Electrical Test & Measurement Instruments

What this category covers

Electrical test instruments fall into two groups that are easy to confuse. Measuring instruments read what a circuit is actually doing: voltage, current, power, resistance, insulation quality. Calibration instruments supply a known value so that another instrument can be verified. This category covers the first group, and the reference-grade sources belong to Calibration & Metrology.

Within the measuring group the deciding questions are consistently electrical: which parameter, over what range, to what accuracy, and what has to be recorded afterwards. Those four answers narrow the field faster than any feature list, and they are the same four questions whether the instrument is a hand-held meter or a panel-mounted analyser.

Most of the cost of getting this wrong sits in interpretation rather than in the instrument. A meter that reads the wrong quantity because the waveform is distorted, or one whose accuracy class is quoted under conditions the site does not meet, produces a number that looks authoritative and is not. Establishing the measurement principle before comparing specifications is the step that avoids it.

Product types in electrical test & measurement

Energy meters, frequency counters, phase meters, voltage testers, current meters, circuit testers and general electrical test equipment are listed under this category too.

How to choose a test instrument

There is rarely a single correct instrument for a duty, but there is always a shortlist that can be justified and a set of answers that eliminates the rest. These are the questions that decide it.

• The parameters that actually have to be measured: voltage, current, active power, power factor, energy, insulation resistance or harmonic content. This one answer eliminates most of the catalogue.
• Whether the readings are taken on clean sinusoidal supplies or on waveforms distorted by drives and rectifiers, which decides whether true-RMS conversion is required or merely desirable.
• Range at both ends of the span, because a wide upper range is usually paid for with resolution at the levels where the judgement is actually made.
• Accuracy class together with the standard it is declared against and the reference conditions it applies to. An accuracy figure without those is not comparable with another.
• Whether results have to be recorded or exported: memory depth, logging interval, interface, and the format a report or an acceptance record needs.
• The safety category of the installation and whether the work is carried out live, since this governs the instrument class rather than any item on the specification sheet.
• Power source for the instrument itself: battery life and field replaceability for site work, mains or loop power for bench and permanent installations.
• Accessories as part of the measurement: leads, clamps, probes and shunts are in the signal path, and an unsuitable clamp undoes an accurate instrument.
• Whether the duty is measurement or verification of another instrument. A measuring instrument reads a circuit, a reference source drives a known value, and the two are bought to different specifications.

Working through the selection in order

1. Write down the parameters required and the range for each, rather than starting from an instrument type.
2. Establish whether the supply is clean or distorted, and choose the conversion principle from that answer.
3. Fix the accuracy needed at the point where the decision is made, not the best accuracy available on the market.
4. Decide whether the reading is instantaneous, logged over time, or part of a formal report, which sets the memory and interface requirements.
5. Check the safety category and whether measurements are taken live, because that governs the instrument class.
6. Confirm the accessories - clamps, leads, probes, shunts, temperature sensors - since they decide whether the set is usable.
7. Decide the calibration and verification routine, and whether certificates are required with the delivery.
8. Only then compare price across instruments that meet the same requirement, because instruments of different classes do not actually compete.

Comparing the technologies

Parameters to confirm before ordering

What is left after the technology is chosen is a list of values that only the installation can supply. These are the ones that change the model, the price or the lead time:

• The parameters to be measured and the range for each, with the smallest value that has to be resolved.
• The nature of the supply: nominal voltage, frequency, and whether drives or rectifiers distort the waveform.
• The accuracy class required, and the standard it has to be declared against where one is named.
• Whether results must be logged, stored or exported, and in what form.
• Environment of use: bench, panel, field or production line, with the ambient conditions.
• The safety category of the installation, and whether measurements are taken on live circuits.
• Accessories needed: current clamps, leads, probes, shunts, temperature probes or a carrying case.
• Calibration and certification requirements, including whether traceability has to be documented for the end customer.
• Quantity, and whether the requirement is a single instrument or a repeat order for a team.
• Any standard the work itself has to satisfy, since the test method can dictate the instrument class.

Where these instruments are used