Multimeters

What is the difference between a True RMS and an average-responding multimeter?

An average-responding multimeter measures the rectified average of an AC waveform and multiplies it by a fixed factor that is only correct for a clean sine wave. A True RMS multimeter calculates the actual heating value of the waveform, so it stays accurate on distorted waveforms. On the chopped waveform produced by a variable speed drive, an inverter, a dimmer or a switch-mode power supply, an average-responding meter can read significantly low. If you work on solar inverters, UPS systems, generators or VSD-fed motors, choose True RMS. The Extech EX411A, EX430A, EX470A, EX350, EX355 and the entire EX5xxA series are True RMS instruments.

What CAT rating do I need for a multimeter in South Africa?

The CAT rating describes how much transient overvoltage energy an instrument can survive at a given point in an installation. CAT II covers plug-connected loads and socket outlets. CAT III covers fixed installation wiring, distribution boards and permanently connected motors. CAT IV covers the origin of the installation, the supply side of the main disconnect, service entrance conductors and outdoor conductors. Work closer to the supply transformer means higher available fault energy, so a higher CAT rating is required. For distribution board work choose at least CAT III; for metering panels, service entrance work and outdoor connections choose CAT IV. The Extech EX5xxA series is rated CAT IV-600 V. Always confirm both the CAT category and the voltage number, since CAT III-1000 V and CAT IV-600 V are different specifications.

Why does my multimeter show a voltage on a wire that should be dead?

This is usually a ghost voltage, also called a phantom voltage. A modern digital multimeter has very high input impedance, so capacitive coupling between an energised conductor and an adjacent unused conductor running in the same conduit or trunking induces a voltage that the meter faithfully displays. The circuit cannot supply meaningful current, so the reading is real but misleading. A LoZ or low-impedance mode places a deliberate low resistance load across the input that dissipates the coupled charge and collapses the ghost reading, while a genuine live circuit continues to read correctly. The Extech EX520A and EX530A include a LoZ function. Always prove dead with an approved method before working on a conductor.

What does display count mean, and how many counts do I need?

Count is the highest number the display can show before it must switch to a coarser range, and it sets the resolution of a reading. A 2 000 count meter reads up to 1999 and resolves 600 V in 1 V steps. A 6 000 count meter resolves the same range more finely, and a 50 000 count meter finer still. Count is resolution, not accuracy, which is quoted separately as a percentage of reading. For general electrical fault finding 2 000 to 4 000 counts is adequate, which covers the Extech EX310 and EX330. For calibration checks, low-level signal work or where small changes matter, the 50 000 count Extech EX530A at ±0.06% basic accuracy is the appropriate choice.

Which Extech multimeter is best for solar PV and inverter installations?

Inverter and battery installations need three things from a multimeter. First, True RMS, because inverter output is a synthesised waveform rather than a clean sine wave. Second, adequate DC capability, because PV strings and battery banks operate at DC voltages that can exceed the AC side. Third, a low pass filter, which strips high frequency switching noise so the meter reads the fundamental rather than the carrier. The Extech EX520A and EX530A provide all three, together with an IP67 rating for outdoor rooftop and ground-mount work. The EX470A is a useful alternative where infrared temperature measurement of connections and busbars matters as much as electrical measurement.

Is the Extech EX410, EX411, DM110, MN16A or DV26 still available?

No. Extech has discontinued all five of these models. The EX410 is replaced by the EX410A, the EX411 by the EX411A, and the DM110 by the DM220. The MN16A is superseded by the MN35 manual-ranging and MN36 autoranging mini multimeters. The DV26 non-contact voltage detector is discontinued and current alternatives sit in the voltage and current tester range. Ecotao supplies the current equivalents throughout South Africa and neighbouring countries. Contact sales@ecotao.co.za for a quotation or for help matching a discontinued model to its replacement.

What does an IP67 rating mean on an industrial multimeter?

IP67 is a two-digit ingress protection rating. The first digit, 6, is the highest solid particle rating and means the enclosure is completely dust tight. The second digit, 7, means the enclosure withstands temporary immersion in water to one metre for thirty minutes. For a multimeter this matters in wash-down areas, food processing plants, water and wastewater works, mining, marine and outdoor installations, where a conventional meter would fail from moisture or dust ingress. The Extech EX505A, EX520A and EX530A are all IP67 rated and additionally drop-proof to 2 metres.

Does Ecotao deliver Extech multimeters outside South Africa?

Yes. Ecotao supplies from its Cape Town base to customers throughout South Africa and to neighbouring countries including Namibia, Botswana, Zimbabwe, Mozambique, Lesotho and Eswatini. Contact sales@ecotao.co.za or telephone +27 21 911 5835 with the model and quantity required and delivery will be quoted to the destination.