Sometimes an entire investment is stopped by the lack of a single piece of paper.

The EU Declaration of Conformity, the Operation and Maintenance Manual, the factory test report and the acceptance documentation are not add-ons to the transformer but part of the entire delivery. In this article, we show which documents should be checked before commissioning the device, who is responsible for their completeness, and how to avoid a situation where a ready transformer station is stopped not by a technical failure, but by a missing signature, serial number or proper test report.

Let us imagine a simple situation.

The transformer is already standing on its foundation. The cables have been prepared, the protections set, the assembly team is finishing the last measurements, and the energisation date is approaching faster than Monday after a quiet weekend. Everything looks good – until during acceptance someone asks:

– We will need the Declaration of Conformity, the factory test report and the current installation manual.

Silence falls.

Someone starts searching through emails. Someone else calls the supplier. After a moment, it turns out that the documentation "was probably in the inbox", "should be with the site manager" or "the manufacturer will definitely send it".

And it is at that moment that the multi-tonne transformer ceases to be the biggest problem on site. The bigger problem becomes the ring binder that no one can find.

Does this sound trivial? Unfortunately, only until the acceptance of the station is delayed due to incomplete documentation – a production line, a photovoltaic farm, an energy storage system or an entire facility. A transformer's technical documentation is not an extra thrown into the delivery out of the manufacturer's courtesy. It is part of the product – just like the nameplate, bushings, tap changer, temperature sensors or cooling system.

The most important rule is simple: a transformer without complete documentation may be a functional device, but for the investor it remains an unrecognised technical, contractual and financial risk.

This article is intended primarily for investors, designers, general contractors, transformer station integrators, maintenance managers and those responsible for accepting power equipment.

After reading it, you will know:

• what the EU Declaration of Conformity actually means in practice and what the CE mark really proves,
• what a good transformer Operation and Maintenance Manual (DTR) should contain,
• the difference between routine tests, type tests and special tests,
• where the manufacturer's responsibility ends and the designer's, contractor's and user's responsibility begins,
• how to check the documentation before signing the acceptance protocol.

Reading time: approximately 11 minutes

Technical documentation does not supply power to the plant. Until its absence shuts down the entire investment.

In power engineering projects, attention naturally focuses on the device parameters. We analyse the rated power, the MV and LV voltages, the level of no-load and load losses, the short-circuit voltage, the connection group, the cooling method, the noise level, the insulation class and the environmental conditions.

This is a correct approach. The problem begins when we treat the documentation as elegant packaging for the actual technology.

Yet the documents are needed at every stage of the transformer's life.

The designer uses them to prepare the foundation, ventilation, electrical connections and protections.

The contractor needs them during transport, unloading and installation.

The person performing measurements checks the reference parameters.

The maintenance staff base their inspection schedule on them.

The service team compares the results of periodic tests with the initial values.

The insurer or expert may analyse the documentation after a failure.

A well-prepared set of documents therefore creates something like a technical biography of the device. It shows what the transformer was like when it left the factory, what condition it arrived in, how it was installed and what happened to it over the years.

Without this history, diagnosing a problem is like visiting a doctor without test results, medication information and a previous diagnosis. Of course, you can start from scratch – it is just usually more expensive, slower and much more stressful.

The CE mark is not a quality medal or a magic sticker

One of the most commonly misunderstood elements of documentation is the CE marking.

CE does not mean that the device received a quality award, passed one universal European test or was "approved by the central CE authority". The marking indicates that the manufacturer declares the product's conformity with the applicable EU regulations and has carried out the appropriate conformity assessment procedure.

This is an important difference.

The manufacturer cannot simply stick on the CE symbol because it looks nice next to the serial number. They must determine which legal acts apply to the product, prepare the technical documentation, carry out the required assessment and issue a Declaration of Conformity. The CE mark relates to the product's conformity at the time it is placed on the market in the European Union.

For power transformers, one of the key acts is Commission Regulation (EU) No 548/2014 on ecodesign requirements for small, medium and large power transformers. It was amended by Regulation (EU) 2019/1783.

The regulations specify, among other things, requirements for efficiency and maximum energy losses, and the second-stage requirements, known as Tier 2, have been in force since 1 July 2021.

This does not mean, however, that the same list of directives found in the first declaration template from the internet should automatically be copied to every transformer.

Which regulations may apply to a transformer?

The scope of regulations depends on the construction, rated voltages, equipment and the way the device is placed on the market.

The Low Voltage Directive 2014/35/EU applies to electrical equipment designed for operation at voltages from 50 to 1000 V AC and from 75 to 1500 V DC. The main medium-voltage side of an MV/LV transformer is therefore outside the LVD voltage range, but the directive may be relevant for certain low-voltage circuits and auxiliary equipment.

