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￭ Identifying the earthing system of the electrical power network
WARNING!
Only a qualified electrical professional may do the work instructed in this section.
Depending on the installation site, the work may even be categorized as live
working. Proceed only if you are an electrical professional certified for the work.
Obey the local regulations. If you ignore them, injury or death can occur.
To identify the earthing system, find out the supply transformer connection. If that is not
possible, measure these voltages at the distribution board, and use the table below to define
the earthing system type.
1. input voltage line to line (U
L-L
)
2. input voltage line 1 to ground (U
L1-G
)
3. input voltage line 2 to ground (U
L2-G
)
4. input voltage line 3 to ground (U
L3-G
).
The table below shows the line-to-ground voltages in relation to the line-to-line voltage for
each earthing system.
Electrical power system type U
L3-G
U
L2-G
U
L1-G
U
L-L
Symmetrically grounded TN system (TN-S sys-
tem)
0.58·X 0.58·X 0.58·X X
Corner-grounded delta system (nonsymmetrical) 0 1.0·X 1.0·X X
Midpoint-grounded delta system (nonsymmetrical) 0.5·X 0.5·X 0.866·X X
IT systems (ungrounded or high-resistance-
grounded [>30 ohms]) nonsymmetrical Varying level
versus time
Varying level
versus time
Varying level
versus time
X
TT system (the protective earth connection for
the consumer is provided by a local earth elec-
trode, and there is another independently installed
at the generator
Varying level
versus time
Varying level
versus time
Varying level
versus time
X
Electrical installation – IEC 55
11