The only standard that is widely used is American National standard IEEE-32: 1972 ‘Requirements, Terminology & Test Procedures for Neutral Grounding Devices’. This is used internationally, although it uses US rather than IEC insulation levels. There is no IEC standard specifically for neutral earthing resistors (or indeed for earthing devices in general). IEC standards that are relevant to the design of neutral earthing resistors include 60529 (degree of protection of enclosures) and 60071 (Insulation Coordination). It is widely accepted that the appropriate insulation class for a neutral earthing resistor should be the next-highest standard voltage above the line to neutral voltage. This is the principle adopted in IEEE-32, (paragraph 10.2 and table 4), which when adapted to International standards suggests for example that the appropriate insulation rating for a neutral earthing resistor used in a 12kV system should be 7.2kV. This class is then used to set the minimum air clearances and surface creepage distances within the NER.
Duty cycle and time ratings
Present practice is to size resistors so that after application of L-N voltage for 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-32 specifies this temperature rise as 760°C and this figure is widely accepted within the industry. It is possible to run to higher temperatures because this reduces cost, but this is then outside the scope of accepted standards.
In the past, 30 and 60 second ratings were appropriate for liquid resistors because of their extended cool-down times and very wide tolerances on ohmic value, even when cold. The time rating commonly used today is 10 seconds. Given typical fault clearance times of a second or less, coupled with the fact that faults only rarely appear as full voltage across the resistor itself, this ensures a very large margin of safety under all conditions. The worst case to be considered when setting an appropriate time rating is that of multiple successive faults, arising either from an auto-reclosing sequence or (seen more recently) from instantaneous resetting of an electronic earth fault relay during a long fault with transient zeroes in the fault current. This is an important consideration. Cressall’s work in the traction field means that we have sophisticated finite-element analysis tools and extensive empirical data which allows us to model this sort of condition very accurately and to produce designs which will meet particular duty cycles as economically as possible. If the customer can suggest a realistic ‘worst-case’ cycle from their own records we can analyse this and then compare the results with the margins available on a standard ten-second rated NER.
Neutral earthing resistors 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 over many years in electric traction rolling stock and under conditions of severe vibration and extreme climate world-wide.