Page 172 - Electrical Safety of Low Voltage Systems

CHAPTER9

IT Grounding System

The lecturer should give the audience full reason
to believe that all his powers have been exerted
for their pleasure and instruction.
michael faraday (1791–1867)

9.1 Introduction
In IT (Isolation Terre) systems, the power source, for example, trans-
formers, is not solidly connected to earth, and therefore, is defined as
ungrounded. Enclosures of ECPs, though, must be grounded, indi-
vidually, in groups or collectively (see Chap. 1).
The insulation of secondary sides of supply transformers from the
earth may be obtained through a high-resistance grounding resistor,
typically in high-/medium-voltage systems. It is not advisable, al-
though not forbidden by technical standards, to distribute the neutral
conductor in order to facilitate its insulation from ground.
As already anticipated in Chap. 2, electrical systems cannot be
completely isolated from ground, even in the absence of any inten-
tional connection to the earth. In IT systems, ground-fault currents
can circulate through the distributed system capacitance to ground
C 0 (Fig. 9.1).
At the system frequency, in fact, cables and earth can be seen
as armatures of a capacitor, whose dielectric is the surrounding air.
In addition, the resistance offered by the cable insulation to ground,
which has the magnitude of a few megaohms per kilometer, consti-
tutes another leaking path in parallel to the system net capacitance.
In addition, connected equipment and devices (e.g., electrical motors
and surge arresters), although to a lesser amount, increases both the
system capacitance and the leakage resistance to ground.
The leakage resistance is, indeed, very high and, therefore, may
be considered as an open circuit in parallel to the system capacitance.

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