NOVELTY - An active isolation circuit comprises current sensor, and switching circuit. The current sensor detects a fault current in the coolant and generates a fault signal when the fault current is detected. The switching circuit redirects the fault current based on the fault signal. The current sensor is immersed in the coolant. The switching circuit monitors the fault signal. The switching circuit comprises operational amplifier and switch. The operational amplifier receives the fault signal and generates an output signal.    USE - The active isolation circuit is useful for fuel cell system, particularly for circuit interconnected with HVDC bus of fuel cell system.    ADVANTAGE - The circuit enables fuel cell stack (14, 24) coolant scheme that includes conductive coolant entering and exiting the fuel cell stack at common voltage potential plate or at any fuel cell voltage location. The circuit reduces or eliminates contamination in the coolant system, thus reducing or eliminating conductivity of low-conductivity coolant over time.    DETAILED DESCRIPTION - The switch selectively enables an alternate path to ground for fault current based on the output signal. The switch includes MOSFET transistors enabling the alternate path when the conductive mode. The current sensor detects the fault current the op-amp signals. The switches enables the alternate paths from high voltage direct current (HVDC) plus to ground or HVDC minus to ground based on the fault current polarity such that the coolant fault current is redirected or compensated towards 0 mA. The high voltage direct current bus is interconnected with the fuel cell. The active isolation circuit is used for implementation with a fuel cell system having a coolant flowing through it. The active isolation circuit comprises first or multiple current sensor, and switching circuit. The current sensor detects a fault current in the coolant and generates a first fault signed when the fault current is detected. The switching circuit redirects the fault current based on the first fault signal. The first current sensor is immersed in the coolant. The switching circuit monitors the first fault signal. The switching circuit comprises operational amplifier, and switching unit. The operational amplifier receives the first fault signal and generates an output signal. The switching unit selectively enables an alternate path from HV+ or HV- to ground for the fault current based on the output signal. INDEPENDENT CLAIMS are included for:    (1) circuit interconnected with HVDC bus of fuel cell system (10); and    (2) fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows electrical schematic of conductive liquid cooled fuel cell system high voltage bus having Y capacitors.    Fuel cell system (10)    HVDC power bus (12)    Fuel cell stack (14, 24)    Coolant system (26)    Capacitors (C1-C3)