Page 27 - Electrical Safety of Low Voltage Systems

10 Chapter Tw o

FIGURE 2.1 Indirect contact.

2.2 Protection Against Direct Contact
It is understood that all electrical equipment must have provisions to
guarantee protection against direct contact. In the following sections,
the fundamental strategies of basic protection are examined.

2.2.1 Insulation of Live Parts
In order to operate, electric equipment contains parts at different po-
tentials, which must be properly insulated from each other and from
their enclosure through the functional insulation.
The basic insulation prevents persons from coming in contact with
live parts and is the fundamental protection against direct contact. To
be effective as a protection, the insulation material must completely
cover the live parts and should be removable only by destruction
(Fig. 2.2).
The basic insulation must be capable of withstanding the possible
stresses during the functioning of the equipment without losing its in-
tegrity. Electric fields, mechanical collisions, high temperatures, and
the aging of the insulating material are the possible causes of failure
of the basic insulation. It is essential, then, that the basic insulation has
sufficient mechanical strength to withstand the stress caused by the
normal operation of equipment. As a consequence, insulating paints,
and similar products, cannot be considered suitable for the basic
insulation; however, they can be used as the functional insulation
(e.g., insulation between windings of transformers or motors).