For a variety of reasons, not all instruments require EMI protection. David Bowring argues that many OEMs could save costs by having the freedom to choose or specify an instrument case, either with or without EMI protectionInstrument Cases range from relatively simple metal or plastic enclosures through to custom designed composite structures for a specific product or market, often containing a wide range of components. Widely used for test, data capture or for portable equipment, they are available as standard (off-the-shelf), or built to suit a particular product or application.One of the major criteria since the early 1990s has been the ability of cases to provide Electromagnetic Interference (EMI) protection when used to house electronic circuits. This became the norm within the electronics industry as electromagnetic compatibility (EMC) was required to assure users that they would not experience interoperability problems with adjacent electronic equipment. However, not all enclosure applications are susceptible to interference, or likely to generate it, as cases are frequently used to house low frequency electrical or analogue electronics rather than the more EMI prone high clock speed digital circuitry.There is generally a cost penalty in providing EMC through the enclosure, either because a metallic surface has to be provided, which acts as the main screen, or due to the necessity of electrically sealing the gaps between adjacent (conductive/metallic) surfaces.With basic metal enclosures EMC can be achieved with an additional fold or ridge between the cover and the sides. This makes the EMI path less direct and thus improves the protection provided. It can only be certain to provide protection if the adjacent surfaces are conductive and the points of contact are relatively close to each other, which again may involve additional cost.The need for the contact points to be close together increases directly with the increase in frequency of generation or susceptibility. EMI loves long thin slit shaped slots (the join between sides and top or the vertical join between adjacent front panels), particularly if the metalwork on either side is at different potentials. If the case has a painted finish, the joints between adjacent panels must be masked to ensure good contact (provided the surface finish under the painting is conductive). These precautions are great if the application demands EMI protection, but an additional expense for those who do not need it. A few years ago EMC was seen as a major threat, with the result that most case users opted for a product that provided at least a basic level of EMI protection. Many customers have since found they do not need the additional cost of an EMC enclosure and one enclosure manufacturer, Rittal, has provided a cost effective solution by re-packaging its instrument cases into two distinct families, standard and EMC. Other changes being introduced at the same time include the supply of top and bottom covers as separate items to allow a choice of perforated (for ventilation), or solid (to prevent entry of foreign bodies) to any model in the range, both EMC and standard. For manufacturers of plastic cases, the cost saving of non-EMC compared to an EMC case may be as much as 40%, thanks to the removal of an additional metal plating process. It is less likely that these cases will utilise gaskets as there is often a locking 'hill and dale' feature built into the carcass joins, which increases the complexity of the entry or exit path for interference. So, while it is accepted that products should be electromagnetically compatible, it is worth checking whether protection is necessary in the first place, otherwise you might just be wasting your money. David Bowring is product manager, Electronic Systems at Rittal