What a Real SPD Type Test Looks Like: Impulse, Thermal Stability and Endurance Testing

Contents

1. From certificate to test evidence
2. The impulse tests: 8/20, 10/350 and combination wave
3. Thermal stability and TOV behaviour
4. Operating duty and endurance testing
5. Test item – standard reference – report field
6. Sample identity and variant coverage
7. What a manufacturer's laboratory typically covers
8. How to request and read a test report
9. FAQ

Ask for a certificate and most suppliers will send one. Ask what tests were actually performed, on which sample, and what the measured values were, and the conversation becomes much more informative. Understanding what an SPD type test programme contains is the practical next step for anyone evaluating suppliers, because it turns "the product is certified" into questions that can be verified.

This article describes the main tests in a type test programme for low-voltage surge protective devices — the impulse current tests, the thermal stability test and the operating duty or endurance sequence — and explains what each one shows, which product class it applies to, and what to look for in the report. It describes general test principles and the kind of capability a manufacturer's laboratory typically holds; it does not claim any specific accreditation for any specific product.

From certificate to test evidence

A certificate, at its core, is a summary. It states that a product of a given type reference was assessed against a given standard edition. What it usually does not show is the body of work behind that statement: the samples, the test sequence, and the measured values recorded along the way.

The test report is where that work lives. Two questions move a supplier evaluation from "checking certificates" to "checking test evidence": which tests were run on the sample, and does the sample described in the report match the product variant being purchased? A certificate can be genuine while the underlying report covers a different pole count or a different internal construction — the certificate alone does not tell you.

The tests themselves follow defined methods. For AC power SPDs the product standard is IEC 61643-11, which sets the test sequences and pass criteria by class; signal and data SPDs follow their own product standard, IEC 61643-21, with a different test programme suited to their interfaces.

Procurement practice has been moving in this direction for some years. Framework agreements and technical audits increasingly ask for the evidence behind the certificate, and repeat buyers compare reports across suppliers rather than comparing datasheets alone. A supplier who can produce a coherent report for the exact reference is, in practical terms, easier to qualify — and easier to re-qualify at the next audit.

None of this makes the certificate irrelevant. The certificate states scope and the standard edition; the report provides the substance. The evaluation works when the two are read together and both name the same type reference.

The impulse tests: 8/20, 10/350 and combination wave

The impulse current tests establish how the device behaves when it carries a surge. The waveform used depends on the product class. Class II devices — the common Type 2 board-level SPD — are tested with the 8/20 μs current impulse, first at the nominal discharge current In and then, for the maximum rating, at Imax. Class I devices, installed at the service entrance where direct lightning current may be shared, are tested with the 10/350 μs impulse current at the declared Iimp.

The difference between the two waveforms is not cosmetic. At a comparable peak, the 10/350 impulse delivers its energy over a far longer tail, which is why a Class I rating and a Class II rating are not interchangeable numbers. Class III devices are tested with a combination wave — a 1.2/50 μs voltage impulse that drives an 8/20 μs current through the device — which models the residual stress seen close to sensitive equipment.

Throughout these tests the residual voltage at the device terminals is recorded. That residual, expressed as the voltage protection level Up, is the number that later gets compared against the equipment's tolerance. The test report records the measured residual at each stage, so the datasheet value can be traced back to a measurement.

Before any impulse is applied, the programme includes its own groundwork: visual inspection, marking verification, and electrical checks that establish the sample's baseline condition. These baseline records are part of what makes the later results interpretable — a residual value measured after the sequence is meaningful only against a residual value measured before it.

The measurement chain itself is also defined by the method: how the impulse is generated, how the residual is captured, and how the values are verified. A report that states these elements allows the numbers to be assessed on their merits, which is the point of reading it.

Thermal stability and TOV behaviour

Surges are not the sole stress an SPD faces. In normal service the device sits on the supply continuously, and a degraded device can begin to draw current at the system voltage. The thermal stability test asks whether the device, stressed under defined conditions at its maximum continuous operating voltage, reaches a safe and stable state — typically by operating its internal disconnector — rather than entering thermal runaway.

Temporary overvoltage behaviour is a related concern. System faults and earth-fault conditions can raise the voltage across the device well above nominal for seconds or longer, and the declared TOV characteristics describe how the device behaves under those conditions. These tests matter for selection as much as for safety, because the required TOV performance depends on the earthing arrangement of the installation.