The same applies to electromagnetic compatibility. A classic transformer is a largely passive device, but electronic temperature indicators, ventilation controllers, communication modules, monitoring systems or switch drives may require assessment for EMC.

The RoHS directives or the Machinery Directive should also not be included in the declaration "just in case". Their application requires an analysis of the product's scope, equipment and intended use.

A good declaration is not a document with the longest list of legal acts.

It is a document with a correct list.

It is a bit like seasoning a soup. More does not always mean better, and emptying the entire spice drawer rarely proves the chef's professionalism.

The EU Declaration of Conformity: what should it really contain?

The EU Declaration of Conformity should answer three very simple questions: who is responsible for the transformer, which exact device the document concerns, and on what basis the manufacturer confirms its conformity with the requirements.

This sounds trivial, but during acceptance, this is precisely where it is easiest to stumble.

The nameplate shows one model, the test report shows a slightly different one, and the Declaration of Conformity vaguely describes "transformers of the X series".

Each document individually looks professional.

Only when placed side by side do they start to resemble a family photo where no one is quite sure who the person in the last row is.

Therefore, the declaration should clearly indicate the manufacturer and their address, and if an authorised representative is involved, also their details.

It must also allow unambiguous identification of the product, for example by type, model, batch number, series or serial number.

It should contain a statement that it is issued under the sole responsibility of the manufacturer, indicate the relevant directives and regulations, and cite the applied standards or technical specifications. Finally, the place and date of issue, the details of the authorised person and their signature are needed.

The serial number deserves special attention here.

In many projects, it is expected to be visible not only on the nameplate but also in the declaration, the product test report, the warranty card and the transport documents.

This immediately confirms that all the papers refer to exactly the unit standing on the foundation, not to its cousin from the same series produced three weeks earlier.

This does not mean, however, that a declaration without a serial number is always automatically invalid. The key is whether the product can be unambiguously identified.

Depending on the production method, this can also be achieved by type, series, batch or another consistent identifier.

From the investor's point of view, however, it is best not to leave room for legal puzzles.

The same number on the nameplate, declaration, test report, warranty and delivery documents means fewer questions, faster acceptance and significantly less risk that someone during commissioning will ask:

"But are these documents definitely for this transformer?"

Five documents, one number and zero guessing. In power engineering, that is luxury.

Factory tests, type test reports, routine tests… one piece of paper, many terms

Let us imagine that a transformer arrives on site.

It looks impressive: a fresh paint coating, a legible nameplate, protected bushings, signed delivery documents. The project manager looks at the device with satisfaction, because this time everything has arrived on time.

An innocent question, however, is asked:

– And where is the test report for this unit?

The driver shrugs. The supplier searches through emails. The manufacturer sends a report after a moment, but the serial number does not match. The power is similar, the voltages are almost the same, and the production date differs by only a few weeks. Almost a success.

Unfortunately, in power engineering, "almost the same transformer" works like "almost the same key to the apartment". It may look right, but the door still remains locked.

The EU Declaration of Conformity confirms that the manufacturer takes responsibility for the product's conformity with the applicable requirements. It is not, however, proof that this exact transformer standing before us achieved the parameters on its nameplate.

For that, a report from tests on the specific unit is needed, most often referred to as the Routine Test Report. It may also form part of the FAT documentation – Factory Acceptance Test.

The declaration therefore says: "this type of device has been designed to meet the requirements". The test report adds: "and we actually checked this particular unit".

This is a small linguistic difference, but a very large technical one.

The basic reference point for power transformers is the IEC 60076 series of standards. Its first part specifies the general requirements for single-phase and three-phase transformers. The mere information that the device was made "in accordance with IEC 60076" does not yet say, however, which specific tests were carried out, whether they applied to every unit, and whether the investor will receive the results for their unit.

This is why it is worth distinguishing three groups of tests.

1. Routine tests – checking a specific unit

Routine tests are performed on every manufactured unit to the extent required by the applicable standard and order specification. Their purpose is to confirm that the specific transformer has been correctly manufactured and achieves the declared parameters.

The report may include, among other things, winding resistance measurement, ratio and phase displacement checks, no‑load and load loss measurement, no‑load current, short‑circuit voltage and the relevant dielectric tests.

For the investor, it is particularly important that the report contains identification of the unit – ideally the serial number matching the nameplate. Without this, we receive test results for some transformer. Perhaps a very good one. Just not necessarily ours.

2. Type tests – can this design do what it promises?

Type tests confirm specific properties of the design or an entire family of devices. They do not have to be performed separately for every transformer produced.

