Page 271 - Handbook of Electrical Engineering

256 HANDBOOK OF ELECTRICAL ENGINEERING

The maintenance procedures for working with Ex ‘d’ equipment need to be exercised with
care so that the machined surfaces are not degraded or damaged. BS5345 was introduced in 1976 to
address this and similar subjects. See Reference 4 for practical view of the problems involved with
maintenance of hazardous area equipment.

10.5.2 Type of Protection ‘e’

Type ‘e’ is also called ‘increased safety’ and intended for apparatus that is to be installed in a
Zone 1 area. Two of the allowable features of the type ‘d’ enclosures, namely permitting sparking
components and no Temperature Class limit to the internal components, cannot be incorporated into
the type ‘e’ designs. The practical aspect of this is the removal of a source of ignition i.e. a spark
or a hot surface. In many types of equipment e.g. luminaries, terminal boxes, junction boxes, some
designs of motor control stations, telephones and public address speakers, the elimination of these
two sources of ignition is not a difficult problem.

For motors the removal of sparking components, such as a commutator is not too difficult, but
the prevention of a hot internal surface is a problem for the designer. Clearly a DC motor cannot be
designed as an ‘e’ type machine. The identification of hot-spots in the windings or core of a motor
at the design stage is extremely difficult. This applies especially to the rotor cage of an induction
motor. Consequently the design of an ‘e’ motor needs to be somewhat conservative. For example
the temperature rise of the windings needs to be reduced. The power output of an ‘e’ type motor
for a given frame size is generally found to be less than for type ‘d’ or ‘p’ motors. There are also
restrictions on the allowable starting current and run-up time. Hence the motor characteristics will
need to be more carefully matched to the driven machine. High inertia rotors in the driven machines
should therefore be avoided. This conservativeness is also supported by the requirement that the
protective relay equipment at the motor control centre shall have special characteristics. Hence the
use of an ‘e’ type motor means that a ‘system’ of components or equipment must be used, not just
the motor by itself. This adds an element of ‘unusualness’ to the circuits in a motor control centre,
and for this reason the use of ‘e’ type motors in the oil industry is not common practice.

10.5.3 Type of Protection ‘i’

Intrinsically safe type ‘i’ protection is not applicable to electrical power equipment. It is mainly
intended for electronic measuring and control circuits i.e. instrumentation and telemetry. The principle
behind ‘i’ protections is that a circuit and its devices do not have sufficient operating energy or stored
energy to cause a spark that will ignite the gas-air mixture. A spark can occur but it must be inherently
too weak to ignite the mixture. There are two sub-divisions of type ‘i’, namely ‘i a ’and ‘i b ’. The type
‘i a ’ has a more stringent specification than ‘i b ’and istherefore allowedtobeusedinaZone 0area.
Type ‘i b ’ equipment cannot be used in a Zone 0 area.
Like the type ‘e’ protection of motors a ‘system’ approach is required for type ‘i’ equipment.
The system includes the source of power and its Zener Barrier, the interconnecting cables which by
their nature have inductance and capacitance, and the connected apparatus or load. If the connected
apparatus has inherent capacitance or inductance then extra attention must be paid to the design and
certification of the system. Reference 1 Chapter 13 gives more information about certifying a system
of components.