- October 24, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
An automatic transfer switch (ATS) test asks one blunt question: when the grid fails, does the generator start and carry the building? After a Retrofit Emission Control Device (RECD) is fitted, that question deserves a fresh answer. The exhaust path has changed, a new controller has joined the electrical system, and new alarms may now flow to the building management system (BMS). None of that should stop the set from doing its main job, but you only know for sure once you have tested it.
The difficulty is that an ATS test involves more people than most facility jobs. The electrical contractor operates the switchgear, the DG maintainer watches the engine, the RECD supplier watches the device, the BMS contractor watches alarms, and the owners of critical loads (server rooms, lifts, medical equipment, cold rooms) need to know exactly when power will blink. This post sets out who coordinates what, then walks through a test sequence, a rollback plan and a worked Sunday-morning scenario where something did go wrong.
The Coordination Matrix
Start here. Before anyone writes a method statement, agree this matrix in a short meeting and circulate it. Each party has a job before, during and after the test.
| Party | Before the test | During the test | After the test |
|---|---|---|---|
| Facility team | Fix date and window, issue notices, chair the planning meeting, appoint a test lead | Run the clock, hold the go/no-go call, keep the occupant contact line open | Collect records, close actions, decide on retest |
| Electrical contractor | Review ATS settings (delays, transfer timing), check isolation points, confirm how mains failure will be simulated | Operate the incomer or simulation switch, read transfer times, watch voltage and frequency | Report transfer times and any switchgear faults |
| DG maintainer | Confirm engine health, fuel, batteries, coolant heaters; state engine-protection limits | Watch start, load pick-up and engine parameters; holds authority to stop the engine | Record running hours and any engine observations |
| RECD supplier | Confirm controller power source and alarm outputs; brief on expected readings | Watch controller status, pressure and temperature readings through each transfer | Confirm logs captured; report any RECD events |
| BMS contractor | Confirm which RECD and DG points are mapped; put alarm routing in test mode if agreed | Watch alarm arrival, timestamps and communication status | Export alarm log; restore normal routing |
| Critical-load owners | Confirm UPS autonomy, plan safe states, nominate a contact | Stand by at their equipment, report problems immediately | Confirm all equipment recovered normally |
Two principles sit behind the matrix. First, one test lead makes the go/no-go calls; everyone else reports to that person. Second, the DG maintainer can stop the engine at any point for engine protection, and that overrides the schedule. Responsibility splits are discussed more broadly in customer and installer responsibilities in an RECD project.
Why ATS Testing Matters After an RECD Retrofit
A load-bank test or a manual run shows the RECD working on a warm, steadily loaded engine. An ATS event is harsher and more realistic. Four things are different.
Cold start straight to load
In a real outage, the engine starts from cold and takes a large block of load within seconds. Exhaust temperature is low, flow jumps quickly, and any restriction in the exhaust path shows up as the engine works to accelerate under load. The DG maintainer should watch for slow pick-up, deep frequency dips or unusual smoke. These may have causes unrelated to the RECD, so judge them against the set’s pre-retrofit behaviour where that is known.
Transfer transients
The ATS opens one source and closes another. Between them there is a dead period, and on closure there are voltage and frequency transients. Electronic equipment that is not on a UPS sees all of this. The RECD controller is one such piece of equipment unless it has been deliberately protected.
Controller power
Where does the RECD controller get its power? If it is fed from a circuit downstream of the ATS, it loses power during the dead period, then comes back when the DG closes on. That may be acceptable if the controller reboots quickly and keeps its logs, or it may cause a reset, a lost record of the start event and a communication alarm. The questions to ask are covered in RECD controller and heater power supply requirements and in RECD monitoring during power failure.
Alarms during transfer
During a transfer, the BMS may see a burst of alarms: mains fail, DG running, ATS position change, and possibly RECD communication loss. If the RECD alarms are badly set up, the real message (the DG has taken load) gets lost in noise. The alarm-management thinking in the ISA-18.2 standard applies: alarms should be meaningful and actionable, and expected transfer events should not flood operators. Our guide on reducing nuisance RECD alerts without hiding faults shows how to keep real faults visible.
The Test Sequence
Each phase has a purpose, a set of electrical and engine checks, and RECD observations. Do not skip phases to save time. If any phase fails, use the rollback rules further down before moving on.
