GB2069776A - Fuse containing boric acid - Google Patents

Abstract

A current limiting fuse comprising an insulating tube (2), a fuse element (4) set in the insulating tube, and an arc extinguisher (3) including boric acid. The boric acid may be mixed with silica or calcium silicates. <IMAGE>

Description

SPECIFICATION Current limiting fuses This invention relates to current limiting fuses.

Conventional current limiting fuses contain a fuse element set in an insulating tube with an electrically conductive cap at each end thereof. The insulating tube may comprise a ceramic or material glass fibre treated with a heat-resisting resin. The insulating tube is fitted with an arc extinguisher comprising a granular mixture of silica (SiO2) and calcium carbonate (CaCO3) and surrounds the fuse element. When conventional current limiting fuses are used for breaking short circuit currents, the fuse element melts and is vaporised by the momentary high temperature of the arc. The metal vapor from the fuse element is diffused and absorbed into the arc extinguisher thereby extinguishing the arc. However, the arc extinguisher is melted by the high temperature of the arc and becomes electrically conductive. As a result current continues to flow through the current limiting fuse.This current is generally termed "secondary current". FIGURE 1 illustrates the time-current characteristics of conventional current limiting fuse of the 7.2 kilovolt (KV) and 50 ampere (A) type. In FIGURE 1 the abscissa is time measured in milliseconds (msec), the ordinate is the current measured in kiloamperes (KA), i is the initial passthrough current and i2 is the secondary current. The dotted line indicates the ideal wave form of the passthrough current it without any secondary current. The secondary current i2 causes the energy generated by the arc within the insulating tube to become so great that the temperature and pressure therein continue increasing over that generated by the initial overload. As a result of this, the insulating tube may explode and/orthe current limiting fuse may not be able to break the current caused by the short circuit.In order to prevent such explosions, the insulating tube is made large and is reinforced.

Additionally, if the secondary current of such a current limiting fuse is large, the electrical apparatus and/or circuits protected by such fuses may be damaged because of the poor current breaking characteristics of the fuse.

Accordingly, there is a requirement for current limiting fuses having low secondary current characteristics.

The present invention seeks to provide current limiting fuses having reduced secondary current characteristics, and which are preferably small in size and can break a large current.

The invention further seeks to provide a current limiting fuse which may not require reinforced insulating tubes.

According to the present invention there is provided a current limiting fuse having an arc extinguisher including boric acid.

Some embodiments of this inventon will now be described with reference to the accompanying drawings, wherein: FIGURE 1 is a wave form chart that shows the cur rentitime characteristics of a conventional current limiting fuse; FIGURE 2 is a partially sectional view of a preferred current limiting fuse of th is invention; FIGURE 3 is a wave form chart illustrating the currentitime characteristics of the current limiting fuse shown in FIGURE 2, corresponding to the chart of FIGURE 1; and FIGURE 4 is a graph illustrating the relationship between the percentage of orthoboric acid in the granular mixture of arc extinguisher, and the secondary current in the current limiting fuse of this invention.

The current limiting fuse 10 shown in FIGURE 2 comprises an insulating tube 2, and a fuse element 4 set in the insulating tube which is fitted with an arc extinguisher 3 around the fuse element 4. The insulating tube 2 comprises an insulating material, such as glass fibre treated with a heat-resisting resin, and has an electrically conductive cap 1 at each end.

The fuse element4 is 3 conventional fuse element and comprises a conventional fuse material, such as silver, silver alloy or the like. The arc extinguisher 3 is a granular mixture comprising boric acid (B2O3.nH2o, wherein n is a positive integer).

In addition to boric acid the arc extinguisher may contain other materials which are stable under the conditions encountered in breaking short circuits.

Such materials include silica (SiO2), calcium silicates, and similar materials.

When the fuse element is vaporized by an excessive amount of current arcing will occur. The boric acid extinguishes this arc by depriving the arc of its energy. First, the boric acid is dehydrated by the high temperature of the arc. This dehydration consumes a large amount of energy since the water of hydration is tightly bound to the boric acid crystal. Secondly, the water released by the dehydration step then consumes additional energy when it is vaporized to steam. Thirdly the dehydrated boric acid (boric oxide) consumes still more energy when it melts. As a result, the arc is extinguished and the flow of secondary current is eliminated or greatly reduced.

The advantage of boric acid over conventional arc extinguishers lies in the large amount of energy consumed in the steps of dehydrating the boric acid and then vaporizing the water released.

The amount of boric acid present in the arc extinguishing composition is not critical provided sufficient boric acid is used to achieve the desired low secondary current characteristics. It is preferred that the arc extinguisher also contain some silica, calcium silicates or similar material. If the arc extinguisher contains only boric acid there is the possibility that sufficient steam may be released to cause the fuse to explode unless it is properly reinforced against the pressures generated or the fuse is vented. Venting may be accomplished by providing the insulating tube with a plug or similar device which melts at elevated temperature thereby, allowing the steam to escape and preventing an explosion. However, by diluting the boric acid with silica or similar material, the quantity of steam generated can be controlled to avoid an explosion hazard.In fuses which are designed for low currents, the arc extinguisher may comprise solely boric acid since there is little danger that sufficient steam will be generated to present an explosion hazard.

The minimum amount of boric acid to be included in the arc extinguisher is that sufficient to yield a fuse having the desired secondary current characteristics.

