to C18 the melting temperature of the glass is below that
of the resistive film.
In describing the prior art, the German publication states (as translated)
"Previously resistors were made with carbon, metal strips or wires and con-
structed to act as safety devices which, in addition to their function as
resistors, self-destruct when there is an electric overload, so that the
circuit into which the resistor is inserted can no longer pass current, and
the remaining components of the circuit are protected against destruction."
According to the publication such resistors are not satisfactory because
"there are cases wherein the overload is not sufficient to destroy the resistor
rapidly; it can merely destroy it slowly, eg., with a gradual disintegration
of the resistor layer. The increase in the resistance value introduced
(among other things) by the disintegration of the layer reaches (in this
connection) frequently orders of magnitude that exceed by far the maximal per-
missible deviation from the rated [required, desired] value of resistance."
The German applicant continues to indicate how he overcomes such difficulties,
and we quote:
The invention solves the problem in such a manner that the metal
layer arranged on the base of a resistor is covered at least in
part with adhering glass whose temperature of fusion corresponds
to a predetermined temperature of the layer that appear at an
inadmissible overload of the resistor and which glass exhibits
ionic conductivity only above the temperature of fusion.
and:
In relation to known safety resistors constructed as layer resistors,
the safety resistors of this invention also offer considerable
advantages in manufacture. For example, any completed metal-layer
resistor can be transformed into a safety resistor by coating it
with a suitable glass, without the necessity of providing narrowed
portions in the resistor layer, such as is otherwise customary in
safety resistors. In accordance with the degree of mechanical stresses
to which safety resistors are subjected in use, ranging from shaking
to positions at rest, the glass can be bonded in powder form to the
metal layer with a silicon resin as binder, depositied on it by means
of silicone oil, powdered on the metal layer, or even fused onto
the layer.
It is clear that the German publication does use a glass whose melting tem-
perature is below the melting temperature of the resistive film. If that
were not so, there would be no purpose in using the glass additive, since other-
wise the metal would fuse first to open ti circuit, as was common in the
prior art. We quote again from the German reference:
The object of the invention is to produce safety resistors that
are not only destroyed automatically at a very high overload
but are self-destructing even at overloads that normally lead
only to an overstepping of the maximal permissible deviation from
the rated value of resistance and not to the destruction of the
resistor. (underlining added)
In this invention, when an overload occurs the glass coating on the resistance
film is fused by the heat evolved from the resistive film. This makes the
fused glass conductive, thereby rapidly changing the resistance value of the
resistive film and causing swift failure of the resistor. The applicant uses
the same means to accomplish the same result as shown in the German publication.
The applicant has stressed that a metal oxide resistance film is not shown
in the reference. However the use of tin oxide resistor films in this art is
well known. See, for example, the admission on page 1, line 2 of the
applicant's own disclosure. Since the conductivity of a metal oxide increases
with current overloads, it would generate more heat before it fuses than a
metal resistor under similar circumstances. There is no suggestion in the
disclosure that a resistance film of metal oxide would provide a superior
fusing resistor than a metal resistor, and in our view does not represent a
patentable advance in the art. Whether the resistor is a metal or metal oxide
is immaterial to the invention we are considering, which is the use of glass
or other materials to decompose the resistor. The applicant was obviously of
the same mind, since the main claim contains no limitation to oxides.
We are satisfied that claims C1 to C18 inclusive fail to show any patentable
advance in the art and we recommend that the decision to refuse these claims
be affirmed.
G. Asher
Chairman
Patent Appeal Board
I concur with the findings of the Patent Appeal Board. Claims C1 to C18
inclusive are refused. The applicant has six months within which to re-
move the claims, or to appeal this decision under the provisions of
Section 44 of the Patent Act.
J.H.A. Gari‚py
Commissioner of Patents
Dated at Hull, Quebec
this 22nd. day of April, 1976
Agent for Applicant
Marks & Clerk,
Box 957, Station B,
Ottawa, Ontario