Page 47 - Electrical Safety of Low Voltage Systems

30 Chapter Three

FIGURE 3.1 Faults compromising safety.

of as a function of time. We can define safety at time t as the proba-
bility (i.e., quantity comprised between 0 and 1) that the item will not
cause a dangerous voltage exposure as a consequence of faults. Elec-
trical safety of an ECP must be referred to as the absence of “superfi-
cial” dangerous potentials on its enclosure and must not be confused
with its functionality. Some faults, in fact, may compromise safety
against electric shock but not the operation of the equipment, which
may keep working. This concept is shown in the Venn diagram in
Fig. 3.1.
Faults falling in set III, but not in set II, create the most hazardous
situation,asthelackofsafetyisnotrevealedbythelossoffunctionality
of the equipment.
In formulas:

N − F(t)
S(t) = (3.1)
N
where N denotes the total number of identical items, while F(t)is
the number of equipment among N, whose enclosure became “hot”
after the time t. The numerator of Eq. (3.1) represents the number of
“safe” items against electric shock after the cumulative time t during
which items have been functioning. As the exposure time t to risk
progresses, the number of items becoming “live” will increase and
safety asymptotically will approach zero. Hypothetically speaking,
after infinite time, electrical accident will surely happen, as the basic
insulation as well as other deployed PMs will no longer carry out
their protective functions because of their inevitable aging. On the
other hand, safety is at its maximum value (i.e., unity) when either
the item is not energized or its failure cannot cause any hazardous
1
situations (e.g., the item functions at extremely low voltages ).
We can link safety to the failure rate of the single PM deployed
on an item (e.g., Class I equipment). If the PM malfunctions (e.g., the
basic insulation fails), the system being protected becomes unsafe.
The reliability of the protective measure equates to the safety against
indirect contact of the entire equipment.