Delta BLD-E1 Series User Manual page 14

TT grounding system:
TT grounding system is usually called 3-phase 4-line grounding system. The feature of TT
grounding system is no electrical connection between the neutral line and protective earthing,
i.e. the grounding of the neutral and PE line is separated. No matter 3-phase load is balanced
or not, the PE line won't be live part as the neutral line is live part when this system is in normal
operation. When only 1-phase grounding is fault, the fault can't be stopped immediately due to
the low sensitive of protective earthing and only equipment case may be live part.
IT grounding system:
IT grounding system is a 3-phase 3-line grounding system. The neutral of the system
transformer is not grounded or grounded by the impedance, no neutral line N and protective
earthing is grounded separately. The advantage of this system is that when only one phase is
grounded, it won't cause greater current in the case and the system will operate normally.
Revision May 2009, 00DE, V0.50
L1
L2
L3
PEN
TN-C-S grounding system
L1
L2
L3
N
TT grounding system
L1
L2
L3
N
L1 L2 L3
IT grounding system
Chapter 1 Introduction| BLD-E1 Series
L1 L2 L3
PE
L1 L2 L3
PE
N
PE
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