• • • ENCLOSURES • • •
Considerations when specifying metal and plastic enclosures
Russell Irvine, sales manager at Hammond Electronics, says that in an ideal world, the enclosure for a new project would have been selected as early as possible in the design process
I
n the real world once the design is finalised the first question is normally, ‘what box do I need for this?’.
Size The fundamental questions. How big are the PCB and other components? How much space is needed on external surfaces for any indicators, switches, connectors, cable entries and similar components? What space is required for board-mounted components, stand-alone electro-mechanical components or multiple horizontal boards?
Installed environment For outside installation, UV stability is a potential issue for plastic enclosures. For road and rail applications, shock and vibration are likely to be a consideration. Will the ingress of dust and water be something to be considered? Are there likely to be oil and chemicals present? Is resistance to impact damage a possible problem? If so, metal enclosures typically offer better resistance than plastic ones. Are particularly high or low temperature expected?
Enclosure material The main choice is between ABS and polycarbonate plastic, aluminium, stainless steel and GRP. For plastic enclosures, fire resistance is a consideration. UL94 specifies a vertical or horizontal burn, the vertical burn test being the more stringent: material will be classified as V0, V1 or V2 where V0 is the highest. Polycarbonate and GRP are better for outdoor use because of their high resistance to UV embrittlement and colour fading.
Aluminium enclosures are either die-cast or
extruded. Aluminium is robust, non-corrosive, lightweight, non-magnetic, easily machined and gives good impact resistance and is electrically conductive. For applications where protection against shock damage is important, where EMC is likely to be an issue or where high temperatures, dust or water are expected to be present, an aluminium enclosure is the ideal choice. Stainless steel is particularly suited for use in
pharmaceutical and food processing plants, where its cleanability, strength, and corrosion resistance are beneficial. It also has high impact resistance. GRP is light weight, stronger than many
metals, non-magnetic, non-conductive and it is chemically inert.
Environmental sealing The relevant standard is IEC 60529. Typically, enclosures rated at IP54 are suitable for general purpose use; for installation in environments where dust and water are likely to be present, IP66, 67 or 68 would normally be specified. The highest rating is IP69, which gives protection against high pressure steam cleaning.
Protection classes for solid object protection First figure
0 1
2 3 4 5 6
Scope of protection No protection
Protection from solid objects greater than 50mm in diameter
Protection against solid objects greater in diameter than 12.5mm
Protection against solid objects greater in diameter than 2.5mm
Protection against solid objects larger than 1.0mm
Protection against the ingress of dust in harmful quantities
Dust tight
EMC Die-cast or extruded aluminium will provide a level of EMC likely to be more than sufficient for the majority of applications. Compared to other metals such as aluminium, copper and silver, stainless steel is a relatively poor attenuator, so it would not normally be chosen as the material for an EMC enclosure. Plastic enclosures are non-conductive, so they
provide no EMC attenuation. If the housed equipment is likely to be susceptible to external EMC or if it will generate high levels of EM
28 ELECTRICAL ENGINEERING • MAY 2022 Protection classes for water protection
Second figure Scope of protection 0 1
No protection
Protected against vertically falling drops of water
2 3 4 5 6 7 8 9
Protection against falling water droplets up to 15° from the vertical
Protected against direct water sprays from up to 60 degrees from the vertical
Protected against splashing water from any direction
Protection against low pressure 6.3mm jets of water sprayed from any angle
Protection against powerful 12.5mm water jets from any angle
Protection against temporary immersion in water up to one metre deep
Protection against continuous immersion in water greater than one metre in depth
Protection against high- pressure, high-temperature jet sprays, wash-downs or steam-cleaning procedures
radiation, the best option is to choose an aluminium die-cast or extruded enclosure.
Manufacturer modification
capability In order to make a standard enclosure suitable for a specific application, it will need to be modified. The best option is for the original manufacturer to provide a modified enclosure configured to the specific requirements of the project, so there is no need to over-order to allow for set-up and wastage quantities using third-party contractors.
Conclusions A superficial glance at any enclosure may give the impression that it is a simple box, but in reality, it is the end result of a great deal of design effort and expertise to create the most useful and feature-rich housing, suitable for a use in a wide variety of applications and environments. Work with the enclosure manufacturer from an
early stage in the project design cycle to tap the wealth of experience behind a standard enclosure.
electricalengineeringmagazine.co.uk
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