What is electromagnetic shielding
Electromagnetic shielding is a conductive barrier that completely envelopes a device to protect it from environmental interference or to stop emissions from the device itself interfering with other devices in the same environment. It is a form of insulation in that it reduces or prevents transfer of energy, in this case electromagnetic energy, between a high output device and the environment or protects a sensitive device from electromagnetic fields in the environment. Environmental electromagnetic conditions are, by their nature, unpredictable. Shielding is designed to remove this performance threat.
What does EMI mean?
Whenever electricity flows in a conductor a magnetic field is generated. The fields generated are related to the amount of power being dissipated, so power hungry devices such as motors and transformers will produce significant fluctuating fields. Motors have electromagnetic fields that switch at high frequencies to keep them running. This is an ideal way of producing interference. Electromagnetic Interference (EMI) is the adverse effect of these electromagnetic fields on other devices. The interference is produced as the fluctuating field traverses other devices or their connecting cables or PCB tracks. Each traverse induces a voltage which may be quite small. However, data processing devices run on low voltages and these induced voltages are relatively large and can easily corrupt input and output data signals.
EMI shielding effectiveness
Conductive materials form a barrier to electromagnetic interference. That is the easy part. An enclosure must completely envelope the device. Usually there is a need to access the device and so the enclosure will be in at least two parts. There must be conductive continuity between these parts or else the interference may ‘leak’ in through the gap between them. Continuous face to face contact between a case and its lid with the precision required is not reliable. Add in other environmental hazards, such as dust and water, requiring flexible seals. Flexible seals in rubber are transparent to EMI and so must be enclosed in knitted metal wire sleeves to contact both parts. Some equipment requires air cooling whilst still maintaining the shield. Holes or slots in the shield would act as aerials and allow the EMI to re-radiate within the enclosure. Generally, if any hole or slot is under 3mm an effective shield can be maintained.
CP Cases ERack Lightweight 19″ Ruggedised Enclosures are on example of this technology in practice.
How to reduce EMI interference
The best known description of an EMI shield is ‘the Faraday Cage’. This is a conductive enclosure that keeps voltages out because charges repel each other sideways. Dimensions of the openings in the cage determine what voltages will be excluded. For electronic devices any opening must be small but do not need to be ‘airtight’.
CP has produced communications enclosures for the new aircraft carriers for the Royal Navy. The problem was that multiple cable entries were required and it would be an advantage to be able to install or change cabling without a lengthy dismantling or assembly procedure. By designing a custom flexible and conductive multiple cable entry port EMI protection standards were met.
To maintain good continuity of contact on flexible seals it is important that they are evenly compressed. CP provides for this with butterfly and toggle catches and have further latch solutions in development.
EMI Best Practise Case Study
CP Cases experience with EMI was put the test through a project with the Aircraft Carrier Alliance (ACA) where Rackmount Enclosures for QE Class Aircraft Carriers were delivered.
CP Cases supplied more than 1200 custom-designed rackmount enclosures and more than 4000 adaptors and brackets, for installation on-board two (QE) Class aircraft carriers to house datacoms enclosures of various sizes. The smallest enclosure accommodates two single units of 19” rack-mounted network switches; the largest cabinet stands almost five feet tall and can accommodate 29 units. Also developed especially for the project were a series of plinths and brackets, which secure telephone handsets to horizontal and vertical surfaces around the ship. Working closely with BAE Systems, one of the four Aircraft Carrier Alliance (ACA) partners, CP also integrated the power supply wiring, voltage filter and cable management systems to ensure the enclosures were ‘plug & play’ ready.
About the product
The primary function of the network equipment enclosures was to provide physical and electromagnetic security to various types of Cisco IE Ethernet network switches and the thousands of accompanying connecting datacoms, Ethernet and fibre optic cables.
The communications requirement was substantial – the carriers need constant contact with all in-flight aircraft, the consort group, HQ and land forces. The network switches process and route this vital communications data.
Each enclosure protects the 19” rack mounted switches from vibration, impact, particle and moisture ingress, and also provide screening from any electromagnetic interference (EMC/EMI) that could disrupt the mission communication systems from radar energy and other strong electromagnetic signals which are emanating from various positions on board, ensuring the ship’s sensors do their job without adversely affecting each other.
CP Cases introduced and utilised novel designs embracing the same materials developed specifically for EMC/EMI screening used as part of the ‘ERACK’ series of portable Anti- Vibration rackmount containers, which are performance certified to provide a high level of electromagnetic shielding.
The solution for providing electromagnetic compatibility throughout all sizes of enclosures, also manages interference which could potentially emanate where the data transfers cables enter and exit the enclosures. CP Cases developed a unique Cable Entry Port (CEP) which allows cables, with accompanying connectors, to be fed into and out of the enclosure through a simple “foam flap” letterbox system.
This specially designed (and EMI/EMC performance certified) CEP, uses conductive closed-cell polyethylene foam, which compresses around individual data cables. Conventionally, each cable would have been installed via an individual four part metallic cable gland, and bearing in mind that many enclosures have up to 144 individual cable entry & exit points, the CEP system provides significant advantages on installation labour hours, providing substantial cost benefits and ensures any future systems upgrades are implemented efficiently.
Cable management rails inside the enclosures with hook-n-loop cable ties ensure that the recommended bend radius for each data-coms cable is never compromised and that network switch gear connections are safe-guarded against accidental disconnections.
In parallel with EMI/EMC security, a primary function was to provide simplicity during equipment installation, maintenance and cable connectivity. Top and front covers are secured with unique tamper-resistant TORX PLUS® fasteners ensuring access is limited to authorised service personnel. Ventilation panels in key areas, provide natural convection that dissipates any heat build-up.
The thermoplastic powder coated surface finish is applied onto an etch primed base; pale grey, it’s applied electrostatically without solvents and cured with heat to provide a hard wearing semi-gloss surface finish. This is tougher and much less hazardous than conventional liquid paint coatings and has exceptionally high corrosion resistance.