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Neutral Earthing

Neutral Earthing Resistors (NERs) – sometimes called Neutral Grounding Resistors – are employed in medium-voltage AC distribution networks to limit the current that would flow through the neutral point of a transformer or generator in the event of an earth fault. NERs limit fault currents to a value that does not cause any further damage to switchgear, generators or transformers beyond what has already been caused by the fault itself.

Cressall manufactures neutral earthing resistors for voltages up to 132kV and for any current and time rating. NERs are a key component of the earth fault protection scheme in the majority of medium voltage electricity generation and distribution systems.
neutral earthing resistor

Features of all Cressall grounding resistors include:
• Current ratings up to 8,000 Amps
• Full compliance with ANSI-IEEE 32, Terminology and Test Procedure for Neutral Grounding Devices
• Stainless steel resistors and enclosures as standard, IP23 to IP54
• Type-tested standard designs (certificates available)

Options include:
• Supply of resistor banks only for incorporation into customers’ own enclosures
• Shock-tested marine designs
• Provision for CTs, heaters, special cabling, etc.
• Interlocked off-load by-pass isolators
• Single or multiple vacuum contactors


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.

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.

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
Cressall metallic resistors cool down very much faster than liquid types after operation; this is why typical industry standard ratings are 5 or 10 seconds for metallic NERs but 30 seconds for liquid resistors.

Ease of installation and maintenance
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.

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 for Cressall’s designs this has been proved over many years in electric traction rolling stock and under conditions of severe vibration and extreme climate world-wide

Downloads: Guide to Specifying NERsNER Type Test 11kVNER Type Test 6.6kVNER DatasheetMultiple Generator Earthing DatasheetNER Types

Cressall Resistors Ltd, Evington Valley Road, Leicester, LE5 5LZ U.K. Tel: (+44) (0)116 273 3633