NORSOK standard E-001
Edition 5, July 2007
5.2.2
Neutral earthing for system up to and including 1000 V (see IEC 61892-2, 5.4)
See IEC 61892-2, 5.4.1
The system earthing methods for the different low voltage levels are shown in Table 2.
The neutral shall be earthed through a resistance with numerical value equal to or somewhat less than 1/3 of
the capacitive reactance between phase and earth.
The resistive current shall for all voltage levels be limited to the maximum values given in Table 3.
Table 2 - System earthing methods
System voltage
690 V
Power source
Generator neutral shall be high
resistance earthed. Maximum earth
fault current shall be limited to 100A
b
per generator.
Transformer
Transformer neutral shall be high
resistance earthed. Maximum earth
fault current shall be limited to 100 A
per transformer.
Transformer neutrals shall be solidly
earthed.
400/230 V
Direct earthed
power systems
c
230 V IT (UPS)
Isolated
Isolated
a
DC
Both poles isolated.
a
DC voltages for telecommunication system may have one pole earthed.
b
Emergency generator neutral(s) shall be isolated.
c
UPS dedicated for a special purpose, can be solidly earthed.
5.2.3
Neutral earthing for system above 1000 V (see IEC 61892-2, 5.5)
See IEC 61892-2, 5.5.1
The system earthing methods for the different voltage levels are shown in Table 3. For high voltage levels,
the system earthing shall be performed as “High resistance earthed neutral”.
The neutral shall be earthed through a resistance with numerical value equal to or somewhat less than 1/3 of
the capacitive reactance between phase and earth.
The resistive current shall for all voltage levels be limited to the maximum values given in Table 3.
Table 3 – System earthing methods
System voltage
11 kV
6,6 kV
Power source
Generator neutral shall be high
resistance earthed. Maximum earth
fault current shall be limited to 20 A
per generator.
Generator neutral shall be high
resistance earthed. Maximum earth
fault current shall be limited to 20 A
per generator.
Transformer
Transformer neutral shall be high
resistance earthed. Maximum earth
fault current shall be limited to 20 A
per transformer.
Transformer neutral shall be high
resistance earthed. Maximum earth
fault shall be limited to 20 A per
transformer.
5.2.4
Earthing resistors, connection to hull/structure (see IEC 61892-2, 5.7)
See IEC 61892-2, 5.7.2
Each power source shall have separate earthing boss for neutral earthing. Common earthing boss for system
earthing and protective earthing can be used for the same unit. Connection on earthing side of resistors is not
required.
5.3
Distribution systems (see IEC 61892-2, Clause 6)
5.3.1
Direct current (DC) distribution systems (see IEC 61892-2, 6.1)
The following voltage levels shall be used:
 UPS 48 V DC (shall only be used as distribution voltage for telecommunication systems);
NORSOK standard
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