The addition of Carbon Nanotubes (CNT) to polymer has paved the way to ultra-low filled nanocomposites characterized with both electrical and mechanical properties. They have, amongst other, made their way into specific packaging with Electrical Static Dissipation protection. The privileged process is the sheets thermoforming of High Impact Polystyrene (HIPS) filled with CNT. This process presents major advantages for packaging such as efficiency and its cost-effective thermoforming tools and molds. However, during thermoforming the parts experience elongation flow which strongly deform the parts and may orientate the filler and then drastically impacts the electrically properties of the final part. Indeed, electron conduction requires filler percolation and good connection between filler aggregates. The challenge is then to reduce to a maximum the CNT amount keeping a percolating enough filler network to obtain complex parts with ESD properties. The deformation of the CNT network in polymer melts has been investigated by a combination of in-situ conductivity monitoring and extensional deformation analysis. For this particular purpose, an Extensional Viscosity Fixture put in a rotational rheometer ARES has been modified to allow the sample conductivity measurement during the extensional experiment. This new setup permits the observation of the conductivity variation under uniaxial deformation. We have experienced conductivity increase under specific condition and observed the conductivity-insulated transition and that before the sample break illustrating the filler network disconnection. Moreover, a stronger sensitivity to both deformation and temperature contribution than extensional rate has emerged. To fully understand the mechanism of the filler network behavior during thermoforming we have in addition characterized the nanocomposites systems through improved contrast charge SEM method and rheological combined with conductivity measurement analysis.