Proper grounding of RF-shielded rooms

27/03/2024

When Michael Faraday invented his 'Faraday Cage' at the end of the 19th century, he could never have imagined how important his invention would become in today’s electrical engineering world. This is very important because for EMC measuring rooms, MRI and TEMPEST/EMP applications, his invention is still indispensable for protecting equipment from outside radiation effects. But not only that. The 'Faraday cage' also prevents important data used at data centres or military installations from being captured through surveillance and eavesdropping. Connecting the RF-shielded room to a high-quality ground is necessary for optimal functionality in almost all applications. We asked expert Jan Kappert, to explain why it is essential and how to prepare it.

“If RF-shielded rooms are not or incorrectly grounded, this can firstly pose safety risks to people. But it can also affect equipment operation and test results’ accuracy, for example. An RF-shielded room can also be ‘leaky’, which means that sensitive information is exposed,” says Jan Kappert, who worked as an International Sales Manager at Comtest Engineering for more than thirty-five years and since three years operating under the name ‘k-PHI Consultancy’ as an independent consultant.

Ensuring safety

“A ‘Faraday cage’ is a construction of steel panels bolted together using an RF-tight gasket in between. This creates a completely closed environment, generally known as a Faraday Cage,” Kappert explains. “There should be no ‘holes’ anywhere. Special measures for feed-throughs, like air ventilation, door entrance, and powerline connections, will be needed to prevent ‘leaks’. Comtest has developed unique solutions to maintain high-quality RF shielding using those penetrations.”

“Steel also means that the RF-shielded room may expose an electric voltage in case of a defect. When not (correctly) grounded, this can pose an immediate shock hazard. Of course, that is already a mandatory reason to look carefully at the grounding. To ensure good grounding, a separate grounding is often provided in RF-shielded rooms in addition to the ‘normal ground’ (the yellow/green wire in the mains cable). A connection point is provided on the RF-shielded room to connect this additional ground.”

‘Clean ground’

Is grounding necessary for all RF-shielded rooms? What type of grounding should be used, and what is a proper ground? Kappert explains, “Grounding the room itself is sufficient for general applications. For this purpose, a connection for grounding is usually made on the same panel of the RF-shielded room, where the EMC power line filters are installed. This is done to avoid any resistance between this grounding point and the ground connection provided in the main voltage filter supply lines. If a potential difference occurs between these two grounds, RF noise can be generated into the RF-shielded room caused by a differential current, which will flow between those grounding points.”

“But that doesn’t mean you can’t get glitches. For example, at a large factory in Germany, it turned out that an interference signal entered the RF-shielded room via the ground connection, but no one knew where it came from. By switching off all electrical appliances, machines and systems over the weekend and switching them on again one by one, they discovered that the noise was generated by a ventilation system two construction halls away from the location of the RF-shielded room. To solve this problem, it was decided to create a separate ground by installing a long metal rod deep into the ground near the position of the RF-shielded room.

Connecting this so-called ‘clean ground‘ to the RF-shielded room ensures it is safe and no outside disturbances can enter the RF room via this ground. To be sure, one can use a current clamp and a spectrum analyser to measure whether there is anything on the ground in the lower frequency range. In demanding cases, you must separate the powerline cables using an isolation transformer to avoid interference signals entering the RF-shielded room through these lines. Installing and ensuring proper ground and installing an isolation transformer is always the responsibility of a certified electric installation company!”

Surveillance and eavesdropping

“For areas that have to meet the so-called TEMPEST requirements, as stated in the SDIP standards, tiny signals are taken into account,” Kappert continues. “TEMPEST refers to the security measures that protect against technical surveillance and eavesdropping of inappropriate electronic devices and systems. This is not so much about the level of interference signals like in EMC certification but about the presence of sensitive information (data) on the signals generated by these devices. For example, if power cables or grounding are not properly shielded, one could pick up data signals, making it easy to obtain important information. That is why it is so important that data centres have good RF shielding. And this is especially important in TEMPEST-qualified rooms where equipment is developed and tested to meet the strict TEMPEST requirements.”

Additional requirements

“Because TEMPEST measurements are very sensitive, additional requirements are imposed on the RF-shielded room and its grounding,” Kappert explains. “For example, the RF-shielded room must be installed electrically isolated from its environment. For TEMPEST measuring facilities, it is also recommended to use a separate RF-shielded room to house the measuring equipment to prevent direct radiation from entering it.”

“The powerline primary filters to be installed must also have excellent attenuation values starting from 10kHz. These filters are connected to the mains via an isolation transformer, while the RF-shielded room(s) is/are connected to a separate, so-called ‘clean ground’. By installing a ‘clean ground’, any interference signals on the building ground are prevented from contaminating the ground potential of the shielded facility. This was the case with the example above from Germany. Furthermore, using an isolated setup, any stray currents between different grounding points of the RF-shielded room are prevented from flowing across the outside of the room, contaminating the ground potential and thus affecting or making measurements impossible.”

Unintended ground points

“You also need to avoid so-called ‘unintentional ground points,’” Kappert warns. “These can be caused, among other things, by metal anchors on the roof or walls of the RF-shielded room. External ventilation ducts can also create such an unwanted ground point if they come into contact with the wall of the RF-shielded room. Therefore, using a so-called Megger instrument during constructing the RF-shielded room is advisable. This allows you to detect and resolve any unwanted ground points immediately.”

Electromagnetic Pulse

Equipment installed in an RF-shielded area must sometimes be explicitly protected against the influence of a so-called Electromagnetic Pulse, or EMP for short. When does this happen?

Kappert explains: “For example, a powerful EM Pulse is released during a nuclear explosion called a NEMP (Nuclear Electromagnetic Pulse). Defense and Civil installations mainly use EMP-shielded rooms with crucial operating systems for national infrastructures. EM pulses are short-lived, very powerful and mainly present in the lower frequency range. These properties place high demands on how an RF-shielded room must be grounded. Given the frequency range and power of such a pulse, the use of a single ground wire is not sufficient, and it is recommended to use a woven copper ground cable. One ground point is used. If the RF-shielded room is huge, ground connections can be made at several points in the RF-shielded room. However, these are all connected to the single ground point outside the room.”

Dotting the i’s

Finally, are there any matters that should also be considered when creating an optimally shielded room? “Yes, certainly,” Kappert responds. “For example, it sometimes happens that screws that are too long are used when mounting entry panels or ventilation assemblies. This should always be avoided because these can act as antennas and pick up (parts of) the electromagnetic pulse. Therefore, all metal penetrations into the RF-shielded room must be kept as short as possible. In addition, all electrical penetrations must be equipped with Transient Arrestors. These devices prevent the penetration of an EM pulse via, for example, the power cables.

In particular, EMP-shielded and TEMPEST measuring rooms are critical, and errors occur before you know it. Particular anchoring systems can cause problems if these are not properly installed. For example, Comtest builds RF-shielded rooms 30 meters long, 15 meters wide and 3 meters high. Special constructions are used to ensure maximum shielding. For now, it is always advisable to consult a specialist company when purchasing an RF-shielded room, especially for critical applications. Comtest has many years of experience in this area and specialist knowledge for designing and realising RF-shielded rooms for various applications.”

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COMTEST ENGINEERING BV

Hoge Rijndijk 205 | 2382 AL Zoeterwoude | The Netherlands | +31 71 541 7531 | Send us an email