GB2166010A

GB2166010A - Electrical fuse

Info

Publication number: GB2166010A
Application number: GB08525439A
Authority: GB
Inventors: Cogan Donard De
Original assignee: Brush Fusegear Ltd
Current assignee: Cooper Bussmann UK Ltd
Priority date: 1984-10-16
Filing date: 1985-10-16
Publication date: 1986-04-23
Also published as: GB8525439D0; GB8426109D0

Abstract

In an electrical fuse, a metal fuse element is embedded or covered either in a filler consisting of a mixture of "sand'' and a thermite material or a thermite mixture alone. When the fuse is subjected to an overload current, molten and/or vapourised metal from the fuse element is deposited upon particles of the sand and thermite mixture to initiate an exothermic reaction within the mixture per se. The filler may comprise aluminium, magnesium, titanium, zirconium or hafnium with an oxide of iron, chromium or nickel.

Description

SPECIFICATION Electrical fuse The present invention relates to an electrical fuse and is concerned more particularly with such fuses known in the art as "sand-filled" or "sand-covered" electrical fuses.

In a typical sand-filled electrical fuse, the fuse element is embedded in an insulating material of granular form, which material will be hereinafter referred to as "sand". When the fuse element is subjected to an overload current, the element first heats since the rate of heat generation is greater than the rate at which heat can be dissipated from the fuse. Thereafter the element melts and then vapourises to create a break. An electrical arc strikes across the break in the fuse element and ablates (known in the art as "burn-back") within the confines of the sand until the arc extinguishes and the fuse finally becomes open-circuit. The term "sand covered" is used to describe a fuse element in which the fuse element is a thin film of metal which is deposited onto a rigid insulating substrate and then covered in "sand".

In the above types of fuses, at the stage when the element melts, metal vapour condenses upon the sand particles to provide an alternative conducting path to the fuse element. An additional quantity of heat will be required therefore both to clear the arc and also the alternative conducting path. In conventional fuses this is provided by additional power supplied from the external circuit.

One method of producing additional heat which has been attempted, involves creating an exothermic chemical reaction between the metal of the fuse element and the surrounding sand filler which reaction takes place at a temperature essentially above the melting point of the fuse element.

It has been found that one disadvantage to the use of an exothermic reaction created in this manner is that it tends not to be initiated at a fixed temperature but rather over a range of temperature including temperatures below the melting point of the fuse element, resulting in a "slow" clearing fuse. This results in possible inaccuracies in the rating of the fuse.

An object of the present invention is to provide an electrical fuse of the kind referred to above with a "faster" reaction time. Such fuses will hereinafter be referred to as "sand containing electrical fuses".

In accordance with the present invention, there is provided an electrical fuse comprising a metal fuse element embedded or covered in a mixture of sand containing a thermite mixture in which when the fuse is subjected to an overload current, molten and/or vapourised metal from the fuse element is deposited upon particles of the sand and thermite mixture to initiate an exothermic reaction within the mixture per se.

The heat generated within the sand maintains the metal in a molten or vaporous state, hence hastening the breaking of the arc.

The thermite mixture may include aluminium or magnesium and oxides of preferably iron, chromium or nickel. However titanium, zirconium or hafnium and metallic oxides such as iron (Ill) oxide, chromium (Ill) oxide and nickel (II) oxide may also be used.

Preferred metals for the fuse element are silver or copper. When the metal element first melts it loses mechanical strength. An arc is formed which quickly gives rapid vaporisation of the metal, which coats onto the sand particles. As the metal atoms pass through the sand mixture they initiate the exothermic reaction within the sand particles which produces heat sufficient to maintain the metal in its vapour phase.

When a fuse according to the prior art operates using an aluminium fuse element; heat is emitted at a low level. For example, if a reference level of heat emission is taken to be that emitted when an oxy-hydrogen flame produces a pre-determined quantity of water molecules at the melting point of aluminium, then the reaction between aluminium (the fuse element) and silica (the sand) emits only 70% of the reference level for the production of the same number of molecules of water at the same temperature.

However, according to the present invention in an example where the sand particles used are haematite (red iron oxide Fe2O3) and the fuse element is aluminium, the heat produced is 250% of the reference level under the same conditions. In this example, the amount of heat emitted rises as a function of the temperature at which the reaction takes place which has the effect of accelerating the heat generation process.

Although the sand may comprise solely thermite material, the following further examples were carried out using a mixture of conventional sand and a stoichiometric mixture of thermite. A standard industrial semi-conductor protection fuse using normal sand was operated as a control in all cases.

Thermite Mixtures having compositions were subjected to standard short circuit test conditions, the balance of the mixture in each case being "sand" and the fuse element silver.

Example 1.

Thermite Mixture Arcing Time 10% Al and Fe2O3 2.7ms 18% Al and Fe2O3 4.lems 25% Al and Fe2O3 4.3ms Control fuse 5.3ms (filter-silica) Example 2.

Thermite Mixture Arcing Time 10% Mg and NiO 5.7ms 10% Al and NiO 4.33ms 10% Mg and Fe2Q 4.0ms Control fuse 5.81ms (filter-silica) Fuse of identical design as above but with copper used as the element metal.

Example 3.

Thermite Mixture Arcing Time 10% Al and NiO 6.0ms 10% Mg and Fe2O3 5.3ms Control fuse 6.27ms (filter silica)

Claims

1. An electrical fuse comprising a metal fuse element embedded or covered in a filler of a mixture of sand containing a thermite mixture in which when the fuse is subjected to an overload current, molten and/or vapourised metal from the fuse element is deposited upon particles of the sand and thermite mixture to initiate an exothermic reaction within the mixture per se.

2. An electrical fuse comprising a metal fuse element embedded or covered in a filler of a particulate thermite material in which when a fuse is subjected to an overload current, molten and/or vapourised metal from the fuse element is deposited upon particles of the thermite material to initiate an exothermic reaction within the particles of the thermite material.

3. A fuse as claimed in claim 1 or 2 in which the filler comprises aluminium or magnesium oxides of iron, chromium or nickel.

4. A fuse as claimed in claim 1 or 2 in which the filler comprises titanium, zirconium or hafniums and metallic oxides.

5. A fuse as claimed in any one of claims 1 to 4 in which the metal fuse element is of silver or copper.

6. An electrical fuse element substantially as herein described with reference to any one of the accompanying examples.