They may concern, for example, temperature rise, noise level or other characteristics that would be time‑consuming, costly or require a special test bay to test every time.

One could say that routine tests check a specific unit, while type tests ask whether the design as a whole passed a more important exam.

It is a bit like with a car. Every unit should pass a final inspection, but not every newly produced vehicle is crashed into a wall again as part of a crash test. That would be an extremely thorough approach, though rather unfavourable for the delivery schedule.

3. Special tests – when the standard package is not enough

Special tests are performed when required by the contract conditions, the operator's standard, the nature of the installation or particular operating conditions.

They may include, for example, extended short‑circuit withstand tests, frequency response measurements, additional environmental tests, specialist noise measurements or in‑depth construction diagnostics.

Such requirements appear especially in projects where the transformer will operate in an unusual environment, with a high share of harmonics, with power electronic converters, or in an installation with exceptionally high requirements for supply continuity.

An energy storage system, a photovoltaic farm, a data centre and a small production plant may need transformers of the same power, but that does not mean they need an identical scope of testing.

A complete test package – two words, five different interpretations

Problems often begin already in the tender specification.

The investor writes: "Complete transformer testing package required."

The manufacturer understands this as standard routine tests.

The designer has in mind routine tests and current type test reports.

The operator expects additional tests according to their standard.

The laboratory, meanwhile, asks whether "complete" also includes special tests.

Everyone uses the same term, but each orders something different.

This is an extremely efficient way of producing later disputes.

That is why it is not enough to rely on a general requirement that the transformer should have a "full package" or "complete testing" in the procurement documentation.

You should clearly specify:

• which tests are to be performed for each unit,
• which type test reports the manufacturer should present,
• whether special tests are required,
• according to which standard and its parts the tests should be carried out,
• whether the investor will participate in the FAT,
• in what form and language the results should be delivered,
• which serial number the report should be linked to.

This way, the transformer arrives not only with a declaration that everything should be fine, but also with specific results showing that it really is fine.

The transformer “DTR”(..wth(eck?)), manual or instruction – what is the difference?

Nothing.

In Polish projects, a requirement often appears: "the transformer must be delivered with a DTR."

A foreign manufacturer replies that they do not have any "DTR", but can send an Installation, Operation and Maintenance Manual, Operating Instructions or the German Betriebsanleitung.

And the paper ping‑pong begins.

The investor waits for the DTR, the manufacturer sends the manual again, and someone in the middle tries to establish whether another document should be ordered.

Most often, it is not necessary, because DTR is a somewhat older, established name in Poland for the documentation describing the method of transport, assembly, commissioning, operation and maintenance of the device.

Modern regulations and technical documents more often use terms such as "instruction manual", "user manual" or "operating instructions".

Polish regulations concerning power equipment also use the term "operating instructions" and do not require a document necessarily titled "DTR".

What counts is therefore not the name on the cover, but the content.

The manufacturer's manual should allow unambiguous identification of the transformer and give its most important parameters. It should also explain how to transport, lift, store, position, connect and prepare the device for first energisation.

Basic information on operating conditions, earthing, cooling, tightening torques, pre‑commissioning checks and subsequent inspections is also needed.

It does not have to be a 400‑page saga about the life of transformer oil and its descendants.

It should be a document from which the designer, installer and user can learn what to do to avoid damaging the device and to operate it safely.

If a foreign manual contains this information, it is in practice the equivalent of the traditionally understood manufacturer's DTR. There is no point in requiring a second document just so that three familiar letters appear on the cover.

You must, however, distinguish the manufacturer's manual from the operating instructions for the entire station or installation, prepared by the user for a specific facility.

Such a document may take into account the local grid layout, work organisation, protections, switching procedures and emergency response rules.

Then it is indeed no longer just a translated transformer manual.

Simply put: the transformer DTR, manual and Betriebsanleitung can be one and the same document. A separate operating instructions document is needed only when it is to describe not only the device but also its operation in a specific installation.

The name is secondary.

The transformer will not be offended that its instructions are called a "manual".

The person receiving it, however, may rightly be frustrated if under an elegant cover they do not find the information needed for installation and commissioning.

Manufacturer, importer, designer, contractor and user – each holds a different piece of the puzzle

When the transformer is working correctly, the boundaries of responsibility are rarely the subject of exciting conversations. The situation changes after a failure, an acceptance delay or a warranty dispute.

Then, suddenly, everyone starts reading specifications, protocols and emails from two years ago very carefully.

Manufacturer

The manufacturer is responsible for designing and manufacturing the device in accordance with the applicable requirements.

