Page 125 - Electrical Safety of Low Voltage Systems

108 Chapter Six

take only an extra residual current comprising between 5 and 20 mA
to unnecessarily operate the device and cause nuisance trippings. To
preserve the continuity of the service, in the presence of known leak-
age currents, sometimes physiological to electrical systems, it is im-
portant that I dNO > I d (see Chap. 2).

6.9 The Neutral Conductor in TT Systems
In TT systems, the neutral wire is connected to the common point of
the three secondary windings in a typical star-connected utility power
transformer and is locally earthed. The neutral wire is shipped to the
customer together with the line conductors. In normal conditions,
this conductor is at zero potential, but due to faults, and also in other
nonfault conditions, can assume a nonzero voltage with respect to
ground.
Any current circulating in the utility’s ground R N , due to ground
faults on both the high and the low-voltage side of the utility’s trans-
former, at the customer’s or along the distribution line, causes a
ground potential rise V N on the neutral. If R N is not low enough,
V N may reach dangerous values.
If the neutral conductor is accidentally interrupted, the neutral
wire downstream of the interruption becomes live because phase and
neutral conductors result at the same potential (Fig. 6.15).
For the above reason, the neutral wire in TT systems must be
considered a “live” conductor and needs to be switched off at the
same time as the line conductors.
Also in three-phase systems, the accidental interruption of the
neutral wire causes hazardous situations (Fig. 6.16).

FIGURE 6.15 Accidental interruption of the neutral conductor in single-phase
TT systems.