Page 332 - Offshore Electrical Engineering Manual

Mixing Hazardous Area Certified Equipment 319

LOGIC OF AREA CLASSIFICATION

Fig. 5.4.5 shows a typical flow diagram for selection of hazardous zones. The flow
diagram is used in conjunction with a questionnaire, such as that shown in Fig. 5.4.6.
The resulting zone boundaries are drawn on to a set of plans and elevations for the
installation in order to produce the required hazardous area boundary drawings.
Throughout the duration of a design project, regular meetings between discipline
engineering heads and safety representatives are held to discuss and revise these
drawings. Two typical hazardous area drawings are shown in Fig. 5.4.7.

SELECTION OF MOTORS FOR HAZARDOUS AREAS

It is common practice in offshore installations to require all motors for use in haz-
ardous areas to be suitable for zone 1 areas, although in many cases the actual area
classification might be zone 2. This allows

• areas to be reclassified from zone 2 to zone 1 without the need to replace motors;
• enables the safer transfer of motors from one hazardous area to another;
• tends to rationalise spare parts holdings.

Where a motor is used as part of a variable speed drive system, the hazardous
area certification will not be valid unless the motor has been tested for safe operation
over the whole speed range through which it is required to operate. At low speeds the
motor will have different heat dissipation characteristics and, if dependent for cool-
ing on a rotor driven fan, will have a reduced cooling air flow rate. Therefore there
is a risk that the enclosure temperature may exceed the required maximum surface
temperature requirement.
It is now known that large motors whose casings are made up of bolted sections
may suffer sparking across the section joints due to voltages produced by stray cur-
rents induced in the sections. To avoid risk of ignition, all such motors should now be
fitted with copper braid bonding straps across each section joint in order to prevent
any potential build-up across the joint.
In general, there are four methods of motor design to achieve suitability for use
in hazardous areas, as follows.

MIXING HAZARDOUS AREA CERTIFIED EQUIPMENT
VARIOUS GLOBAL AND NATIONAL STANDARDS

The certification concept, for example, Ex ‘d’ or Ex ‘e’ can be selected based on the
most suitable and economic design. However, mixing between different national and
international standards would be ill-advised and dangerous as they are not always
compatible. This is also likely to lead to confusion in the maintenance workforce
and will require separate storage of the two types of spares for certified equipment.