EMI and shielding data centres and enclosure

What is Electromagnetic Interference and what does EMI stand for?

Electromagnetic interference (EMI) happens when one electromagnetic field interferes with another, causing distortion of both fields. Think of the static you hear from a radio when switching between frequencies. EMI can create hazards in critical applications – it’s why, when flying, all electronic devices must be turned off during take-off and landing. Otherwise, EMI can interfere with communication between pilot, ground control or other systems crucial to the plane and safety of everyone on board.

That is an extreme example, but it makes the point. At the very least, EMI can cause disruption. Its origins can be both man-made and natural sources. Common causes include:

• Television transmissions, both analogue and digital
• Radio AM, FM, and Satellite
• Solar magnetic storms
• Lightning, which is a very high-voltage and high-current event
• Utility power grid transmission lines. These have high-voltage, low-current, and low-frequency characteristics. This also includes Broadband over Power Lines (BPL) digital signals.
• Airport port radar, telecom transmissions, electrostatic discharge (ESD), and white noise. The earth’s magnetic field flux, which has a value of about 500 milligauss, also falls into this category.
• Switching-mode power supplies, arc welders, motor bushes, and electrical contacts

As an engineer, you need to anticipate electromagnetic energy in your application’s intended environment. This will enable you to design the proper protection into your application. Failure to do this can be a nightmare, resulting in rushed fixes at the final stage that add weight and cost to the final product.

EMI and data centres

In data centres, power sources produce low-frequency EMI. This damages the hardware and corrupts data. It can even wipe out the entire hard drive. The problem is, cables near EMI sources, unless properly protected, are exposed to currents and can suffer a surge in voltage. It’s the high-voltage current that then generates electrical noise, which interferes with data and voice applications that the cabling supports.

Other problems occur in transmission, analogue voice communication, IT security, network traffic problems, and ultimately, employee productivity. After all, how can anyone work without the internet?

The Uptime Institute in the U.S. developed the Tier Standard, which the UK and most of the world have adopted. These standards evaluate the quality and reliability of data centers. Tier 1 is the simplest infrastructure, while Tier 4 is the most complex. Each tier contains the required components of all the tiers below it. Design rules come into play for Tiers 3 and 4 to address potential threats to uptime of the systems.

Hewlett Packard outlines the tiers like this:

Tier 1: Has a single path for power and cooling and few, if any, redundant and backup components. It has an expected uptime of 99.671% (28.8 hours of downtime annually).

Tier 2: Has a single path for power and cooling and some redundant and backup components. It has an expected uptime of 99.741% (22 hours of downtime annually).

Tier 3: Has multiple paths for power and cooling and systems in place to update and maintain it without taking it offline. It has an expected uptime of 99.982% (1.6 hours of downtime annually).

Tier 4: Built to be completely fault tolerant and has redundancy for every component. It has an expected uptime of 99.995% (26.3 minutes of downtime annually).

Power equipment and devices can cause low and high electromagnetic fields of interference which can harm IT equipment and cable communications, reducing the quality of service and causing downtime. This can be anything from shaky computer screens to periodic failures and even loss of data.

EMI Shielding - why is it important?

Shielding is your best protection against EMI in higher-tier data centres. Shielding is the practice of surrounding electronics and cables with conductive or magnetic materials. This protects against incoming or outgoing emissions of electromagnetic frequencies.

When designing your data center, you must always consider the regulatory environment. While reducing electrical noise and its impact on signals and lowering electromagnetic radiation,
shielding also complies with legal requirements set forth by bodies such as CE in Europe and the FCC in the U.S.

