Ensuring Safe Operation

Ensuring the safe operation of your product

Hioki measuring instrument has a maximum rated voltage to earth and operating environment requirements to ensure it can be used safely. This information can be found in the "Specifications" section of the instruction manual.

Maximum rated voltage to earth

Voltage of the measurement location relative to ground

Operating environment


Measurement categories

Safety standards (the EN61010 series and the JIS C 1010 series) define a series of measurement categories II through IV based on the rated voltage to earth, current capacity (i.e., the magnitude of the current that would flow in the event of a short-circuit), and anticipated transient overvoltage for the location being measured.

Measurement category II (CAT II)

Measurement at a point from the power plug to the equipment's power circuits, where equipment is directly connected to an outlet.

Measurement category III (CAT III)

Measurement at a point on the power distribution cabling or power supply circuits, or at a point from the distribution panel to a distribution terminal behind an outlet, where equipment (for example a fixed installation) takes electricity directly from a distribution panel.

Measurement category IV (CAT IV)

Measurement at a point on a service drop to a building, or on the line from the drop connection to the power meter or distribution panel.

Never measure a location with a higher category number than the category indicated on the measuring instrument. Doing so may result in serious equipment damage or bodily injury such as electric shock.

Anticipated transient overvoltage

Power lines at factories and similar facilities may carry a transient overvoltage (impulse voltage) of up to 10 times the supply voltage. Instruments are designed to withstand these transient overvoltages based on a prediction of the transient overvoltage that could be encountered at a given type of measurement location. Safety standards define the following transient overvoltage values based on the rated voltage to earth and measurement categories.

Rated voltage to earthTransient overvoltage
300 V2500 V4000 V6000 V
600 V4000 V6000 V8000 V
1000 V6000 V8000 V12000 V

For a measurement location that has a rated voltage to earth of 600 V, a measurement category IV location could include a transient overvoltage of up to 8000 V. CAT IV instruments are designed to withstand a transient overvoltage of up to 8000 V. If a transient overvoltage of 8000 V entered the internal circuitry of a CAT III instrument that is only designed to withstand up to 6000 V, the voltage could cause an insulation breakdown, leading to electric shock.

Pollution Degree

Contamination of the surface of a measuring instrument with pollutants can cause the instrument’s insulation performance to degrade, increasing the risk of electric shock. Safety standards define four pollution levels (numbered 1 through 4) that characterize the environments in which instruments are used.

Pollution Degree 1

Environments that will not affect an instrument’s insulation performance because they lack pollution or have only dry pollutants (non-conductive dirt, dust, etc.)

Pollution Degree 2

Environments that have only dry pollutants (non-conductive dirt, dust, etc.) but whose pollutants could temporarily degrade the insulation performance of an instrument that is exposed to them.

Pollution Degree 3

Environments that could affect an instrument's insulation performance due to the adhesion of conductive pollutants (water, soil, etc.) Alternatively, environments in which the surface of an instrument could remain wet for a comparatively long period of time due to high humidity, causing non-conductive pollutants to condense onto the instrument.

Pollution Degree 4

Environments that could degrade insulation performance continuously due to the adhesion of conductive pollutants (water, soil, etc.) or rain to large areas of the instrument.

Instruments designated for use at pollution level 2 can be used without compromising safety in environments with a pollution level of 1 or 2, while instruments designated for use at pollution level 3 can be used in environments with a pollution level of 1 through 3.


Atmospheric pressure decreases as altitude (elevation) increases, making discharges (caused by the insulation breakdown of air) more likely. Consequently, safety standards require a design that anticipates use at or below an altitude of 2000 m. When using an instrument at an altitude exceeding 2000 m, it is necessary to leave more space between circuits carrying dangerous voltages and surfaces with which people could come into contact.