FORM 2
The Patents Act, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A low electrical resistance element and a method of manufacturing the same
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Datta Partha, Nikam Murlidhar Narayan and Singal Vivek, all Indian nationals and all of Crompton Greaves Ltd, Advanced Materials and Process Technology Centre (AMPTC), CG Global R and D Centre, Kanjur Marg (E), Mumbai - 400042, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to a low electrical resistance element and a method of manufacturing the same.
PRIOR ART DESCRIPTION
Electrical resistance elements are used for current control, especially in electrical devices like circuit breakers, particularly high voltage SF6 gas circuit breakers which have to handle high to very high currents during switching operations thereof. Electrical resistance elements are used as shunts in circuit breakers to provide a parallel path for the high to very high currents and to assist current control during switching operations of the circuit breakers. UK Patent No 1286815 describes an electrical resistance element comprising 40-90%) of particles of refractory material such as calcined clay or alumina as high resistivity material and 0.5 - 40% of carbon as low resistivity material and 3-50% of clay such as ball clay and/or bentonite as basic binder. Qualities like low electrical resistance, good load bearing capacity and mechanical strength and good thermal dissipation efficiency are all desirable characteristics of a good electrical resistance element. There is, therefore, scope for development of electrical resistance elements of increased desirable characteristics.
OBJECTS OF THE INVENTION


An object of the invention is to provide a low electrical resistance element having high thermal dissipation efficiency and high load bearing capacity and mechanical strength and thermal shock resistance.
Another object of the invention is to provide a method of manufacturing a low electrical resistance element having high thermal dissipation efficiency and high load bearing capacity and mechanical strength and thermal shock resistance.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a low electrical resistance element of nodular butter chemical structure comprising a high resistivity material comprising 30-65%) of calcined clay, 15-30% of reactive alumina and 20-40% of tabular alumina, all of particle sizes of 2.5 to 250 microns, a low resistivity material comprising 0.02-2% of graphite of particle sizes of 2-8 microns and 0.1-5% of a polymeric binder.
According to the invention there is also provided a method of manufacturing a low electrical resistance element of nodular butter chemical structure, the method comprising the following steps:
i) mixing a high resistivity material comprising 30-65% of calcined clay, 15-30% of reactive alumina and 20-40% of tabular alumina, all of particle sizes of 2.5 - 250 microns, a low resistivity material comprising 0.02 - 2% of graphite of particle sizes of 2-8 microns and 0.1-5% of a polymeric binder;


ii) granulating the mixture and sieving the granules to obtain granules of 500-
1000 microns sizes;
iii) compacting the granules under a pressure of 250-350 MPa;
iv) iv) sintering the green electrical resistance element in an inert atmosphere at
1300-1500°C;and
v) allowing the electrical resistance element to cool down to ambient
temperature.
Preferably the raw material mix comprises 45-60% of clay, 18-25% of reactive alumina, 22-30% of tabular alumina, 0.05% to 0.15% of graphite and 0.1-2% of polymeric binder. Preferably the graphite is of particle sizes 4-7 microns. Preferably the polymeric binder is selected from polyvinyl alcohol, polyethylene glycol or polyester resin or mixtures thereof. Preferably the polymeric binder comprises polyvinyl alcohol and polyethylene glycol. Still preferably the material mix comprises 54% of calcined clay of particle size of 2 microns, 21% of reactive alumina of particle size of 2.7 microns, 25% of tabular alumina of particle sizes of 45-250 microns, 0.75% of polyvinyl alcohol and 0.25% of polyethylene glycol and 0.05 to 0.15% of graphite of particle sizes of 6 microns; still preferably 0.1% graphite, 0.08% graphite or 0.12% graphite; and the above mix is compacted under a pressure of 340 MPa and sintered at 1450°C in nitrogen atmosphere. The raw material mix is mixed with or without water. The raw materials mix is granulated in a granulator, the granules are compacted, for instance, in a hydraulic press and the green electrical resistance element is sintered, for instance, in an electrical arc furnace. Preferably the inert atmosphere is nitrogen or argon.

The graphite may be in powder or colloidal form. A compaction pressure of 250-350 MPa ensures a fired density of 2.5± 0.03 gram/cc for the electrical resistance element. The electrical resistance element of invention is found to have a nodular butter chemical or mullite structure which has the distinct advantage of thermal shock resistance. Further the electrical resistance element of the invention has low resistivity of the order of 0.5 to 17 ohms, high load bearing capacity and mechanical strength (because of the high cold crushing strength) and high heat dissipation efficiency. Therefore, it is ideal for use in current control applications especially in electrical devices like circuit breakers particularly high voltage SF6 gas circuit breakers.
The following experimental examples are illustrative of the invention but not limitative of the scope thereof:
Example 1
Calcined clay particles of 2 micron size (54%), reactive alumina particles of 2.7 micron size (21%), tabular alumna particles of 45-250 micron size (25%), graphite powder of 6 micron size (0.1%) and polyvinyl alcohol (0.75%) and polyethylene glycol (0.25%) were dry mixed and granulated. The granules were sieved to obtain granules of 500-1000 micron sizes. The granules were compacted at 350 MPa and the green electrical resistance element was sintered at 1450°C in nitrogen atmosphere. The electrical resistant element was allowed to cool down ambient and had the following characteristics. Electrical resistance - 4±0.4 ohms.
Cold crushing strength - 90-95 MPa.


