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 a packaged ATS is that the contacts are typically listed for a maximum of 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 two or three 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.

This automatic transfer switch uses a ComAp ATS Controller combined with contactor switching provides an ATS with full metering & logging, which can also be remotely monitored.

An Auto Mains Fail Transfer Switch using relay logic is also documented here.

(DSN) files can be edited with TinyCAD.

Contactors K-U [3] and K-G [5] 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 [3], K-U [4]: Utility & Generator Power Contactors, 400 Amp 3-pole

1. Starting with Utility Power being supplied to the load: Contactor K-U [3] is closed, and all timers have expired.
2. Power is removed from the coil for K-U [3] if:
   1. Utility power fails
   2. A phase is lost
   3. Voltage drops below the setpoint limit
   4. Voltage goes above the setpoint limit
3. When K-U [3] opens:
   1. Load is disconnected (Preventing damage to equipment caused by low voltage.)
4. If utility power fails to return within 5 seconds:
   1. InteliATS closes KGS [6] to start the genset
5. Once the generator is running, the InteliATS:
   1. Qualifies all phases live, phase rotation, and minimum and maximum voltage
   2. Illuminates L-G (Blue) indicating generator power is OK.
   3. Begins countdown (5 seconds).
6. K-G [5] then closes:
   1. Supplies generator power to the load
   2. Opens the neutral connection for the coil on K-U [3], preventing K-U [3] from closing by accident.
   3. Illuminates L-KG (Blue) indicating that generator is supplying the load.

1. When Utility Power returns, InteliATS:
   1. Qualifies input for phase (all phases live, correct phase rotation), minimum, maximum voltage
   2. Indicator L-U (Green) illuminates to confirm utility grid voltage is acceptable
2. 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 operating temperature
3. Opens the coil for K-G [5]:
   1. Disconnects load from Generator
   2. Connects neutral of K-U [3] coil
4. K-U [3] closes after setpoint time-delay (as short as 1mS)
   1. Connects load to Utility Grid
   2. Disables K-G [5] by disconnecting neutral of coil
   3. Illuminates L-KU (Green), indicating Utility/Grid is supplying load
5. Begins time delay (2 minutes) for Generator cool-down (running with no load)
   1. Disconnects the generator auto-start signal
   2. Genset stops

• Operating S2, Manual Bypass, forces selection of Utility or Generator power and bypasses InteliATS controls
  • S2 is mounted inside protective shroud to prevent casual operation
• Operating S1, Auto/Gen, forces selection of Generator by InteliATS
  • InteliATS will start generator, qualify power, then turn off K-U [3] and turn on K-G [5] after setpoint delay (as short as 1mS).
• Switching S1 back to Auto will switch back to Utility power if it is qualified based on normal setpoint limits.

• Operating the Emergency Stop (E-Stop) mushroom switch immediately disconnects all power sources and shuts down genset, if it is operating.
• The E-Stop must be turned clockwise to release the latched condition, and the alarm reset on the InteliATS in order to return to normal operation.

• BOM 117 -- Google Sheets
• BOM 117 -- Web Page