Electrical Safety - Letting the CAT out of the Bag

Consider this scenario: An engineer believes he has found a good bargain for a digital multimeter (DMM) by deciding to buy a CAT II 1000 V rated DMM over a CAT III 600 V rated DMM because it was few hundred bucks cheaper and had a higher rated voltage. The truth is, instead of finding a bargain, he could be taking on unnecessary risk. A proper understanding of the CAT ratings for electrical safety can help prevent premature device failure and possibly serious injury.


CAT (category) ratings essentially classify the ability of electrical tools to withstand over-voltage impulse transients applied through specified resistance levels. The ratings are specified under the IEC 61010 standard, ranging from CAT I - IV. The Roman numerals after CAT denote the environment which the electrical tool is designed to operate in. The higher the category, the less  the risk that a high voltage can overload a circuit and cause electrical and physical damages. Consequently the higher the CAT rating on the electrical tool, the safer it is.

CAT I is meant for protected secondary circuits which are not intended to be connected to the mains supply, such as electronics and circuits powered by regulated low voltage sources. The IEC no longer specifies protection levels for CAT I instruments. Therefore it is usually not marked on the instrument - instead simply rated voltages and currents are stated.

CAT II refers to the local-level electrical distribution, such as standard mains socket and plug-in loads. This also includes household appliances and portable tools.

CAT III instruments are designed for use in distribution circuits, including cables, bus bars, junction boxes, switches, socket outlets in the fixed installation, and stationary motors with permanent connections to fixed installations.

CAT IV offers the ultimate overvoltage protection level for those working at the origin of installation or utility level works. This goes all the way until before the circuit breakers in the building.

The following table shows the corresponding maximum transient voltage across specified resistance and rated voltage:


Going back to the instruments in our opening scenario, the CAT II 1000V DMM can withstand a 6 kV transient, while the CAT III 600 V DMM can also withstand a 6kV transient. However, due to the lower test resistance for the CAT III 600 V, it can withstand up to 6 times higher current from the transient compared to the CAT II 1000 V DMM. That means the CAT III is able to protect the user from a higher transient current. Did he choose the instrument with the right CAT ratings for his application?

Understanding the ratings can mean the difference between finding a bargain and taking on unnecessary risk. Have you had to make this type of decision? If so, please let us know in the comments section, below.

Image source: Wikipedia