From June 30th 2006 anyone specifying weighing equipment in hazardous areas must comply with ATEX regulations states Steve Cox of Avery Weigh-Tronix.  Having a clear understanding of your safety requirements will ensure compliance and avoid the expensive mistake of overspecifying on a ‘better safe than sorry’ basis.

Any potentially explosive area below ground, on the surface and on offshore fixed facilities is considered hazardous and falls under the legislation.  For an explosion to occur three elements are needed:

• Dust, gas or mist
• Sufficient air or oxygen
• A source of ignition.

ATEX consists of two directives. 

The first, ATEX 1999/92/EC requires the end user to take appropriate measures to ensure the safety of all employees working in explosive atmospheres.  You must only use equipment that has formally met the safety criteria in these areas.

The second, known as ATEX 94/9/EC, details the requirements suppliers must meet in order to achieve the necessary accreditation.  It relates to all electrical and mechanical equipment and systems for hazardous areas.

To comply the end user has an obligation to complete a formal risk assessment of all hazardous areas by the end of June. This must be documented and updated regularly to demonstrate that the equipment, working environment and practices are designed, operated and maintained properly.  A qualified person must complete this.

Hazardous areas are divided into zones.  These relate to the chance of an explosive atmosphere existing and are summarised in table 1 below.  Example 1 illustrates a typical application.

Table 1 Hazardous area classification

For gases and vapours the type of hazard also needs identifying.  These are classified into gas groups IIA, IIB and IIC, where gases in IIC are potentially the most explosive.  IIC contains gases such as hydrogen and acetylene.

Temperature classification is also important for selecting equipment used in hazardous areas.  This specifies the maximum surface temperature that any part of the equipment can reach under the worst circumstances, typically fault conditions. It assumes that there is an ambient temperature of +40oC.  Products certified to meet T6 meet the most stringent requirements. The classifications are detailed in table 2 below:

Table 2:  Temperature classification

For a weighing system the list of equipment intended for use in hazardous areas is extensive.   It might include control equipment and sensors, transformers, pumps, motors, lights, trips and relays.

Each piece of equipment and interconnecting cable or lead is a potential source of ignition, which needs protection.  The system may also include non-electrical items such as weighing platforms, which can be a source of sparks resulting from contact with other material.  Frictional contact or static discharge can also cause ignition.

In addition just because each component is individually certified, it does not mean that a weighing system based on those components meets the requirements.  Compatibility has to be established before a system can be certified safe.  It is best to work with a supplier who has full ATEX accreditation.  Such suppliers should be able to demonstrate full system traceability to ensure that any bought-in parts also meet the regulations.

There are several methods for a supplier to protect its equipment and meet the ATEX requirements.  These will vary according to the zone that the equipment is installed in.  For weighing systems they will be mostly be in zones 1 and 2 for gases and zones 21 and 22 for dusts.

Letters on the equipment marking designates details of the protection technique used, as detailed below:

Flow restricting enclosure - fr
Flameproof enclosure – d
Inherent safety in the design – g
Constructional safety – c
Control of ignition sources – b
Pressurisation – p
Liquid immersion - k

The exact type of protection will vary according to the equipment and also according to its application.

Electrical equipment typically uses intrinsic safety for protection.  This technique limits the available energy (spark or heat) to very low levels that are incapable of causing ignition.  Equipment protected to ia can be used in zone 0.  ib can be used in zones 1 and 2 only

By July you must have identified and classified any hazardous areas in your plant.   In addition any new weighing or other equipment used in these areas must meet ATEX requirements and be installed correctly.  Finally check that suppliers can offer full ATEX accreditation and are certified to provide installation and service support in hazardous areas.

It is best to seek expert advice since ATEX is a complex process.  There is no room for error, especially when lives are potentially at risk.

For a free guide to ATEX and weighing contact Avery Weigh-Tronix Telephone 0870 90 50061 or e-mail info@awtxglobal.com