Electrical cables that dissipate spurious static electric charges, in addition to performing their main functions of conducting signals, have been developed. These cables are intended for use in trapped-ion or ionizing-radiation environments, in which electric charges tend to accumulate within, and on the surfaces of, dielectric layers of cables. If the charging rate exceeds the dissipation rate, charges can accumulate in excessive amounts, giving rise to high-current discharges that can damage electronic circuitry and/or systems connected to it.
The basic idea of design and operation of charge-dissipative electrical cables is to drain spurious charges to ground by use of lossy (slightly electrically conductive) dielectric layers, possibly in conjunction with drain wires and/or drain shields (see figure). In typical cases, the drain wires and/or drain shields could be electrically grounded via the connector assemblies at the ends of the cables, in any of the conventional techniques for grounding signal conductors and signal shields. In some cases, signal shields could double as drain shields. To be suitable for use in a charge-dissipating cable, a dielectric material must be inherently lossy throughout its bulk, and not, say, an insulating polymer with a conductive surface film or containing embedded conductive particles. Conductive surface films can be rendered ineffective by flaking off or cracking, especially when cables are bent. Embedded particles can act as defect sites that initiate arcing within dielectric layers. The concept of lossiness can be quantified: Dielectric materials can be broadly categorized, as either "excellent" or "lossy" according to their volume electrical resistivity (p) values. Excellent insulators may be roughly categorized as having ρ of the order of 1016Ωm, while lossy or dissipative insulators may be categorized as having r of the order of 109 Ωm. In designing for a specific application, one must choose the lossy dielectric material and the configuration of grounding conductors to be capable of dissipating a sufficient proportion of static charge within an acceptably short time. For a typical cable that handles signals of sufficiently low frequencies (having wavelengths much greater than the length of the cable), the effective charge-dissipating admittance or conductance must be much less than the nominal signal admittance or signal conductance of the circuits connected with the cable, so as not to adversely affect the transmission of signals. For a typical cable that handles signals of sufficiently high frequencies (having wavelengths comparable to or less than the length of the cable), the effective charge-dissipating admittance or conductance must be taken into account as part of the overall signal-propagation cable impedance, and the signal attenuation caused by loss in the dielectric must be acceptably low. These requirements could be difficult to satisfy if a cable is too long and, hence, imposes either a limit on the allowable length of the cable or else a requirement to pay closer attention to interactions between the charge-dissipation and signal propagation aspects of the cable design.
This work was done by John R. Kolasinski and Edward J. Wollack of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. GSC-14648-1