GB2163612A - Electrical components - Google Patents

Abstract

An in-line fuse holder 10 incorporating as a thermal barrier an insert 53 comprising a square piece of paper 52 rolled into tubular form. The inner surface of the paper is printed with a metallic coating 54 in a pattern of squares separated by discontinuities 56 which prevent shorting. The heat-insulating properties of the paper 52 and the reflective properties of the coating 54 protect the holder 10 when the element of the fuse 47 heats up under overload and improve its capability to fail when loaded above its rating. Other forms of thermal shield cut for example from adhesive-coated sheet are used to protect fuse housings and printed circuit boards. <IMAGE>

Description

SPECIFICATION Improvements in and relating to electrical components This invention relates to electrical components such as, for example, fuses and RF chokes incorporating a thermal barrier; and to sheet material for forming a thermal barrier in such components, the material also having other applications, for example in protecting fuse housings and printed circuit boards from exposure to excessive temperatures.

Conventional in-line fuse holders having bodies and cups moulded from thermoplastics material tend to suffer from distortion, melting or degradation of their components parts caused by excessive heat released by an overloaded fuse for a sufficient period of time to exceed the temperature limits of the material.

The total wattage dissipation and subsequent heat generation within an in-line fuse holder is dependent on the quality of the contacts and the composition of the fuse element material. Many fuses employ a high temperature copper element and under overload conditions the copper element can glow red hot, emitting infrared radiation to cause rapid overheating and failure of the thermoplastics fuse holder body. Until now the only way to prevent this failure was to make the fuse body of a thermosetting plastics material or ceramic and it is desirable to achieve a similar performance in the face of the heat generated by higher current carrying fuses, whilst using a thermoplastics material which provides a cheaper form of moulding, and would effect substantial cost saving.

According to the present invention there is provided an electrical component comprising an electrically conducting element, a housing formed of plastics material which is liable to suffer degradation from heat generated by the element when subject to prolonged overload conditions, and a thermal barrier interposed between the element and the housing.

By thermally insulating the housing from the radiation emitted by the conductive element not only is the housing shielded but also some of the radiation is reflected back to the element, thus elevating its temperature and shortening the time until open circuit occurs.

The thermal barrier may comprise a heatreflecting and/or a heat-absorbing layer and e.g. paper or fibre glass matting be in the form of a loose or secured insert or lining.

Alternatively the layer or layers constituting the thermal barrier may take the form of surface layers coated on the housing.

Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through an in-line fuse holder according to the invention in assembled condition; Figure 2 is a transverse section on an enlarged scale on the line 2-2 in Fig. 1; Figure 3 is a plan view of the heat-reflective insert shown in Figs. 1 and 2 in the flat condition; Figure 4 is a perspective view of a fuse holder base protected by heat-reflective sheet material according to the invention; Figure 5 is a partial section through a holder for blade-type fuses, similarly protected.

As shown in Figs. 1 and 2, an in-line fuse holder 10 comprises a barrel-shaped body 12 having a collar 14, and a cap 16 moulded from a 6:6 nylon thermoplastics material. The cap 16 has two lugs 18 and engages the collar end of the body 12 by means of a bayonet-type fixing comprising two axial slots (not shown) formed inwardly of the collar 12 and each allowing passage of a respective lug 18 into a connected circumferential arcuate slot 20. As the bayonet-type fixing is conventional it will not be described in further detail.

Passing through an orifice in the outer end of the body 12, i.e. the end remote from the collar 14, is a lead 22 having a core 24, the bared end of which is secured as by crimping within a hollow stem 26 of a contact 28 having a generally cup-shaped body 30. The base of the body 30 is provided with a cylindrical surface 32 over which engages in a tight fit an end of a compression spring 34, the other end of which seats against a shoulder 36 formed in the outer end of the body 12. The spring 34 is thus retained on the contact 28 when the holder 10 is disassembled.

The inside of the cup-shaped body 30 of the contact 28 is formed with a major internally frusto-conical surface 38 having a taper angle of 2 and stepped at its inner end to a minor internally frusto-conical surface 40 having a taper angle of 5 and terminates in a flat floor 42 having a central conical recess 44.

The mouth of the cup 30 is formed with a chamfered lead 45.

A similar contact 28 is housed within the cap 16 and has a shorter spring 46 engaged over the cylindrical surface 32 thereof.

Recessed between the two contacts 28 is a standard Continental cylindrical fuse 49, each pointed end cap 48 of which engages the conical recess 44 of the respective contact 28.

In Fig. 3 is shown a square piece 50 of heat-reflective material comprising a backing layer of 0.005" (0.013 cm) thick white paper 52 having printed on the glossy surface thereof in metallic silver a pattern of spaced squares 54 forming a grid-like pattern of rectilinear discontinuities 56 therebetween. On the opposite, matt surface of the paper (not shown) is printed wording indicating that the square 50 is a heat shield and not to be removed, and any other desired wording.

