VA-TEK Engineering Three different numbers get called “accuracy” in relay testing. A 0.1 or 0.2 class is a calibration classification. A 0.2% figure in a datasheet is a manufacturer’s accuracy statement. A 0.5 class is the minimum instrument class that DL/T 995 allows for field setting verification. They are related, but they are not interchangeable, and a test report that confuses them cannot be defended.
Key takeaways
- Class is evidence: the calibration certificate, not the brochure, establishes the class and the range over which it applies.
- Field minimum: DL/T 995 requires instruments of at least 0.5 class for setting verification.
- Error budget: a 0.5-class set can consume 10% of a ±5% relay tolerance before the relay error is counted.
The three numbers that get confused
| Number | What it means | Where it comes from | How it is proven |
|---|---|---|---|
| 0.1 class / 0.2 class | Calibration or verification class of the test set | JJG 1112-2015; DL/T 1153-2025 | Calibration or verification certificate |
| 0.2% AC accuracy | Manufacturer’s accuracy statement for a defined range and condition | Datasheet and type-test report | Type test and factory calibration |
| 0.5 class | Minimum instrument class for field setting verification | DL/T 995-2016, 5.2.1.1 | Valid calibration/verification certificate |
A test set described as “0.2% accuracy” is not automatically a “0.2-class” instrument. The class is a metrological statement with defined limits, ranges and uncertainty; the percentage is a specification claim. Ask for the certificate that maps the claim to a class.
The standard chain: product, calibration, verification
| Standard | Role | What it controls | Current edition |
|---|---|---|---|
| DL/T 624 | Product technical condition | Classification, technical requirements, test methods, acceptance rules | DL/T 624-2023 |
| DL/T 1153 | Calibration specification | Metrological characteristics, calibration conditions, items, methods, result expression, recalibration interval | DL/T 1153-2025, effective 2026-06-18 |
| JJG 1112 | National verification regulation | First verification, subsequent verification, in-service check | JJG 1112-2015 |
The three documents answer different questions. DL/T 624 tells a manufacturer what the product must do. DL/T 1153 tells a calibration laboratory how to characterize it. JJG 1112 tells the legal metrology system how to decide whether it passes verification. A procurement file should contain the type-test report, the calibration certificate and the verification certificate, not one document standing in for all three.
What the classes actually mean
| Class | AC error limit | DC error limit | Typical use |
|---|---|---|---|
| 0.1 class | ±0.1% | ±0.2% | Reference measurements, type-test comparisons |
| 0.2 class | ±0.2% | ±0.5% | Laboratory and high-precision field work |
| 0.5 class | ±0.5% | As specified | Minimum for field setting verification under DL/T 995 |
The distinction matters because the test set is part of the measurement chain. If the relay is allowed a ±5% pickup tolerance and the test set contributes ±0.5%, the instrument alone uses 10% of the allowed tolerance. At 0.2 class, it uses 4%. Both can be compliant, but the 0.5-class result leaves less room for lead burden, waveform distortion, reading resolution and temperature effects.
How accuracy changes a test verdict
Consider a pickup test with a setting of 100 A and an acceptance band of ±5%, or 95 to 105 A. A test set with a 0.5% amplitude error can report 100.5 A when the true output is 100.0 A. That 0.5 A displacement is small against the 5 A band, but it is not the only error. Lead resistance, current-channel burden, waveform distortion and the relay’s own measurement algorithm add their own contributions.
The practical rule is to keep the test set at least one accuracy step better than the tolerance being judged. For a ±5% field criterion, a 0.5-class set is the floor and a 0.2-class set is comfortable. For a ±1% laboratory comparison, a 0.2-class set may be marginal and a 0.1-class reference is appropriate.
What a calibration certificate must show
- Instrument identity: model, serial number and asset number that match the unit on site.
- Ranges and test points: the certificate is valid only for the ranges and points actually calibrated. Low-current points are often the ones used for relay pickup tests and are sometimes omitted.
- Uncertainty: the measurement uncertainty at each point, not only a pass/fail statement.
- Validity: issue date, calibration date and recommended recalibration date.
- Traceability: the reference standard and its own calibration status.
- Condition: any repair, adjustment or limitation noted by the laboratory.
Record the certificate number and the test-set serial number in every test report. If a result is challenged later, the certificate is the evidence that the instrument was capable at the time of the test.
Checks before a job
- Warm up. Let the set reach its specified operating condition; a cold set can drift during the first tests.
- Zero and self-check. Run the built-in diagnostic and confirm zero offset on the current and voltage channels.
- Verify the leads. Measure lead resistance and check the current-channel burden; a long, thin lead can dominate a low-voltage test.
- Confirm the certificate covers the range. A certificate for 1–120 A does not prove performance at 0.2 A.
- Record the setup. Note the set, serial number, certificate, lead set and any burden used.
Frequently asked questions
Is a 0.2% test set a 0.2-class instrument?
Not necessarily. The class is established by calibration or verification against JJG 1112 or DL/T 1153. A datasheet percentage is a manufacturer’s statement; the certificate defines the class and the range over which it holds.
How often should a relay test set be calibrated?
Follow the interval in the applicable calibration specification and the owner’s metrology procedure. DL/T 1153-2025 defines recalibration requirements; JJG 1112-2015 covers verification. Any repair, drop or module replacement should trigger a new calibration before the set returns to acceptance work.
Can a 0.5-class set be used for commissioning?
Yes, where the field criterion is the DL/T 995 setting-verification requirement and the set is within its valid certificate. It is not suitable as a reference for type testing or for a laboratory comparison with a tight tolerance.
What if the certificate does not cover the low-current points?
Treat the uncovered points as unverified. Either use a set whose certificate covers them or add a calibrated reference measurement. Relay pickup tests often sit at low currents, so this gap matters more than the headline range suggests.
Where VA-TEK test sets fit
The VAE-430 and VAE-660 are specified with AC accuracy of 0.2% or better. The datasheet figure describes the product; the calibration certificate supplied with the unit is the evidence of its metrological status. For CT and PT characterization, the VACTP200P measures ratio to ±0.05% and phase to ±2 minutes, with knee-point voltage up to 40 kV. The full range is listed on the relay test set hub.
For selection by channel count and power, see how to choose a relay test set. For budget planning, see relay test set price. For the standards that set the criteria, read relay testing standards compared.
Free whitepaper: the standards, test items and test-set requirements discussed in this article are consolidated in Relay Protection Testing: Standards, Test Items and Test-Set Requirements — a 29-page PDF, free to download without registration.
Standards and references
- DL/T 624-2023, Technical conditions for microprocessor-based relay protection test equipment.
- DL/T 1153-2025, Calibration specification for relay protection test equipment.
- JJG 1112-2015, Verification regulation for relay protection test equipment.
- DL/T 995-2016, clause 5.2.1.1 (field instrument class).
