There are two Industrial standards specifically covering Neutral Earthing Resistors.
The European standard is IEC 60076-25:2023 “Power Transformers, Neutral Grounding Resistors” and the more established American standard IEEE-C57-32a (2020) “Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices”.
Cressall can offer in line with either standard, although insulation coordination and IP ratings for both will (unless agreed differently) be in accordance with IEC 60071 and IEC 60529 respectively.
NERs are sized so that after application of the phase voltage for the rated time the hottest part of the resistor elements does not exceed an agreed temperature rise above ambient.
This temperature limit is selected to be within the performance limits for the resistor material, the insulators and the mechanical construction used. IEEE-C57.32a (2020) specifies this temperature rise as 760°C and this figure is widely accepted within the industry.
It is possible to operate at higher temperatures and this could reduce the size and price of the resistor however this will then be outside of the accepted standards.
Resistors are usually rated for 10 seconds, however most modern protection systems will operate well within this time, the 10 seconds allows for multiple operations, which could happen when auto-reclosers are used.
It also allows for the operation of an upstream backup protection device, if the local protection relay fails.
Times of 30 seconds usually indicate an old specification based on liquid resistors. The long duration reflects the extended cooling time associated with this old technology.
NERs are generally rated for occasional use and only have a limited capacity to handle continuous current. This limited capacity is typically 5 to 10% of the 10 second current rating.
Where necessary, NERs can be specified to handle significant continuous current. This may significantly increase size, weight and cost of the NER. Enclosures with restricted ventilation can make these requirements especially difficult and expensive to achieve due to the restricted ability to dissipate the heat
NERs may only be called upon to operate a few times in their service life, which may be 25 years or more.
Reliability is imperative, and has been proved with thousands of Cressall units installed worldwide under conditions of severe and extreme climate.