Phase 1: No-load test
Start the generator from the ATS panel (test mode) without transferring load.
- Purpose: prove the start signal, the engine and the RECD controller all respond.
- Electrical: confirm start signal, voltage and frequency build-up, no ATS faults.
- RECD: controller recognises engine running, pressure readings rise from zero, no alarms.
- Duration: run for 5–10 minutes, then stop or proceed as planned.
Phase 2: On-load transfer
Simulate a mains failure. The electrical contractor opens the incomer or uses the agreed simulation method.
- Purpose: prove the full chain under real conditions: detection, start, voltage build-up, transfer, load pick-up.
- Electrical: record time from mains loss to DG closing on. Record voltage and frequency dip on load pick-up.
- Engine: DG maintainer watches pick-up, smoke, coolant and oil.
- RECD: watch controller status through the dead period and after closure. Note whether it reset. Record ΔP and back-pressure once load settles.
- Duration: hold on DG for at least 20–30 minutes so the exhaust warms and readings stabilise.
Phase 3: Retransfer
Restore mains. The ATS should wait for its set retransfer delay, then switch back.
- Purpose: prove the return path, which is a second transient for every connected load.
- Electrical: record retransfer delay and transfer time.
- RECD: confirm controller behaviour on the second transfer. If it reset on the first, it may reset again.
- BMS: confirm alarms clear in the right order.
Phase 4: Cool-down
The DG runs unloaded for the period set in the ATS or controller, then stops.
- Purpose: protect the turbocharger and engine, and prove the stop sequence.
- RECD: readings fall smoothly, no alarm on shutdown, run logged as ended.
- Records: controller and BMS logs exported before anyone leaves.
A note on full-load back-pressure
An ATS test rarely puts the set at full load, because it runs on whatever the building is drawing. Back-pressure at full load against the OEM limit should already have been proven during commissioning, ideally with a load bank (see Coordinating RECD Commissioning with Generator Load-Bank Testing). Use the ATS test reading as a cross-check at part load, compared with the commissioning reading at a similar kW. The method for comparing readings at different loads is in comparing RECD pressure readings at different generator loads, and the engine side is explained in how exhaust back-pressure affects a DG set.
RECD Observation Checklist
The RECD supplier’s engineer, or a trained member of the facility team, should hold this checklist during the test.
| Moment | What to observe | Normal | Investigate if |
|---|---|---|---|
| Before start | Controller status, ΔP at rest | Healthy, near-zero ΔP | Alarm present, ΔP offset |
| Engine start | Controller sees run signal | Run state shown within seconds | Run state missing or delayed |
| Mains dead period | Controller display and comms | Stays on (if protected) or reboots cleanly | Repeated reboots, lost settings |
| Load pick-up | ΔP and back-pressure trend | Rises smoothly with load | Sudden spike, alarm |
| Settled on DG | ΔP, inlet and outlet temperature | Stable, consistent with commissioning data | Far above earlier readings at similar kW |
| Retransfer | Controller and comms | As on first transfer | New or worse behaviour |
| Shutdown | Run end logged, alarms clear | Clean stop | Alarm on stop, log gap |
Record each observation with its time. If the controller and BMS clocks differ, note the offset. It saves hours when the logs are compared later.
Communicating with Occupants
The technical work is often easier than the people side. A transfer means a power break for everything not on a UPS, and two breaks once retransfer happens.
Notice
- Give at least a week’s notice for a planned on-load transfer, longer for hospitals, data rooms or production lines.
- State the date, the window, and the number of expected breaks (normally two).
- Name the equipment likely to be affected: lifts, lighting not on emergency circuits, non-UPS sockets, access control.
On the day
- Confirm lifts are empty and parked before the transfer. Some sites put lifts on hold for the window.
- Use one announcement channel (PA, WhatsApp group, radio) and stick to it.
- Announce “transfer in five minutes”, “transfer now” and “test complete”.
Critical-load owners
Owners of critical loads should confirm their UPS autonomy covers the dead period with a margin, and that equipment without UPS is put into a safe state. The sector guides on RECDs for hospitals and RECDs for data centres go further into this.
Rollback: What If Something Fails?
