Shielding Design for Facilities

Engineering for Architects and Space Planners

Electromagnetic Interference (EMI) can cause system malfunction, communications interruptions, loss of data and erroneous measurements. With the increased quantity and mobility of RF threats, EMI management has become a critical issue in facilities design. Protection in an urban environment, where systems and users are in close proximity to potential threats, requires an understanding of the coupling and physics to find the most effective solutions.

Examples of interference sources in the urban environment include:

  • TV/Radio and communications transmitters
  • WiFi and Wireless communications systems
  • Rail and subway systems
  • Motors, lighting and electrical equipment
  • Power distribution

Protection

Protection for Systems

Data, communications, voice, video and broadband services require protection against Electromagnetic Interference (EMI). Shielding systems are often employed in building facilities to protect sensitive systems from radio frequency sources in the urban environment. Communications security must be robust and free from eavesdropping or disruption.

Protection in the Laboratory
Research hospitals, biotechnology and life sciences organizations deal with extremely sensitive sensors and systems during research and development. The small signals that are detected in animal-cell activities are measured in billionths of amperes, which can be overwhelmed by intruding signals from mobile networks, WiFi access points, TV/AM/FM broadcast and other sources of man-made noise.

Protection in the Workplace
Electromagnetic Fields (EMF) are a concern for worker safety and health, as well as potentially interfering with sensitive electrical equipment. High-amplitude AC power currents can generate low frequency magnetic fields that are difficult to control and to reduce. Shielding is one of the options to reduce these fields.

A System Approach

  1. Quantifying the threat
  2. System sensitivity and probability of interference
  3. Reduction and control of potential sources
  4. Prudent space configuration to minimize interference coupling
  5. Specifying appropriate materials and construction methods
  6. Quantifying the performance

Design Considerations and Methodology

The potential for interference depends on the level of interference and the level of susceptibility. To solve an interference problem, one may suppress the source or harden the victim. Our approach is to specify a degree of both, as appropriate for the situation.For many installations, it is not possible to predict the possibility of interference; however, based on judicious selection of equipment and an appropriate level of appropriate shielding, a high degree of confidence can be achieved.

Source of Interference and Mitigation

There are two primary considerations when considering “low-noise” design for facilities. The two concerns are based on frequency and nature of potential threats to equipment operations and accurate measurements.

It is important to separate the two concerns because the nature of the design and mitigation techniques are different.

Magnetic fields

Magnetic fields are primarily generated by 60Hz AC power currents. The major sources are from long conductors, particularly distribution cabling. From experience, the major source of magnetic fields are from legacy wiring that has may have multiple ground conductors and are not in compliance with current codes that require single-point-grounding.In addition, older equipment may have significant “leakage current” that causes ground currents to flow along presently uncontrolled return paths.To mitigate this type of situation often requires expensive and extensive shielding. For new installations, compliance with NEC requirements often preempts this source of interference. In addition, newer equipment is more likely to have low leakage current because safety standards are more strict than in the past. Higher leakage currents constitute a safety risk as well.

  • Other methods of construction can be implemented to reduce the potential for magnetic fields, including separation and placement of equipment with respect to potential interference sources.
  • Avoid the use of bus duct risers. Use conductors in conduit risers in electrical distribution. The close bundling of phase conductors results in a net cancellation of the fields that radiate from current-carrying conductors.
  • Locating high-current conductors as far as possible from sensitive equipment.
  • Locate power distribution transformers as far away as possible from sensitive equipment

With the above technique in place, it is possible that magnetic field threats may be entirely mitigated without special measures. However, the exact arrangement equipment and systems needs to observe critical noise-reduction measures, such as proper shielding of cables, equipment grounding and minimization of pickup loops.

Radio Frequency Fields

High frequency interference is of concern in urban environments. The potential for applying shielding around the space is a consideration. The exact determination will be made after measurements of the existing fields are performed.

A shield system that would be adequate to protect the equipment may involve a foil-based shield material up to frame and panel designed installations. There are several possible options for achieving the necessary attenuation. The ultimate level is dependent on an assessment of the existing field levels and the sensitivity of the equipment.

Shielding Effectiveness is used to describe the amount of reduction offered by a shield to an impinging source. The levels are normally specified in terms of decibels (dB) over some frequency range. Proper shield design must take into account the following electrical and mechanical penetrations:

  • Electrical outlets, lights and switches
  • Air handling units, HVAC
  • Plumbing
  • Structural members
  • Fire Alarm Systems
  • Floor/wall/ceiling interfaces
  • Communications cabling
  • Doors, window and other ingress/egress

A complete and comprehensive study and review of a facility is recommended for cost-effective and practical design to protect electrical and electronic systems from Electromagnet Interference.