Factory electrical interface register: close MV/LV and OEM boundaries before procurement

An FDI factory electrical project can have sound individual technical drawings and still fail at the points between contracts. The imported production machine has a defined incoming supply; the Vietnamese electrical contractor has a feeder drawing; the civil contractor has a foundation; and the utility or industrial-park distributor has a connection agreement. The design can fall apart when no document identifies who owns the last five metres of cable, the gland plate, neutral arrangement, control interlock, trench opening or final energization record.

A useful electrical interface register is not a directory of subcontractors. It is a controlled decision log connecting a physical boundary to the correct design revision, an accountable producer, an acceptance criterion and a delivery milestone. This guide explains how to build that register across MV, LV and process equipment, with a worked hypothetical example, sample hold points and an RFQ structure for industrial owners and EPC procurement teams in Vietnam.

Prepared for QuangAnhcons · Editorial focus: cross-package electrical delivery for factories · Sources checked: 10 October 2026. The example roles, sizes and dates are illustrative discussion tools and not a real project contract.

Why a package boundary needs an equipment point, not a vague phrase

Statements such as “EPC to provide complete power” or “machine supplier to connect all equipment” leave too much open. An engineer needs a physical point or document deliverable that can be inspected. The upstream endpoint could be the transformer LV bushings, MSB outgoing terminals, an MDB incoming ACB or the machine control cabinet incomer. At every boundary, specify whether the responsibility includes supply, design, installation, cable termination, protective coordination, testing, labeling and acceptance. A contract may assign different aspects to different parties; the register must not imply that every aspect of a physical asset belongs to whoever supplies its hardware.

Start from the single-line diagram rather than the organization chart. Identify the electricity seller’s metering/PCC interface, MV owner boundary, RMU/transformer supply scope, LV bus and feeder circuits, distribution boards, machine incomers, BMS/SCADA communications, earthing/PE continuity, and integrated tests. For a new factory, add the civil interface for transformer foundations, trenches, wall openings and cable risers. For a brownfield factory, add existing-system ownership, planned shutdown windows and conditions for restoration of service. Treat site access, lifting and energized-work constraints as delivery interfaces, not invisible preliminaries.

The source of authority matters. The landlord, factory owner, EPC, distribution utility and equipment supplier may each control different interfaces. A register cannot assign authority on their behalf when contracts or approved documents say otherwise. Label an unresolved allocation PROPOSED, not APPROVED, and record who is required to make the decision.

Minimum columns that make an interface register actionable

A robust row includes: unique interface ID; system/area; upstream and downstream package; drawing and revision; physical boundary; required technical data; producer; reviewer; contractual approver; downstream consumer; required date; status; decision; evidence link; residual assumption; change event and last update. Additional columns can track tag numbers, source type, safety criticality and testing witness. Avoid placing dozens of possible questions in free-text cells without a responsible person or due date: the register becomes a list nobody can close.

Use a finite status vocabulary: OPEN, INPUT_RECEIVED, UNDER_TECHNICAL_REVIEW, APPROVED_WITH_ACTIONS, CLOSED, REOPENED_AFTER_CHANGE. The last status matters: an OEM changes machine current or terminal location after IFC drawings are released, and yesterday’s closed boundary can become wrong. An audit trail should preserve who closed it and which revision made that close-out valid.

Worked interface: a production machine supplied at 400 V

Assume an OEM machine has a three-phase 400 V incoming panel rated at 160 A, with a 25-metre intended cable route from an existing factory MDB. The two parties agree only that “power will be brought to the machine”. Nothing yet states whether the OEM has a neutral requirement, the terminal bar accepts two parallel conductors, the machine has a built-in main isolator, the protection device must have a specific characteristic, or the cable entry is top or bottom. A tender that locks the cable cross-section now is not only uncertain in price—it risks a physical installation that cannot terminate safely.

At an idealized balanced power factor of 0.90, the electrical active power corresponding to 160 A at 400 V would be P = sqrt(3) × 400 × 160 × 0.90 / 1000 ≈ 99.8 kW, with apparent power S ≈ 110.9 kVA. This is a simple arithmetic check to make the interface scale intelligible. It does not establish that the machine will actually draw that steady current, nor specify its inrush, harmonic spectrum, cable grouping, conductor temperature rating or short-circuit protection. The input 160 A is a stated fictional panel rating, not evidence that 160 A is its measured operating demand.

