This Auto Mains Fail or Automatic Transfer Switch uses "Relay Logic" to control two motor contactors (rated 100A or above) to transfer power to a load circuit between utility power and a standby generator.

One or more multi-meter(s) can be included but is not shown.¹⁾

The appropriate size of motor contactors should be selected based on the load served and the genset size. The 400A frame is a good general purpose selection, but smaller or larger can substituted. Recommended source: FactoryMation.com (225A frame and larger have 100-240VAC/DC coils)

• See the discussion at ATS Pro & Con for reasons why this may or may not be an appropriate choice for your application.

Contactors K-U [13] and K-G [14] are mechanically and electrically interlocked to ensure that only one can be closed at a time.

• Utility Connected: Two Green indicators (L-U, L-KU)
• Generator Connected: Two Blue Indicators (L-G, L-KG)
• Load Connected: White Indicator(s) (one per phase, if desired)
• K-G [13], K-U [14]: Utility & Generator Power Contactors, 400 Amp 3-pole
• K-PHM-U [2], K-PHM-G [10] : Phase Monitor – Loss of phase, phase rotation, and low voltage relay
• K-VM-UA [3], K-VM-GA [9] : Over voltage relay
• K-US [4], K-GS [8] : Time Delay (Voltage stabilization)
• K-U ENABLE [5] : Time delay to ensure utility/grid power stability & minimum generator run-time
• K-G STOP [6] : Generator cool-down delay
• K-G START [7] : Generator start delay after utility power failure

1. Starting with Utility Power being supplied to the load: Contactor K-U [13] is closed, and all timers have expired.²⁾ Timer K-G STOP [6] has expired and its contacts are closed.
2. Power is removed from the coil for K-U [13] if:
   1. Utility power fails
   2. A phase is lost or the voltage drops below the limit set by K-PHM-U [2]
   3. Voltage goes above the limit set by K-VM-UA [3]
3. When K-U [13] opens:
   1. Load is disconnected (Preventing damage to equipment caused by low voltage.)
   2. Removes power from timer K-G START [7], which starts its timer.
   3. If Utility/Grid returns before K-GS [8] time delay closes K-G [14]:
      1. K-U [13] will close after K-US [4] delay
      2. K-G STOP [6] will time out, potentially running the genset for up to K-G STOP [6] time delay.
4. When timer K-G START [7] expires (30 seconds), its contacts close, which causes the selected genset to begin its autostart sequence.
5. Once the generator is supplying power:
   1. K-PHM-G [10] qualifies all phases live, phase rotation, and minimum voltage
   2. K-VM-GA [9] qualifies maximum voltage
   3. Illuminates L-G (Blue) indicating generator power is OK.
   4. K-GS [8] begins its countdown (30 seconds).
6. K-G [14] then closes:
   1. Supplies generator power to the load
   2. Opens the neutral connection for the coil on K-U [13], preventing K-U [13] from closing by accident.
   3. Illuminates L-KG (Blue) indicating that generator is supplying the load.

1. When Utility Power returns, it must meet the constraints:
   1. K-PHM-U [2] for phase (all phases live, correct phase rotation) and minimum voltage
   2. K-VM-UA [3] maximum voltage
   3. Indicator L-U (Green) illuminates to confirm utility grid voltage is acceptable
2. K-US [4] begins voltage stabilization delay (10 seconds).
3. K-U ENABLE [5] begins time delay (15 minutes), which ensures:
   1. The utility grid has time to stabilize
   2. Delays adding loads to the grid after an outage
   3. Eliminates short-cycling if the grid drops again shortly after returning
   4. Provides a minimum run-time for the generator to ensure it gets up to temperature
4. When K-U ENABLE [5] turns on, it opens the coil for K-G [14]:
   1. Disconnects load from Generator
   2. Connects neutral of K-U [13] coil
5. K-U [13] closes immediately
   1. Connects load to Utility Grid
   2. Disables K-G [14] by disconnecting neutral of coil
   3. Illuminates L-KU (Green), indicating Utility/Grid is supplying load
   4. Provides power to K-G STOP [6]
6. K-G STOP [6] begins time delay (2 minutes) for Generator cool-down (running with no load)
7. K-G START [7] turns on, disconnecting the generator auto-start signal
   1. Genset stops

• BOM-116 (G Sheets)
• BOM-116 (PDF)
• BOM-116 (XLSX)
• Alternate selector switch choice