Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Fusible element actuated
Patent
1987-03-26
1988-07-12
Broome, H.
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Fusible element actuated
337273, 337280, H01H 8518, H01H 8538
Patent
active
047572965
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to electrical fuselinks and, more particularly, to fuselinks having improved surge-resistant characteristics, for example, a 1 mS delay factor>200.
The delay factor or D.F. is a measure of a fuselink's surge resistance and is defined by the ratio of I.sub.s, /I.sub.f, where I.sub.s is the current required to blow the fuse in a short specified time (1-10 mS), and I.sub.f is the minimum fusing current, that is, the least current which will ultimately blow the fuse if allowed sufficient time.
It has been discovered that one parameter which significantly influences the D.F. of a fuselink is the heat loss from the fuse element. The greater the heat loss, the less is the delay factor. In a conventional cartridge fuselink, for example, some heat is conducted axially along the fuse element to the end caps and a small amount is radiated from the surface of the fuse element but, in an air-filled fuselink, most of the heat loss is by convection to the surrounding ceramic or glass barrel. For example, an increase of 2.7:1 in the D.F. of a 20.times.5 mm cartridge fuselink could be expected if it were practicable to reduce the heat loss by evacuating the air-space within the insulating barrel.
Moreover, it has been discovered experimentally that the introduction of any of the conventional solid thermal insulants into the air-space within the insulating barrel of a cartridge fuselink (e.g. a 20.times.5 mm fuselink) has the surprising detrimental effect of increasing and not decreasing the heat loss. The thermal conductivity of the solid material with its entrapped air is greater than that of free air in a fuselink of this size. The materials evaluated included fibreglass, polystyrene foam and vermiculite. Of course, the provision of a vacuum or reduced air pressure within the space in the insulating barrel would provide for reduced heat loss in relation to that achieved with free air but such a provision is not generally a practical or economical proposition for cartridge fuselinks.
SUMMARY OF THE INVENTION
The present invention has as an object to provide an electrical fuselink having reduced heat loss, and hence improved surge resistance, in relation to hitherto known fuselinks of the same type, and is based on the discovery that in order to decrease heat loss from a fuselink by the use of a solid insulating material, the latter must have certain other characteristics besides a low intrinsic thermal conductivity.
To this end, the invention consists in an electrical fuselink comprising a fuse element and a solid thermal insulating material arranged to reduce heat loss from the fuse element, characterised in that the insulating material includes a multiplicity of cavities or cells which are sufficiently small in size so that the maximum distance apart of the walls of each cavity or cell is less than the mean free path of a molecule of the gas, usually air, occupying the cavities or cells. For example, with an insulating material in which the voidage is occupied by air, the maximum distance apart of the walls of each microcavity or cell must be less than 0.1 microns at normal temperature and pressure or N.T.P. Hence, the cavity or cell size is such as to inhibit conduction by inter-molecular collision of the gas molecules and convection currents are not set-up.
A suitable solid insulating material which has low intrinsic thermal conductivity and which uses this microporous principle to reduce heat transference by conduction and convection is an ultra fine powder of amorphous silica structured and bonded to give an extremely small cavity size which is less than the average inter-molecular collision distance of air. This material is commercially marketed under the trademark "Microtherm" by Micropore International Ltd. of Hadzor Hall, Droitwich, Worcester, WR9 7DJ, Great Britain.
The invention may be applied to a cartridge or other fuselink in which the fuse element is enclosed within an electrically insulating barrel or housing. In either event, all or part of
REFERENCES:
patent: 1016443 (1912-02-01), Sachs
patent: 1480225 (1924-01-01), Snook
patent: 3492619 (1970-01-01), Hagler et al.
Brown Russell
Flindall John D.
Broome H.
Dubilier PLC
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