A voltage block device, for preventing the negative electric potential from transferred to the coating material source, has a switching device including a slider which is selectively slidable between first and second positions and has an inlet port fluidly communicated with the coating material source and an outlet port fluidly communicated with the spray, a reservoir including first and second chambers, the inlet and outlet ports are fluidly communicated with the first and second chambers, respectively when the slider is at the first position, and the inlet and outlet ports are fluidly communicated with the second and first chambers, respectively when the slider is at the second position.