A single-ended probe is a ground-referenced probe. The alligator clip connected to the side of the probe is a ground reference. This allows the scope and device under test have a common reference to ground. Typically, a single-ended probe should only be referenced to ground; but if the scope has isolated inputs, then it does not have a reference to earth ground. In this case, the probe may be floated up to the rated reference level. The P5122 and P5150 probes can float up to 600 Vrms on CAT2 systems when used in conjunction with a TPS2000 or THS3000 series oscilloscope.