Electrical resistors – With mounting or supporting means – Compensates for or permits resistor
Reexamination Certificate
1999-09-16
2001-12-11
Easthom, Karl D. (Department: 2832)
Electrical resistors
With mounting or supporting means
Compensates for or permits resistor
C338S280000, C338S289000, C338S290000, C338S295000, C338S317000
Reexamination Certificate
active
06329900
ABSTRACT:
The present invention relates to resistors and also to resistor elements joined together to form a resistor.
Resistors are used in many applications and are frequently designed for modular construction in order to enable the desired resistor value or power handling capabilities to be achieved.
Examples of applications of resistors of the type described herein are neutral earthing resistors or test resistors. In neutral earthing resistors a small current may be carried all the time the circuit in which the resistor is connected is in operation. In the event of an earth fault or circuit failure the resistor is required to safely carry large amounts of current usually for a few seconds until the other safety devices incorporated in the circuit can operate.
One of the significant problems with neutral earthing resistors is providing a construction with the ability to dissipate in a controlled manner the heat generated in passing high currents.
Resistors all suffer from problems arising from a temperature increase, for example thermal expansion. The extent of the thermal expansion would depend upon the nature of the material and the temperature rise experienced by the material. In some cases the temperature rise can be significant, so too can the thermal expansion. Controlling the resistive qualities of a resistor over a wide temperature range, and also restraining and supporting the resistor elements when they expand thermally is a difficult problem. This is particularly difficult because in most cases the dimensions of the resistor element will change with temperature and the resistivity also changes with temperature.
According to the present invention there is provided a resistor element including an elongate length of electrical resistance material, the element being supported at or near an end portion by support means and arranged such that the element is suspended substantially vertically, said support means arranged to permit the unsupported part of the element to move in a substantially vertical direction.
A particular advantage of the present invention is that the resistor element is supported by its upper end and allowed to hang vertically so that it may move freely from its supporting means in a vertical direction. The thermal expansion will tend to be in a vertical direction and thus subject to the force of gravity along its axial length. This tends to keep the resistor elements straight so preventing the problems that arise from their twisting and distortion, as can occur in other arrangements.
Resistor elements are frequently grouped together in large numbers to form a resistor matrix. Since they all hang vertically the risk of resistors expanding, then distorting and so contacting one another causing a short circuit are significantly reduced.
According to the second aspect of the present invention, there is provided a resistor including at least two resistor elements, each element including a generally elongate length of resistor material, each element being suspended by support means and suspended in a substantially vertical orientation and supported at or near an upper part of the element so that the element may move in a substantially vertical direction, the elements being connected together by electrically conducting means.
A further advantage of this invention is the provision of a resistor to provide both the appropriate resistor value and the thermal capacity, by use of a plurality of resistor elements. By having many elements close together and suspending them vertically as provided for in the present invention, the problems arising from elements distorting or sagging either during or as a result of a repeated heating and cooling is minimised. The resistor elements all tend to expand primarily in a length wise direction. Normally the surrounding enclosure will be sufficiently large that there is a minimal risk of the elements contacting each other and generating a short circuit. The spacing requirements are normally governed by the voltages being handled and the requirement to have a margin for a protection against electrical creepage caused by accumulated dust being able to generate a conduction path to earth.
In a neutral earthing or test resistor this is particularly advantageous because a large current may be passed very suddenly and over a short period of time. In these circumstances the heat builds up in the resistor very quickly. In such conditions the individual resistor elements are more likely to twist and buckle, with a consequently deleterious effect on the long term life of the resistor. In the present invention the resistor elements all expand primarily in a longitudinal fashion, this helps to keep them straight since they need only be suspended from one point and any expansion can be assisted by gravity. The arrangement also facilitates the gradual dissipation of the heat over a period of time.
In a matrix comprising a plurality of resistor elements all of which are linked together to form a single resistor, the components used to connect the individual elements together are sized and dimensioned such that they provide paths of negligible resistance between the individual resistor elements and therefore are not a component which should be a source of local heat or stress. This can be achieved, of course, by appropriate selection of material as well as dimensioning of the structure.
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Cressall Resistors Ltd.
Easthom Karl D.
Watts, Hoffmann, Fisher & Heinke Co. L.P.A.
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