Resistor materials and tolerances
The resistance alloys used are high temperature stainless steels capable of withstanding temperature excursions to 1100°C whilst retaining their strength – unlike 304 or other structural grade stainless steels which are limited to much lower temperatures. Earthing resistors designed for operation to higher temperatures require less active mass, resulting in more compact and economical designs.
All the stainless steels used to make earthing resistors elements increase in resistance as they get hotter, typically between 1% and 15% per 100°C rise. There is a cost penalty when low-coefficient materials are specified; to what extent this is worth paying is not always clear. Many earthing resistors are over-specified in this respect, as much because engineers are intuitively uncomfortable with the idea of an earthing resistor with a very wide current range than as a result of a performance analysis. There is certainly scope for cost saving by accepting the use of higher temperature coefficient, lower cost steels.
We make extensive use of grade 430 stainless steel; this is a low-cost 18% chrome alloy with little or no nickel content but with good corrosion and high-temperature performance. It has a temperature coefficient of resistance of about 14% per 100°C , which makes it unsuitable for continuous applications, but it is widely used in resistors for dynamic braking, motor starting and neutral earthing for which the key property is high heat capacity rather than close resistance tolerance.
Testing: type tests
Realistic, full-size type tests are an essential part of the development process and are the point of departure for all our design rules and methods. It is reasonable for any customer to demand sight of such documents. Because of the expense, type testing is usually only required when new designs have fundamental changes or extensions to the operating envelope.
For high current ratings the most suitable earthing resistor element is the type RP oval edge-wound coil. RP resistors are manufactured from a continuous stainless steel strip, wound edgewise into oval coils. Each coil is supported by ridged ceramic insulators mounted on a stainless steel centre support, with stainless steel terminals welded at each end. This arrangement allows free expansion and contraction at operating temperatures up to 1000°C without imposing strain on the assembly.
HPR grid resistors
For medium current ratings (30-100A) the HPR grid is used; this is a versatile, robust construction with a high ratio of surface area to active mass and can be manufactured in a very wide range of resistor materials.
For low currents the most suitable elements are the coiled-coil, type HP, which has a compact construction, excellent overload rating and good shock resistance.