Closing the EMC enclosure gap

For the most part, electrical enclosures have relied on conducting grooves and gaskets to eliminate problems with EMC. Now, however, a more innovative solution has been developed

Electrical enclosures are usually simple affairs, typically consisting of a base and a lid. Usually, the lids of the enclosures are fastened by screws or quick fasteners to ensure that no dust, humidity, oil or water can enter the enclosure interior. To achieve this, the enclosure base has a tongue, and the lid has a groove with a gasket of rubber or plastic in it. After assembly, the tongue presses against the gasket to seal the enclosure lid to the base.

Enclosure manufacturers are well aware of the need for high frequency shielding, and a variety of schemes have been developed, both for metal and plastic enclosures, for providing such enclosures with the appropriate shielding properties.

Aluminium enclosures give some protection against high frequency interference because of the nature of the aluminium, but even here, shielding can cause problems. For example, conventional groove and tongue sealing structures have to be free of paint and expensive conducting gaskets have to be installed.

An apparent way to enhance the shielding capability of enclosures is to create paint free contact areas, but this succeeds only when the contact areas are protected against corrosion. Otherwise, the contact areas will corrode after a short time and the high frequency screening effect will be lost.

What is more, the contact between the lid and the bottom is only made on the outer area of the enclosure. Only in the outer area is the contact metal to metal, but this area is not protected against corrosion. It is also expensive to hold this area free of paint, but the majority of users want the area painted for corrosion resistance or decorative reasons.

Because of this, the use of conducting gaskets and paint free tongues and grooves has been the only effective method available.

Now, there is an alternative, developed and patented by German based Rolec. Rolec’s design is based on the fact that the best possible electrical contact between the top and the bottom of an aluminium enclosure is via butt-type contact between two bare metal surfaces.

By making this contact occur inside, instead of outside the enclosure, Rolec has removed the need for metallised seals and reduced the likelihood of moisture, causing corrosion that will degrade electrical isolation. An external seal protects the contact surface from ingress.

When the top and the bottom of the new Rolec electrical enclosure are brought together, a gasket is compressed between two surfaces. At that time, the inside wall of the top of the enclosure is brought into contact with the inside part of the bottom of the enclosure. But, because of the way the enclosure has been designed, the outside lip of the enclosure cover is never in contact with the outside of the base.

The new enclosures, called Conform, are claimed to cost the same as traditional enclosures. And independent testing at the University of Hanover has demonstrated that the enclosures perform as well as traditional enclosures with conductive seals.