Page 186 - Electrical Safety of Low Voltage Systems

IT Grounding System 169

FIGURE 9.14 Fault resistance in IT systems.

With reference to Fig. 9.14, in IT systems the prospective touch
voltage becomes

V IT R G V 1N (R G + R F ) × R G
ST = V 1G = 2 2
R G + R F [1/(3 C 0 ) ] + (R G + R F ) R G + R F
V 1N R G
(9.28)
=
2
[1/(3 C 0 ) ] + (R G + R F ) 2
In order to graph Eqs. (9.27) and (9.28) as a function of R F , let us
assume R N = 1 , C 0 = 2 nF, and R G = 10 . The result, for a nominal
voltage line-to-ground equal to 230 V, is shown in Fig. 9.15.
In IT systems, the presence of a fault resistance R F plays in favor
of safety. The prospective touch voltage, in fact, decreases with R F ,
unlike in TT systems.

24

20

16

TT (V x 10)
V ST 12 IT (mV)

8

4

0
0 5 10 15 20 25 30 35 40 45 50
R F
FIGURE 9.15 V ST as a function of R F in TT and IT systems.