METREL MI 3399, Instruction Manual With Exercises

Page: 37 / 272

Electrical Installation Safety Trainer AD 1 - MI 3399
37
-
Earth Resistance R
E
.
-
Ground resistance from earthing electrode R
E
to power transformer.
-
Resistance of power transformer’s earthing system R
O
.
The Line and Loop Impedances can simply be measured at each end of the circuit:

Line impedance in a TT system:
Z
Line
= Z
Line conductor
+ Z
Transformer
+ Z
Neutral conductor
+ Z
Local neutral conductor

Loop impedance in a TT system:
Z
Loop
= ZL
ine conductor
+ Z
Transformer
+ R
O transformer earthing
+ R
Earth of the object
+ Z
Local PE conductor
Line Impedance in TT system is much lower than Loop Impedance because of the
fault loop current flowing through the ground.
1.3 IT system
The IT system has the supply part of the power source separated from earth or is
earthed through a sufficient high impedance at the source. Exposed conductive parts
are autonomously earthed or connected to the PE conductor and are locally earthed
at the installation input.
Figure 1-5: IT system
Fault loop impedance in IT- systems is not in function until a first fault occurs. After the first
fault, the system must generate a sound and visual alarm and start operating as a TN or
TT system.
The Line Impedances can simply be measured at each end of the circuit: