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Summary:
Applying Motors in Hazardous Areas -- What are the Rules? With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
Where explosive gas or combustible dust may be present, a motor must
be designed to minimize the possibility of fire or explosion. Such
construction is costly, and economically justified only for
locations that are clearly hazardous during normal circumstances.
(Motor starting has been considered an abnormal condition because of
its short, infrequent duration.)
In the United States, judging area classification according to the
degree of hazard is the responsibility of the authority enforcing
the applicable safety code, and is the subject of several standards
of the National Fire Protection Association (which also issues the
National Electrical Code defining the requirements for apparatus to
be used in hazardous areas).
Areas of potential hazard are divided into two Classes (flammable
vapors and gases, or combustible dusts). Each is subdivided into
either two Divisions (1 and 2) or three Zones (0, 1, and 2. In
Division 1, motors must be designed for explosion-proof or
dust-ignition-proof service.
The Zone concept, standard in Europe, was adopted by the NEC in 1996
as an alternative to Divisions. Zone 0 involves such a high risk
that only intrinsically safe equipment is permitted (no motors
qualify).
In Class I Zone 1 locations, "increased safety" motors (based on IEC
Standard 60079-7) are allowed. Such motors and their controls are
designed to ensure that surface temperatures cannot reach the
atmosphere ignition point.
In either Division 2 or Zone 2, explosion-proof or
dust-ignition-proof motor construction is not required. But motor
surface temperature limits apply, and accessories or components that
produce arcs or sparks are not permitted.
As a guide to applying motors in Division 2 areas,
IEEE Standard 1349 was
published in 2000. Experience in the petrochemical industry
indicates that standard open or enclosed motors have no significant
history of causing ignition of flammable atmospheres even with
gas/air mixtures of the lowest commonly-encountered ignition
temperatures.
From
"Applying Motors in Hazardous Areas --
What are the Rules?" ...by
Richard L. Nailen, EA Engineering Editor - published in
Electrical Apparatus November 2003.
© 2003
Barks Publications, Inc. All Rights Reserved.
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