To form a heat shield the square 50 is sim ply rolled up to form a tubular insert 53 and inserted into the body 12 (see Figs. 1 and 2).

The Continental-type fuse 49 shown in Figs.

1 and 2 is one of the three standard forms of fuse that can be accommodated in the holder 10, the other types being cylindrical caps fuses having diameters of 0.25" (6.5 mm) and 5 mm. The range of fuse lengths that the holder 10 will accommodate is from 20 to 32 mm and it has a continuous rating of up to 20A: the holder is thus very versatile. Moreover when the fuse received in the holder 10 is one having a cylindrical end cap, the taper on the frusto-conical surface 38 or 40 allows the fuse to travel into the cup during its working life, thus always maintaining good conduct even if the fuse is passing a high level of current as is the case if it is protecting a circuit supplying a cellular telephone or an electrically-aperated car window.Additionally, the maintenance of good contact reduces the voltage drop and incidence of corrosion with consequent arcing between the fuse cap and contact. It will be appreciated that the use of two springs 34,46 reduces contact voltage and subsequent dissipation and restricts heat flow from the fuse 49 through 28 to the body 12 and caps 16.

The tubular insert 53 acts in two ways as a heat shield to protect the thermoplastic body 12 and cap 16 from damage by heat dissipated by the fusible element in the fuse 49 up to a rate of 6-8W, especially under prolonged overload conditions when the element emits infra-red radiation. Firstly, the paper 50 acts as a thermal insulator, preventing heat from being transmitted to the body 12 and cap 60.

Secondly, the metallic silver coating 54 reflects back some of the infra-red radiation towards the element. Accordingly not only are the body 12 and cap 16 protected from distortion or melting thus prolonging their life and allowing re-use of the holder, but also the temperature of the element is raised by the containment of the heat and by the reflected radiation, thus causing it to fail sooner, thereby reducing the time during which it is operating at excessively high temperature and lessening the risk of fire.

The electrical breaks in the insert 53 constituted by the discontinuities 56 ensure that the fuse 49 is not shorted out before or after failure.

When the holder 10 fitted with a soda glass automobile cartridge fuse having a copper wire element designed to blow within 10 seconds at 50A was run for two minutes at 40A, the energy dissipation was 18.5W and the heat generated sufficient to distort the glass barrel of the fuse. However, no distortion or damage to the body 12 or cap 16 was observable.

In the case of a lower wattage dissipating fuse of low current rating a thermal barrier consisting of a layer of heat insulating material is effective to enhance the performance of a conventional holder moulded from cheap plastics material and having a single spring and pressed end contacts.

In Fig. 4 is shown a holder 60 for a cylindrical fuse 62, the holder comprising a base 64 moulded from thermoplastics material and having two spring clips 66 mounted thereon. To protect the base 64 from thermal degradation the upper surface lying between the clips 66 has adhered thereto with a suitable adhesive a rectangular piece 68 of the same sheet material as shown in Fig. 3. The base 64 is thus protected from short-term softening due to heat radiated by the fuse 62 when overloaded.

As an alternative to the use of a separate adhesive the piece 68 of sheet material may be derived from a sheet having its non-reflective surface coated with a pressure-sensitive adhesive protected by a peelable protective layer.

Other uses of heat-reflective sheet material according to the present invention are to protect hot spots on printed circuit boards and whenever a heatdegradable material is exposed to excessive heat generated by a conducting element.

Fig. 5 is a partial section through a fuse box 70 accepting blade fuses one ofwhich is designated 72. The box 70 has a base 74 and a cover 76, the upper surfaces of the base 74 and the inner surface of the cover 76 being covered with pieces 78 of heat-reflective material of the same type as described with reference to Fig. 4.

My earlier Application No. 2 094 077A disclosed an in-line fuse holder having the interior surface of the board provided with a reflective metal coating to enhance heat-build up and ensure earlier blowing of the fuse. In the following Claims I make no claim to such a fuse holder having only a continuous metal coating on its interior surface.

Claims (42)

1. An electrical component comprising an electrically conducting element, a housing formed of plastics material which is liable to suffer degradation from heat generated by the element when subject to prolonged overload conditions, and a thermal barrier interposed between the element and the housing.

2. A component as claimed in Claim 1, in which the housing is of the type represented by an in-line fuse or RF choke holder, and the thermal barrier is constituted by a tubular insert, lining or internal coating.

3. A component as claimed in Claim 2, in which the insert or lining comprises a tube or a rolled up sheet of heat-insulating material.

4. A component as claimed in Claim 2 or 3, in which the insert, lining or coating comprises a heat-reflective layer on the inner face thereof.

