© Fortress Systems Pty Ltd 2000, 2001, 2002, 2003
NEUTRAL EARTHING (GROUNDING)
Neutral Earthing Resistors (NER'S) are employed in AC distribution networks to limit the current that would flow through the neutral star point of a transformer or generator in the event of an earth fault. The rating of the NER is chosen so that the fault current is limited to that necessary to operate the protection relays within the required time.
Why use Fortress NER's?
Choice
An extensive range of resistor elements allows selection of the most efficient and cost effective solution for any required duty. Fortress can manufacture NERs with rated times from a few seconds to continuous, any required initial current, and for system voltages up to 66kV.
Stability
Resistance values are set at the manufacturing stage and remain constant throughout the life of the NER. Resistance values of liquid resistors by contrast are strongly affected by changes in ambient temperature.
Predictability
The change in resistance with temperature during operation is small and predictable. This means that protection levels can be predetermined accurately. An Fortress NER will not induce unpredictable phase changes or resonances into the fault current, as may occur with an earthing reactor.
Rapid Cooling
Faster cooling after operation means that it is safe to use a short time rating. Liquid resistors take a longer time to cool down after operation, and therefore their time rating needs to cater for the possibility of several successive faults.
Reliability
NERs may only be called upon to operate a few times in their service life, which may be 25 years or more. Reliability is essential; elements from Fortress have been proved over many years in electric traction rolling stock and under conditions of severe vibration and extreme climate worldwide.