Their duties include carrying out the appropriate conformity assessment procedure, preparing the technical documentation, performing the required tests, issuing the declaration and providing instructions enabling safe use.

Their responsibility may include, among other things, material defects, design errors, non‑conformity of parameters with the order, incorrectly made windings, tank leaks or incorrect information in the documentation.

If the manual gives the wrong connection diagram or incorrect tightening torque values, the problem does not cease to be the manufacturer's problem just because it is on paper instead of in steel.

Importer and distributor

An importer introducing a product from outside the European Union should not limit their role to organising transport and issuing an invoice.

They must verify whether the manufacturer carried out the required conformity assessment, whether the device has the proper marking and whether the required documents are available.

This does not automatically mean that the importer in every situation "becomes the manufacturer" in the full legal sense.

They may, however, be treated as the manufacturer if they place the product on the market under their own name or trademark, or modify it in a way that may affect conformity.

A distributor should also act with due diligence.

If they see that the documentation is incomplete, the markings are inconsistent, and the device numbers do not match the test report, they should not pretend that the transformer has a mild paper hiccup.

The obligations of manufacturers, importers and distributors, and the principles of product traceability, are described in the EU "Blue Guide" on the implementation of product rules.

Designer

The designer is responsible for selecting the device for the grid, the load and the environmental conditions. They should take into account, among other things, voltage levels, short‑circuit currents, protections, ventilation, fire resistance, floor loading, service access and the operator's requirements.

A transformer may be perfectly manufactured and still operate incorrectly if it is placed in a compartment that is too small, poorly ventilated, or selected without considering the harmonics generated by inverters, rectifiers and converters.

Good equipment does not automatically fix a bad design.

Contractor and installer

The contractor is responsible for how the device was transported to the site, unloaded, positioned, connected, earthed and prepared for commissioning.

Common problems include damage during unloading, incorrect support, stress on bushings from rigid busbar connections, omission of protective conductors, incorrect connection of sensors or lack of functional protection tests.

The contractor should also hand over to the investor the as‑built documentation: measurement protocols, test results, diagrams after changes and confirmation that the protections have been checked.

Without this, acceptance is like buying a house without knowing where the cables, pipes and valves run. Theoretically, you can live in it. Practically, the first failure turns into a treasure hunt.

Investor and user

After acceptance of the device, responsibility for its correct operation passes largely to the user.

They should ensure adequately qualified personnel, maintain operating documentation, carry out inspections, monitor operating conditions and respond to abnormal temperatures, noise, leaks, contamination or protection operation.

The manual should not lie in a cupboard for the next 23 years in its factory‑fresh condition, still smelling of the print shop. It should be a document that is used, supplemented with inspection results and accessible to those responsible for the device.

What about Poland’s technical inspection authority, the DSO and the insurer?

These three parties are often mentioned in the same breath, although they perform completely different jobs. Put simply: one supervises selected categories of technical equipment, another decides whether an installation meets the requirements for connection to the distribution network, and the third becomes particularly interested when something has already gone wrong.

Poland’s Office of Technical Inspection

The Office of Technical Inspection, known in Poland as Urząd Dozoru Technicznego or UDT, is a state institution responsible for supervising technical equipment that may pose a risk to people, property or the environment.

A standard power transformer does not automatically become equipment requiring UDT registration simply because it is installed inside a substation. However, other devices used at the facility — such as lifting equipment or certain pressure devices — may fall under technical inspection rules.

The practical rule is simple: check the actual equipment installed at the site rather than assuming that the entire transformer station is either “subject to UDT” or completely outside its scope.

Distribution System Operator

In Poland, the abbreviation OSD means operator systemu dystrybucyjnego. In English, the correct term is Distribution System Operator — DSO. Polish DSOs include companies such as PGE Dystrybucja, TAURON Dystrybucja, Enea Operator, Energa-Operator and Stoen Operator.

The DSO may define technical parameters, protection requirements, tests and documents needed before a transformer installation can be connected to the distribution network. These requirements are not identical in every project. They depend on the operator, the connection conditions, the ownership structure and the design of the installation.

A missing test report or a transformer that does not meet the relevant DSO standard may delay acceptance. This is why the required documentation should be agreed with the designer, contractor and operator before the transformer is ordered — not when it is already standing in the substation and everyone is searching through old email attachments.

Insurer

The insurer has yet another role. It does not approve the connection and does not perform technical inspection of the transformer. After a failure, however, it may examine the policy terms, the cause of the damage, the maintenance history and the records showing how the equipment was operated.

