Cressall
Earthing Resistors
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Cressall earthing resistors are
installed all over the world. They can
be found indoors and outdoors. The
climate can vary from arctic cold to
the heat and humidity of the Middle
East.
The earthing resistors are built
around a standard design with a
high level of flexibility for each installation. The design meets all major
standards like IEC, ANSI, NEMA and
IEEE.
Long life with a minimum of maintenance is expected from an earthing
resistor and is an important part of
the design. Cressall is an ISO 9001
registered company.
Cressall Earthing Resistors - Cost-efficent in Design - Reliable in Operation
Earthing System
The main purpose of earthing is to protect
life and property in the event of an earth
fault. There are basically three ways of neutral earthing in a star connected three phase
system
• Un-earthed neutral, fig 1
• Direct earthed neutral, fig 2
• Impedance earthed neutral, resistive or
inductive, fig 3
In case of an earth fault, systems with unearthed neutral exhibit a low fault current,
but high over-voltages will arise on the other
two phases and the fault may remain undetected. By comparison, direct earthed neutral will exhibit lower over-voltages on the
other two phases, but the earth fault current
will be very high. Neither of these charac-
teristics is desirable. Therefore, depending
on local regulations and practice, system
voltage and the overall design of the
system, the compromise solution of impedance earthed neutral, either by inductance
or by resistance is usually selected.
Furthermore, a resistive earthing impedance - earthing resistor - is in many cases
the most economical solution. It is also
worth noting that the presence of an earthing resistor tends to reduce the magnitude
of switching surges on the system by damping the oscillations resulting from cabling
capacitance and transformer reactance.
The choice of earthing method is depending
on local regulations and practice, voltage
level and finally the size and design of the
distribution network.
Impedance earthing by resistance, in form
of an earthing resistor with suitable accessories, is in many cases the most common
and economical solution.
Fig 1
Fig 2
Fig 3
Earthing systems
Cressall Air Cooled Earthing Resistors
Earthing resistors are used for protecting
transformers, generators and distribution
networks.
The earthing resistors are designed in
accordance with IEEE 32-1972. The resistor
elements will be made from high temperature stainless steel or nickel-chrome
alloy which allows the earthing resistor
to safely operate up to 1100°C. Higher
operating temperatures require less active
mass, resulting in more compact and economical designs. Modern materials guarantee long and reliable service even if the
temperature exceeds 760°C as specified in
IEEE32-1972.
The earthing resistors can be supplied
enclosed, ingress protection IP 23 to IP 55
or mounted in an open frame, IP 00. Enclosures for outdoor usage are made of stainless steel.
Enclosures rated up to 72,5 kV are constructed as a fully earthed main frame.
The earthing resistors are designed to meet
all major international standards as IEC,
ANSI, NEMA and IEEE.
Resistor element
GP Coil
Plain wire-wound resistors on a solid
ceramic former, using nickel-chrome resistance material. Mainly for very high impedance.
CC Coil
Spiral wire-wound resistors on a ceramic
former, using nickel-chrome resistance
material. Mainly for fault currents #50 A.
CC Coil
GP Coil
RP Coil
Oval edge-wound resistors on ridged
ceramic insulators and supported on a central stainless steel strap, using high grade
stainless steel resistance material. Mainly
used for fault currents for 50+ A.
RP Coil
Connections
The earthing resistor is designed to permit
ease of access with various options for main
neutral connection like
•
•
•
•
External air insulated connection box
External bushing
External plug contact
Direct on resistor element, IP 00 versions
Overtemperature protection
System earthing connection is separated
from the enclosure/framework earth.
Auxiliary circuit terminals are located in an
external terminal box.
For some applications, overtemperature
protection may be required. The short timerating of the earthing resistor demands
that a fast-acting protection device be
used. Cressall’s Minotaur protection system
detects the infrared radiation from the resistor elements, thereby providing the necessary speed of response.
Complete Earthing Systems
Earthing resistors are frequently used in
conjunction with auxiliary devices such as
•
•
•
•
•
Manual or motorized knife switches
Vacuum contactors
Current transformers
Voltage transformers
Lightning arresters
Traditionally, such auxiliary devices have
been considered to be part of the switchgear, but considerable space and cost
advantages can be achieved by combining them with the earthing resistor.
The earthing resistor and its associated
devices then arrive on Site as one integrated unit, with considerable savings
in cabling requirements and installation
times.
RP Coil open style earthing resistor
Example on earthing systems consisting of three
vacuum contactors, three current transformers and
one earthing resistor. The apparatus inside the
dashed lines are installed in the same enclosure.
General Data
Ambient temperature
Cooling method
Resistor elements
Current range
-40 - +50°C and according to customer specification
Air cooled, self convection
GP Coil, CC, Coil, RP Coil
# 4000 A
Connection time, typical
5 s, 10 s, 30 s and continuous duty
Resistor Element Data
GP Coil
Resistor material
Mean temperature coefficient
Temperature rise
Ni-Cr alloy
0,00014% per °C
#300°C
CC Coil
Resistor material
Mean temperature coefficient
Temperature rise
Ni-Cr alloy
0,00014% per °C
#760°C
RP Coil
Resistor material
Mean temperature coefficient
Temperature rise
Stainless steel Fe-Cr-Al
0,0355% per °C
#1000°C
Enclosure Data
Insulation voltage, IEC 71
Enclosure/frame material
Surface treatment
Ingress protection, IEC 529
0,8; 3,6; 7,2; 12; 17,5; 24; 36; 52 and 72,5 kV
Galvanised steel frame, stainless steel enclosure
Mild steel: galvanised or painted
Stainless steel: none or painted
IP 00, IP 23, IP 54 and IP 55
Standard
IEEE 32-1972 and customers specification
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CHS Controls AB
Florettgatan 33
SE-254 67 Helsingborg, Sweden
Phone +46 42 386100, fax + 46 42 386129
chs@chscontrols.se
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Leaflet PM4036E, May 2000
Technical Data for Earthing Resistors