This paper presents an experimental study of partial discharges (PD's) carried out with a Silicon Nitride Si3N4 substrate embedded in silicone oil. Optical PD detection is feasible with such non-electroluminescent materials, and can be done whatever the shape of applied voltage (dc, ac, fast impulses), whereas conventional electrical PD measurements can be obtained only with ac or dc. Optical PD recordings show that PD inception is greatly favoured under fast rise impulse compared to ac. The influence of pulse risetime and dc prestress shows that PD inception is strongly influenced by space charge injection, in the case of slow high voltage waves such as ac or dc.