Know your IP from your Exd and Exe
01 June 2012
When it comes to specifying enclosures for hazardous area electrical applications, designers must ensure they fully understand IP protection ratings, as well as the implications of modifying an Exd or Exe enclosure prior to, or after, installation.
As a manufacturer of enclosures for harsh industrial and hazardous environments, Gary Johnson says his company often deals with customers who don’t fully understand what they require in terms of the IP protection rating of an enclosure and whether what they are asking for meets the end user’s technical specification.
The function of an enclosure is to protect internally housed components from excess heat, humidity, dirt, dust or water. It might also have to be sited in a hazardous gas or dust environment such as an oil platform or petrochemicals plant, a flour mill or sugar production plant. In either case, it is crucial that the customer fully appreciates the differences between the IP ratings along with the requirements for an enclosure that is fully certified for use in a hazardous gas or dust environment.
And when modifying an enclosure – either prior to or after installation – it is equally important that the installer and end user fully appreciate what they are allowed to do in terms of an Exe or Exd enclosure to avoid adversely affecting its protection rating or, indeed, invalidating the certification.
With an Exd enclosure, the box cannot be modified once it has been manufactured and assembled, otherwise the protection rating will be affected. For example, drilling new cable entries must only be carried out by the manufacturer or by an approved local assembler (distributor).
However, once an Exe enclosure leaves the factory, although this is in a certified state, end users may then modify that box prior to or after installation within certain guidelines. Any alterations need to be communicated to the manufacturer or local assembler to ensure that the correct certification is given or maintained.
These modifications normally involve adding some extra cable entries or terminals; reputable suppliers normally provide a range of standard hazardous area Exe enclosures (in stainless steel, GRP or painted steel), leaving any modifications to their networks of approved local assemblers who can carry out the work without adversely affecting the certification or protection rating.
Many customers don’t fully appreciate that an Exd or Exe enclosure has been subjected to a series of arduous tests, including thermal conditioning (accelerated ageing) and mechanical impact tests at sites considered to be the weakest points of the enclosure. If the material is considered to have a reduction in resistance to impact at lower temperatures, the box is also impact tested at the lower end of the box’s operating temperature range.
These tests are important because, once installed in a harsh hazardous area, access for servicing might be difficult, requiring hot work permits or even a plant shutdown. The enclosure is also very likely to suffer knocks and bangs during its lifetime in duty, but it must remain intact in order to continue performing the intended function for protection.
In recent years, European and International standards for hazardous area enclosures have become even stricter in terms of impact and thermal tests. In the latest standards, all openings on the enclosure are tested (opened and closed) prior to their IP test. Many companies can offer industrial-grade enclosures with an IP66 rating, but for hazardous areas, customers need to be absolutely certain that the enclosures they purchase are fully certified boxes that have been tested in terms of temperature ratings and impact protection.
It is a general requirement in the oil & gas industry that hazardous area enclosures are manufactured from 316L stainless steel as a minimum specification. The author’s company’s hazardous area enclosures are mostly made from electro-chemical polished 316L high-grade stainless steel as standard, as this offers enhanced corrosion resistance over other finishes.
GRP enclosures also offer technical advantages. The material is often graphite-loaded in order to avoid electrostatic charge accumulation, which is important for hazardous area applications. Moulded, non-metallic enclosures also enable the manufacturer to construct very modular or flexible design features for the customer, including quick-fitting of components such as pushbuttons, switches, lamps, and so on.
Non-metallic enclosures also facilitate the design low-sided enclosures for easier, faster wiring. With stainless steel fabricated boxes, it is a little more difficult to offer these features, but fabricated steel enclosures are very flexible in the size ranges offered, special sizes can be quickly included - and they can also be provided in much larger sizes if required.
Gary Johnson is technical manager, Cooper Crouse-Hinds UK
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