DG Expansion and Synchronisation: Set the Backup Boundary First

Adding a second or third DG set is the usual answer when a factory outgrows its backup. Whether it delivers resilience depends on decisions made before the set is ordered: which loads the sets must carry, whether they can accept those loads in the steps in which they will be switched on, how the sets synchronise and share load, and what happens when one of them trips. Two sets that cannot run together — or that run together but take each other down — give less backup than the kVA on their nameplates suggests.

Use the DG sizing calculator for a first estimate of kVA; this guide covers the boundary and the engineering checks around it.

Set the backup boundary: essential and non-essential loads

Start with a schedule of every load the DG system might feed, and put each into a category before anyone sizes a set:

• Life-safety loads — fire pumps, fire alarm, emergency lighting, smoke control. Their power-supply arrangements follow fire-safety requirements and the fire authority; confirm them separately instead of folding them into the general DG plan.
• Process-critical loads — those whose interruption scraps product or damages equipment. Many also need a UPS for the seconds before the DG picks up (see UPS planning for critical factory and office loads).
• Essential but restartable — compressors, chillers, pumps and lighting that can wait seconds or minutes.
• Non-essential — loads that stay off on DG, or are shed first when capacity runs short.

Essential and non-essential loads checklist

Check the OEM's transient limits, not just the kVA

A DG set's kVA rating says what it can carry once loaded; it does not say how much load it can pick up in one step without the voltage and frequency dipping too far. That is a transient question:

• Rating type. ISO 8528-1:2018 defines several power ratings — continuous, prime, limited-time running, emergency standby and others — each with limits on running hours and average load. A set rated only for standby duty is the wrong basis for an expansion that will spend long periods on DG.
• Transient performance. ISO 8528-5 (current edition 2025) sets the requirements for voltage and frequency deviation and recovery after a load step, grouped into performance classes (G1 to G4 in recent editions; check which edition your OEM quotes). Ask the OEM for the set's load-acceptance data — the largest step it accepts within its class — and plan the restart sequence around it.
• Motor starting. Direct-on-line starting draws several times running current, which on a DG becomes a deep voltage dip. Soft starters or drives reduce the step but add non-linear load.
• Non-linear and regenerative loads. Large drive and UPS loads distort the voltage and heat the alternator, and a regenerative load on a lightly loaded set can trip it on reverse power. Both belong in the sizing data you give the OEM.

Synchronisation: one system, not two purchases

Running sets in parallel needs a synchronising and load-sharing system: an automatic synchroniser that matches voltage, frequency and phase angle before the breaker closes, a check-synchronising relay that blocks an out-of-phase closure, and controls that share real power (through the governors) and reactive power (through the voltage regulators) in proportion to each set's rating. Sets bought years apart, from different manufacturers or with different controllers, can be paralleled — but only when the synchronising panel is engineered for that pairing and the governors and regulators are compatible.

• Neutral earthing. Regulation 43 of the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023, which covers systems above 50 V and up to 650 V, requires the neutral point of every generator and transformer to be earthed by at least two separate and distinct connections. Where generators above 650 V run in parallel with neutral switching, Regulation 47 requires an interlock so that a generator breaker cannot close unless one of the neutrals is connected to earth. Paralleled low-voltage sets with several earthed neutrals can also circulate current between them, so the neutral scheme is a design decision.
• Fault level. Paralleled sets feed a fault together. Check the breaking capacity of the DG and LT switchgear for the combined fault current (see fault-level coordination failures).
• Grid and solar. Where a DG is not meant to run in parallel with the grid, interlocks must prevent it; for installations above 650 V, Regulation 47 requires supplies not intended to operate in parallel to be interlocked against inadvertent paralleling or backfeed. Paralleling with the grid needs the DISCOM's permission and protection designed for it. Rooftop solar on a DG-fed bus needs a controller that limits solar output so the sets never see reverse power (see factory microgrids: solar, DG and grid).

Synchronising protection to specify

• Check-synchronising, blocking closure outside voltage, frequency and phase-angle limits.
• Reverse power, so a set whose engine fails is not motored by the others.
• Loss of excitation.
• Over- and under-voltage; over- and under-frequency.
• Overcurrent and earth fault on each set, coordinated with the outgoing feeders.

For installations above 650 V, CEA 2023 Regulation 47(2) requires earth-fault protection on generators of 100 kVA and above, and restricted earth-fault or differential protection on generators of 1,000 kVA and above. Many factory DG sets generate at 415 V; confirm with your Electrical Inspector what applies to yours.

