Page 61 - Electrical Safety of Low Voltage Systems

44 Chapter Three

where R i (t)isthe reliability function, which represents the number of devices
that survived up to time t (100 in our case). t equals 1 year. We solve for
R i (1):
R i (1) = 99.48.
Q. Is traveling on an airplane more hazardous than riding a motorcycle?
A. The feared/expected damage to persons in case of failure of the airplane
systems during a flight is very high, unlike the case of the motorcycle, whose
failure will not necessarily cause a fatal outcome.
On the other hand, because the technology employed on an airplane is
much more sophisticated than the one used on motorcycles, the probability
of an aircraft failure is lower. Ergo, the airplane may be considered safer than
the motorcycle.

Q. Would a piece of equipment in which, hypothetically, the basic insulation
and the conductive enclosure have been inverted between each other be safer?
A. Hypothetically speaking, Class I equipment with the basic insulation cov-
ering the outside of the metal enclosure would not be safer! Eq. (3.10), in fact,
does not take into account the mutual positions of the protective measures.
However, the risk offered by this hypothetical piece of equipment would
decrease, as the probability of touching the energized enclosure would greatly
reduce.

Endnotes

1. See Chap. 10.
2. IEEE Std. 493-2007, “IEEE Recommended Practice for the Design of Reliable Industrial
and Commercial Power Systems.”
3. See Chap. 15 for further details.
4. Nuclear facilities, aircraft, and spacecraft may be the exception. Such installa-
tions typically use a radial distribution and the connecting wires are redundant
(i.e., doubled).
5. In installations, a component with the sole basic insulation (e.g., cables in air)
may become Class 0 equipment due to deposit of conductive dust or moisture
eventually present in the environment, which may act as a metal enclosure.
6. We have already discussed in Chap. 2 that in supervised locations (e.g., noncon-
ducting locations) Class 0 equipment can be used.
7. Bonding/grounding connections mechanically link protective conductors to
metal enclosures, ground rods, steel structures, the neutral point of the source,
etc.
8. The North American National Electrical Code states, instead, that double insulated
pieces of equipment are not required to be bonded. Therefore, their bonding is
permitted.