In actual operation, when a fault occurs on a fused circuit, this circuit will probably be carrying a load current, so that the fuse element will be at some temperature above ambient, and this will affect its operating characteristics under the fault conditions. Standard specifications of test do not take this into account, so that fuses are tested when initially at ambient temperature. Tests carried out on fuses with single-wire elements over a wide range of load and fault currents are described. These have shown the range and amount of the reduction in pre-arcing time caused by preloading; allowances for this reduction suggested previously do not appear to have been complete. The arcing time is also affected, and the transient voltage produced by the fuse when interrupting the circuit is reduced by pre-loading. Tests at a constant prospective current, with the instant of circuit closure varied throughout a half-cycle, have shown large variations in pre-arcing times and in transient voltages, both for pre-loaded and unloaded fuses. The temperature rise of the element centre for any load current has been calculated and verified by tests using temperature-sensitive paints. With a knowledge of this temperature, the reduction in the current-heat integral with pre-loading has been calculated and checked experimentally.