Based on your current slider settings, here's how the timeline breaks down. Total blackout = detection (1s) + start/crank + warmup + ATS transfer. All times are in simulated seconds (the simulator runs at approximately 10× real time).
The 10-second benchmark: NEC Article 700 (700.12) requires emergency power to be available within the time required for the application, not exceeding 10 seconds. Under NFPA 110, generator systems serving such loads are typically classified Level 1, Type 10 (“Level” is NFPA 110 terminology, not NEC). This simulator uses the 10-second figure as a learning benchmark only; full code compliance depends on many factors beyond timing.
Try it: Toggle utility power off in the sidebar and watch the full sequence unfold. Then check the Timing Breakdown panel above to see how your settings compare to the 10-second benchmark.
Each event in the power transfer sequence, with timing and delays made visible. Times are simulated seconds; the simulator runs at 10× real time.
Active sources and power paths highlighted in real time. Click any icon for details. Simplified single-ATS model: NEC requires emergency-system transfer equipment to supply only emergency loads — legally required standby and optional loads use separate transfer switches. This simulator collapses them into one ATS for clarity.
The decision states the ATS controller moves through. Think of it as the system's brain.
The ATS controller continuously monitors utility voltage and frequency. When utility power is present and acceptable, no action is needed. When utility fails, the controller sends a start command to the generator and waits for the generator to reach acceptable voltage and frequency before transferring.
Click any card to auto-configure settings and watch the scenario play out. Each scenario adjusts the sidebar controls and runs automatically.
Power Outage → Generator Transfer
Utility fails. Generator starts, warms up, ATS transfers. The core emergency sequence.
What to watch: Total blackout time, each state transition in order
Generator Fails to Start
Utility fails but the generator won't crank. Emergency loads stay dark. A worst-case scenario.
What to watch: State machine stops at FAILED_START — no transfer possible
Bad Voltage / Frequency
Generator starts and runs, but output is unstable. The ATS correctly refuses to transfer.
What to watch: Generator reaches BAD_VF state — ATS protects loads from bad power
Full Cycle (Outage → Recovery)
Complete sequence: utility fails, generator takes over, utility returns, ATS retransfers, generator cools down.
What to watch: All states from start to finish, including retransfer delay and full cooldown
Overloaded Generator
Generator capacity below total demand. Non-essential loads are shed to protect critical systems.
What to watch: Load bars show priority shedding — life safety stays at 100% with priority ON
Reset to Normal
Restore all defaults (sliders, toggles) and return to normal utility service.
Use the sidebar controls to set up any combination of conditions, then hit Simulate Outage. Try these challenges:
- Set start/crank to 30s — watch how long loads wait in darkness
- Enable "Bad V/F" — see the ATS refuse to transfer even though the generator is running
- Enable "Failed Start" — understand what happens when backup fails the backup
- Set generator capacity to 50% and disable load priority — see proportional load reduction
- Set retransfer delay to 120s — learn why patience prevents double-transfer failures
Why This Matters
When utility power fails, the difference between a managed transition and chaos is a sequence of timed, logical steps. Emergency power systems don't just "turn on" — they follow a precise state machine that balances speed against safety.
The Core Sequence
- Normal Operation — Utility feeds all loads through the ATS (Normal position)
- Utility Failure Detected — ATS senses loss of acceptable voltage/frequency (small detection delay)
- Start Signal — ATS sends start command to generator
- Generator Cranking / Starting — Engine cranks and attempts to start
- Warmup Period — Generator runs but must reach acceptable V/F before transfer
- ATS Transfer — ATS switches from Normal to Emergency position
- Emergency Operation — Loads served from generator
- Utility Returns — ATS senses utility restoration, starts retransfer timer
- Retransfer — ATS switches back to Normal (utility) position
- Cooldown — Generator runs unloaded to cool down before shutdown
ATS Logic & Timing
The Automatic Transfer Switch is the decision-maker. It monitors both sources and only transfers when conditions are met:
- Transfer to Emergency: Only when generator voltage AND frequency are within acceptable limits
- Retransfer to Normal: Only after utility has been stable for the retransfer delay period — adjustable by controller/facility requirements; use manufacturer settings and AHJ/project requirements
- Break-before-make: The ATS disconnects from one source before connecting to the other — a brief interruption is expected during the switch
- Transition types: This simulator models an open-transition (break-before-make) ATS. Closed-transition (make-before-break) and delayed-transition types also exist. The transfer-time slider represents total programmed transfer timing — the mechanical switch operation itself takes only a fraction of a second
Common Mistakes
- Assuming instant transfer — There are real, physical delays at every step
- Forgetting warmup — A running generator is not the same as a ready generator
- Ignoring retransfer delay — Transferring back too quickly risks double-outage if utility flickers
- Skipping cooldown — Cooldown is manufacturer/controller dependent and is most relevant to engine/turbocharger thermal management
- Not planning for failed start — If the generator does not start, you need a backup plan (UPS, redundant gen)
What to Notice
- The total blackout time is detection delay + start/crank + warmup + ATS transfer — often 15–30 seconds with default settings
- Load priority determines which loads come back first (life safety, then critical, then equipment, then non-essential)
- Load shedding protects the generator from overload by dropping non-essential loads
- The retransfer delay prevents "bouncing" — if utility flickers back briefly then fails again
Load Shedding & Priority
When a generator can't handle the full load, the system must decide what stays powered. This simulator uses an example share split:
- Life Safety: 20% (egress and emergency lighting, exit signs, fire alarm) — never shed when priority is ON
- Critical: 30% (operating rooms, patient-care areas, critical process loads)
- Equipment: 30% (critical HVAC, elevators)
- Non-Essential: 20% (general lighting, convenience outlets) — shed first
Fire pumps are intentionally not listed: they take power through a dedicated path under NEC Article 695 / NFPA 20 and are not part of building load-shed schemes.
Quiz: Test Your Understanding
1. The utility fails and the generator starts, but the ATS does not transfer. What is the most likely reason?
2. After utility returns, why doesn't the ATS transfer back immediately?
3. The generator starts and the ATS transfers, but emergency loads are still partially off. Why?
4. What is the correct sequence after utility failure?
5. Why does the generator run unloaded after loads transfer back to utility?
6. In this simulator (detection delay 1s), the generator takes 7s to crank, 8s to warm up, and 2s for ATS transfer. Total blackout time?
7. During load shedding with priority ON, which loads should be dropped FIRST?
8. The utility flickers back on for 5 seconds then fails again. What prevents the ATS from transferring back and forth rapidly?
Configuration Challenge
Read the scenario requirement below, configure the sidebar sliders to match, then click Check Configuration to see if you got it right.