A method performed in a welding or cutting system configured to deliver weld power to an electrode tip extending from a torch to create an arc on a workpiece, comprises: sampling a sensed voltage indicative of a weld voltage provided to the electrode tip, to produce voltage values; computing a time derivative of the voltage values to produce voltage derivative values that represent a contact resistance between the electrode tip and the workpiece; and upon detecting a decrease in the voltage derivative values from above a time-derivative threshold to below the time-derivative threshold as an indication of a weld start event, increasing the weld power supplied to the electrode tip to initiate the arc on the workpiece.