FIGURE 4 illustrates the results obtained for an extinguisher containing various quantities of orthoboric acid for two fuse ratings. Curve LA illustrates the results obtained for a 10 ampere fuse and curve LB a 50 ampere fuse. As can be seen, minute quantities of orthoboric acid markedly improve the secondary current characteristics of these fuses.

For low currentfuses, the optimum amount of boric acid in the arc extinguishing composition is generally less than 50 wt. per cent. Larger amounts may be used, up to 100 wt. per cent, but such large quantities of boric acid do not significantly improve the secondary current characteristics over that obtained through use of lesser amounts.

With fuses designed to carry large currents the use of an arc extinguisher comprising 100 wt. per cent boric acid may be used provided the fuse is reinforced to prevent explosion caused by the large amount of steam generated by the arc when the fuse capacity is exceeded. However, the use of 100 per cent boric acid does not significantly improve the secondary characteristics of the fuse over that obtained with lesser quantities of boric acid. Significant improvement in the secondary current characteristics of the fuse may be obtained with small quantities of boric acid, in particular at amounts of 1 per cent or less of the total arc extinguisher composition. Thus, 0.1, 0.001 or even lower weight percentage of boric acid may be used in the arc extinguisher.The lower limit of boric acid is determined solely by the secondary current characteristics desired and may be readily determined by those skilled in the art.

The total amount of arc extinguisher used in the fuse corresponds to that conventionally employed in the art and may be readily determined by those skilled in the art.

Orthoboric (B2O3.3H2O) is the preferred form of boric acid for use in this invention. This acid decomposes upon heating by the arc to metaboric acid (B2O3.H2O), tetraboric acid (2B303.H20) and finally at about 300"C to boric oxide (B2O3). Each of these conversions absorbs significant energy from the arc as does the vaporization of the water released. However, either metaboric acid ortetraboric acid may be used if desired or mixtures of orthoboric acid with metaboric acid and/ortetraboric boric acid may be used. The term boric acid as used herein encompasses all these forms of boric acid. When either metaboric ortetraboric acid is used as the boric acid larger quantities may be used since these acids release less water and, hence, generate less steam.

The optimum amounts may readily be determined by those skilled in the art.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for the purposes of illustration only and are not intended to be limiting unless otherwise specified.

EXAMPLE 7 Two types of fuses were prepared containing vari ous quantities of orthoboric acid and silica. One is designated fuse A, which is 7.2KV, 1 OA type fuse and has a single fuse element in the insulating tube. The other fuse is designated fuse B, which is 7.2KV, 50A type and has five fuse elements in the insulating tube. The fuse element and the insulating tube in fuse B are the same as in fuse A. FIGURE 4 illustrates the relationship between the percentage of orthoboric acid in the mixture of arc extinguisher and secondary current. In FIGURE 4 the abscissa is the percentage of the orthoboric acid measured by weight percent and the ordinate is the secondary current 12 measured in kiloampere (KA). The quan tity of the secondary current 12 is as in FIGURE 3.In FIGURE 4 curves LA, LB indicate the relation of the secondary current case of fuse A, B respectively.

As can be seen in FIGURE 4, for fuse A, the secondary current was significantly reduced at 10 wt. per cent of orthoboric acid, and was further reduced at more than 20 weight percent. In the case of fuse B, the secondary current was greatly reduced at greater than 1 weight percent of orthoboric acid. FOP fuses A and B of this example, the percentage of orthoboric acid should be less than 50 and 10 weight percent respectively.

EXAMPLE2 A current limiting fuse rated at 7.2 KV and 50A was prepared using a granular mixture of orthoboric acid and silica. The mixture contained 5 weight per cent orthoboric acid. The current characteristics of this fuse are setforth in FIGURE 3. FIGURE 1 illustrates the current characteristics for a 7.2 KV, 50A fuse not containing boric acid. As can be seen from a comparison of FIGURES 1 and 3, the presence of boric acid in the arc extinguisher markedly improved the current characteristics of the fuse.

The percentage of orthoboric acid of the mixture of the arc extinguisher can be determined for other fuses by using the method of these examples. That is, the lower limit of the percentage of orthoboric acid is determined by the relationship between the secondary current and the boric acid content, and the upper limit is determined by the ability of the insulating tube to withstand the pressures generated by the water vapor released.

As described above, this invention can provide current limiting fuses having reduced secondary current characteristics. This invention also provides current limiting fuses which are small in size and can break large currents without the need for reinforcing the insulating tube. Moreover, this invention provides current limiting fuses which are superior in current breaking characteristics.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims (9)

1. A current limiting fuse containing an arc extinguisher including boric acid.

2. A current limiting fuse according to claim 1, wherein said arc extinguisher comprises a mixture of boric acid and silica.

3. A current limiting fuse according to claim 2, wherein said boric acid is orthoboric acid.

4. A current limiting fuse according to claim 3, wherein the percentage of said orthoboric acid in said arc extinguisher is less than 50 weight per cent.

5. A current limiting fuse according to claim 2, wherein said boric acid is metaboric acid.

6. A method of reducing secondary current in a current limiting fuse which comprises incorporating boric acid into said fuse.

7. The method of claim 6 which comprises incorporating silica and boric acid into said fuse.

8. The method of claim 6 wherein said boric acid is orthoboric acid.

9. A current limiting fuse substantially as herein described with reference to FIGURE 2 and 3 of the accompanying drawings.