Plenty of facilities run their generator on a schedule, watch it start, hear it run, and check the box. The engine cranked, it made power, everyone moves on. The problem: that exercise proves the engine works. It says nothing about whether the transfer switch will actually move the load when the utility drops.
A healthy generator can sit right next to a transfer switch that will not transfer — and a generator-only exercise will never reveal it. That gap is the whole point of this guide.
What the transfer switch actually does
An automatic transfer switch (ATS) sits between a normal (utility) source, an emergency (generator) source, and the load. Its job is to transfer the load between those two sources.
Under normal conditions it monitors the utility source. When that source drops out of tolerance, the ATS signals the generator to start, waits for voltage and frequency to come up, then transfers the load to the emergency source. When utility power returns and stabilizes, it re-transfers the load back to normal and commands the engine to cool down and shut off.
That sequence — sense, start, transfer, run under load, re-transfer, cool down — is the thing that has to work when the lights go out. Notice how little of it a no-load engine exercise touches.
Exercising the generator vs. testing the ATS
Running or exercising the generator proves three things: the engine cranks, it runs, and the alternator produces power. That is genuinely worth confirming. But it does not move the load, and it does not test the transfer switch's sensing and transfer functions. The switch never has to do its job.
A transfer test is different. It simulates loss of normal power so the ATS performs the full sequence:
- Sensing the loss of normal (utility) power.
- Sending the start signal, and the engine starting.
- Transferring the load to the emergency source.
- Stable operation under load.
- Re-transferring the load back to normal when utility returns.
- Engine cooldown and shutdown.
That is the real test. An engine exercise checks the generator; a transfer test checks the system that decides whether the generator ever carries your building.
Operator input — SME-1
Why the timing is worth capturing
It is worth capturing more than pass/fail. Record the time from simulated outage to transfer, and the time from utility return to re-transfer. Those numbers are an early-warning indicator: drift in either direction over successive tests can signal a failing controller, a tired relay, or a weak battery long before the switch outright fails.
The ATS failure modes a generator exercise hides
Here is what a no-load run will not catch, because the switch never operates:
Starting battery
A weak, sulfated, or dead starting battery is a common cause of a no-start. If the engine won't start, none of the rest of the transfer sequence can happen. Battery condition is why this is the first thing checked on most standby PMs.
Transfer controller or relay
A failed or sticking transfer controller or relay can prevent the switch from commanding a transfer at all — or it can transfer to the generator and then fail to re-transfer back to utility when power returns, leaving you running on standby longer than you should.
Power contacts
The power contacts that carry the load through the switch wear, pit, and overheat over time. That can cause a poor transfer, nuisance trips, or heat damage inside the cabinet.
Terminations and corrosion
Loose terminations and corrosion create resistance. Resistance creates heat and unreliable operation. None of it shows up while the switch is sitting idle.
Settings that got changed and never restored
Exercise timers, transfer delays, and sensing thresholds can be changed during troubleshooting and then never put back. Wrong settings can defeat real-world response. Confirming the settings is part of testing, not an afterthought.
The point
How often to test
There is no single universal interval. ATS test cadence depends on the facility type, the governing standard, and the authority having jurisdiction (AHJ).
A common industry pattern is a more frequent operational exercise (often monthly) paired with a less frequent, more thorough test (often annual, often under load). Life-safety occupancies — health care in particular — carry stricter and more prescriptive requirements for frequency, load, and duration than a typical commercial building. Critical facilities such as data centers and municipal infrastructure may also carry additional internal or contractual testing requirements on top of code.
Confirm the exact intervals against the governing standard for the specific site and the AHJ. Do not assume one building's schedule applies to the next.
Operator input — SME-2
The record is the deliverable
Standards for emergency and standby power systems expect operators to keep permanent, attributable records of tests — the kind you can put in front of an auditor or AHJ. A transfer test you ran but cannot prove you ran is worth very little when someone asks.
A defensible ATS test record should capture:
- The named technician who performed it.
- Date and time.
- The equipment tested by identity — make, model, serial, location.
- Readings and timing, including transfer and re-transfer times.
- Pass/fail on both transfer and re-transfer.
- Any deficiencies found.
ATS testing belongs inside the preventive-maintenance scope for the standby system, alongside battery checks, load tests, and inspection of contacts and terminations. It is part of the system, not a separate errand.
Where PowerOps fits
This is exactly the accountability gap PowerOps is built to close. In PowerOps, completed PMs, load bank results, and transfer tests become attributed, timestamped records written by a named, authenticated technician — not a checkbox someone can backfill later.
Field data — readings, photos, serials — is captured in the field rather than retyped at the office, and the canonical data (customer, site, the equipment list, access notes, prior findings) auto-flows so the tech isn't re-entering it every visit. The result is a test record attributed to a real person at a real moment that becomes a permanent record you can hand an auditor.