4000 Series Vacuum Fault Interrupter The Trayer 4000 Series Single-Phase VFI is patterned off the 3000 Series VFI product line. This single-phase mechanism is powered from the current transformers, thus eliminating the cost of a potential transformer. The 4000 Series Single-Phase VFI is available with liquid or SF6 insulation and is available for 15 and 25 kV submersible, vault or padmount applications.