Standards-based testing
A power supply that must give 5.00 V ± 1% measures 5.045 V with an uncertainty of 10 mV. It seems to comply, but there is almost a 16% probability that it does not.
01A test anyone can repeat
A standards-based test is a measurement with a procedure fixed in advance: what is measured, with which instruments, under which environmental conditions, with which setup and by which criterion it is accepted. The goal is that two different laboratories, testing the same product, reach the same result. This is how a product is certified as safe, as not interfering with other equipment, and as withstanding the environment where it is going to operate.
International standards are issued by the IEC (electrotechnology) and ISO (everything else), and in Argentina IRAM (the Argentine standards institute) adopts them and issues its own standards. Some are mandatory by regulation: low-voltage electrical products sold in the country must certify their safety under the regime of the Secretariat of Commerce, and radio equipment must obtain homologation from ENACOM (the national communications regulator).
02The families of tests
| Family | Reference standards | What is verified |
|---|---|---|
| Electrical safety | IEC 61010 (measuring equipment), IEC 62368-1 (audio, video and IT), IEC 60335 (household appliances) | Dielectric strength, insulation resistance, leakage current, ground continuity, temperatures |
| Electromagnetic emissions | CISPR 32, IEC 61000-3-2 (harmonics) | That the equipment does not disturb others, by conduction and by radiation |
| Electromagnetic immunity | IEC 61000-4-2 (electrostatic discharge), -4-3 (radiated fields), -4-4 (bursts), -4-5 (surges) | That the equipment keeps working under standardized disturbances |
| Environmental | IEC 60068-2 (temperature, humidity, vibration, shock) | That it withstands transport and operating conditions |
| Ingress protection | IEC 60529 (IP code) | Protection against solids and water |
In immunity tests the result is not a number but a behavior, classified into criteria: A, it operates normally during the test; B, it degrades temporarily and recovers on its own; C, it recovers with operator intervention. Each product standard sets which criterion it requires.
03How it is carried out and reported
A valid test requires calibrated instruments with traceability, the environmental conditions set by the standard, the exact setup it describes (distances, cables, ground plane) and a record of everything. The report, according to ISO/IEC 17025, identifies the item tested, the standard and its version, the method, the equipment, the conditions, the results with their uncertainty and, if applicable, the statement of conformity with the decision rule that was used. That rule is what connects the measurement uncertainty, seen in errors and uncertainty, with the decision to accept or reject.
04Deciding with uncertainty
If a power supply must give 5 V ± 1% and the laboratory measures 5.045 V with an expanded uncertainty of 10 mV, the true value may be between 5.035 and 5.055 V: outside the tolerance. Does it comply or not? Decision rules (ILAC-G8 guide, JCGM 106 document) resolve it in two ways:
- Simple acceptance. It is accepted if the measured value is within the tolerance. The risk of accepting something that does not comply is shared between the laboratory and the customer, and is acceptable only if the uncertainty is small compared with the tolerance.
- Guard band. The acceptance limits are shrunk by \( U \) on each side. Conformity is declared only if the measured value falls within them; between the guard band and the limit, neither one thing nor the other can be stated.
A power supply specified at 5.00 V ± 1% (from 4.95 to 5.05 V). You choose the measured value and the expanded uncertainty (k = 2). The graph shows the tolerance, the acceptance zone with guard band and the probable distribution of the true value.
05Common safety tests
| Test | How it is done | What it protects |
|---|---|---|
| Dielectric strength | A high AC or DC voltage is applied between the mains and the accessible parts for a fixed time, without breakdown occurring | That the insulation withstands overvoltages |
| Insulation resistance | Measured with 500 V DC | The condition of the insulation |
| Leakage current | The current that would flow through a person touching the equipment is measured, with a network that imitates the human body | Against electric shock |
| Ground continuity | A high current is passed through the protective conductor and its resistance is measured | That the ground acts in the event of a fault |
The voltage, time and limit values are set by each product standard according to the equipment class and the overvoltage category.
06In the laboratory
Write the test plan for a laboratory power supply: variables, instruments with their uncertainty, conditions, acceptance criteria and decision rule.
With an ESD simulator, apply 4 kV contact discharges to a device with a microcontroller according to IEC 61000-4-2 and classify its behavior under criteria A, B or C.
Write the report of a measurement made in another activity with all the elements the standard requires.
07Common mistakes
- Declaring conformity without considering the uncertainty. Near the limit, the decision may be wrong.
- Citing a standard without its version. Requirements change between editions.
- Improvising the setup. In electromagnetic compatibility, a cable moved changes the result.
- Testing with instruments without a valid calibration. The result loses traceability and validity.
08Self-assessment
What TUR does a tolerance of ±0.5% on 10 V measured with U = 10 mV have?
\( 50\ \text{mV} / 10\ \text{mV} = 5 \).
With a guard band, up to what value is an upper tolerance of 5.05 V accepted with U = 10 mV?
Up to 5.04 V.
What does criterion B mean in an immunity test?
That the equipment temporarily loses some function during the test and recovers on its own, without intervention.
Which document defines the competence requirements for a testing laboratory?
The ISO/IEC 17025 standard.
09Further reading
- JCGM. Evaluation of Measurement Data: The Role of Measurement Uncertainty in Conformity Assessment (JCGM 106:2012). BIPM, 2012. Decision rules with uncertainty and the calculation of consumer’s and producer’s risks.
- ILAC. ILAC-G8: Guidelines on Decision Rules and Statements of Conformity. ILAC, 2019. The guide used by accredited laboratories to declare conformity.
- Tim Williams. EMC for Product Designers. 5th ed., Newnes, 2017. The electromagnetic compatibility standards and how to test and design to meet them.