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Good EMC design techniques: EM mitigation and zoning (Part 4)06 September 2010Keith Armstrong provides more practical detail on segregation techniques, known as ‘EM Zoning’In the last three editions I introduced the concept of EM Zoning – an essential EMC engineering technique if filtering, shielding (screening), galvanic isolation, surge and transient suppression are to work as intended at frequencies above about 100kHz.
Zoning is an effective EMC technique, and an important one, whether we are constructing a cabinet, a system, or an installation of any size – even up to the size of a national network.
In the last issue, I showed how electronic modules and their cabling could be arranged on the zinc-plated backplate of an industrial cabinet, using the detailed techniques described in previous articles [1]. References [2] and [3] provide more detail.
Although the example used here is of an industrial-type control cabinet, this approach is appropriate for all metal boxes that contain two or more electronic and/or electro-mechanical/hydraulic/pneumatic/optical modules connected together by wires. This is true regardless of the application – from domestic HVAC, through industrial automation and robotics, vehicles of all types, professional audio, video and lighting control “desks” or “consoles”, etc. – whether civilian or military. Note that in EMC terms a vehicle of any sort (road, rail, sea, subsea, air, space, etc.) is just a box containing interconnected electronic and/or electro-mechanical/hydraulic/pneumatic/optical modules.
Figure 1 shows some additional detail for the layout of the example cabinet used last time, in the motor drive area, using a “bookshelf” type mains filter and shielded (screened) motor cable. Detailed information on the various techniques for suppressing variable-speed motor drives, and why they work, is given in the new REO guide [4]. Some motor drives provide arrangements for clamping motor cable shields (screens), but not the drive used in this example – so the cable shield is saddle-clamped metal-to-metal directly to the backplate as close as possible to the drive.
Some motor drive manufacturers recommend fitting external ferrites or other types of filters (sometimes called “dV/dt filters”), like the one shown in Figure 1. Where such additional components are specified, the manufacturers’ instructions should be followed exactly, and they should be fitted as close as possible to the drive to minimise the length of unfiltered cable, and grounded metal-to-metal directly to the backplate.
It has become commonplace to fit emergency stop buttons or other isolators in the shielded cables between drives and their motors. When these use plastic enclosures and connect the cut motor cable shields by a length of wire, they ruin the shielding of the motor cables and make it difficult to meet emissions specifications. These E-stops or isolators should be designed and installed to maintain shielding effectiveness up to at least 100MHz, which will require metal boxes with conductively-gasketed metal covers, and 360° shield-bonding “EMC” cable glands.
Don’t forget that, when using shielded motor cables for DC or AC motors, the cable shields must terminate to the motors’ metal terminal boxes using 360° shield-bonding “EMC” cable glands. Motors with plastic terminal boxes are unsuitable for use with shielded motor cables, see [4] for alternative suppression techniques.
Figure 2 shows a different example, a sketch of the module (rack unit) segregation in a rack cabinet, and the cable routing in its rear. It helps to design the various rack units to segregate their connectors according to a pre-arranged scheme, to make the cable segregation in their cabinets easier. When some or all rack units are purchased from other suppliers there is no such scheme, so it may help to provide metal shelves at the rear of the units to provide a “cable marshalling” area where cables can be grouped into their classes. The metal shelves should, of course, be RF-bonded to the cabinet frame at all fixings to carry the cables’ “leaked” common-mode (CM) currents, to help prevent them contributing to radiated emissions.
I recently saw a major installation where someone had the bright idea of powder-coating all the elements of the frames of the bolted-together rack cabinets, a different colour for each type of cabinet. Of course, this meant that RF-bonding the cabinet frames was impossible. It seems a great shame to waste such a powerful and free EMC resource, when a similar effect could easily have been achieved with tin or zinc-plated frames that automatically achieved good RF-bonding, and used coloured “skin” panels to mark the different types. Figure 2 shows an example of segregation in a 19in rack cabinet.
Figure 3 shows an example of module layout inside a rack’s chassis unit, which should follow the same segregation rules for modules and cable classes as for the example cabinet shown in my previous PSB article. All internal cables should be routed close to the metal chassis at all times, to help control CM currents, in accordance with good EMC engineering practice. All connectors and other cable penetration points should use a single connector panel, which has no seams or joints in it so that CM and ground-potential-equalising currents can more easily flow from one external cable to another without flowing through the chassis unit.
References:
[1] Previous PSB columns in this series are archived at: www.psbonthenet.net/company.aspx?CompanyID=12242. I also plan to post my complete set of columns (with figures) on my website http://www.cherryclough.com/ by autumn 2010.
[2] “Good EMC Practices in the Design and Construction of Electrical Cabinets”, 2007, www.reo.co.uk/knowledgebase
[3] “EMC for Systems and Installations”, Tim Williams and Keith Armstrong, Newnes, 2000, ISBN: 0-7506-4167-3, www.bh.com/newnes, RS Components Part No. 377-6463
[4] “Complying with IEC/EN 61800-3 – Good EMC Engineering Practices in the Installation of Power Drive Systems”, www.reo.co.uk/knowledgebase
[Readers - to view the figures, please visit the digital issue by clicking on the cover image on the home page]
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