The mechanisms of acceleration, injection and propagation of relativistic solar protons that are occasionally detected at the Earth (Ground Levels Enhancements or GLEs) have not been uniquely identified so far. These GLEs are always accompanied by energetic solar flares and CMEs, both of which can accelerate particles to high energies. For temporally well-structured GLEs, a detailed comparative analysis using multi-instrument observations of energetic particle signatures can constrain the acceleration mechanism of relativistic particles. A detailed comparative analysis of the GLE on the 20 January 2005 led us to conclude that the first beam of relativistic particles is flare accelerated whereas the second one is mostly related to a shock in the low corona. How could these relativistic particles, being accelerated in the low corona, be injected in open flux tubes connecting the Earth and the acceleration site? A coronal null-point topology with an open outer spine could accelerate particles by reconnection process at the null point, and inject particles along the open outer spine. So as to investigate the mechanisms implied during a 3D coronal null point reconnection, we studied an observed C-class flare where the shape of the ribbons is clearly due to reconnection process in a 3D coronal null point topology with a closed outer spine. Comparing the temporal evolution of an MHD simulation of this flare with the dynamic of ribbons, we investigated the 3D properties of magnetic reconnection. This work highlights that the evolution of the TRACE's ribbons is due the association of a null-point topology embedded in Quasi-Separatrix layers inducing sequentially slipping/null point/slipping reconnection. In an open outer spine configuration, these two coupled regimes may allow the injection of particles in a broad beam, and could provide new perspective for injecting accelerated particles in flux tube connecting the Earth, even if the acceleration site is not.