This power is partially stored in the mass of the resistor elements before being transferred to the surrounding environment through convection cooling. Not only is the overall mass of the current carrying material important, but also:
- The surface area of the resistor elements
- The temperature difference between ambient air and the heating elements
- The cumulative effect of the resistors on each other
The overall filter design dictates the required resistance. As such, it is imperative the ustomer specify the nominal value of the resistor as well the desired tolerance.
Additionally, the inherent inductance of the resistor may affect the performance of the filter. Fortress engineers their resistor assemblies to not only minimize this inductance, but also limit the change in resistance as the elements heat up to guarantee a stable operating profile.
The resistor must be designed in accordance with local standards (IEEE, IEC, etc.) and consider line to line voltage, arcing distances, creepage distances, BIL withstand level and any expected voltage transients.
The resistive elements, along with the internal and external connections must be carefully considered and engineered to carry not only the rated current, but also periodic high current transients.
Other factors can contribute to the design of the resistor, including physical conditions of the site such as the level of atmospheric pollution, seismic zone, corrosive environment, wind and altitude.