Service · Factory execution · FAT
Partial discharge (PD) test witnessing at FAT in China
Partial-discharge measurement can reveal insulation defects that a withstand test does not expose. On a buyer-direct order, Sinospect's engineer witnesses the measurement at the Chinese factory as the buyer's independent representative: circuit calibration verified, background noise recorded, voltage sequence followed, and the measured level read against the applicable contractual limit. When Sinospect supplies the equipment, the same checks are supplier-side quality control and any required independent or accredited witness remains separate.
- Measurement basis
- Project-specified IEC 60270 edition; limits from the approved product standard or purchase 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?
On a buyer-direct order, the independent witness verifies measuring-circuit calibration and background noise, confirms the voltage sequence against the approved project protocol, observes the measurement, and records the PD level against the contractual limit with the tested serial numbers. On a Sinospect-supplied order, these checks form part of supplier-side quality control; any required independent witness remains separate.
| Verification point | What is checked | Why it decides credibility |
|---|---|---|
| Circuit calibration | Calibration under the project-specified IEC 60270 edition, 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 calls for witnessed PD testing at FAT?
Equipment whose governing standard or purchase specification sets a PD limit: power and distribution transformers, MV and HV switchgear including gas-insulated assemblies, instrument transformers, and HV cable systems and accessories. For these classes, PD reveals the insulation voids, contamination and workmanship defects that survive a pass/fail withstand test.
| 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 can reveal an active insulation defect that may develop 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, witnessing that measurement makes the contractual limit reviewable.
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.
- A lender, insurer, EPC or end client requires independent evidence behind the PD line on the test certificate.
- Your own HV engineers can cover the first unit at most, and the order has many.
02 · Controls and evidence
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 under the project-specified IEC 60270 edition 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.
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.
Evidence reviewed
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 recording the units, measurements and results established at FAT.
- 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 recorded as input to the contract-defined decision, inside the signed FAT witness record.
03 · Deliverables
Deliverables issued
Deliverable
For · Engineering and procurementPD witness plan
The agreed limit, voltage sequence, measurement conditions and witness scope before the visit.
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 recommendation for the contract-defined dispatch and payment decision.
04 · Risks controlled
Risks controlled
Risk
The certificate shows a passing picocoulomb figure measured on an uncalibrated or unverified circuit.
Control response
The witness verifies and records calibration at the actual setup under the project-specified IEC 60270 edition before measurement.
Risk
Background noise close to the limit hides real discharge activity.
Control response
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.
Control response
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.
Control response
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, while the units shipping stay untested.
Control response
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.
Control response
Exceedances are logged with the limit and the supplier's response, and carry a hold recommendation until a witnessed retest passes.
05 · 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 & GIS partial-discharge witnessingThe switchgear-specific layer: which object in the assembly was tested, and the defect classes a phase-resolved pattern separates.
- 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.
06 · 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 under the project-specified IEC 60270 edition with the record made at the actual setup; the background noise is measured; the voltage sequence follows the approved protocol; and the PD level is read against the contractual limit for that equipment class. The report records this basis instead of treating the certificate alone as acceptance evidence.
Which instruments are used for PD testing?
PD measurements use the manufacturer’s or test laboratory’s calibrated equipment. Sinospect specifies, arranges and witnesses the agreed tests for buyer-direct assignments, and carries out supplier-side quality control on equipment it supplies. Any contractually required independent or accredited witness is appointed separately.
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. On a buyer-direct order, the buyer makes the release decision under its contract. On a Sinospect-supplied order, Sinospect records a supplier-side quality-control disposition; buyer acceptance and any payment or shipment consequence follow the signed contract.
Can PD witnessing be combined with witnessing the rest of the FAT?
Yes, and that is the efficient shape. PD is one hold point inside the wider FAT scope — identity checks, routine electrical tests, function and documentation review. One witnessed visit covers the full agreed protocol; this page exists because PD is the point where independent verification of the measurement conditions matters most.
07 · 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.
Send the documents available. Sinospect confirms the governing limit and sequence, and what is missing.
Arrange witnessing for your PD hold point
Send the specification, equipment class and expected test window. On a buyer-direct order, Sinospect can propose an independent China-side witness; on a Sinospect-supplied order, the work is supplier-side quality control. Sinospect acknowledges the request within one business day. When the file is sufficient, the initial written assessment follows within two business days with the proposed scope, a quotation and the earliest feasible attendance. The measurement runs on the laboratory's calibrated instruments, verified on the bay.