Selecting EMP protection for enclosures

Using an electromagnetic pulse to disrupt electronic circuitry is no longer a theoretical exercise just for engineers

BY PRAVEEN POTHAPRAGADA
Chief Engineer, Equipto Electronics
www.equiptoelec.com

As proven by George Clooney and his crew in Ocean’s Eleven, even Hollywood knows that electronics can be manipulated and even destroyed by being subjected to an electromagnetic pulse (EMP). And as electronic devices have become portable, so too have EMP generators — today they are as small as a briefcase.

The harm that can be caused by these EMP briefcases includes destruction of electronic equipment and industrial production lines as well as neutralization of surveillance systems, television, radios, and telephones. U.S. military forces routinely face EMP threats in the fight against terrorism — imagine the damage an EMP could cause to a Humvee carrying mission critical electronic equipment, or to the data storage equipment in a US embassy, or to the security and surveillance systems on a military base.

As our dependence on electronics increases, the need to protect our data and electronic equipment also should increase. Fortunately, such threats can be averted with proper electromagnetic shielding of electronics. The solutions of electronic packaging and EMC protection are not something that can be effectively implemented after the system is designed. The threat of the portable EMP suitcase forces us to bring EMC to the forefront of the systems design process. Simple calculations and application of physics prior to the design of shielded enclosures can save time and effort during later phases, such as testing.

Protection options

Depending on their intended application, electronic systems can be packaged in three ways:

1. Put the electronic system in a shielded room such as anechoic chamber.

2. Shield each of the system’s electronic components individually.

3. Use shielded enclosures to shield a group of electronic components.

While shielded rooms can provide a high degree of immunity, they preclude mobility; if a US embassy needs to be relocated, the shielded room that’s an integral part of the old building must stay behind. Shielding individual components can permit a high degree of mobility, but is often cost prohibitive in many applications, such as a telecom server room.

Shielded enclosures offer the convenience of mobility and a cost savings over shielding individual components. The design of shielded enclosures is very application specific and involves knowledge of many disciplines. However, there are some basic steps that are common to most applications.

In the following sections of this article, we provide some of the steps and calculations used in designing a shielded enclosure.

Determining shielding effectiveness

The first step is to determine which standard is appropriate to the enclosure. Some of the common standards used by the Department of Defense are as follows:

• IEEE-299-2006.

• MIL-STD-461.

• NSA 94-106.

• FCC Part 15.

• Tempest.

• MIL-STD-188 (HEMP).

The next step is to determine the shielding effectiveness of the enclosure: is it designed to limit the emissions or to protect the equipment from outside electrical and magnetic fields to the level required by the standard.

To determine the shielding effectiveness, let us use our example of an EMP suitcase. The EMP suitcase can emit a radiated electric field of 120 kV/m at 300 MHz. Based on testing, we determine that the unprotected electronics of our surveillance equipment will start to have stressed operation when subjected to an electric field of 5 V/m. So the Shielding Effectiveness (S.E.) of our enclosure needs to be:

S.E.(dB) = 20log10 (Field strength w/o enclosure/Compliant field strength)

S.E.(dB) = 20log10 (120×103 /5) ≈ 88 dB at 300 MHz

Enclosure material selection

Once the required Shielding Effectiveness has been determined, the next step is to determine the type and thickness of the material needed to make the shielded enclosure. .

The ability to protect against electrical and magnetic fields for an enclosure material is the sum of Absorption (A), Reflection (RE ), and secondary reflection coefficients. However, for all practical purposes we can neglect the secondary reflection coefficient. The following equation can help narrow the selection:

S.E.(dB) = A + R

A(dB) = (3.338 × 10-3 ) × t√μfG

RE (dB) = 353.6 + 10log10 [G/(f3 μr1 2 )]

RH (dB) = 20log10 [(0.462/r1 )√(μ/Gf ) + 0.136r1 √(fG/μ) + 0.354]

RP (dB) = 108.2 + 10log10 [(G×106 )/μf]

where G is relative conductivity (copper), f is frequency (Hz), µ is relative permeability (free space), r1 is the distance between source and shield (in.), and t is thickness (mils).

