IEC 60076-25:2023 applies to dry type natural air-cooled resistors, for neutral grounding of transformers and generators, in order to limit the earth fault current in power systems by means of metallic resistive elements.
For the purposes of this document, the resistor can be:
- used alone or in combination with other electrotechnical products not covered by this document, such as (but not limited to): a step-down single-phase transformer, an open triangle or zig-zag transformer (where the neutral point is not available) and a Petersen coil reactor (in order to increase active power contribution to the fault or reduce time constant for proper protection operation or both);
- designed, manufactured and verified on a one-off basis or fully standardized and manufactured in quantity.
Both terms "neutral grounding resistor" (NGR) and "neutral earthing resistor" (NER) can be used. However, for the purposes of this document and in order to avoid any confusion with "neutral earthing reactor" (NER), the term "neutral grounding resistor" (NGR) is used.
This document specifies:
- the characteristics of the NGR;
- the service conditions requirements for NGRs;
- the tests and test methods for confirming that these conditions have been met;
- the requirements relating to marking for NGRs.
Annex A provides guidance on how to consider the effect of resistance variation with temperature.