Site Tools


Auto Mains Fail Transfer Switch

Pro & Con

Generator transfer switches using industrial motor-control contactors have the advantage of simplicity and long-life, typically over 500,000 operations at rated load. They also use standard off-the-shelf components that can be purchased almost anywhere.

The primary disadvantage is that they do not meet National Electrical Code requirements in the United States for Automatic Transfer Switches for emergency and legally required standby power use. They do not satisfy the two requirements of NEC [700.5(C)] and [701.5(C)] (2011):

  1. They are not manufactured as Automatic Transfer Switches (ATS) or approved by a testing lab for emergency power system use.
  2. They are not mechanically held.1)

Optional Standby systems as defined in [702.4] are not bound by these legal requirements for transfer switches.

The biggest disadvantage of an ATS is that the contacts are typically rated for less then 5000 transfers at rated load, at which point the contacts must be replaced, and frequently the electro-mechanical actuator as well. In locations where electric power is reliable, this is typically not an issue as 5000 transfers will take several decades to accumulate. In locations with unreliable power this number of transfers can be accumulated in just a few years.

An ATS is also physically large compared to a set of contactors, which can otherwise be carried in an airline suitcase. That in particular makes this design more suitable for remote locations, as the parts can be carried in and assembled on location in a short time.

One-Line Diagram

Auto Mains Fail Transfer Switch

Auto Mains Fail Transfer Switch -- Selector Switch

(DSN) files can be edited with TinyCAD.

Description of Operation

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

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.

Utility Power Return

  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

Bill of Materials

116 ATS -- Automatic Transfer Switch-Contactor

Time delays listed are defaults, and can be adjusted as desired.
cp/ats.txt · Last modified: 2023/12/31 01:55 by Alan Shea