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Material choices for enclosures

Material choices for enclosures

At first glance electrical enclosures are simple boxes, but choosing the best material of construction from the many available requires some consideration. Chris Lloyd of Spelsberg offers some advice.

Specifying a product is always a balance between price and performance, but in the engineering world performance should always be the key consideration. The physical properties of any enclosure are primarily determined by the material of construction, and the wide range of options indicates that there are many parameters to review.

The first decision is likely to be metal or plastic? Metal enclosures offer very high-impact resistance and excellent explosion protection, so are a good choice in exposed locations and in potentially hazardous applications. However, for many applications the main issues are more prosaic: ingress protection, ease of installation, cable retention, moderate impact protection, UV protection and cost. Plastic enclosures can satisfy all these requirements; and in many cases they are better than the more traditional metal units. So how do you choose the material for a given application? What are the different strengths and characteristics of each option?

A good idea is to look first at the most common choices made by other people and work out the attraction of each. For enclosures we can summarise this as:
* PVC - a budget option: ideal for small simple installations
* Polystyrene - offers medium impact requirements and up to IP67 ingress protection as standard/IP68 with gelling compound
* Polycarbonate - withstands substantial knocks, offers UV protection for outdoor use and up to IP67 ingress protection as standard/IP68 with gelling compound
* GRP (glass reinforced polyester) - robust material suited to extremely harsh industrial environments and can be UV-stabilised

Now let us look at each option in a bit more detail. PVC is one of the most widely produced plastics in the world. For the enclosures spectrum it is typically used in smaller junction boxes, which are used to house terminal blocks and connectors. PVC enclosures are extremely low cost, but contain highly toxic dioxins, which can bleed slowly into the atmosphere, particularly if they are exposed to UV light. An increasing number of enclosure companies choose not to supply PVC products.

For the majority of applications, the choice comes down to polystyrene or polycarbonate. Neither material contains halogens and both are strong enough to withstand knocks, and provide high cable retention and IP68 protection when used in conjunction with suitable cable glands or a gelling compound.

However, polystyrene can show signs of UV decay long before polycarbonate. Therefore, polycarbonate is far more suitable for external applications, or where direct sunlight may be experienced. Polycarbonate is also thermally stable, a major consideration in say, inverter installations, where heat generation is inevitable. Finally, polycarbonate's strength makes it more suitable for applications where impact is likely.

GRP enclosures are for use in industrial environments where extremely high levels of protection are required. Offering a high level of rigidity at minimal thickness, equating to increased protection from impact in a lightweight, aesthetic design, GRP enclosures provide effective corrosion resistance for electrical and electronic controls in the harsh environments that are typical of many industries, such as marine, offshore, petrochemical, paper and water treatment.

There is an extremely wide range of possible applications and environments in which an electrical enclosure may be necessary; each presents different challenges and different environments from which to provide protection. This is why manufacturers have invested so heavily in developing different material technologies. The intention is to make sure that each customer can specify an enclosure that is cost-effective and ideally suited to the environment in which it will be placed.

Of course, the list of materials mentioned is 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, which 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 specialty applications.

As with all electrical equipment, it is important that you consult an expert before specifying enclosures for your application. All reputable manufacturers and distributors should provide easy access to a sales force for technical support, which can answer questions on halogens, resistance capabilities and material comparisons.

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