Every test plan needs a pre-agreed answer to “what do we do if this goes wrong?”. Write it down before the day so nobody improvises under pressure.
| Failure | Immediate action | Who decides | Follow-up |
|---|---|---|---|
| DG fails to start | Restore mains straight away; abandon test | Test lead with electrical contractor | DG maintainer investigates start system |
| DG starts but ATS does not transfer | Restore mains; stop DG after cool-down | Test lead | Electrical contractor checks ATS logic |
| Engine distress on load pick-up | Restore mains; DG maintainer may stop engine | DG maintainer | Engine and exhaust investigation, including back-pressure |
| RECD high-pressure alarm | Restore mains, unload DG, follow RECD alarm response | DG maintainer with RECD supplier | Check device, sensors and tappings |
| RECD controller resets or loses comms | Continue if engine is healthy; note times | Test lead | Review controller power supply and BMS mapping |
| Critical load problem | Restore mains at once | Test lead on owner’s request | Critical-load owner reviews UPS and safe-state plan |
Note the difference between a fault that threatens the engine or the building and a fault that only affects monitoring. A high back-pressure alarm is a reason to unload the set. A controller reset is a reason to fix the power supply, but not necessarily to abandon the test. The distinction is set out in RECD alarm vs generator trip: control response and in the RECD alarm response matrix for operators.
Also keep the real world in mind. If the grid fails during your test window, the test becomes a real outage. Your rollback plan should say how the team switches from test mode to normal outage response.
Worked Scenario: A Sunday-Morning ATS Test
Illustrative scenario. The building, times and readings below are invented to show how the coordination works in practice.
Background
An office building in Gurugram has a 750 kVA DG set, rated at 600 kW at 0.8 PF (750 × 0.8 = 600). An RECD was fitted three weeks earlier and commissioned with a resistive load bank. At commissioning, ΔP across the RECD at 300 kW was recorded as 2.6 kPa. The facility team plans an ATS test on a Sunday morning, when building load is low but still meaningful. Expected load on the DG is about 330 kW, which is 55% of rating (330 ÷ 600 = 0.55).
The RECD controller is fed from a lighting and small power distribution board downstream of the ATS. Its alarm outputs and Modbus data are mapped to the BMS.
Timeline (IST)
| Time | Event |
|---|---|
| 06:30 | Briefing in the DG room. Test lead confirms roles, rollback table and the DG maintainer’s stop authority. Lifts parked; critical-load contacts confirm UPS ready. |
| 06:45 | Phase 1. DG started from ATS test mode. Engine normal. RECD controller shows run state. No alarms. |
| 06:55 | DG stopped. Team agrees to proceed. |
| 07:00 | Announcement: “Transfer in five minutes.” |
| 07:05:00 | Phase 2. Electrical contractor opens the mains incomer. |
| 07:05:03 | ATS detects mains failure after its 3-second delay and sends start signal. |
| 07:05:11 | DG at rated voltage and frequency. |
| 07:05:12 | ATS closes onto DG. Total break: 12 seconds. Frequency dips briefly, recovers within limits. |
| 07:05:12 | RECD controller display is blank. It lost power when the mains opened at 07:05:00 and starts rebooting once the DG closes on. |
| 07:06:40 | RECD controller completes reboot, about 88 seconds after closure. BMS shows “RECD communication fail” from 07:05:04 to 07:06:45. |
| 07:07 | RECD engineer checks the controller log: the start of the run is missing. Run recorded as starting at 07:06:40. |
| 07:30 | Readings settled at 330 kW. ΔP 2.8 kPa. Compared with 2.6 kPa at 300 kW during commissioning, the small increase is consistent with the higher load. Engine normal. |
| 07:35 | Test lead decides: engine and RECD are working; the issue is controller power during transfer. Continue to Phase 3. |
| 07:36 | Mains restored. Retransfer delay set at 5 minutes. |
| 07:41 | ATS retransfers to mains. RECD controller resets again. |
| 07:41–07:46 | Phase 4. DG cool-down at no load for 5 minutes, then stops. Controller back online by 07:43. |
| 07:50 | Debrief. Root cause agreed: controller supply is downstream of the ATS and drops during both transfers. |
The fix
The team reviews options with the RECD supplier and the electrical contractor:
- Supply from a circuit that stays live through transfers. Options include a small UPS for the controller or another supply arrangement that the RECD supplier confirms is suitable.
