Service · Factory execution · FAT
Partial discharge (PD) test witnessing at FAT in China
Partial discharge is the measurement that separates insulation that merely survived the withstand test from insulation that will survive twenty years of service — and it is also the easiest measurement to get wrong, or to report kindly. Sinospect's engineers witness PD measurement during the FAT at Chinese factories as the buyer's independent representative: circuit calibration verified, background noise recorded, voltage sequence followed, and the measured level read against the limit your contract actually sets. Witnessing and specification only — Sinospect does not sell PD test instruments.
- Measurement basis
- IEC 60270 measuring practice; limits per product standard or your specification
- Equipment class
- Transformers, MV/HV and gas-insulated switchgear, instrument transformers, HV cables
- Where it runs
- Manufacturers' test bays and HV laboratories across China
- Engagement output
- Witnessed PD record inside a signed FAT witness report, with release recommendation
Quick answers
What does independent PD test witnessing during a FAT in China cover?
The witness verifies the measuring-circuit calibration and background noise level, confirms the test-voltage sequence against the standard or agreed protocol, observes the measurement, and records the PD level against the applicable acceptance limit — tied to the serial numbers of the units under test, inside a signed witness record.
| Verification point | What is checked | Why it decides credibility |
|---|---|---|
| Circuit calibration | Calibration in the sense of IEC 60270, performed at the actual test setup | An uncalibrated circuit makes the picocoulomb reading meaningless |
| Background noise | Measured noise level recorded before the test | Noise near the limit can mask or fake a pass |
| Voltage sequence | Pre-stress and measurement voltages, durations and steps per the standard or protocol | PD at the wrong voltage proves nothing about the acceptance criterion |
| Recorded level vs limit | PD magnitude read against the limit for this equipment class and specification | Pass/fail must reference the correct limit, not a generic one |
| Unit identity | Serial numbers and nameplate tied to the measurement record | The result must belong to the unit you are releasing |
Which equipment from China typically requires witnessed PD testing at FAT?
Power and distribution transformers where the standard or specification sets a PD limit, MV and HV switchgear including gas-insulated assemblies, instrument transformers, and HV cable systems and accessories. For these classes, PD reveals insulation voids, contamination and workmanship defects that a pass/fail withstand test does not.
| Equipment class | What PD reveals before dispatch |
|---|---|
| Power and distribution transformers | Voids, moisture and assembly defects in the insulation system |
| MV/HV and gas-insulated switchgear | Cavities, particles and stress points in solid or gas insulation |
| Instrument transformers | Insulation workmanship on units that guard metering and protection |
| HV cables and accessories | Workmanship in insulation and prefabricated accessories before installation |
01 · Scope
When witnessed PD measurement is the right control
A withstand test answers one question — did the insulation survive this voltage for this minute. PD measurement answers the question buyers actually care about: is there a defect in the insulation that will grow in service. Because the result is a sensitive measurement rather than a survived stress, everything depends on the conditions: calibration, noise, voltage sequence, and reading the right limit. That is why specifications make PD a witnessed hold point — and why the witness must understand the measurement, not just attend it. If your contract sets a PD limit for transformers, switchgear, GIS or cables from a Chinese manufacturer, independent witnessing of that measurement is the control that makes the limit real.
It becomes useful when
- The product standard or purchase specification sets a PD acceptance limit for the units on order.
- Transformers, MV/HV switchgear, GIS, instrument transformers or HV cable systems are bought for a project where insulation failure is intolerable.
- A previous unit, supplier or factory has a history of marginal PD results or kindly-reported certificates.
- The acceptance limit, test voltage or measurement practice differs between the supplier's routine and your specification — and someone must hold the line.
- A lender, insurer, EPC or end client requires independent evidence behind the PD line on the test certificate.
- Your own HV engineers cannot attend the Chinese test bay for every unit of the order.
02 · Verification
What the witness controls at the PD hold point
Control point
Test basis and limit agreement
The applicable PD limit, test-voltage sequence and measurement conditions confirmed in writing before the visit — from the product standard, the purchase specification or the agreed FAT protocol, whichever governs.
Control point
Measuring-circuit calibration
Calibration in the sense of IEC 60270 witnessed at the actual test setup, with the calibration record and injected charge noted before any measurement counts.
Control point
Background noise verification
The background noise level measured and recorded before energization — and challenged when it sits close enough to the limit to mask a real result.
Control point
Voltage sequence and measurement
Pre-stress and measurement voltage steps, durations and the recorded PD magnitudes witnessed against the agreed sequence; unstable or marginal readings recorded as observed, not as summarized afterwards.
Control point
Result, identity and disposition
The measured level read against the correct limit, tied to the serial numbers under test; exceedances logged as non-conformities with the supplier's response and the witnessed retest tracked to closure.
03 · Evidence
Evidence captured during witnessed PD measurement
- Measurement basis record
- Applicable standard clauses, specification limits, test-voltage sequence and measurement conditions agreed and documented before the test.
- Test-bay setup, measuring instrument references and laboratory calibration certificates noted.
- Witness scope stated: which units, which measurements, what the FAT does and does not prove.
- Witnessed measurement record
- Circuit calibration and background noise recorded at the setup, before energization.
- Voltage steps, durations and PD magnitudes recorded as observed, with the unit serial numbers photographed on the bay.
- Anomalies — unstable readings, noise excursions, repeated runs — captured as they happen.
- Disposition and release record
- Exceedances logged with measured level, limit, voltage step and supplier response.
- Witnessed retests after drying, reprocessing or repair recorded against the same criteria.
- Release, hold or retest recommendation prepared for the buyer's decision, inside the signed FAT witness record.
