A bstra ct-Supercritical helium is proposed as a coolant for a.c. superconducting fault current limiters, because it provides better electrical insulation than conventional pool-boiling helium. However, the effect of SHE cooling on the current carrying capacity of a x . devices is presently unknown. Quench currents were measured for two single-strand NbTi conductors and a tightly-wound two-layer coil at 0-200 Hz frequency and 0-1 T d.c. background magnetic induction in boiling atmospheric helium (LHE) and 6-atm, 4.2-K pressurized helium (SHE).The a.c. quench currents for the Cu30%Ni matrix wire were 2 0~3 0 % lower in SHE, while the CulO%Ni matrix composite showed almost identical quench currents in both coolants. Transient thermal analysis suggests that higher internal temperatures resulting from the Cu30 %Ni matrix's smaller thermal diffusivity may be responsible for its reduced current capacity in SHE.