Terrorist-proof cables: Shielding electromagnetic pulses

May 1, 2012
We've discussed shielding VFD cables, but what about more exotic situations?

In applications where the accidental or terrorist detonation of a nuclear device is a concern, engineers design equipment to withstand electromagnetic pulses (EMPs). Cables for such applications were manufactured during the Cold War, and in some industries, there is renewed interest in them.

EMP shielding has superior attenuation, effective from Hz to GHz frequencies, to protect electrical and electronic systems against both electromagnetic pulses and interference (EMI).

How does EMP shielding work?

A high and low permeable alloy material is used for each layer of the cable for maximum deflection. The input signal is interference-free until it comes in contact with the power-line magnetic field (or other magnetic field), which in turn causes a distorted signal in the equipment. In contrast, copper provides very little shielding at low frequencies.

In some designs, an EMP/EMI shielding outer braid provides moderate shielding at low frequencies and good shielding at high frequencies. Meanwhile, the inner braid provides excellent shielding at low frequencies — for a shielding combination effective over a broad frequency range.

Electromagnetic frequency shielding (as illustrated below) provides protection for input signals susceptible to magnetic field induction and electrical fields until the signal is safely within the controlled electrical or electronic equipment. Cables with conductors to be protected from external EMP/EMI sources have an outer low-permeability shield braided over a high-permeability shield; here, incident radiation from external EMP/EMI strikes the outside surface of the outer shield (the first boundary reflection loss area) where some of the radiation is reflected. The radiation then penetrates the material, where it is absorbed.

One caveat: The backshell of an EMP-resistant connector must make solid EMP-resistant contact with the shields, to ensure that the black box, connector, and EMP-resistant cable keep electromagnetic pulses from interfering with circuits critical to system operations.

For more information, call (800) 905-7161 or visit calmont.com.

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