Multimeter Safety

Overvoltage Installation Categories
The most important single concept to understand about the new standards is the Overvoltage Installation Category. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (See Figure 1). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (acresistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards. Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1000 V meter has superior protection compared to a CAT III-600 V rated meter..

When is 600V more than 1000V?

Table 2 helps us understand an instrument’s true voltage withstand rating:

1. Within a category, a higher “working voltage” (steadystate voltage) is associated with a higher transient, as would be expected. For example, a CAT III-600V meter is tested with 6000V transients while a CAT III-1000V meter is tested with 8000V transients. So far, so good.
2. What is not as obvious is the difference between the 6000V transient for CAT III-600V and the 6000 V transient for CAT II-1000 V. They are not the same. This is where the source impedance comes in. Ohm’s Law (Amps = Volts/Ohms) tells us that the 2Ω test source for CAT III has six times the current of the 12Ω test source for CAT II.

The CAT III-600V meter clearly offers superior transient protection compared to the CAT II-1000 V meter, even though its so-called “voltage rating” could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage withstand rating. A note on CAT IV: Test values and design standards for Category IV voltage testing are addressed in IEC 1010 second edition.

Table 2. Transient test values for measurement categories. (50 V/150 V/300 V values not included.)