Page 36 - Electrical Safety of Low Voltage Systems

Fundamentals of Electrical Safety 19

measures, active and passive, against this type of fault situation must
be considered.

2.3.1 Protection by Automatic Disconnection of Supply
The automatic disconnection of the faulty circuit from its source is an
active protective measure aimed to limit the persistence of prospective
touch voltages on an ECP to a time that the human body can withstand
without incurring harmful physiological effects. The protective device
must promptly trip in accordance with the magnitude of the touch
voltage: the higher this value, the faster it must trip. As explained
in Chap. 5, the time–voltage safety curve describes the permissible
prospective touch voltage for persons as a function of the contact
duration in any type of earthing system (e.g., 50 V a.c. can be withstood
for no more than 5 s).
As later shown, maximum disconnection times of protective de-
vices have been elaboratedasafunctionofthenominalvoltageandthe
type grounding of the electrical system, rather than of the perspective
touch voltage.
Disconnection of supply upon faults is a measure that requires
an efficient bonding of the ECPs to the earthing system so that pro-
tective devices, by sensing the leakage to earth, can intervene at the
inception of the ground fault even before a person comes in con-
tact with energized objects. This protection is suitable if electrical
items are equipped with bonding terminals (i.e., Class I equipment of
Fig. 2.2). Under this point of view, the disconnection of supply can be
considered as a preventive approach to safety.
If the ground-fault current is high enough (e.g., TN systems), au-
tomatic circuit breakers can be employed to switch off the faulty cir-
cuit. RCDs may also be used to disconnect the voltage source upon
ground faults (especially in TT systems). In this regard, it is important
to underline the importance of the inclusion of the neutral conductor
through the RCD’s toroid (Fig. 2.8).
If the neutral is not included, any unbalanced load could cause
nuisance tripping of the device. In fact, the vector sum of the phase
currents circulating through the toroid would not be compensated by
the neutral current, causing a nonzero result. If the system does not
carrytheneutralconductor,thevectorsumofthelinecurrentsthrough
the toroid is normally zero, even if the load is unbalanced, but becomes
nonzero in the case of a ground fault, allowing the operation of the
device.
On the other hand, the protective conductor PE must be excluded
from the RCD, otherwise the device would never trip. The fault cur-
rents over the PE, in fact, would return to the source passing through
the toroid, thereby causing the vector sum of the currents to be zero,
despite the presence of the fault.