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Legislation: Good RF bonding techniques for cabinets01 January 2009Previous editions of this column introduced the six basic good EMC engineering practices for shielded cabinets. Keith Armstrong continues with some more practical details on how to 'RF-bond' filters and cable shields. Lack of attention to these issues has ruined the EMC performance of many a costly shielded enclosure! RF-bonding’ means achieving a lowimpedance electrical connection at the highest frequency that must be controlled. For compliance with normal commercial/industrial EMC standards this is 1GHz, likely to become 2.7GHz or more in the near future.
Most of the electronic units used in commercial/industrial control panels and systems do not (yet) suffer from significant EMC problems at such high frequencies. Variable-speed motor drives and other switch-mode power convertors (e.g. inverters, UPSs, ac-dc and dc-dc converters, etc.) tend to have problems up to about 2,000 times their switching rate, so for example a 100kW motor drive switching at 10kHz would most likely have ‘a highest frequency to be controlled’ of about 20MHz. But note that where a cabinet contains powerful computers or other digital processing, problems can exist up to the highest frequencies in the test standards, and beyond.
By a ‘low impedance electrical bond’ we mean much less than 0.1 ohm, ideally less than 10 milliohms. When we consider that 30mm of a wire with cross-sectional area 4 sq.mm has an inductance of about 20 nanoHenries, giving it an impedance of about 2.5 ohms at 20MHz, and 125 ohms at 1GHz, it shows us that we can’t use wires for these bonds.
The ‘skin effect’ makes all RF currents travel on the surfaces of conductors and metalwork, rather than through its bulk metal thickness. To get good EMC performance from any metal cabinet we use this effect to encourage the noise currents created by the equipment inside our cabinet to stay inside, to reduce its RF emissions. And we can use it to encourage the noise currents flowing in the environment outside of our cabinet, to remain on its outside, to improve RF immunity.
These two key issues - the need for low impedance bonding up to the highest frequency to be controlled, plus using skin effect to keep internal noises inside, and external noises outside, show us how to bond filters and cable shields to our cabinets:
a) Direct metal-to-metal electrical bonds between the filters or cable shields and the cabinet’s metal structure. Rather than a single point of contact, we need complete circumferential (360º), or at least multi-point bonding.
b) These electrical bonds must be at the points where the cables enter/exit a cabinet surface. These should be in an RF-bonded part of the cabinet’s structure. For a small cabinet made from folded steel, this would simply be one of its walls. But for a larger cabinet consisting of a frame with cladding panels, it should be a metal connector panel that is RFbonded (with spot welds or screw fixings) at multiple points around its perimeter, to the cabinet's frame.
If errors or malfunctions in the electronics or software of your project could increase safety risks, complying with the EMC Directive will not be enough to show ‘due diligence’ in ensuring tolerable safety risks despite the possibilities for electromagnetic interference. For a suitable EMC methodology, download the IET’s new free guide on “EMC for Functional Safety” from www.theiet.org/factfiles/emc/index.cfm.
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