The analysis of quiescent discharges exhibiting edge harmonic magnetoydrodynamic activity in the JET-Carbon wall machine is presented. It is observed that the quiescent phase with multiple-n harmonic oscillations is sustained until a threshold in pedestal electron density and collisionality is crossed. The macroscopic pedestal parameters associated with the quiescent phase are compared with those of a database of JET-ELMy discharges with both Carbon and ITER-like wall condition- ing. This comparison provides the identification of the existence regions in the relevant pedestal and global plasma parameters for edge harmonic oscillations in JET plasmas. It is found that quiescent scenarios operate at the very boundaries of the parameter space scanned by the database (both C and ILW), though global and pedestal paramenters of these discharges are not intrinsically incompatible with the ones observed in quiescent shots.