Neutral Earthing

Neutral Earthing Resistors (NERs) are employed in AC distribution networks to limit the current which would flow through the neutral point of a transformer or generator in the event of an earth fault.

The rating of the NER is chosen to meet the requirements of the system protection scheme; this will include consideration of maximum acceptable fault current, earth potential levels, and preventing damage caused by the fault.

Built in benefits of Cressall NERs

Choice

An extensive range of resistor elements allows selection of the most efficient and cost effective solution for any required duty. Cressall can manufacture NERs for any system voltage or initial current and with rated times from a few seconds to continuous.

Stability

The resistance value is set at the manufacturing stage and remains constant throughout the life of the NER. This value is unaffected by changes in ambient temperature, unlike liquid resistors.

Predictability

The change in resistance with temperature during operation is small and predictable. This means that protection levels can be accurately pre-determined.

In contrast to an earthing reactor, a Cressall NER does not induce unpredictable phase changes or resonances into the fault current and does not require matching to the associated transformer.

Rapid cooling and short time ratings

Modern protection systems typically clear faults in less than a second. However, the time rating of an NER usually needs to cater for the possibility of several successive faults. Cressall metallic resistors cool down very much faster than liquid type after operation; this is why typical industry standard ratings are 5 or 10 seconds for metallic NERs but 30 seconds for liquid resistors.

Cressall NERs are compact and do not require site calibration or auxiliary power supplies. Maintenance can be limited to periodic inspection and cleaning only. Anti-frost and anti-condensation heaters are not required.

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 imperative, and has been proved over many years in electric traction rolling stock and under conditions of severe vibration and extreme climate world-wide.

Construction Standards

At present the only standard for NERs is American National Standard specification Standard Requirements, Terminology and Test Procedure for Neutral Grounding Devices, ANSI/IEEE-32, 1972. Cressall Resistors NERs can be designed and tested to comply with this standard, or with individual client specifications.

Resistor Elements

For the majority of ratings the most suitable element is the type RP oval edge-wound coil. RP resistors are manufactured from a continuous stainless steel strip, wound edgewise into oval coils. Each coil is supported by ridged ceramic insulators mounted on a stainless steel centre support, with stainless steel terminals welded at each end. This arrangement allows free expansion and contraction at operating temperatures up to 1000°C without imposing strain on the assembly.

Type RP oval Edgewound Coil

The resistance alloys used are high temperature stainless steels capable of withstanding temperature excursions to 1100°C whilst retaining their strength - unlike 304 or other structural grade stainless steels which are limited to much lower temperatures. NERs designed for operation to higher temperatures require less active mass, resulting in more compact and economical designs.

Unlike lower grade stainless steel alloys such as 304 and 430, Cressall Type RP resistor elements have a resistance change with tempearture of <3% per 100°C. This means that it is easier to set protection devices.

For guidelines on specifying Neutral Earthing Resistors, click here