CT4299 Fused Probe with Locking Safety Cap Cal Test Electronics CT4299 Fused Probe with Locking Safety Cap easily connects to 4mm sheathed banana plug leads, which are in turn connected to a DMM or other equipment. The small, compact body of the Cal Test Electronics CT4299 Fused Probe features an internal fast-acting ceramic fuse that can cut power in microseconds during a transient overvoltage event. This fast-acting fuse feature protects equipment and personnel working in high-energy environments. The closer the fuse is to the test point, the higher the level of protection.