Hazardous Area Classification

Hazardous Areas
What is a Hazardous Area?
A potentially explosive atmospheres exist where there is a risk of explosion due

to mixtures of gas/air, vapour/air, dust/air or other flammable combinations.
Any area that could have a potentially explosive atmosphere is known as a
Hazardous Area.
Why are Hazardous Areas Important?
Instrument engineers introduce a potential ignition source into a Hazardous

Area when they locate an electrical or electronic instrument, or a electrical or
electronic final element e.g. valve actuator. Understanding the degree of risk,
and design of instrument allows the instrument engineer to minimise the
likelihood of an explosion occuring.
How are Hazardous Areas Classified?
Process plants, refineries, oil and gas platforms etc are divided into Zones
(European and IEC method) or Divisions (North American method) according
to the likelihood of a potentially explosive atmosphere being present, e.g.
What Standards are used for Hazardous Areas Classification?
The following international standards are used to define the classification:

- EN 1127-1:1997; Explosive atmosphere - Explosion prevention and
protection. Part 1: Basic concepts and methodology
- EN 60079-10:2003; Electrical apparatus for explosive gas atmospheres. Part
10: Classification of potentially explosive atmospheres
- EN 50281-3:2002; Electrical apparatus for use in the presence of combustible
dust. Part 3: Classification of areas where dusts are or may be present
- N.E.C. Chapter 5
It is worth noting that many - though by no means all - countries outside of
Europe and North America use the IEC Standards as a basis for their own
national standards.
Hazardous Area Drawings
Hazardous area zones are usually marked on a drawing of the plant, the drawing
being referred to as the Hazardous Area drawing. These drawings also
include temperature classification information and gas group information. By
using these drawings the engineer can specify instruments and equipment
suitable for the area in which it is to be located.
Temperature Classification
What is Temperature Classification?
Temperature classification (also known as temperature class, or T class) defines

the maximum surface temperature that a product destined for use in a
potentially hazardous atmosphere is allowed to operate at, relative to an ambient
temperature of -20°C to +40°C.
According to the type of protection used on the product e.g. Exd, Exe etc, the
temperature corresponds either to the maximum temperature of the external
surface of the product, or to the maximum temperature of the internal surface of
the product. Generally, Temp-class is based on fault conditions or, at the very
least, worst case normal operating conditions.
Why is Temperature Class Important?
All flammable gases have an auto-ignition temperature. If a flammable mixture

of the gas is exposed to a component above the auto-ignition temperature then
the mixture will ignite. Therefore, when selecting equipment, the Temperature
class must be below the auto-ignition temperature of the potentially explosive
atmosphere where it will be installed.
If several different flammable materials may be present within a particular area,
the material that gives the lowest auto ignition temperature dictates the overall
area classification, and hence T-class.
As can be seen from the Temperature Classes table below; a T6 certification
allows a maximum permissible surface temperature of 85°C therefore a T6
instrument can be used in T5, T4, T3, T2 and T1 environments.
T-Class and Equipment Marking
The ATEX directives typically require all products certified suitable for use in a
hazardous area, e.g. instruments, enclosures, luminaries etc to be marked with
their temperature class. Look out for a T number at the end of the protection
concept marking on the product's label e.g. EEx de IIB T3 indicates that this
explosion proof apparatus has a temperature classification of T3 which
corresponds to a maximum surface temperature of 200°C. See our page on
the ATEX Directive for further background on ATEX.
Temperature Class for Group I Applications
Temperature classes do not apply to Group I applications, i.e. equipment for use
in the mining industry. Mining equipment has either a rigid 150°C or 450°C
limit.
Temperature Class for Group II Applications
The temperatures corresponding to each temp class are shown below. It is worth
noting that North American and Canadian standards differ from European
standards by having sub groups e.g. T3A

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Hazardous Areas

What is a Hazardous Area?

A potentially explosive atmospheres exist where there is a risk of explosion due

Why are Hazardous Areas Important?

Instrument engineers introduce a potential ignition source into a Hazardous

How are Hazardous Areas Classified?

The following international standards are used to define the classification:

What is Temperature Classification?

Temperature classification (also known as temperature class, or T class) defines

Why is Temperature Class Important?

All flammable gases have an auto-ignition temperature. If a flammable mixture

T-Class and Equipment Marking

Temperature Class for Group II Applications

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