Page 28 - Electrical Safety of Low Voltage Systems

Fundamentals of Electrical Safety 11

FIGURE 2.2
Diagrammatic
representation of
functional and
basic insulations in
Class I equipment.

Figure 2.2 shows a piece of Class I equipment, that is, an ECP out-
fitted with a bonding terminal to allow the grounding of the enclosure.

2.2.2 Enclosures and Barriers
Both enclosures and barriers are constructions, firmly held in their
positions, intended to prevent persons from intentionally, or acciden-
tally, touching live parts without the aid of tools.
As the term suggests, enclosures provide protection in any ap-
proaching direction to the equipment by “enclosing” it. Live parts
are inside the protective construction. Barriers, instead, may offer the
same defined degree of protection against direct contact, but only in
a limited number of approaching “routes” to the equipment. Safety
is equally achieved if live parts are kept “behind” barriers, instead of
inside of an enclosure.
For instance, barriers may be used around an open-type piece of
equipment when, due to its height, the access from above is naturally
precluded to persons. The “top” is, therefore, deemed unnecessary for
safety and the enclosure is not strictly required.
Removal of barriers, or opening of enclosures, must be possible
only by using keys or tools so as to prevent the accidental elimination
of the fundamental protection against direct contact. The necessity of
keysortoolsasa“ruleofengagement”totheequipmentcanbewaived
if removal/opening of protection can occur only after the supply is
disconnected.
The minimum insulation requirement for enclosures and barriers
is that live parts be inaccessible to a person’s finger. This requirement
limits the size of openings in equipment, for example, vents.
2
The IEC International Protection Code has standardized designa-
tions composed of the letters IP followed by two characteristic nu-
merals, which describe the degree of protection offered by different
types of enclosures and barriers. The first characteristic numeral (0 to