- Check controller settings retention. The supplier confirms settings were retained through both reboots; only the run-start record was lost.
- Revise BMS alarm handling. The BMS contractor adds a short, documented delay on the “RECD communication fail” alarm while the ATS is in transfer, so a brief comms loss during a transfer is logged but does not page the duty engineer. A loss that lasts beyond the delay still alarms.
The electrical contractor has a spare 1 kVA online UPS in the stores. With the supplier’s agreement, it is installed temporarily that morning as a controller feed. The permanent arrangement is added to the punch list with an owner and a target date.
The retest
| Time | Event |
|---|---|
| 09:00 | Briefing repeated, shorter. Occupants notified of one more test. |
| 09:10:00 | Mains incomer opened. |
| 09:10:12 | ATS closes onto DG. Total break again 12 seconds. |
| 09:10:12 | RECD controller stays on through the transfer. Run start logged at 09:10:05, a few seconds after the start signal. |
| 09:10:15 | BMS shows DG running and ATS on DG. No RECD comms alarm. |
| 09:30 | Readings settled. Retransfer initiated. |
| 09:35 | ATS retransfers. Controller stays on. |
| 09:40 | Cool-down complete. DG stopped. Logs exported. |
What the scenario shows
- The engine and RECD were never the problem; the controller supply was. Without the test, the first anyone would have known was a missing log entry after a real outage, perhaps during a GRAP period when records matter.
- The rollback table helped. Because a controller reset was listed as “continue if the engine is healthy”, nobody stopped the test in panic.
- Timestamps mattered. The BMS showed the comms alarm from 07:05:04, one second after the ATS start signal, which pointed straight at a supply drop rather than a network fault.
Records to Keep
An ATS test is evidence: of backup readiness, of the RECD working with the DG, and of a disciplined maintenance regime. Keep:
- The method statement and coordination matrix as issued.
- Signed attendance list with roles.
- Transfer and retransfer times, with voltage and frequency observations.
- Engine observations from the DG maintainer.
- RECD controller log export and the observation checklist.
- BMS alarm log export, with timestamps in a consistent format. Using the ISO 8601 date and time format avoids DD/MM and MM/DD confusion when files are shared.
- Occupant notices and any complaints received.
- Issues found, actions, owners and closure dates.
File these with the RECD commissioning data pack and the DG maintenance record. If a pollution control board inspection asks how you know the device works in service, these records help answer.
Frequently Asked Questions
Do we need to repeat ATS testing after an RECD is fitted if we test the ATS every month anyway?
Your routine test may already be enough, provided the first one after the retrofit is treated as a special test with the RECD supplier and BMS contractor present and the observation checklist used. After that, routine tests can carry on, with the RECD checks added to the normal record.
Can the RECD stop the generator from starting?
An RECD sits in the exhaust path and does not normally control the engine. However, if the controller’s outputs are wired into the DG controls (for example, to raise an alarm or trip on very high back-pressure), the interaction should be understood and tested. Engine-protection decisions sit with the DG maintainer and engine maker.
Should the RECD controller be on a UPS?
Not always, but someone should decide deliberately. If losing power during transfer causes resets, lost logs or nuisance alarms, a supply that stays live through transfers is usually worth it. Ask the RECD supplier what supply arrangement suits the controller.
Is a Sunday morning always the best time?
It is common because load and occupancy are low. But the load still needs to be meaningful, and some sites (hospitals, hotels, data centres) have no quiet time. Choose the window with the critical-load owners, not just the facility team.
What if the grid actually fails during our test?
Treat it as a real outage. The test lead should switch the team to the normal outage procedure, and the records from the event become part of the test evidence.
Related Reading
- Coordinating RECD Commissioning with Generator Load-Bank Testing
- How to Include an RECD Fault in a Business Continuity Plan
- Managing RECD Projects in Buildings with Several Equipment Owners
- Integrate RECD Monitoring with a Building Management System
- Preserve Backup Power During an RECD Retrofit
Plan Your Post-Retrofit Testing with Aceget
Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA. If you are planning an ATS test after an RECD retrofit, our team can talk through what to observe and what to record. See our certifications, read the background on the CPCB website, or contact our team to discuss your site.