Preference for plastic? How to choose wisely

28 February 2012

So, you’ve decided on a plastic enclosure but how do you decide which material type is best suited to your application. Chris Lloyd looks at what’s available, describing the limitations and advantages of each in terms of the environments and duties to which your enclosures will ultimately be exposed

A quick online search for electrical enclosures shows just how many options and variations there are on today’s market; there is a myriad of apparent choices for material, and each seems to offer ‘unique’ benefits and features. It can be hard to pin down the enclosure type that is best for your application, both in terms of performance and cost, so what are the key considerations when specifying an enclosure’s material of construction - and how can you be sure that the products you choose will meet the application’s requirements?

Metal versus plastic
The term ‘electrical enclosure’ is incredibly broad. It can cover a vast spectrum of possible applications and requirements; from a small junction box that protects a terminal block located in the ceiling of an air conditioned office, to a large housing that protects vital industrial control equipment in a hazardous environment. To complement the scope of possible applications, there is huge choice in terms of materials when specifying an enclosure. Initially, this may seem daunting; but with the correct guidance it is quick and simple to find an enclosure that is suitable for almost any job.

What may appear to be the first and most fundamental choice - metal versus plastic - is also possibly the simplest. While metal enclosures certainly have their place when it comes to applications that require extremely high impact resistance or explosion protection, most applications require a combination of ingress protection, ease of installation, cable retention, impact protection, UV protection, cost effectiveness and aesthetics. With these considerations in mind, plastic variants are likely as not your best option.

So, assuming that your application is compatible with a plastic enclosure, the next step is to choose the material type. It is at this stage that decisions become more personal and it is important to understand the different strengths and characteristics of each option.

Of the choices available, the four most common that best cover the range of protection requirements are PVC (polyvinyl chloride) – a cheaper option, that is often used for low-end, small junction boxes; polystyrene – suitable for most internal applications with medium impact requirements and up to IP68 ingress protection; polycarbonate – a tough material that can withstand substantial knocks, offers UV protection and up to IP68 ingress protection; and GRP (glass reinforced plastic) – a very robust material suitable for extremely harsh industrial environments that can be manufactured with UV stabilisers for outdoor use.
PVC: does budget have its place?

Thanks to its low cost of production and the ease with which it can be worked, PVC is one of the most widely produced plastics in the world. It is found towards the budget end of the enclosure spectrum and is typically used in smaller junction boxes which are used to house terminal blocks and connectors. PVC enclosures are extremely cheap, which is often appealing to budget conscious contractors.

However, it has one drawback. PVC contains chlorine, one of a group of elements known as halogens which, when they burn, release dioxins – known to be carcinogenic. Dioxins are also slowly released into the atmosphere as PVC breaks down as a result of UV exposure.
While EU legislation has long banned a number of halogens (such as PBB and PBDE) it is yet to clamp down on PVC. In spite of this, however, the author’s company has taken the decision to use completely halogen free plastics such as polystyrene, polycarbonate and GRP in the construction of its products.

Polystyrene versus polycarbonate
For the majority of applications, the choice comes down to polystyrene or polycarbonate. Both materials are halogen-free and, while polystyrene is typically the least expensive material, both are strong enough to withstand knocks, and provide high cable retention and IP68 protection when used in conjunction with suitable cable glands or sealing method. They are both easily worked to produce aesthetically pleasing enclosures, and most cable entry options can be accommodated.

The similarities end, however, when these plastics are compared against each other in certain application environments. Installed indoors in an environment subject to wash-down, both materials would be capable of providing long-term protection from water ingress. However, placed in an outdoor environment, a polystyrene enclosure would suffer the effects of UV exposure long before a polycarbonate alternative, which offers a far higher level of UV protection.

Polycarbonate is also tolerant of high - or fluctuating - operating temperatures. When housing a drive, for example, polycarbonate would be the most suitable material of construction. Polycarbonate also has higher impact resistance than polystyrene, making it more suitable for applications requiring mechanical robustness, such as vehicle-mounted equipment.

But when the going gets tough….
In some applications, particularly in manufacturing industry, enclosures need to be able to withstand more than submersion in water, exposure to sunlight and occasional knocks. They may be subjected to corrosive environments (such as sea air) or chemicals (such as cleaning agents), vibration and extreme temperatures.

Where very high levels of protection are necessary, GRP constructed enclosures offer a high level of rigidity for minimal wall thickness. This means you get high impact resistance from a lightweight material, without compromising aesthetics. GRP enclosures also provide effective corrosion resistance for electrical and electronic controls sited in harsh industrial and maritime environments.

Of course the materials listed above are by no means exhaustive; they have been selected as a representative group that demonstrates how, beneath a similar surface, there can be stark contrasts between performance capabilities. There are many highly specialised materials, that have not been mentioned, that are designed for extreme environments and offer protection levels beyond the capabilities of the materials listed above, but these are only necessary for a minority of speciality applications.

Chris Lloyd is sales and marketing manager for Spelsberg els UK

Environmental compatibility
Electrical enclosure applications pose a variety of challenges and this is why manufacturers, such as Spelsberg, have invested so heavily in developing different material technologies. The intention is to make sure that each customer can specify an enclosure that is not only cost effective but also compatible with the environment in which it will be placed.

As with all electrical equipment, it is important that you consult an expert – an individual or a manufacturer - before specifying enclosures for your application. All reputable manufacturers and distributors should provide easy access to a sales force for technical support, as well as answers to your questions about materials of construction and their limitations.