Page 35 - Electrical Safety of Low Voltage Systems

18 Chapter Tw o

FIGURE 2.9
Direct contact
phase-to-neutral.

The three-phase RCD is a transformer whose primary winding
is constituted by the line conductors themselves. The vector sum of
the line currents and the neutral current in healthy three-phase cir-
cuits is always zero, and therefore, in the secondary winding, which
has the task of switching off the supply, no current will circulate. If a
fault occurs, the vector sum becomes nonzero due to the current leav-
ing the system through the PE not passing through the toroid. The
RCD, then, activated by its secondary winding, will trip the circuit
breaker.
RCDs must be considered as an additional means of protection
and do not substitute for the other fundamental protective measures
against direct contact previously examined. RCDs, in fact, can pro-
tect persons by disconnecting the supply only in the case of contact
between energized objects and the ground. They can sense only fault
currents not returning to the source through the legitimate path. Con-
sequently,directcontactbetweenthephaseandtheneutralconductors
may not activate the RCD, as there may not be enough ground current
circulation for it to sense (Fig. 2.9).
The RCD will only sense the component I 3 , while the larger current
I 1 will circulate through the person’s body. I 3 may not be large enough
to exceed the RCD’s operating threshold, which cannot disconnect the
supply.

2.3 Protection Against Indirect Contact
The failure of the basic insulation may cause electrocution owing to
the accidental presence of voltage-to-ground over metal parts not nor-
mally live (Fig. 2.1). This condition is particularly dangerous as it is
not under a person’s control despite any prudent conduct. Protective