pumps and compressors throughout the plant.
They can also be positioned along the external
boundaries. Some applications may benefit from a
mix of detectors � a grid of detectors can easily
locate a leak, whereas open path systems provide
greater coverage per unit. Used appropriately, the
right combination can serve as an early warning
system to detect potentially hazardous situations
and ensure sufficient time for counteraction to be
initiated and damage to installations to be either
avoided or at least minimised.
As well as the up front capital expenditure, the
installation costs, on-going maintenance require-
ments and general running costs, the benefits that
the system can bring in terms of time savings and
potential loss of life, plant and end product should
all be used in the lifetime costs evaluation.
However, the cost of ownership is just one of
many considerations. Fixed gas detection systems
are also required to conform to both ATEX
(Directive 94/9/EC*) and Safety Integrity Level (SIL)
legislation. ATEX is concerned with the ability to
perform safety shutdowns and the use of
approved equipment from sensors right through
to control panels and cards that can handle
multiple results. SIL is also making demands on
users and suppliers alike in that it covers not just
the system and the components, but the whole
installation including cabling and processes.
ATEX
The ATEX Directives affect detection systems in
two ways: the 1999/92/EC Directive applies to the
safety of installation whilst the 94/9/EC Directive
applies to equipment.
1999/92/EC Directive:
The aim of this Directive is to ensure that users
fully understand, document and control explosive
atmospheres. Explosive atmospheres should be
removed but, where this is not possible, it states
that ignition should be avoided. If an explosion
were to occur then the effects of the explosion
should be mitigated. In order to do so, it requires
that the overall risk is assessed and that explosion
protection documentation (EPD) is produced.
This documentation should clearly outline the
risk in terms of probable explosive atmosphere
(zone), minimise the risk by identifying possible
ignition sources (equipment) as well as the nature
of flammable materials (Group, T class) and ensure
that users understand the impact damage if an
explosion did occur (in health and safety, financial
and environmental terms).
In addition, there is a requirement to train and
warn personnel about the risks and to continue
to monitor major changes to either the plant
equipment or process.
94/9/EC Directive:
This Directive is clear to understand when consid-
ering equipment located in the hazardous area. It
is also clear that any associated galvanic (zener)
isolating device, even though it may be mounted
in the safe area, must also be suitably certified
for use. What is not quite so clear is that, when
monitoring and protecting (primary protection)
against the risk of explosion, the associated
control equipment must also be certified.
This means that when a gas detection system or
other system is considered to be a primary protec-
tion system or have a measurement function, the
control equipment, which is located in the safe
area, requires certification over and above the
normal conformance to general standards.
As far as ATEX is concerned, gas detection
systems can be categorised as:
Those that are only used to emit an alarm:
In these applications, the gas detector must be
ATEX certified as safe for use in explosive
atmospheres. However, neither the gas detector
nor the associated control equipment requires
a performance test and a second ATEX
certificate.
INTERNATIONAL FIRE PROTECTION 55
Draeger Sensor IR
SAFETY IN GAS DETECTION � MORE THAN MONEY CAN BUY
GAS DETECTION

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