SPD Factory Audit Checklist: 10 Items to Verify Before Approving a Supplier
Qualifying a new SPD manufacturer involves more than reviewing a product datasheet and a certificate scan. The certificate confirms that a specific sample passed a test under controlled conditions. The audit confirms that the factory producing your order operates the processes, equipment, and controls needed to deliver that standard consistently across every batch.
This checklist covers the ten areas most likely to reveal the gap between a supplier's claims and their actual production capability. It applies equally to in-person factory visits and documentary audits conducted remotely.
The Checklist
What to verify:
- Does the supplier hold original IEC 61643-11 test reports — not scanned copies or certificates alone?
- Are reports issued by an accredited third-party laboratory (CNAS, DAkkS, UKAS, or equivalent)?
- Does the report cover the exact model being ordered, including terminal configuration and protection mode (L-N, L-PE, N-PE)?
- Are the reports current? IEC 61643-11:2011+A1:2017 is the current edition; reports referencing superseded editions need clarification.
Why it matters:
A CE certificate or IEC mark on the product label only confirms that a declaration was made. The test report is the evidence. Reports from accredited laboratories include the laboratory's accreditation number and scope, which can be independently verified. Reports issued by the manufacturer's own in-house laboratory do not meet third-party verification requirements for most B2B procurement standards.
What to request:
Full test report PDF, including the laboratory cover page with accreditation number, test results for all applicable clauses (including thermal stability and endurance tests), and the specific test class (Type 1, 2, or 3; Imax; In).
What to verify:
- Are MOV components manufactured in-house or sourced from external suppliers?
- If sourced externally, who are the MOV suppliers, and what incoming inspection process applies?
- Is 100% parametric testing (varistor voltage V₁mA, clamping voltage) performed on incoming MOV batches?
- Are batch traceability records maintained — linking finished SPD serial numbers back to specific MOV production lots?
Why it matters:
The MOV is the primary protection element in most SPDs. Batch-to-batch variation in varistor voltage directly affects the device's clamping performance and end-of-life behaviour. Suppliers who source MOVs from spot-market vendors, or who do not perform incoming inspection, cannot guarantee consistent performance across production runs. For OEM orders where the SPD carries your brand, the MOV supply chain is part of your product quality liability.
What to request:
MOV supplier documentation or, for in-house production, a summary of process controls. Incoming inspection procedure and sample test records from recent batches.
What to verify:
- Does every SPD model include a thermal disconnector (also called a thermal cutoff or TPNR)?
- Is the thermal disconnector integrated into the MOV assembly, or is it a separate component in series?
- What is the rated operating temperature of the thermal disconnector, and how was it validated?
- Does the disconnector trigger the fault indication mechanism (window indicator or remote signalling contact)?
Why it matters:
IEC 61643-11 requires compliant SPDs to include a means of disconnection that operates before the MOV reaches a destructive temperature during thermal runaway. This is the primary safety mechanism that prevents an end-of-life MOV from causing a fire. SPDs sold without a verified thermal disconnector — or with a disconnector that does not reliably trigger the status indicator — create liability for the installer and the distributor. This item requires physical inspection of a sample unit, not documentation review alone.
What to request:
Cross-section or exploded diagram showing thermal disconnector placement. Type test records showing thermal disconnector activation temperature and response time.
What to verify:
- What soldering process is used for PCB assembly — wave soldering, reflow, or manual?
- Is the process controlled by documented temperature profiles (for reflow) or by operator qualification records (for manual soldering)?
- What workmanship standard applies? (IPC-A-610 is the most widely used reference for electronics assembly.)
- Is in-process inspection performed at the soldering stage, and what are the rejection criteria?
Why it matters:
Cold solder joints and insufficient solder coverage are common causes of SPD field failures unrelated to surge events. Under thermal cycling from repeated surge absorption, marginal solder joints develop intermittent resistance or open completely. These failures are difficult to detect in final inspection because the device may pass a functional test at room temperature but fail under operating conditions. Process control at the soldering stage is more reliable than end-of-line detection.
What to request:
Soldering process specification or reflow profile documentation. Sample of in-process inspection records. Factory may demonstrate the soldering process during an in-person visit.
