Portable digital multimeter

01

Match the CAT rating, leads and probes to the circuit

The UK Health and Safety Executive's guidance GS38 explains that BS EN 61010 measurement categories reflect the overvoltage expected at the point of measurement: CAT IV at the source of the low-voltage installation such as meters and primary overcurrent devices, CAT III in the building installation such as distribution boards and sockets, and CAT II on circuits connected to the installation such as appliances and portable tools. Using CAT II equipment on a CAT III installation is listed as a cause of accidents. [1]

GS38 says test probes, clips and leads should conform to BS EN 61010-031, be marked with their CAT rating, have finger barriers, and leave an exposed metal tip of no more than 4 mm, preferably 2 mm or less; with a multimeter they should have high-breaking-capacity fuses rated, in GS38's words, "usually not exceeding 500 mA"; for loop impedance or RCD testers, GS38 instead says "10 A is typically used" or a current-limiting resistor and a fuse. Such fuses can add up to 2 Ω to a lead, which matters for resistance readings, and glass fuses are not suitable. [1]

Accidents also come from a meter set to a current or resistance range while measuring voltage, a lead falling off a terminal, damaged leads and makeshift equipment. To prove a circuit dead, GS38 prefers a proprietary voltage detector to a multimeter, proved before and after use. [1]

Give the supplier the circuits and CAT level, maximum voltage and current, functions needed (AC/DC volts and amps, resistance, capacitance, frequency, temperature), accuracy on your ranges, leads and probe tips, fuse ratings, calibration certificate and environment.

02

Check impulse withstand and the meter's protection against wrong connections

Hioki, a test instrument maker, shows why the CAT rating matters: in its recorded example a solenoid valve switching on a 100 V AC line produced an impulse of close to 1,000 V. Under IEC 61010-1 a meter must withstand the impulse voltage expected for its category and voltage to ground; Hioki's example is that a CAT II (300 V) multimeter must withstand an impulse of 2,500 V. [3]

Hioki's category descriptions are: CAT II from the power plug of a device on an outlet to its power supply circuit; CAT III from the distribution panel to fixed equipment connected directly to the supply and to outlets; CAT IV the building's lead-in and the circuit from the service entrance to the meter or distribution panel. It gives two of its own instruments as examples of dual ratings: a pocket tester rated CAT III 300 V and CAT II 600 V, and a digital multimeter rated CAT IV 600 V and CAT III 1,000 V. [3]

Hioki explains that the current and resistance circuits have low impedance, so connecting the meter across a supply on those settings drives an overcurrent that can burn out the meter and, through the short, the distribution panel; protections include a fuse, omitting the A terminal, a rotary-switch interlock on the terminals, and, on Hioki meters, an overcurrent protection circuit rated to withstand overvoltage for up to 1 minute. It also notes that capped test leads with a shorter exposed tip reduce short-circuit accidents, the same point GS38 makes with its 4 mm limit. [3] [1]

03

Read the percentage and count terms on the selected range

Fluke’s 87V overview lists DC voltage accuracy as ±(0.05% + 1) and AC voltage as ±(0.7% + 2), with a 6,000-count display and 19,999 counts in high-resolution mode. These overview figures need the function/range conditions and calibration interval from the applicable manual before use as an uncertainty allowance. Its safety ratings are CAT III 1,000 V and CAT IV 600 V. [2]

Editorial calculation: assuming an AC range with 0.1 V per count and an applicable ±(0.7% of reading + 2 counts) specification, an assumed 230.0 V reading gives ±(230.0 × 0.007 + 2 × 0.1) = ±1.81 V. Assuming a DC range with 0.01 V per count and an applicable ±(0.05% of reading + 1 count) specification, an assumed 24.00 V reading gives ±(24.00 × 0.0005 + 0.01) = ±0.022 V. The percentage/count format comes from Fluke’s overview; the readings, resolutions and application of those coefficients are explicit example assumptions, not manufacturer-published test results or measured errors. Check the selected range and test conditions before applying them. [2]

Ask suppliers for the accuracy table per range and function, the temperature and calibration period it applies to, and the count of the display.

Fluke’s Environmental Specifications table states −20 to +55°C for operation. Its FlukeCorp Q&A answer dated 25 October 2024 instead gives 5–131°F and adds operation down to −40°F for 20 minutes. The stated cold limits differ; retain both positions and ask Fluke to confirm applicability to the exact instrument and conditions rather than treating the Q&A as an extension of the table. [2]

04

Separate basic handheld ranges from function-specific accuracy

The MM720 example below lists handheld measurement ranges and protection ratings. Its product-page overview does not replace the accuracy table for the function being measured. [4]

A trade-electrician meter shows which headline figures appear on a maker's specification page. Klein Tools lists its MM720 as a true-RMS auto-ranging meter with a 6,000-count reverse-contrast LCD, 1,000 V AC/DC, 10 A AC/DC, 60 MΩ resistance, 6,000 µF capacitance and 1 Hz–500 kHz frequency ranges, a thermocouple temperature range of −40 to 1,000 °C (−40 to 1,832 °F), a low-impedance (LoZ) mode for identifying stray or ghost voltages, CAT IV 600 V and CAT III 1,000 V ratings, IP42 ingress protection, a 2 m (6.6 ft) drop rating, two AAA batteries, and a mass of 346 g; test leads are supplied with CAT III/CAT IV safety caps. Its product page does not list accuracy per range. [4]

Editorial note: the MM720 and Fluke's 87V share the same CAT IV 600 V / CAT III 1,000 V pair and a 6,000-count display, so compare quotes on the accuracy table per range, the input fuse ratings and the lead set supplied rather than on these headline figures; GS38's limits on exposed probe tips apply to the leads whichever meter is chosen. [4] [2] [1]

05

Identify the input protection and displayed count resolution

GS38 addresses the protection of leads, probes and input connections; Fluke’s display count identifies resolution rather than measurement accuracy. [1] [2]

06

Record the chosen voltage range, current limit and supplied mass

Keep the Fluke 87V overview values tied to their function and physical configuration. Operating-temperature applicability requires clarification of the table and Q&A positions described above. [2]

07

Send the circuit, accuracy and safety data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Circuits: [CAT level; maximum voltage; current]
Functions: [V AC/DC; A; Ω; capacitance; frequency; temperature]
Accuracy: [ranges used; required accuracy]
Safety: [CAT rating of meter and leads; fuses; probe tips]
Supply: [calibration certificate; quantity; branding]
Environment: [temperature; humidity; drop]

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