Index
Neutral earthing resistors and transformers
SECTION 6
Neutral earthing resistors
and transformers
Introduction
Neutral earthing
resistors
Neutral earthing
transformers
Page 6/2
Network class NERs
Page 6/3
Distribution class NERs
Page 6/3
Neutral current transformers
Page 6/3
CT terminal box
Page 6/3
Packing and transport
Page 6/4
Voltages
Page 6/4
Characteristics
Page 6/4
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Page 6/5
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Introduction
Neutral earthing resistors (NERs)
SECTION 6
Neutral earthing resistors
and transformers
We manufacture two basic types of neutral earthing resistor in our New Zealand factory:
Transmission class NERs
c as supplied for transmission networks, and critical applications, and
Distribution class NERs
c as supplied for distribution networks, and less critical applications.
General
Neutral earthing resistors are designed to limit the current flow to earth under fault conditions. This NER
limited fault current is low enough to prevent damage to the generating, distribution and other associated
equipment yet high enough to operate fault clearing relays.
Our New Zealand NERs enclosures are manufactured to our type designs to meet New Zealand seismic
and structural standards. These type designs have been analysed and approved for seismic and structural
strength by consultant structural engineers.
Each NER consists of a single main enclosure arranged for the HV connection via a bushing and the LV
connection internally.
Two types of HV arrangement are available: either with a top mounted bushing projecting above the roof
of the NER for aerial cable connection, or with a side mounted bushing within an HV cable box for
underground cable termination.
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The resistor banks are mounted in the enclosures with support and inter bank insulators. All internal
electrical connections between the banks and the bushings are made with copper or stainless steel bar
and stainless steel bolts.
A section of mesh in the floor allows entry of cooling air while the roof lip allows the heat to exhaust.
The enclosures have 200mm high legs that allow free movement of cooling air up through the floor mesh
into the NER.
The legs are reinforced to allow for seismic forces and normal freight handling.
Pads on the bottom of the feet allow the NER to be bolted down onto a suitable level foundation provided
by the customer.
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Neutral earthing resistors (NERs)
Network class NERs
Distribution class NERs
Neutral current transformers
CT terminal box
SECTION 6
Neutral earthing resistors
and transformers
Network class NERs
Our transmission class NERs a designed and built for critical applications requiring a relatively robust NER.
These NERs include an enclosure consisting of a mild steel welded angle frame galvanised after
machining and welding, and clad in stainless steel panels and roof.
These NERs typically use solid stainless steel edge wound resistor elements which are manufactured in
the UK to approved designs by our principals.
These resistor elements are welded and tested before despatch, and we import them as assemblies, or
banks. The NER enclosures are manufactured in New Zealand and the complete NER is assembled in
our Christchurch Works.
The NER’s complete resistor consist of multiple banks connected in series or parallel as necessary to
achieve the assigned ratings.
The elements are of the oval edge wound type. The coils are manufactured from a continuous stainless
steel strip wound edge wise into elliptical coils. Each coil is mounted on a stainless steel centre core with
porcelain insulation. Stainless steel terminals are welded to each end.
This arrangement allows free expansion and contraction with temperature change without imposing strain
on the assembly.
The resistance alloy used is a high temperature stainless steel with 13% chrome and 4% aluminium
content. This alloy is able to withstand temperature excursions up to 1000°C while retaining its strength.
The banks are high pot tested to meet the requirements of IEEE 32-1972 section 10 prior to despatch
from the UK.
Distribution class NERs
Our distribution class NERs a designed and built for less critical applications requiring a relatively
economic NER.
These NERs include an enclosure consisting of a mild steel welded angle frame galvanised after
machining and welding, and clad in aluminium panels and roof.
These NERs typically use RL style nickel-chromium stainless steel alloy resistor elements.
In addition, the resistor elements and banks used in our distribution class NERs are generally of a lighter
construction to those used in our transmission class NERs, described above.
The resistor elements are welded and tested before despatch, and we import them as assemblies, or
banks. The NER enclosures are manufactured in New Zealand and the complete NER is assembled in
our Christchurch works.
Neutral current transformers
Where specified by the customer neutral current transformers (NCTs) are provided. These are normally
ring type transformers connected at the earth end of the resistor. The earth end cable to the resistor
normally provides the primary bar for the NCT.
Alternatively, especially if there is more than one NCT, a copper bar with suitable bolted links and
supported by stand-off insulators may be provided for the primary through the ring-type NCT(s).
Core details are supplied to customer requirements and are detailed on the drawing.
The NCT is normally housed inside the main enclosure, clamped in position, in one corner near the
resistors earth terminal. Access to the NCT is via one of the main enclosure removable side panels.
CT terminal box
Where there are NCTs, a suitably sized small weather proof box with hinged door is provided to house the
CT terminal blocks. The NCT secondaries are cabled to these terminals via trunking.
For our transmission class NERs, this CT terminal box is normally a stainless steel “CRSX” type with an
IP65 rating.
For our distribution class NERs, this CT terminal box is normally a polyester “PLM” type with an IP66 rating.
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6
Neutral earthing resistors (NERs)
Packing and transport
Voltages
Characteristics
SECTION 6
Neutral earthing resistors
and transformers
Packing and transport
Each resistor consists of a cubicle or cubicles with dimensions as shown on the relevant drawings issued
for the project.
The resistors are fully assembled in the works with all internal connections and bar work completed prior
to testing.
External bushings, if used, may be removed to prevent damage during transport and installation.
The NER enclosures are wrapped in protective material to prevent damage to surface panels during
handling and storage.
The NERs are provided with four lifting eye bolts on each top corner for lifting with a crane, hoist or hiab.
Alternatively a forklift can be used, lifting under the base. The forks must be long enough to reach both
sides so weight is not carried on the floor or mesh.
Please note that the NERs must be kept upright at all times. Damage is likely to the frame or insulators if
they are tipped or laid on their sides.
Voltages
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We are able to supply NERs for virtually any power system voltage.
We have supplied NERs for 11kV, 22kV, 33kV, and 66kV systems most recently.
Characteristics
Please specify the following characteristics when requesting an NER offer:
c power system voltage, for the neutral to be earthed,
c continuous current, if any, normally flowing in the neutral, with resistor connected,
c neutral fault current, with resistor connected,
c duration in seconds of the fault current,
c resistance required, at ambient temperature, before the onset of fault current,
c transmission or distribution class type NER required.
Please contact us to discuss your specific requirements.
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Neutral earthing transformers (NETs)
Neutral earthing transformers (NETs)
BC Transformateurs
SECTION 6
Neutral earthing resistors
and transformers
Neutral earthing transformers (NETs)
NETs for indoor applications. These units are imported from our principals in France and are indoor dry
type (cast resin) type neutral earthing transformers.
If your application calls for an outdoor installation we may be able to provide you with a weather proof
cubicle or enclosure with a suitable IP rating and thermal rating to satisfy your project’s requirements.
Please contact us to discuss your specific requirements.
BC Transformateurs
Our principal for the supply of Neutral Earthing Transformers is BC Transformateurs, a company of
Schneider Electric since 1983, with two factories in France.
Please see section 11 for more information about these specialty transformers.
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Notes
SECTION 6
Neutral earthing resistors
and transformers
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