What to verify:
- Does the factory have surge test equipment capable of generating the test waveforms required by IEC 61643-11 (8/20 μs combination wave generator)?
- Is the test equipment calibrated, with current calibration certificates traceable to national standards?
- Are production samples subjected to endurance testing — or is testing limited to type-test samples submitted to external laboratories?
- What percentage of production units undergo electrical testing before shipment?
Why it matters:
Type test results confirm that the design meets the standard. Production test results confirm that specific units leaving the factory perform as designed. Suppliers who rely solely on type tests and perform no electrical verification on production units cannot provide meaningful quality assurance for individual orders. At minimum, 100% continuity and functional testing at rated voltage should be standard practice.
What to request:
List of test equipment with calibration status. Description of production testing scope — which tests are applied to 100% of units, which to sampling.
What to verify:
- Is the production area designated as an ESD-protected area (EPA)?
- Do operators handling PCBs and sensitive components use grounded wrist straps or ESD-safe footwear?
- Are ESD-sensitive components stored in appropriate packaging before and after assembly?
- Is ESD equipment (wrist strap testers, surface resistance meters) tested regularly, with records?
Why it matters:
SPDs contain semiconductor components — particularly in signal line protectors, Type 3 devices, and SPDs with remote signalling electronics — that can be damaged by electrostatic discharge during handling. ESD damage is latent: the device passes functional test but fails prematurely in service. Suppliers without formal ESD control are unlikely to produce consistent results for signal-line or sensitive electronics protection applications.
What to request:
ESD control procedure document. Record of wrist strap testing frequency. For in-person visits, observe whether EPA signage, heel straps, and grounding stations are present and in use.
What to verify:
- Are finished SPDs marked with a serial number or date code that allows traceability back to the production batch?
- Does the factory maintain production records linking serial numbers to: MOV batch, PCB assembly batch, test results, and inspection records?
- How long are production records retained?
- In the event of a field failure, can the supplier identify all other units from the same production batch?
Why it matters:
Traceability is the foundation of any corrective action process. Without it, a field failure cannot be analysed at root cause, and affected units cannot be identified for recall or inspection. For B2B OEM products carrying your brand, the absence of traceability at the supplier level means that any quality incident becomes a full-production exposure rather than a bounded batch issue.
What to request:
Example of a production batch record (redacted if necessary). Description of the serialisation process and record retention policy.
What to verify:
- Does the factory perform outgoing quality control (OQC) inspection before shipment?
- What AQL level applies — 1.0, 1.5, or 2.5?
- Are OQC inspection records retained and available to buyers on request?
- Does the factory permit third-party pre-shipment inspection (PSI) by agencies such as SGS, Bureau Veritas, or Intertek?
Why it matters:
AQL 2.5 is a common standard for industrial components. For surge protective devices, where end-user safety is directly at stake, AQL 1.5 or 1.0 is more appropriate. A supplier's willingness to permit third-party PSI, and to share their own OQC records, is a reliable indicator of process confidence. Suppliers who resist third-party inspection without a clearly stated reason introduce procurement risk that is difficult to quantify until a problem occurs.
What to request:
Most recent OQC inspection report for a similar product. Written confirmation that third-party PSI is permitted, and the name of the inspection standard used.
What to verify:
- Does the factory hold a current ISO 9001 certificate, issued by an accredited certification body?
- Are export licences and customs registration current for your target market?
- For CE-marked products sold in Europe, is the Declaration of Conformity issued under the correct EU directives (Low Voltage Directive 2014/35/EU, EMC Directive 2014/30/EU)?
- Does the factory's business registration match the entity named on the certificate and test reports?
Why it matters:
ISO 9001 certification confirms that a quality management system is in place and has been audited by a third party. It does not guarantee product quality — but its absence in an established manufacturer is a flag. More importantly, the entity names on the business licence, ISO certificate, CE declaration, and test reports should be consistent. Discrepancies may indicate that certificates belong to a related entity, not the factory actually producing the goods.
What to request:
ISO 9001 certificate (front page and scope page). Business registration document (unified social credit code for Chinese manufacturers). Declaration of Conformity for CE-marked products.
