Digital Volt Meters (DVMís) are categorized depending on the risk and severity of transient over-voltage that might occur at the point of test. Transients are short-lived burst of energy induced in a system, e.g. caused by lightning strike on a power line.
On high-energy circuits this leads to hazardous situations as these circuits might provide enough current to feed an arc-over resulting in a plasma breakdown or even explosion. Higher CAT-identification refers to electrical environments with higher power and possibly higher energy transients.
A CAT I rated meter is suitable for measurements on protected electronic circuits that are not directly connected to mains power. <100V (Battery operated devices, Audio Circuits, Education)
A CAT II-rated meter is suitable for measurements in CAT I-environments, as well as for measurements in single-phase appliances that are connected to the mains by means of a plug. Measurement of normal domestic circuits, provided that the circuit is at least 10m apart from any CAT III- or 20m apart from any CAT IV-environment. < 240V (Household Appliances, Portable Tools, Sound and Lighting Equipment)
A CAT III-rated meter is suitable for measurements in CAT I- and CAT II-environments, as well as for measurements on (fixed) single and three phase appliances which are at least 10m apart from of a CAT IV-environment, and for measurements in or on distribution level equipment. <440V (Fuse boxes, Lighting circuits, Ring main, Electric ovens).
A CAT IV-rated meter is suitable for measuring in CAT I-, CAT II- and CAT III-environments as well as on the primary supply level. Note that for all measurements on equipment for which the supply cables run outdoors (either overhead or underground) a CAT IV meter must be used.
ESR Electronic Components Ltd Cullercoats Tyne and Wear NE30 4PQ