Operating duty and endurance testing

A single impulse says little about service life. The operating duty test applies a declared sequence of impulses while the supply voltage is present on the device — stress, measure, stress again — so that the change in protection characteristics across the sequence becomes visible. An SPD endurance test of this kind is what separates a datasheet value taken on a fresh sample from behaviour the product sustains after repeated events.

When reading this part of a report, the trend matters more than any single point. A device whose residual voltage drifts upward across the sequence is telling a different story from one whose values stay flat, even if both pass. End-of-life behaviour after the sequence — whether the device fails safe and how that state is indicated — is assessed as part of the same programme.

Test item – standard reference – report field

Clause numbering differs between standard editions, so the table references test names rather than clause numbers. The report itself should state the standard edition used; where a report omits the edition, that is worth a question to the issuer.

Sample identity and variant coverage

The strongest test programme is only as useful as the link between the tested sample and the product being purchased. A report should identify the sample unambiguously — type reference, marking, construction, photos — and the buyer's question is whether that identity covers the exact variant in the quotation. Pole count, Uc rating and internal options can all create variants within one family.

This is also the point where certificates and reports work together. The certificate on our certificates page states the assessed type reference; the report shows the underlying tests and measured values. Asking for both, for the same reference, is a reasonable and increasingly standard part of supplier evaluation.

What a manufacturer's laboratory typically covers

Manufacturers' own laboratories generally serve two roles: development and type-testing support, where new designs are characterised against the standard's sequences, and routine verification, where production samples are checked against the type-tested reference. The equipment involved — impulse current generators for the declared waveforms, supply sources for thermal and duty testing, and measurement chains for residual voltage — is defined by the test methods rather than by any single instrument brand.

Our testing laboratory page describes the test categories our laboratory works with. Specific statements about which tests are performed in-house, and which are carried out at external accredited laboratories, are confirmed by our technical department for the model in question — capability claims are made per product, not in the abstract.

When evaluating any surge protector testing laboratory, whether in-house or external, the useful questions are the same ones this article has followed: which test categories are covered, under which standard editions, and whether the reporting links the tested sample to the production product. A laboratory's scope statement answers the first two; the report itself answers the third.

One distinction is worth keeping in mind throughout: type testing characterises the design, while routine verification confirms that production still matches it. Both belong in a complete evidence chain, and a supplier who can describe how production samples are checked is usually also a supplier whose type test reports are worth reading.

How to request and read a test report

The request is simple: name the exact type reference, including poles, Uc and options, and ask for the corresponding test report. The reading follows the structure of this article — sample identity first, then the standard edition, then the measured values per test, then variant coverage.

Where a supplier cannot produce a report, or produces one for a different variant, that is information in itself. It does not necessarily mean the product is untested — test programmes are sometimes held at group level or at an external laboratory — but it does mean the evidence chain for that specific model has a gap, and the gap is worth closing before the order.

A final practical note from experience: keep the report with the order documentation. When the same model is re-ordered later, the earlier report becomes the baseline for checking whether the product itself, or the test evidence behind it, has changed in the meantime.

Engineering review note. This article describes general test principles defined by IEC product standards and the typical scope of a manufacturer's laboratory. Publishable statements about our own test capabilities are confirmed by our laboratory and technical department before release. It does not constitute a test certificate for any product, and it does not claim any specific accreditation. Test reports are issued per type reference and are available on request.

FAQ

What is the difference between a certificate and a test report?

The certificate summarises: a type reference was assessed against a standard edition. The test report shows the underlying work — the sample, the test sequence and the measured values. Asking for both, for the same reference, gives a complete picture.

Which products need the 10/350 impulse test?

Class I SPDs, installed where a share of direct lightning current may enter the installation, are tested with the 10/350 waveform at the declared Iimp. Class II devices use the 8/20 waveform; the two ratings are not interchangeable.

What does an SPD endurance test actually show?

It shows how the protection characteristics change across a declared sequence of impulses applied with the supply present. The trend of measured values — and the end-of-life behaviour afterwards — matters more than any single point.

Why does thermal stability matter if the SPD only sees surges occasionally?

Because the device sits on the supply continuously. A degraded device can draw current at system voltage, and the thermal stability test verifies that it reaches a safe state through its disconnector rather than entering thermal runaway.

Does a report for one model cover the whole product family?

Generally it covers the tested sample and closely related variants as stated in the report. Pole count, Uc rating and internal options create variants, so coverage of the exact variant being purchased is confirmed per document.

How do we get the test report for a specific model?

Send the exact type reference — poles, Uc and options — through our contact page. The report for that reference, or a statement of how it was tested, will be provided by the technical department.