Shielded control cables and shielded power cables are good choices, but to prevent the potential for noise loops, the cables need to be grounded at one end. Cable shielding comes in three designs:

• Braided: EMI resistance in power, control and data applications. Protects against low frequencies, up to 15 KHz
• Spiral: excellent flexibility, but prone to lose performance if individual strands separate after too many flex cycles. Also effective at protecting against low frequencies
• Foil shield: lightweight and low cost. Protects against frequencies greater than 15 KHz

Another solution you should use are ferrites, which ensure strong electronic signals through cables in environments where EMI is a threat. Ferrite cores, for example, are handy because they can suppress EMI without grounding. They absorb noise and allow signals at necessary frequencies to pass through. Here’s how it works:

• The cable is passed through the ferrite core
• The magnetic fields produced by the cable stay inside the core, thanks to the attraction of the magnetic flux to the ferrite
• The magnetic energy concentrated in the ferrite is converted to heat and dissipated by the magnetic loss of the ferrite.

Ferrite sleeves and ferrite beads are other useful solutions that reduce EMI output from devices. They prevent the cable from behaving as an antenna for other sources responsible for EMI.

EMI-shielded enclosures:

When designing an EMI-shielded enclosure, getting a balance between cost and performance is a must. Some of your considerations should be:

• Budget: this should include your tooling costs
• Galvanic compatibility: metals should be compatible with each other to avoid galvanic corrosion
• The environment: is corrosion from the elements a possibility?
• Material thickness: this will depend on what frequencies you are shielding your circuit boards from
• Enclosure geometry: the height of the shield and how it will correspond to formability

Materials commonly used are made from steel and aluminium. Any ferromagnetic metal will work, so we’re talking about metals with nickel, iron or cobalt. Most steels are ferromagnetic. Steel is perhaps the most commonly used shielding material, not only because it works well, but it’s inexpensive and easy to obtain. (Be aware, not all stainless steels are ferromagnetic.)

Because of its aesthetic design capabilities, plastic is rapidly becoming a popular choice. However, the plastic should be coated with a metallised compound. Another option is an aluminium or copper foil lining, which is especially effective at shielding.

Material thickness

You’ll find that a thickness of 0.1 mm should provide shielding from frequencies above 1 MHz. If you’re dealing with lower frequencies, such as 30 kHz and less, you’ll need materials with a good magnetic conductivity, meaning it should have a high nickel content of at least 50%. You can also use a thicker material or multiple layers of material.

Shielding gaskets: critical for data centres and enclosures

Never underestimate the importance of gasketing. Always design your enclosure or data centre as though you’ll need EMI gaskets. If, for some reason, you don’t, no problem. Don’t use them. But if you plan your project from the onset without gasketing, discovering that you need then later will eat up valuable costs and money in reconfiguring your design.

The purpose of the gasket is to seal any differences between surfaces. Preventing gaps is especially important at frequencies above 5kHz. High frequency ranging between 100 MHz and 40 GHz are particularly sensitive to small gaps in surfaces.

A rule of thumb is, the higher the frequency, the more you need to be mindful of preventing holes or gaps in your shield. Soft and flexible gaskets can do a good job in these instances. How stiff you gasket should be depends on the rigidity of the construction and the distance between the fixings. If the gasket is too stiff, there’s a danger of a door, lid or panel deflecting, and creating gaps instead of preventing them. The enclosure then loses its shielding ability against higher frequencies.

What type of gasket material and thickness?

The type of gasket material and thickness you should use depends on the tolerances with which the enclosure or panel is fabricated. First, the conductive layer on the outside of the gasket must be in the same galvanic range as the enclosure’s construction materials. You need to prevent galvanic corrosion, which undermines the structure’s electrical conduction and hampers the shielding performance.

Use this as your guideline:

Environment:	Volts
Salt spray/weathering	Maximum of 3.0
Indoors, with only salt-free condensation	Maxiumum of 0.5

Your design should include metal-to-metal contact. Where possible, you want to stop the gasket from over compressing, ensuing an even pressure. Your gasket should not only give you high conductivity, but also provide constant electrical contact with low compression force.

Note: Before fitting EMI gaskets, make sure that all contact surfaces are cleaned and free from greases, paints and other contaminants. The contact surface should be highly conductive.

In conclusion, prevent problems caused by EMI with shielding. Otherwise, be prepared to pay the price not just in money, but time.