Page 121 - Electrical Safety of Low Voltage Systems

104 Chapter Six

FIGURE 6.9 Second fault phase-to-ground in TT systems.

disconnect the supply to the load before any attempt to turn it on. This
may cause nuisance tripping and disruption of the continuity of ser-
vice.
If the load is on, the RCD may sense an unbalance, as the earth
will return part of the neutral current to the source, and a nuisance
tripping may occur, if the operating threshold is exceeded (i.e., R G
must be low enough for this to happen).
Uponaphase-to-groundsecondfault,acurrentdividertakesplace
at node A (Fig. 6.9) between R G and the impedance of the neutral
conductor Z N , which is much lower than R G .
Only if the current through R G exceeds the RCD residual operating
current, the protection against indirect contact offered by the RCD is
effective.
In addition, the presence of a neutral-to-ground first fault may
void the additional protection offered by RCDs against direct contact,
by creating a latent hazardous situation for persons, even when the
load is off.
If a person comes in direct contact with a live part and, simul-
taneously with the ECP, which the neutral is faulting to, the neutral
conductor establishes an alternative, and additional, path to the fault
current, which circulates through the person (Fig. 6.10).
Such a path will carry the component I 1 of the ground current to
the source through the RCD’s toroid and, therefore, desensitizes it.
The RCD will sense only the earth current I 4 = I 2 + I 3 . I 4 may not be
large enough to cause the RCD to intervene and persons are exposed
to the risk of electric shock despite a perfectly functioning protective
device.

6.6 Independently Grounded ECPs in TT Systems
ECPs protected by the same RCD must not be connected to indepen-
dent grounds, because a neutral-to-ground fault can lead to hazardous
situations (Fig. 6.11).