The physical length and purchased conductor length must also be distinguished. A 25-metre route may require a three-phase and neutral system plus separate PE depending on the approved design, different termination allowances, spare length and different cross-sectional selections. The theoretical conductor quantity is route length × number of required conductors/runs; a 25 m route does not mean that a procurement line for multiple single-core conductors should order only 25 m total. The detailed conductor take-off cannot be finalised until the actual number and arrangement of conductors, routing, grouping, voltage drop and fault/earthing conditions are approved.

The appropriate response is a short interface RFI with evidence fields: (1) confirm full-load/maximum input current and starting behaviour; (2) confirm neutral and PE topology; (3) provide exact terminal/gland drawing with lug sizes and direction of entry; (4) identify main isolator and upstream protective coordination requirement; (5) declare earthing/EMC requirements; (6) confirm responsibility for termination tests and first energization. Each question has a recipient and decision deadline, not a blanket “contractor to verify on site”.

Interface engineering is not only about equipment—it changes the schedule

Late interface decisions can generate rework that is more expensive than the component itself. If the civil contractor casts a plinth before receiving verified transformer footprint and anchor data, a later vendor change may require demolition or redesign. If cable routes are closed before machine coordinates are frozen, feeder length and termination locations can change. If utility ownership is unresolved until energization week, even technically complete equipment may not be operable. Put the work-package dependency into the register so a decision is due before an irreversible activity.

These are illustrative work packages, not guaranteed durations or statutory approval steps. Actual local electricity connection arrangements, contracts, competent parties and applicable Vietnamese requirements must be confirmed separately. A contractor cannot legitimately solve a missing approval by simply assigning it to itself in a spreadsheet.

Turn the register into RFQ and change-control discipline

The register should accompany the BOQ. Give bidders the same current drawing revisions and the same open questions. Request pricing both for a confirmed baseline and for separately identified provisional items where the scope is genuinely unknown. A provisional cable route, unidentified CT chamber, unknown incoming panel terminal or uncertain night shutdown should be labelled provisional and linked to a measurable adjustment mechanism. Comparing a cheap quotation with unlimited exclusions against a complete quotation is not a meaningful tender evaluation.

Changes after award should state which interface ID moved, which design documents changed, whether equipment has been manufactured, the effect on safety/protection tests, expected cost/time consequence and who authorized the change. An “approved” drawing is not permanent if a machine vendor issues a changed rev after procurement. In that case, reopen the interface, assess the affected cable/CB/protection or control and obtain a new close-out record before the work advances.

Digital tools can support this process, but a detailed platform is not essential. A controlled spreadsheet with IDs, owners, drawing links, dated decisions and a weekly action meeting often suffices for a smaller package. Larger EPC programs may integrate the same fields into a document management and RFI workflow. What matters is a consistent single owner for each decision, not necessarily one contractor performing all the work.

Quality checks, examples of errors and resolution paths

One recurring failure is to record CLOSED when the owner merely acknowledges receipt of a supplier email. Receipt is not technical acceptance. Another is to place a generic diagram under the equipment boundary, then later discover that the terminal height or bending space differs from the purchased cabinet. A third is to treat a successful transformer insulation test as proof that downstream machine protection has been fully coordinated. Those are separate technical deliverables with different evidence and test conditions.

For each critical interface, verify that the diagram tag exists physically, the latest document is known, the recorded producer and checker are real and the acceptance record corresponds to the specific revision. Reinspect high-risk rows after site changes, especially live-plant cutovers or changes to supply owner boundaries. Keep civil/electrical, data/electrical and OEM/EPC boundaries visible instead of reporting only overall percent completion.

If a dispute arises, the register should show what was requested, what was provided, what was accepted and under which contract or design basis the decision was made. It should not pretend to amend legal contract allocations on its own. A technical interface row is useful evidence for project management; the signed contract and applicable legal approvals still govern responsibilities.

References and applicability

Related resources and next action

For the high-level allocation see Who owns each electrical design deliverable?. For comparable scope in bidding, use What are you really buying in an electrical EPC tender?. For procurement sequencing consult Long-lead electrical equipment: plan a factory schedule. Each addresses a different part of the project decision, while the interface register connects them at the equipment boundaries.

The QuangAnhcons substation commissioning archive video illustrates practical site work and energization context; it does not prove the boundary allocations in this fictional example.

An owner preparing a split-package electrical RFQ can send the latest SLD, package list, OEM machine drawings, scope notes and open interface questions to projects@quanganhcons.com or +84 919 758 191. QuangAnhcons can discuss a defined scope-review and BOQ preparation service without claiming that unapproved project responsibilities have already been assigned.

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