The Home of Diesel Generators

01829 863497

Automatic Transfer Switches

FW Power Guide

Automatic Transfer Switches

Transfer Switches transfer the load from the mains supply to the generator supply and back again. An Automatic Transfer Switch (ATS) does this with no user input whereas a Manual Transfer Switch requires the user to physically “Switch” over the supplies. Transfer Switches are generally used to split supplies between a Prime supply (usually the Grid) and a Standby supply, usually a generator. They can also be used to transfer the load between two generators in equally the same manner. Mechanical and electrical interlocks between the contactors prevent the contactors (i.e. the mains and generator) from connecting to the load at the same time. This prevents the larger infinite supply of the Grid from “back feeding” the generator and ultimately causing severe damage.

Within the ATS a voltage sensing relay monitors the mains supply across one or three phases.  When the voltage drops outside of a preset limit a countdown timer commences before switching the supply over to the generator. Upon sensing that the mains voltage has returned the opposite process occurs. The countdown timers are used to prevent the unnecessary switching over of supplies due to a minor outage of a few seconds. As well as voltage sensing some ATS’s have frequency sensing and follow the same process as voltage instability.


Please note, in this system the mains and generator are never connected together. As such there is always a power outage on mains fail and mains restoration whilst the supplies are switches over from one source to another. On larger systems Motorised Circuit Breakers replace the contactors.

Step by Step Processes for an Automatic Transfer Switch.

  • Mains healthy – mains contactor engaged, generator contactor disengaged.
  • Power failure is detected by the voltage relay.
  • Count down timer of 0-2 minutes commences.
  • If the mains returns to healthy during the timer then no further processes occur and the mains contactor remains engaged.
  • If the mains does not return then a signal is sent to the generator to start.
  • The generator will start and reach its running speed of 1500 rpm for a 50Hz system.
  • Optional – an up to speed signal is sent back to the ATS from the generator.
  • ATS switches over the contactors.
  • Generator healthy & mains failed – mains contactor disengaged, generator contactor engaged.
  • Mains power restoration is sensed by the voltage relay.
  • Count down timer commences to ensure the mains is stable.
  • ATS switches over the contactors.
  • Mains healthy – mains contactor engaged, generator contactor disengaged.

Note: the generator will continue to run for a preset time period to allow it to cool down after which it will shut down and go back into standby mode. If a further mains failure occurs during this cool down period then the same steps as above are still followed.

Simplified Drawing of a System With and Without a Transfer Switch.

Drawing of how an ATS works

For more information on Diesel Generators please View the FW Power Guides to Generator Sizing and Installation as well as The Differences Between Prime and Standby Generators.

FW Power can assist you to decide the best generator for your application. We offer a range of Automatic Transfer Switches from 20 Amps to 6,000 Amps.

Please contact us for more details:


Phone: 01270 821419.

Picture of ATS's

FW Power – The Home of Diesel Generators.


The information provided in this document is for guidance only and represents the views of the author. Only suitable qualified personnel should be used to size, install and commission generators.