Assess a dry-type transformer electrostatic shield through two separate records: the construction offered and the evidence for any claimed electrical-noise attenuation. A bare decibel value cannot tell a buyer whether the claim addresses the project’s disturbance or system arrangement. Ask for the disturbance mode, frequency range, test conditions and relationship to the proposed transformer. This guide addresses U.S. commercial and industrial low-voltage dry-type procurement. It does not establish that a shield is necessary for every installation or that any particular shielded design is available from HEAKOR.
AI-generated illustration for electrostatic-shield evidence review; the pictured enclosure does not demonstrate shield construction or attenuation performance.
Does “isolation transformer” specify an electrostatic shield?
Do not rely on the word isolation to specify a shield. Published low-voltage transformer ordering guidance distinguishes separate primary and secondary windings from a separately specified electrostatic shield. That guidance concerns the documented product scope, rather than establishing the construction of another supplier’s offer. Read the low-voltage isolation and shield ordering guidance.
Write the requirement in the enquiry and ask the bidder to answer it directly. A useful response states whether the requested construction is included, excluded or offered as an alternative requiring review. If the project specification only says “isolation,” ask the responsible designer whether shielding is intended before purchasing assigns meaning to the term.
Define the attenuation claim before comparing numbers
Electrical-noise claims need a measurement context. In the cited transformer guidance, common-mode noise is measured line to ground and transverse-mode noise is measured line to line. Use the stated convention when interpreting that guidance and ask the bidder to identify the convention used in its report. Published technical guidance also notes that attenuation depends on frequency and recommends supporting claims with actual test data, a procedure and a test circuit. Read the shield attenuation evidence guidance.
Use those distinctions to make a clear document request. Ask which quantity the quoted figure represents and whether it is a measured result or a calculated expectation. Do not treat two values as equivalent when they describe different modes, frequencies or conditions. Avoid turning a single best result into an unstated performance requirement across the project’s entire operating range.
Dry-type transformer electrostatic shield: bid evidence
The following is a suggested purchaser’s matrix, not a prescribed testing standard. The responsible engineer should determine which details are necessary for the project and whether the proposed evidence is adequate.
| Claim or requirement | Evidence to request | Decision supported |
|---|---|---|
| Shield included | Construction statement tied to the offered design | Does the proposal actually include the specified feature? |
| Attenuation figure | Quantity, units, disturbance mode and frequency range | Does the number answer the stated requirement? |
| Measured performance | Test report, circuit, procedure and relevant conditions | Can the technical reviewer interpret the result? |
| System relevance | Grounding assumptions and application conditions | Does the evidence address the intended installation? |
| Design applicability | Tested design identity and explanation of applicability | Can the report support this offered configuration? |
| Bid exception | Explicit deviation and proposed acceptance route | Who must approve the alternative before order? |
Treat system grounding as a review input
Provide the designer’s system-grounding description with the requirement. Published low-voltage guidance qualifies the usefulness of an electrostatic shield according to secondary-grounding conditions; that is a reason to obtain a project-specific assessment, not to change grounding arrangements to match a sales claim. See the application conditions in the ordering guidance.
Procurement should record the design assumptions used in the response and identify who has accepted them. If those assumptions differ from the project documents, return the issue for clarification. This article gives no grounding or wiring instructions, and an attenuation discussion does not override installation, protection or safety requirements.
Close a proposed substitution in writing
When a bidder proposes to omit a specified shield or replace the requirement with a different performance statement, keep the original requirement visible. Ask the bidder to describe the alternative, explain what evidence supports it and list any conditions attached to the proposal. Send that package to the responsible technical reviewer.
Record the disposition as accepted, rejected or awaiting named information. If the alternative is accepted, carry its exact conditions into the purchase documents and approved design records. Do not let a commercial clarification disappear between quotation and order while the original technical requirement remains unchanged elsewhere.
| Unclear bid wording | Practical clarification |
|---|---|
| “High noise reduction” | Request a defined quantity and supporting evidence. |
| A dB value without frequency information | Request the frequency range and conditions represented. |
| A report for a different design | Request the technical basis for applying it to the offer. |
| “Equivalent isolation” instead of a shield response | Request a direct answer to the specified construction and performance requirements. |
Do not broaden the evidence into unrelated claims
Keep the review centered on the specified electrical disturbance. A shield statement is not evidence of harmonic mitigation, acceptable audible sound, surge-protection coordination or suitability for a medical system. Each claimed outcome needs its own applicable requirements and evidence. If the original concern is described only as “noise,” have the designer identify the actual problem before selecting a transformer feature.
That distinction also improves supplier enquiries. It is more useful to provide an approved disturbance requirement and its evidence expectations than to ask for the largest available attenuation number. Where the requirement is unresolved, mark it as an engineering question instead of creating a purchasing guarantee that no one has defined.
Frequently asked questions
Can a construction drawing replace an attenuation report?
A drawing can support a construction review, but it does not by itself establish a measured performance claim. Ask separately for the evidence required by the project’s attenuation specification.
Should a report identify the tested transformer?
Yes, request enough identity and configuration information to establish what the evidence covers. If it is design-family evidence, ask the supplier to explain its applicability to the proposed unit.
What if the requirement names no test method?
Ask the responsible engineer to agree the evidence and acceptance basis before the order. Record the accepted procedure and conditions rather than allowing each bidder’s unexplained number to become the comparison basis.
Can an attenuation calculation be submitted?
A calculation may be useful background if clearly identified as such. Have the engineer decide whether it meets the contract or whether measured evidence is required; do not relabel a prediction as a test result.
Does a shield guarantee that connected equipment will operate correctly?
No general guarantee follows from the presence of a shield. The project team must assess the transformer, disturbance requirement and connected system together, with any remaining performance conditions stated explicitly.
Prepare a focused HEAKOR enquiry
For a dry-type transformer enquiry, use HEAKOR’s dry-type transformer category and technical document checklist.
Submit Your Project Specification. Include destination; dry-type duty, kVA, voltages, phase and frequency; designer-approved system-grounding description; disturbance requirement; requested shield construction; evidence expectations; proposed deviations.
Request a direct construction response and the scope of any applicable attenuation evidence. Confirm whether the requested shield and supporting evidence can be offered for the proposed configuration.