5. A component as claimed in Claim 4,in which the heat-reflective layer is a metallic layer.

6. A component as claimed in Claims 4 or 5 in which said layer is formed with electrical breaks.

7. A component as claimed in Claim 5, in which said breaks are constituted by regularly or irregularly arranged discontinuities.

8. A component as claimed in any one of Claims 2 to 7, in which said insert or lining bears indicia on the outer face thereof.

9. An electrical component substantially as herein described with reference to Figs. 1 to 3 of the accompanying drawings.

10. A component as claimed in Claim 1, in which the housing comprises a base provided with mounting means for receiving a blade or cartridge fuse, choke or other circuit interrupting element, and optionally a cover, and the thermal barrier is provided as one or more layers present on at least part of the surface of the housing exposed to radiation from said element.

11. A component as claimed in Claim 10, in which the thermal barrier comprises a layer of heat-insulating material.

12. A component as claimed in Claim 10 or 11, in which the thermal barrier comprises a layer of heatreflective material.

13. A component as claimed in Claim 12, in which the layer of heat-reflective material is a metallic layer.

14. A component as claimed in Claim 13, in which the metallic layer is formed with electrical breaks.

15. A component as claimed in Claim 14, in which said breaks are constituted by regularly or irregularly arranged discontinuities.

16. A component as claimed in any one of Claims 10 to 15, in which the layer or layers constituting the thermal barrier are adhered to said inner surfaces.

17. An electrical component substantially as herein described with reference to Fig. 4 or 5 of the accompanying drawings.

18. Adhesive heat-reflective sheet material for shielding a surface from heat generated by a current-carrying electrical element, the sheet comprising a backing layer having on one face thereof a layer of heat-reflective material and on the opposite face thereof an adhesive layer.

19. Material as claimed in Claim 18 and additionally comprising a peelably removable protective layer on the adhesive layer.

20. Material as claimed in Claim 18 or 19, in which the heat reflective layer is a metallic layer.

21. Material as claimed in Claim 20, in which the metallic layer is formed with electrical breaks.

22. Material as claimed in Claim 21, in which said breaks are constituted by regularly or irregularly arranged discontinuities.

23. Material as claimed in Claim 22, in which the discontinuities have the form of a grid.

24. Material as claimed in any one of Claims 18 to 23 and bearing indicia visible from the non-heatreflective side thereof.

25. Adhesive heat-reflective sheet material substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

26. Heat-reflective sheet material for forming an insert receivable in a holder for an in-line fuse or an RF choke or for shielding a surface from heat generated by a current-carrying electrical element, the material comprising a backing layer having on one side a metallic coating formed with electrical breaks.

27. Material as claimed in Claim 26, in which the breaks are constituted by regularly or irregularly arranged discontinuities.

28. Material as claimed in Claim 27, in which the discontinuities are arranged in the form of a grid.

29. Material as claimed in any one of Claims 26 to 28 and bearing indicia on the other face thereof.

30. Heat-reflective sheet material substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

31. A contact for an in-line fuse or RF choke holder comprising a cup formed of electrically conducting material and having a wall defining an inwardly converging frusto-conical cavity, the mouth of which is of a diameter sufficient to accept the cylindrical cup of a standard cartridge fuse.

32. A contact as claimed in Claim 31, in which the cup wall defines a further inwardly converging frustoconical cavity, the mouth of which has a diameter which is less than the smallest diameter of the first-mentioned cavity and is dimensioned to accept a cup of smaller diameter than the first-mentioned cup.

33. A contact as claimed in Claim 31 or 32, in which the cup has an internally conical bottom for receiving the end of a standard continental-type fuse.

34. A contact as claimed in any one of Claims 31 to 33, in which the cone angle of the or each cavity is from 2" to 5".

35. A contact as claimed in any one of Claims 31 to 34 in which the rim of the cup is provided with an internal chamfer.

36. A contact substantially as herein described with reference to Fig. 1 of the accompanying drawings.

37. A contact as claimed in any one of Claims 31 to 36 in which the cup has an external annual surface and which additionally comprises a spring an end of which grips said surface to retain the spring thereon.

38. An in-line fuse or choke holder incorporating at least one contact as claimed in any one of Claims 31 to 36.

39. An in-line fuse or choke holder incorporating two contacts as claimed in Claim 37, the spring fitted to one contact being larger than that fitted to the other contact.

40. A holder as claimed in Claim 38 or 39 and incorporating a lining or insert as defined in any one of Claims 2 to 8.

41. A component as claimed in any one of Claims 2 to to 8 and incorporating at least one contact as defined in any one of Claims 31 to 37.

42. The features as herein disclosed, or their equivalents, in any novel selection.