Incomplete documentation or missing inspection records do not automatically mean that compensation will be refused. Much depends on the wording of the policy, the circumstances of the event and whether any negligence contributed to the loss. Regular inspection and maintenance nevertheless make it much easier to demonstrate that the transformer was operated responsibly. Insurers themselves emphasise the importance of systematic transformer maintenance as part of industrial risk management.

In practical terms, the difference is straightforward: UDT deals with technical inspection obligations, the DSO deals with network connection requirements, and the insurer deals with the financial consequences of a loss.

They may all ask for documents, but definitely not for the same reason.

Two transformers, two failures and completely different outcomes

Let us consider two hypothetical plants.

In the first, an oil‑immersed transformer has been operating for six years. Since the day of delivery, a history of test results, temperatures, inspections and minor repairs has been kept. During a periodic oil analysis, an alarming change appears. The service compares it with previous results, extends the diagnostics and detects a developing problem before a major failure occurs.

The transformer is de‑energised on a planned date. The plant arranges backup power. The repair is costly, but controlled.

In the second plant, a similar transformer has also been operating for six years. The documentation is in several places, some protocols were lost after a change of service company, and the last oil analysis was "probably done".

A temperature alarm appears, but no one knows whether the sensor previously indicated similar values. There is no trend, no reference values and no complete load history. Every decision requires additional tests, and the plant does not know whether it can safely continue production.

In both cases, the device may have the same power, manufacturer and year of production. The difference is made by information.

In power engineering, historical data is often cheaper than steel, copper and oil. The problem is that its value is usually appreciated only when it is already missing.

What to check before accepting a transformer?

Before signing the acceptance protocol, you must ensure that the documents actually refer to the device standing before you.

The type, model, power, voltages, connection group, short‑circuit voltage, tap range and above all the serial number should match the nameplate, the design, the EU Declaration of Conformity, the test report and the warranty card.

The package should also contain the current dimension drawing, the connection diagram and the manufacturer's manual describing transport, installation, commissioning and basic operating principles. If the order included additional type tests or special tests, their reports must also be delivered.

At the commissioning stage, measurement protocols, confirmation of correct tap setting and the results of protection, alarm and signalling tests should be collected.

It is also worth recording the initial values, which will later serve as a reference point during inspections.

If any of these elements is missing before acceptance, it is better to clarify the matter immediately.

After the protocol is signed, lost documents and unfinished arrangements have a remarkable talent for turning into "the investor's scope".

Documentation should be selected together with the transformer, not after delivery

Most problems arise when the transformer is ordered as a device, and documentation is only discussed during acceptance.

Yet its scope should be included already in the request for quotation or specification. It is worth specifying the required language, file format, number of paper copies, scope of testing, operator's standard, drawing approval procedure and the document delivery deadline.

In projects carried out for industry, renewables, BESS and critical infrastructure, a good solution is to prepare a document register. Each item receives a number, status, version, acceptance date and information on who is responsible for its approval.

Does this sound corporate? Perhaps.

But it still sounds better than: "No one knows where the final drawing is, but I think we installed according to version three."

A good transformer arrives with a full history from day one

A transformer without documentation is a bit like a car without a registration certificate, manual and service history. It may look excellent. It may even work. But at the first inspection, failure or attempt to sell it, the show begins – and no one bought a ticket.

The EU Declaration of Conformity confirms the manufacturer's responsibility for the product's conformity with the applicable requirements. The test report shows the parameters of the specific unit. The DTR explains how to transport, install, commission and maintain the device. The as‑built documentation, in turn, proves that the transformer has been correctly integrated into the installation.

Only together do they form a complete system of technical and organisational safety.

At Energeks, we look at transformer delivery more broadly than just through the lens of rated power. We help select a solution for the grid conditions, environment, load character, investor's requirements and project standards.

Our offer includes MarkoEco2 oil‑immersed transformers as well as TeoEco2 cast‑resin transformers intended, among other things, for industry, photovoltaic installations, energy storage systems, containerised stations and critical infrastructure facilities.

Selected units are available off‑the‑shelf, which helps reduce waiting time without compromising the full documentation package and technical support. We invite you to contact the Energeks team – together we will deliver every project.

Also join us on LinkedIn!

Because a transformer should change voltage levels – not the stress level of the project team ;)

sources:

1. European Commission, The Blue Guide on the implementation of EU product rules 2022

2. EUR-Lex, Commission Regulation (EU) No 548/2014, amended by Commission Regulation (EU) 2019/1783

3. International Electrotechnical Commission, IEC 60076-1:2011 – Power transformers, Part 1: General

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This article is technical and informational in nature and does not replace legal analysis, contract terms, manufacturer documentation or the requirements of the relevant distribution system operator.