Load shedding: decide now what happens when one set trips

Two synchronised sets sized so that both are needed for the essential load (N) are not redundant: when one trips, the other is overloaded unless load is shed quickly. True N+1 means any one set can trip and the rest still carry the essential load. Between the two, a load-shedding scheme — tripping non-essential feeders in priority order on frequency or kW signals — is what keeps the surviving sets on line.

Witnessed tests before you accept the system

• Factory tests at the OEM and the synchronising-panel builder, with recorded results.
• Site acceptance tests: dead-bus closing; synchronising to a live bus; load sharing at several load levels, checking both kW and kVAr balance; a trip of one set with load shedding operating; mains failure and restoration through the automatic changeover; and a load-bank run if site load cannot be used.
• Statutory inspection: under CEA 2023 Regulations 34 and 45, generating units above the capacity notified by the appropriate government (usually the state) must be inspected by the Electrical Inspector before commissioning, and Regulation 48 requires site tests to the relevant standards before apparatus above 650 V is commissioned. See what the Electrical Inspector checks on DG sets.
• Emissions, noise and fuel: a new set must meet the emission norms in force for its rating (see CPCB IV+ DG sets), and a larger fleet changes fuel storage and its siting (see diesel storage safety distances).

Keep the test records with the protection settings; they are the baseline for every later fault investigation. Secured Engineers' electrical infrastructure scope covers DG sets with AMF panels and synchronisation; to have an expansion plan reviewed, request a scope review.

Restoring power is not the same as restarting production

Develop a restart sequence around process dependencies. Identify what must be running before production equipment is released, what can wait and what needs an operator check. Coordinate the sequence with machine suppliers and the electrical design; automatic restart is not appropriate for every machine or process.

The consultant should distinguish electrical restoration from operational readiness. A live distribution board does not prove that cooling, compressed air or control systems are ready. Define the signals and permissions that connect those stages. Verification should be planned safely, with the operating team involved, rather than improvised through a disruptive whole-site outage.

What to confirm before the next decision

• Map utility dependencies for each production line.
• Identify permitted automatic restarts and required manual approvals.
• Agree how recovery time will be measured during controlled testing.

Ask for this review output: A recovery dependency schedule identifying utility readiness, machine permissions, manual checks and an agreed method of verifying restart performance.

Discuss the consultancy scope

Provide the critical equipment list and outage history. Request a coordinated utility and production restart strategy. Request a scoped MEP review. Disciplines, inputs and deliverables are agreed before an appointment; installation and authority decisions are separate.

Background guidance: Schneider Electric — Characteristics of Particular Sources and Loads, Schneider Electric — Downstream LV Network Protection. These inform the review principles; local requirements, project applicability and any necessary approvals must be confirmed by the responsible specialists.

Critical-load segregation should follow the business recovery plan

Turn each proposed backed-up load group into a business recovery record. Name the process owner, the consequence of an interruption, the time the operation can tolerate without that service and the utilities needed to recover. A production line may have power yet remain unavailable because its cooling pump, air supply or controller is excluded. Life-safety obligations must be established separately by the responsible designers; commercial priorities cannot override them.

Have operations and design teams review the record together before procurement. Capture the basis for loads deliberately left off backup, and the conditions that would reopen that decision: a changed product, longer shift, different outage expectation or new machine. This gives later capacity changes a traceable reason instead of letting every department label its equipment critical.

What to confirm before the next decision

• Identify the consequence of losing each load or utility.
• Include auxiliaries needed for safe shutdown and restart.
• Record what the backup system deliberately does not support.

Ask for this review output: A load-to-recovery record with a business owner, outage consequence, permitted interruption, supporting utilities and explicit exclusions for each load group.

Discuss the consultancy scope

Share the production recovery priorities and essential services list. Request a critical-load classification workshop. Request a scoped MEP review. Disciplines, inputs and deliverables are agreed before an appointment; installation and authority decisions are separate.

Background guidance: Schneider Electric — Characteristics of Particular Sources and Loads, Schneider Electric — Downstream LV Network Protection. These inform the review principles; local requirements, project applicability and any necessary approvals must be confirmed by the responsible specialists.

Sources and how to verify

• Central Electricity Authority, CEA (Measures relating to Safety and Electric Supply) Regulations, 2023, notified 8 June 2023: Regulations 34, 43, 45, 47 and 48.
• International Organization for Standardization, ISO 8528-1:2018 (application, ratings and performance) and ISO 8528-5:2025 (generating sets).

Confirm locally: the capacity above which your state requires the Electrical Inspector to inspect generating units, the emission norms in force for the set's rating, whether the DISCOM permits any grid paralleling, and the OEM's load-acceptance data for the exact set and controller.