04 · Deliverables
Deliverables issued
Deliverable
For · Engineering and procurementPD witness plan
The agreed limit, voltage sequence, measurement conditions and witness scope before the visit — so the argument about criteria happens before the test, not during it.
Deliverable
For · HV engineering and external reviewersWitnessed PD measurement record
Calibration, background noise, voltage steps and recorded PD levels tied to unit serials — the evidence behind the certificate line.
Deliverable
For · EngineeringPhoto and setup evidence
The test setup, instruments and unit identity documented on the bay.
Deliverable
For · Procurement and the supplierNon-conformity and retest log
Exceedances and marginal results tracked through supplier response, correction and witnessed retest.
Deliverable
For · Procurement and financeRelease recommendation
Release, hold or retest — the PD hold point resolved into a written basis for your dispatch and payment decision.
05 · Risks reduced
Risks this control closes
Risk
The certificate shows a passing picocoulomb figure measured on an uncalibrated or unverified circuit.
How the witness closes it
No measurement counts until the witnessed calibration at the actual setup is recorded — IEC 60270 practice, not a paper reference.
Risk
Background noise close to the limit hides real discharge activity.
How the witness closes it
Noise is measured and recorded first; a noise floor that compromises the limit is challenged before the test proceeds.
Risk
PD is measured at a kinder voltage or a shortened sequence than the standard requires.
How the witness closes it
The voltage steps and durations are witnessed against the agreed sequence, and deviations are recorded as non-conformities.
Risk
A marginal or unstable reading becomes a clean number by the time it reaches the certificate.
How the witness closes it
Readings are recorded as observed during the test, including instability and repeated runs — the record is written on the bay.
Risk
The passing result belongs to a different unit, or to a type-test reference rather than the units shipping.
How the witness closes it
Serial numbers are photographed and tied to each measurement; every unit with a specified PD hold point gets its own recorded result.
Risk
Elevated PD is waved through as "within normal variation" and becomes an in-service insulation failure.
How the witness closes it
Exceedances are logged with the limit and the supplier's response, and normally carry a hold recommendation until a witnessed retest passes.
06 · Background resources
Related
- Resource · Partial discharge testing at FATThe specifier's guide: when PD belongs in the FAT, how it differs at SAT, and what a credible result records.
- Service · Factory acceptance test in ChinaThe parent FAT service — PD witnessing usually runs inside this wider witnessed scope.
- Service · Transformer FAT witnessingThe full transformer witness scope: routine tests, accessories, documentation and release.
- Service · Switchgear & panel FAT witnessingThe sibling scope for LV/MV switchgear and control panels, where PD applies to MV assemblies.
- Resource · Factory acceptance test checklistThe twelve-section FAT checklist, downloadable as a printable PDF.
- Sector · EnergyHow Sinospect supports energy-sector buyers across supply, inspection and delivery.
07 · Questions
Frequently asked questions
When does PD measurement belong in a FAT for equipment from China?
When the product standard or your purchase specification calls for it — typically for power transformers, MV/HV switchgear, gas-insulated equipment and HV cable systems, where insulation workmanship decides service life. If PD is specified, it should be a witnessed hold point in the FAT with the limit, test voltage and measurement conditions agreed in writing beforehand.
What makes a PD result credible rather than just a number on a certificate?
Four things the witness verifies on the bay: the measuring circuit is calibrated in the sense of IEC 60270 with the calibration record made at the test setup; the background noise level is measured and low enough to make the acceptance limit meaningful; the test-voltage sequence follows the standard or agreed protocol; and the recorded PD level is read against the correct limit for that equipment class. A certificate that omits any of these proves little.
Does Sinospect sell or supply PD test instruments?
No. Sinospect specifies, arranges and witnesses PD testing as the buyer's independent representative and supplies the HV equipment itself where engaged for procurement — but it does not sell PD test instruments. The measurement is performed on the manufacturer's or test laboratory's calibrated equipment; the witness verifies the setup and records the result.
What happens if the PD level exceeds the limit during the witnessed test?
The exceedance is recorded with the measured level, the applicable limit, the voltage step and the background noise, and logged as a non-conformity. Typical supplier responses — locating the source, drying, reprocessing, repair and retest — are tracked, and the witnessed retest must pass the same criteria. Elevated PD normally supports a hold recommendation; the release decision stays with you.
Can PD witnessing be combined with witnessing the rest of the FAT?
Yes, and it usually should be. PD is one hold point inside the wider FAT scope — identity checks, routine electrical tests, function and documentation review. One witnessed visit covering the full agreed protocol is the efficient shape; this page exists because PD is the point where independent verification of the measurement conditions matters most.
08 · Getting started
What to send — start with what you have
The purchase specification and the equipment class are enough to start; the PD limit and sequence are confirmed from the governing standard where your documents do not state them.
Purchase specification or datasheet
The equipment class, ratings and the standards the units were bought against.
PD requirement, if stated
The limit, test voltage and any measurement conditions your contract sets — or the standard it defers to.
FAT protocol or test plan, if one exists
Where PD sits in the wider test sequence, so one visit witnesses the whole hold-point scope.
Purchase order and manufacturer details
Units, serial quantities and the test-bay location, so the visit can be arranged.
Expected test-readiness window
When the manufacturer expects the units on the bay, so witnessing is booked before dispatch pressure builds.
Partial information is fine. Sinospect confirms the governing limit and sequence, and what is missing, before any commitment.
Put an independent witness on your PD hold point
Send the specification, the equipment class and the expected test window. Sinospect replies within one business day with the proposed witness scope, a quotation and the earliest feasible attendance. Witnessing and specification only — the measurement runs on the laboratory's calibrated instruments, verified on the bay.