Metallic enclosure: purpose

In the context of systems EMC, a metal enclosure can have a number of purposes as well as its primary one of providing physical protection and mounting:

• to provide a local ground/earth reference for the internal equipment
• to provide and demarcate a zone of increased EM protection
• to prevent radiated field coupling to and from the internal equipment.

You will notice that the conventionally understood function of a metal cabinet- to provide shielding- is placed last in the above list. This is deliberate. As we will see, many examples of metal housings are likely to function very poorly as shields, because of their surfeit of inadequately treated apertures and seams. But this doesn't mean they have no EMC function. In fact, the first purpose - to provide a local ground/earth reference - is nearly always the most important. This can be achieved by an enclosure which is effectively unshielded, provided that care is taken to use the bulk metal of the cabinet in the right manner. Such an approach is very cost effective, since many of the assembly, installation and maintenance implications of a fully shielded enclosure can be dispensed with.

Simply constructing a metal cabinet or cubicle to house the equipment in the physical sense, is not adequate for electromagnetic purposes. The constructional and assembly methods of the individual structural parts, and the provisions for cable entry/exit, are at least as important as the mere fact of a conductive enclosure.

Interference currents in the structure are expected and have to be controlled. So, electrical interconnections between the parts, and any discontinuities in the form of apertures, seams and cable penetrations, must be fully specified.

This section will first discuss the merits of right use of the structural components for local ground/earthing, and then consider the important aspects of a classically shielded enclosure, before going on to look at architectural shielding.

Transfer impedance of the ground/earth reference

The function of providing a local ground/earth reference is critical to the effectiveness of a metal enclosure, whether this enclosure is deliberately intended as a shield or not.

Section 5 has already looked at the principles of ground/earthing and Section 7 examines cable layout and termination in detail. In between is the area where the cables and equipment are terminated. This area must provide the lowest possible transfer impedance to the internal circuits and equipment, so that interference currents do not couple between the enclosure and the sensitive or noisy circuits within it.

Transfer impedance was discussed prev. --- the hierarchy of structural shapes for improving transfer impedance, from which you might deduce that if all cabinets were cylindrical with the connections taken out from each end, all EMC problems would be over. Life is not quite this simple, but the principle remains that the cabinet structure has an important part to play in providing the best overall EMC performance.

Cabinet backplate ground/earths

That figure showed that, after a cylindrical or rectangular enclosure, a large flat plate over which the relevant circuits are mounted gives the best transfer impedance performance. Industrial control cabinets in general include a backplate for physical mounting, whilst smaller enclosures provide some sort of metal chassis. Telecomms and IT cabinets have an internal support structure. Mounting all electronic modules on, and terminating all cable screens and parallel ground/earthing conductors to, a backplate or similar chassis provides the lowest achievable transfer impedance in practical terms.

But since using the backplate in this way means that it must carry interference currents, the way in which contacts are made to it, and its conductivity, become significant. Zinc plating, and clamp-style cable screen connections, are both recommended.

At high frequencies only a metal area (mesh or plate) can give a reliable low- transfer-impedance ground/earth, so you are best advised to use a solid backplate or chassis of an enclosure as the ground/earth for all internal electronic equipment instead of using green/yellow wires to a star point. This calls for heavy zinc-plated metalwork, not painted; if any passivation is applied it should be the yellow type.

At the cost of some inconvenience, you can continue to use painted metalwork as long as the following precautions are taken for all the ground/earth connections:

• remove the paint
• use star washers to bite into the metal
• apply suitable corrosion protection after the joint is made.

Terminations of screened cables to the backplate or chassis should be carefully planned and implemented so that all common mode interference currents flow directly through it, and not into the circuits mounted on it. The methods, particularly the use of clamps rather than pigtails, should be followed rigorously.

Layout and placement within the enclosure

Cable runs

Coupling of external fields to internal cables, and of local electric and magnetic fields between cables, is greatly affected by the route a cable follows around a system. To minimize coupling of cables with external fields, run the cables close to a well-bonded metal structure which can act as a low impedance ground/earth reference -- this is usually the backplate or chassis. Where a cable leaves the backplate/chassis, ensure that it follows a conductive structure which is electrically bonded to the backplate/chassis. Avoid running cables near to apertures in the structure or enclosure or near to breaks in the bond continuity, as the localized fields around these points are high. (This advice is really the same as saying that the internal construction of the enclosure acts as a continuation of the PEC for the cables.) To minimize coupling of cables with each other, segregate different classes of cable and run them with at least 150mm of separation. Do not allow long runs of closely spaced cable of different classes.

--- Cable runs within the enclosure

---Example of backplate layout for a simple industrial control panel

Module placement

Carefully position the various electronic, electrical, pneumatic, hydraulic, etc., units on the backplate/chassis to keep sensitive units such as PLCs, computers or analogue instrumentation away from electrical noise sources (e.g. switches, relays or contactors), and to help achieve segregation of the different cable classes. The important principle here is to assess each item for its interference potential, and to specify the internal layout accordingly.

Have no internal trunking at all for Class 4 cables by minimizing their internal lengths as much as possible. This means fitting Class 4 associated units such as inverter drives near to the edge of the enclosure and to their cable entries, and /or filtering any Class 4 incoming supplies at their point of entry to make them Class 3 or even 2.

--- an industrial enclosure, showing the cable route to door mounted equipment, with cables strapped along the short ground/earthing braid between door and cabinet wall; and an example of backplate layout in a motor drive area. The purpose of the cable following the ground/earth strap across the door opening is to minimize coupling of the cable with the door aperture, which would compromise the transfer impedance of the cabinet for this cable.

Because of their extremely aggressive emissions, inverter drive motor connections are always important. The example should be regarded as a generic approach, and suppliers' instructions should always be followed if they are available. The important issue here is to provide a local return path to the filter for the switching noise currents which are flowing back down the ground/earthing conductor and /or the screen of the cable to the motor. These currents can easily pollute the rest of the cabinet and even other equipment in the immediate area of the drive-to-motor circuit, if proper high-frequency ground/earth bonding between the cable screen and the filter is not provided. Similar considerations apply for other "noisy" transducer drivers, such as RF- stabilized welding, spark erosion, and Class D audio amplifiers.

---Detail of cable routing internally in cabinet: (a) cable running to door (b) motor drive cabling

The clean-dirty box approach

A frequent and effective approach for industrial and other enclosures is to segregate the enclosure into a "clean" compartment and a "dirty" compartment. Either the cabinet can have a partition welded into it ( ---4), or an additional "dirty" enclosure can be bolted or welded to the side of the main "clean" cabinet instead of a dividing plate.

The "clean" compartment is then used for all the electronics which must be shielded from the external environment. All apertures in this part of the enclosure are rigorously controlled. Connections through to the clean volume must be made via 360-degree screen bonds to the partition plate, or via effectively ground/earthed filters - no untreated cables are allowed. Through-bulkhead filters present no problem, but chassis-mounting filters must keep the leads passing through the partition plate as short as possible and possibly treated with ferrite sleeves to minimize HF propagation across the partition.

The great advantage of this approach is that the dirty volume can be used for many or all of the field-installed connections. The interface through the partition can be pre- wired and checked before the system is shipped from the factory. Then, all of the strictures in this guide about ensuring correct installation practices are taken out of the hands of the installation technician and given to the system assembler and designer.

This is of particular benefit if the system supplier has no control over the installation methods at all - a regrettably common problem.

---The "clean/dirty" segregated shielded cabinet

---The transmission line theory of shielding effectiveness

Next: Shielding theory

Prev: Building a meshed facility ground (earth)

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