In a church, I've seen a large generator and ATS panel (automatic transfer switch) so that the generator turns on when there is a failure in the mains. If there is no power in the mains, How does the control circuit of ATS work and send a signal to the generator to start?

I know that in power distribution rooms (stations) which contains switch gear panels, There are DC batteries (UPS). So that the control circuits still works even if there is no power in the mains.

Does the church have at least one battery? or there is another technique?


3 Answers 3


I was right about capacitors, partly. Some models of ATS have a center position, in which they wait until either supply is stable. In order for this to work, the ATS is powered by a capacitor during this time. For this particular model, it takes 10 minutes for the capacitor to charge. Based on this long time, I suspect it's some kind of supercapacitor.

And it's possible to have the ATS' own battery (or supercap) send the signal and power to start the generator. And the generator has its own built-in battery too in that particular setup (the charge needed to start is substantial). So yes there are batteries or supercaps involved in the transition, but they may be built-in the devices involved.

A more serious setup puts UPS after the ATS so the consumers don't lose power while the transition happens.

Someone even wrote a paper on The importance of the generator starting battery.

Re: "Do you mean that ATS can be supplied from the built-in battery of the generator?" Some ATS models normally do that, for example:

M2G units require a DC power supply which is normally taken from the generator’s starter battery. If connection to this battery is not convenient then an alternative must be found, otherwise the ‘integrated DC supply’ package must be added to the ATS panel.

Actually the whole bit on ATS types in that brochure is worth reading:

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There are more details on the internals of a cheap/household ATS in this paper. That one needs/uses a 12V battery.

For some [more expensive] ATS units, no-external-DC-power-requirement is the standard option. This one from ABB doesn't detail how it powers its own circuitry, but it says

The new ATS family is designed to work without auxiliary power supply. Auxiliary power supply is only required when Dialogue Modbus RS485 is used or in networks with rated frequency of 16 2/3 Hz.

It probably has internal battery or supercap but its charging circuitry needs at least 50Hz to operate; that ATS is not rated for operating on anything in between 16.7Hz and 50Hz mains frequency.

  • \$\begingroup\$ Thank you very much for your useful answer. I have a question "but they may be built-in the devices involved". Do you mean that ATS can be supplied from the built-in battery of the generator? sorry about my english :) \$\endgroup\$ Oct 20, 2015 at 12:42
  • \$\begingroup\$ or you mean the batteries or super-caps are built-in the ATS itself ? \$\endgroup\$ Oct 20, 2015 at 12:43
  • 1
    \$\begingroup\$ @MichaelGeorge: Both methods are possible. See expanded answer. \$\endgroup\$
    – Fizz
    Oct 20, 2015 at 16:26

The church has at least one battery, and a contactor with its coil wired across the mains so that the normally open contacts connect the mains to the church's downstream electrical loads. When the mains fail, the contactor's normally closed contacts then connect the church's downstream electrical loads to the generator, which is started with the battery.


It's simple, in the company where I work The ATS receives DC power to turn on its display lights and to control/ operate the generator.

It gets its DC power from the plus and negative wires in the generator's control panel (which come from the starter) and the starter gets these same wires from the battery.

In other words, the generator's battery is giving DC power to the ATS.

And to ensure that the generator battery can Continually keep powering the ATS and the generator's own display screen without the battery going dead, then what is done is that 220 volts is taken from the building cables (which are connected in the ATS) and this 220 volts is given to a battery charger, which receives 220 volts on one side and on the other side it sends wires that charge the generator battery. And that is how the ATS always has power because the generators battery always has power to give to the ATS because the battery charger ensures this.


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