For a vacuum spark gap with initiation by an electric spark plasma, the limited possibilities of controlling the delay time of switching by varying the energy of the auxiliary discharge are demonstrated. For a vacuum spark gap with initiation by a laser plasma, the threshold value of the radiation energy is detected at which the change in character of the dependence of the recorded temporal parameters of switching on the radiation energy takes place. The threshold energy of radiation is determined by the thermodynamic parameters of the cathode material. Based on the experimental data, an assumption is proposed: under the action of a laser radiation pulse in the interelectrode space, glow discharge is initially ignited in the the electrode erosion products, which then as a result of the development of ionization-overheating instability, undergoes a contraction of the current channel and switches to an arc discharge.