What to verify:
- Does the factory maintain a signed golden sample for each OEM or custom product, agreed before production begins?
- Is the golden sample stored in a controlled manner and retained for the duration of the supply relationship?
- Is the golden sample referenced during production and outgoing inspection?
- What is the process if production units deviate from the golden sample specification?
Why it matters:
The golden sample is the physical contract between buyer and supplier on what "correct" looks like. Without a retained, agreed sample, disputes about colour, finish, label placement, component substitution, or assembly variation have no objective reference point. For custom-branded OEM products, the golden sample is especially important because it defines what your brand is associated with. A supplier without a formal golden sample process cannot offer meaningful quality assurance on first production runs or repeat orders.
What to request:
Confirmation of the golden sample process in writing, including storage responsibility, retention period, and the procedure when deviations are found.
Summary Table
| # | Item | Key Question | What to Request |
|---|---|---|---|
| 1 | IEC Test Reports | Original, accredited lab, correct model? | Full test report PDF with lab accreditation number |
| 2 | MOV Sourcing | In-house or external? Incoming inspection? | MOV supplier docs + incoming inspection records |
| 3 | Thermal Disconnector | Present, verified, linked to indicator? | Cross-section diagram + type test activation records |
| 4 | Soldering Process | Controlled process, in-process inspection? | Reflow profile or process spec + inspection records |
| 5 | Test Equipment | Calibrated surge generator? Production testing scope? | Equipment list + calibration certificates |
| 6 | ESD Controls | EPA in place, equipment tested? | ESD procedure + wrist strap test records |
| 7 | Traceability | Serial number to MOV batch linkage? | Example batch record + retention policy |
| 8 | AQL Inspection | OQC records available? PSI permitted? | Recent OQC report + PSI confirmation in writing |
| 9 | Factory Documents | ISO 9001, export licence, entity consistency? | ISO cert + business registration + DoC |
| 10 | Golden Sample | Retained, referenced, deviation process? | Written golden sample policy confirmation |
How to Use This Checklist
For a remote documentary audit, request items 1, 2, 5, 7, 8, 9, and 10 as documents before committing to a purchase order. Items 3, 4, and 6 are best verified during an in-person visit or through a third-party factory audit service.
Not every supplier will pass all ten items on a first review — particularly smaller manufacturers who may have informal versions of these controls rather than documented procedures. The goal is not to find a perfect score, but to identify which gaps are process maturity issues (correctable over time) versus fundamental capability gaps (not correctable without significant investment). A supplier who acknowledges gaps and has a plan to address them is more trustworthy than one who claims to pass every item without supporting documentation.
For OEM and custom-branded orders, items 2, 3, 7, and 10 are non-negotiable. These four areas are where undisclosed substitutions and quality shortcuts most commonly occur — and where the liability most directly transfers to the brand whose name appears on the product.
Yes. A documentary audit — reviewing certificates, test reports, inspection records, and quality procedures remotely — can complete items 1, 2, 5, 7, 8, 9, and 10 from the checklist. Items 3 (thermal disconnector), 4 (soldering process), and 6 (ESD controls) are best verified through physical inspection or a third-party audit service.
A type test is performed once on a representative sample at an accredited laboratory to confirm the product design meets the standard. A production test is performed on units as they come off the line to verify that specific production batches match the tested design. Suppliers who rely solely on type testing and perform no electrical verification on production units cannot provide meaningful batch-level quality assurance.
AQL 2.5 is common for industrial components. For surge protective devices, where end-user safety is directly at stake, AQL 1.5 or 1.0 is more appropriate. The AQL level should be confirmed in writing before the first order.
A golden sample is the agreed physical reference unit that defines what 'correct' looks like for a production run. Without a retained golden sample, disputes about colour, finish, label placement, or component specification have no objective resolution point. For OEM products carrying your brand, the golden sample is especially important.
For safety-critical components like SPDs, a minimum of 5 years is reasonable. Records should link serial numbers to MOV batch, PCB assembly batch, test results, and inspection records, enabling root-cause analysis if a field failure occurs.
