DC String Reverse Process — CN to UK Corridor (R14)

1. When to use this guide

Use this guide when a DC string is found to be reversed in polarity (positive and negative conductors swapped) at the string combiner or at the inverter DC input, on an asset whose modules, inverters or combiner BOM trace back to the CN to UK corridor. Common triggers: commissioning IV curve anomaly (negative Voc reading) on a LONGi Hi-MO 5 / Jinko Tiger Neo / Trina Vertex S+ string, R14 ticket logged during handover, or a polarity mismatch identified during a periodic thermographic inspection at the Sungrow SG125HV DC input.

The R14 ticket definition used in this guide is an internal maintenance ticket category scoped by the O&M provider for DC string polarity corrections on corridor-sourced assets; the exact ticket schema is held in the CMMS and is not a public document.

2. Step-by-step process

Each step has a single named owner. Inputs and outputs are literal document filenames held in the project EDMS or shared drive. Equipment basis: LONGi Hi-MO 5 (LR5-72HBD) module family, Jinko Tiger Neo (JKM-N-78HL4R-V) family, Trina Vertex S+ (TSM-NE19RC10) family, Sungrow SG125HV string inverter, K2 Systems SingleRail / Dome 6 mounting and combiner hardware, Stäubli MC4-Evo 2 connector family. Specific manual revisions and torque values are confirmed at RFQ against the OEM datasheet PDF on file.

Equipment basis (real, publicly marketed models)

Where the site uses Huawei SUN2000-330KTL-H1 string inverters or Growatt MID 30-50KTL3-XH inverters instead of Sungrow, the same step sequence applies; torque values, Voc limits and SOC handling follow the OEM datasheet and inverter manual named at RFQ. These are listed as model assumptions, not site claims.

3. Document checklist (literal filenames)

• R14_ticket_intake.json — incoming ticket payload from CMMS.
• R14_scope_confirmation.pdf — O&M scope confirmation, signed.
• corridor_doc_index.xlsx — index of corridor supply documents.
• corridor_doc_pack_<site>_<date>.zip — zipped corridor document pack.
• BOM_<site>.xlsx — bill of materials, modules and combiner.
• MTC_<module_model>.pdf — manufacturer test certificate per module model (LONGi Hi-MO 5 / Jinko Tiger Neo / Trina Vertex S+).
• CIQ_<shipment>.pdf — China Inspection and Quarantine certificate.
• BoL_<shipment>.pdf — bill of lading for the CN to UK shipment.
• UKCA_DoC_<product>.pdf — UKCA Declaration of Conformity — placeholder: site-specific issuer and document number added to the on-site file at handover; not a public per-asset number.
• MCS_certificate_<site>.pdf — MCS cert where the asset is MCS registered — placeholder: MCS installation standard is MCS 021 (PV systems) at MCS standards directory; the per-site certificate number is held in the project file.
• RAMS_R14_v<ver>.pdf — risk assessment and method statement for R14.
• LOTO_permit_<date>.pdf — permit-to-work for lock out / tag out.
• asbuilt_single_line.pdf — as-built single line diagram.
• polarity_check_<string>.csv — polarity verification readings.
• OEM_MC4_datasheet.pdf — Stäubli MC4-Evo 2 connector datasheet with torque spec.
• recrimp_record_<string>.csv — recrimp event log per string.
• torque_log_<string>.csv — torque log per connector.
• baseline_IV_<string>.csv — baseline IV trace.
• post_reverse_IV_<string>.csv — post-reverse IV trace.
• R14_closeout_<site>_<date>.zip — closeout evidence pack.

4. Timeline (indicative, single shift, dry conditions)

Times are indicative for one trained pair under dry, daylight conditions. Add time for roof access, permit-to-work issuance, and weather contingency.

5. Indicative cost slots

Cost slots are indicative only and labelled as such. No firm prices are stated. UNCERTAIN rows have no currently named source and are placeholders for RFQ response.

6. HS code and duty block

Spare parts and tooled consumables for a DC string reverse may be declared under headings relating to photovoltaic cells, boards, modules, or electrical connectors. Rates are marked PENDING until verified against the UK Global Tariff lookup at https://www.trade-tariff.service.gov.uk/.

Verify exact classification and rate on the UK Global Tariff lookup before declaring. Rates may vary under preferential origin treatment where the supplier has a verified statement on origin.

7. Health, safety and competence

1. Work must be carried out by personnel competent for DC work at the system maximum DC voltage (up to 1500 V DC for the Sungrow SG125HV string basis), in line with site RAMS and the BS 7671 IET Wiring Regulations for any LV AC interaction at the inverter.
2. PPE must meet the site standard, which references the IEC 61730 module-safety qualification framework (IEC webstore catalogue for related standards, e.g. IEC 61851-1:2017 for adjacent EV charging scope; cite IEC 61730-1 / IEC 61730-2 module-safety parts by full designation in the site RAMS).
3. Live DC work is prohibited; isolation and LOTO are mandatory at the inverter, combiner and DC isolator.
4. Tools and connectors used must match OEM datasheet specifications; MC4-Evo 2 torque values are taken from the Stäubli MC4-Evo 2 datasheet.

Note: IEC 60950 is withdrawn (superseded by IEC 62368-1) and is not cited as a PPE source for DC PV work in this guide. Module electrical safety is governed by IEC 61730-1 and IEC 61730-2.

8. Frequently asked questions

What is an R14 ticket in this context?

An internal maintenance ticket category used by the O&M provider to log DC string polarity corrections on corridor-sourced assets. The exact ticket schema is held in the CMMS and is not a public document; it is classified in this guide as a model assumption.

Does a string reverse always require module-side re-termination?

Not always. Where the polarity error is at the combiner landing only, correction can be made at the combiner. Where module leads are reversed at the LONGi Hi-MO 5 / Jinko Tiger Neo / Trina Vertex S+ MC4-Evo 2 output, re-termination at the module MC4-Evo 2 is required.

Will a polarity reverse affect inverter warranty?

OEM inverter warranties (Sungrow SG125HV and equivalent Huawei / Growatt families) generally treat polarity correction as a recoverable maintenance event provided it is logged with evidence (IV trace, torque log, photos). Confirm with the specific OEM manual named at RFQ; inverter SOC handling during the outage follows the OEM datasheet.

Can the same guide be used for AC wiring corrections?

No. AC corrections are a different scope with different isolation rules (BS 7671 IET Wiring Regulations apply) and should follow the site AC switching procedure.

How is the corridor proven for a string?

By tracing the affected modules or combiner back through the BOM, MTC, CIQ and bill of lading to the CN to UK shipment.

9. Next steps

Send a structured RFQ to TradVolt at the /rfq/ intake with the corridor documents attached. This page is the printable checklist substitute; no separate PDF is published.