Example 2
The procedure of example 1 was followed with 0.08% graphite to obtain electrical resistance element of the following characteristics:
Electrical resistance-16 ±2 ohms. Cold crushing strength - 90-95 MPa.
Example 3
The procedure of Example 1 was followed with 0.12% graphite to obtain electrical resistance element of the following characteristics:
Electrical resistance -1±0.4 ohms. Cold crushing strength - 90-95 MPa.


We claim:
1. A low electrical resistance element of nodular butter chemical structure comprising a high resistivity material comprising 30-65% of calcined clay, 15-30% of reactive alumina and 20-40% of tabular alumina, all of particle sizes of 2.5 to 250 microns, a low resistivity material comprising 0.02-2% of graphite of particle sizes of 2-8 microns and 0.1-5% of a polymeric binder.
2. The low electrical resistance element as claimed in claim 1, which has resistance of 0.5 to 17 ohms.
3. The low electrical resistance element as claimed in claim 1 or 2, which comprises 45-60% of clay, 18-25% of reactive alumina, 22-30% of tabular alumina, 0.05% to 0.15% of graphite and 0.1-2% of polymeric binder.
4. The low electrical resistance element as claimed in any one of claims 1 to 3, wherein the graphite is of particle sizes 4-7 microns.

5. The low electrical resistance element as claimed in anyone of claims 1 to 4, wherein the polymeric binder is selected from polyvinyl alcohol, polyethylene glycol or polyester resin or mixtures thereof.
6. The low electrical resistance element as claimed in anyone of claims 1 to 4, wherein the polymeric binder comprises polyvinyl alcohol and polyethylene glycol.


7. The low electrical resistance element as claimed in claim 1, which comprises 54% of non-calcined clay of particle size of 2 microns, 21% of reactive alumina of particle size of 2.7 microns, 25% of tabular alumina of particle sizes of 45-250 microns, 0.75% of polyvinyl alcohol and 0.25% of polyethylene glycol and 0.5 to 0.15% of graphite of 6 microns compacted under a pressure of 340 MPa and sintered at 1450°C in nitrogen atmosphere.
8. The low electrical resistance element as claimed in claim 7 which comprises 0.1% of graphite.
9. The low electrical resistance element as claimed in claim 7, which comprises 0.08% of graphite.
10. The low electrical resistance element as claimed in claim 7, which comprises 0.12% of graphite.
11. A method of manufacturing a low electrical resistance element, of nodular butter chemical structure, the method comprising the following steps:
vi) mixing a high resistivity material comprising 30-65% of calcined clay, 15-30% of reactive alumina and 20-40% of tabular alumina, all of particle sizes of 2.5 - 250 microns, a low resistivity material comprising 0.02 - 2% of graphite of particle sizes of 2-8 microns and 0.1-5% of a polymeric binder;
vii) granulating the mixture and sieving the granules to obtain granules of 500-1000 microns sizes;


viii) compacting the granules under a pressure of 250-350 MPa;
ix) sintering the green electrical resistance element in an inert atmosphere at
1300-1500°C;and x) allowing the electrical resistance element to cool down to ambient
temperature.
12. The method as claimed in claim 11, wherein the raw material mix comprises 45-60% of clay, 18-25% of reactive alumina, 25-30% of tubular alumina, 0.08-0.15% of graphite and 0.1-2% of polymeric binder.
13. The method as claimed in claim 11 or 12, wherein the graphite is of particle sizes 4-7 microns.
14. The method as claimed in anyone of claims 11 to 13, wherein the polymeric binder is selected from polyvinyl alcohol, polyethylene glycol or polyester resin or mixtures thereof.
15. The method as claimed in anyone of claims 11 to 13, wherein the polymeric binder comprises polymerizing alcohol and polyethylene alcohol.
16. The method as claimed in claim 11, wherein the raw material mix comprises 54% of calcined clay of particle size of 2 microns, 21% of reactive alumina of particle size of 2.7 microns, 25% tabular alumina of particle sizes of 45-250 microns, 0.75% of polyvinyl alcohol and 0.25% of polyethylene glycol and 0.05 to 0.15% of graphite of 6 microns and


the raw material mix is compacted under a pressure of 340 MPa and sintered at 1450°C in nitrogen atmosphere.
17. The method as claimed in claim 16, wherein the raw material mix comprises 0.1% of graphite.
18. The method as claimed in claim 16, wherein the raw material mix comprises 0.08% of graphite.
19. The method as claimed in claim 16, wherein the raw material mix comprises 0.12% of graphite.




Abstract
A low electrical resistance element of butter chemical structure comprising a high resistivity material comprising 30-65% of non-calcined clay, 15-30% of reactive alumina and 20-40% of tubular alumina, all of particle sizes of 2.5 to 250 microns. It also comprises a low resistivity material comprising 0.02-2%) of graphite of particle sizes of 2-8 microns and 0.1-5% of a polymeric binder.