Generally, for radiated emissions we start with a material that absorbs, and for susceptibility we start by choosing a material that reflects more. Other considerations in choosing the material are its ability to be formed and welded, its availability, and its affordability.

Gasket selection

Shielded enclosures need openings for installation and maintenance of electronic equipment and for cable entry and exit. Enclosures typically will have doors, side panels, top and bottom panels, input/output panels, and so on. Gaps between the panel and the cabinet frame result in lost continuity, leading to leakage of electric and magnetic waves. Since having perfectly machined mating surfaces at the opening is cost prohibitive, conductive gaskets provide a cost effective way to seal the openings.

The gasket material should be galvanically compatible with the material of the enclosure. When the gasket material is dissimilar to the enclosure metal, suitable protection against galvanic corrosion must be applied. Care should be taken to protect the anodic member by proper electrical insulation of the joint or, if feasible, by excluding the electrolyte (Fig. 1 ). Although difference in potentials of dissimilar metals is not the only reason for galvanic corrosion, it is better to select materials whose potentials are close to each other. Be wary of the cabinet designs where zinc-plated frames or doors mate with a copper finger gasket.

Fig. 1. This table from MIL-STD-14072D shows the standard potentials of the common metals used for enclosures and gaskets, and material compatibilities.

Most of the gasket applications involve two types of closure forces on the gasket: compression and shear. When gaskets are installed under a flat cover panel, in a compression configuration, the pressure is used to preserve the shielding effectiveness of the seam. The alternative is a shear application where a flange or a channel arrangement shields by shearing against a gasket before the enclosure is closed.

All gaskets are porous to some extent. The porous spots in the gasket can act as slot radiators at high frequencies. It is important to calculate the shielding effectiveness of the gasket given the porousness of the gasket.

Most of the gaskets have a limited lifetime. Normally, it is defined in cycles for a particular compression limit. Based on the usage of the particular opening, the longevity of the gasket can be determined.

Gaskets are mounted to the flanges in different ways, the most common being adhesives or mechanical fasteners. Depending on the particular type of application, the correct mounting method should be used. There are two types of adhesives: conductive and non-conductive. Non-conductive adhesive is the most commonly used, but conductive adhesives may help give better shielding performance.

Calculations for vents

With electronics becoming more compact and densely configured, the need for forced convection in electronics packaging solutions is on the rise. The challenge in designing shielded enclosures is to move the air in and out of the cabinet without compromising the shielding effectiveness of the enclosures. In other words, containing or restricting the electromagnetic waves while allowing the movement of air. The answer lies in the design of the waveguide.

A waveguide in simplest terms is a tube. At the right length (L) and the diameter (D), the tube can block electromagnetic waves while allowing air to flow. The first step is to calculate a cutoff frequency (C).

C = 6.92/D

If the shielded frequency is less than the cutoff frequency, then move on to calculating the maximum shielding effectiveness of the honeycomb vent. As a rule of thumb the length is at least five times greater than the diameter.

S.E.(dB) ≈ (32 × L)/D

With Hollywood doing such a great job highlighting the threat of EMP, it is now up to the electronics packaging industry to come up with solutions to protect against the threat. ■

References

Design of Shielded Enclosures: Cost-Effective Methods to Prevent EMI by Louis T. Gnecco, Newnes (an imprint of Butterworth-Heinemann), 2000.

EMI Shielding Theory , Chomerics, www.chomerics.com/products/documents/emicat/pg192theory_of_emi.pdf.

MIL-F-14072D, Military Specification: Finishes for Ground Based Electronic Equipment , U.S. Military Specifications and Standards, 04 Oct 1990.