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Mechanical engineering

Application of the ATEX Directive 94/9/EC to transportable, pressurised cabins ("modules") intended for use in hazardous areas

This Consideration Paper deals with the application of the ATEX Directive 94/9/EC to transportable pressurised cabins (or "modules"), as such goods are considered to be in-scope of 94/9/EC.

Cabin description

Such cabins are often intended for use in a Zone 1 or Zone 2 hazardous area and are used at both onshore and offshore sites (e.g. fixed drilling platforms). The cabins are pressurised with breathable air both to exclude ingress of flammable gas and to provide a safe atmosphere for operators to work inside the cabins.

The cabins are supplied to meet a variety of purposes, e.g. laboratory, control cabin, office or workshop. They are placed on the market in their finished state, ready for use, but "empty" - suitable for the customer to install and use their own equipment inside the cabin.

The design includes the incorporation of a number of ATEX-certified items, such as fire and gas detection systems, automatic shutdown systems and ventilation fans.

The design ensures positive pressure inside the module to prevent the ingress of flammable gas. As long as there is no internal source of release, this allows the enclosed area to be regarded as a non-hazardous area (reference, for example, UK Institute of Petroleum Code IP 15, section 6, 2002).

The cabin interior is often fitted with unprotected electrical fittings such as luminaires, switches and socket outlets. Should the cabin suffer a loss of pressure, these items have the potential to be exposed to an ingress of flammable gas which may form a potentially explosive atmosphere.

Therefore, the cabins are designed with many safety features to prevent this situation, such as smoke and gas detectors and alarms and the automatic shut down of non-certified electrical equipment being used inside the cabin.

Application of 94/9/EC

Where a manufacturer assembles and supplies a product which includes a number of ATEX-certified items (as is the case here), they are responsible for assuring that the process of design/ manufacture has not introduced any additional ignition sources or other relevant hazards and for having the assembly ATEX certified.

The ventilation system for these cabins should be viewed as a protection device, from the definitions in the directive. A pressurisation fan and a long length of ducting are incorporated, so that the fan can be deployed in a remote, safe area. Therefore, the safe operation of the pressurisation system also needs to be assured. The module itself forms the pressurised enclosure and must therefore be subject to conformity assessment, which would need to demonstrate the effective application of the pressurisation concept (including consideration of the safety integrity of the pressurisation control system) so as to meet the essential requirements of Directive 94/9/EC.

EN 50381:2004 ("Transportable ventilated rooms with or without an internal source of release") covers the essential health and safety requirements of 94/9/EC and applies to the cabins covered in this paper. "Ventilation" in this standard is used as a means of explosion protection. ("Ventilation" is also being used in these particular modules to ensure adequate air quality for personnel working inside. However, this aspect is out with the scope of the standard.)

An alternative route to compliance is for the manufacturer to demonstrate that the essential health and safety requirements of 94/9/EC have been met. This would include demonstrating the effective application of the pressurisation concept, including consideration of the safety integrity of the pressurisation control system.

It is stated in EN 50381:2004 that it is not the intention of this standard to cover stationary analyzer houses according to EN 61285:2004.

User's responsibilities

The installation of the cabin in accordance with the manufacturer's instructions is common to many ATEX products and shouldn't require further certification by/for the end user. The only circumstances when the end user would need to undertake ATEX conformity assessment under 94/9/EC would be if they made any changes which affected the explosion safety features of the product or if they needed to install it in a manner which was not in accordance with the manufacturer's instructions. These instructions should also state any limitations on the use of (non-protected) equipment inside the cabin.

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