Magnetic Effects from Lightning Transients in Shielded Telecommunication Cables

Abstract

Metallic shielded telecommunications cables have been damaged by exposure to magnetic fields characteristic of lightning discharge. Varying degrees of damage occur from the resulting ’’magnetic crush" as a function of cable design, shield/armor materials and geometry, and transient magnitude and duration. Results clearly show that shields or armors that allow circumferential conductivity are subject to severe damage while those without such conductivity are effectively immune from such effects. Magnetic crush involves no direct electrical arc or discharge to the cable, yet damage created can have all the apparent effects of the direct arc condition. Additionally, the longitudinal transient current capacity of present shields and armors used in such cables is shown to be sufficient; little, if any, damage occurs to cables passing or carrying such currents. INTRODUCTION In the 1950s, air-dielectric coaxial cables buried in the earth suffered severe physical and circuital damage under lightning discharge. This discovery led to work that was to determine the actual mechanisms at work and to offer cable designs that would provide immunity to such damage. Some progress was made. The standard test for modeling such conditions evolved and is termed the ’’sandbox" test in which a cable is buried in a box containing compacted wet sand and subjected to direct electrical discharge/arc by placing the high current generator output probe in very close proximity to the cable's shield or armor. The shield/armor is further exposed by removing a small portion of the cable jacket directly opposite the discharge probe. Upon discharge of the generator, an electrical arc forms creating heat damage to the metallic components. A hole is often burned in the cable shield/armor. At times, an additional effect of denting or crushing of the cable structure occurred. The now classical explanation of this latter effect is that the arc created steam which then propagated as a "steamhammer" and caused such physical damage, i.e. a sufficiently high pressure steam front is generated along the arc path and impacts upon the cable. This apparent phenomena^ has now been termed the "steamhammer effect" . Another and more popular "explanation" of the steamhammer is lightning striking a tree and the tree splits. Moisture is' supposedly "vaporized" creating such a result. Recently, data shows that the fundamental mechanism is acoustical s^ock whose remote effect is well known as thunder . Water­ bearing sand possibly produces the same effect — disassociation of water, hot the creation of steam, and the attendant acoustic-hydrodynamic Shockwave may produce effects sufficient to cause cable damage. Conclusive proof of this has yet to be presented. The authors present data of considerable magnitude on magnetic field effects which demonstrate a much more plausible cause for such cable crushing or denting damage.