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What is an automatic transfer switch, or ats for short? An ats is usually used to automatically connect backup power from a standby engine generator, or genset, to a building when normal power fails. It also signals the genset when to run and when to shut down.
To perform this function, the controller in the ats monitors the normal power source. If the Voltage or frequency falls outside of acceptable limits for more than a preset time then a control contact is closed. The controller in the genset senses this run contact closure and starts the genset. The ats controller also monitors the genset Voltage and frequency output, and when it reaches acceptable values and has stabilized it transfers the main power contacts of the ats to connect the building load to the genset.
An ats also comes standard with a programmable automatic exerciser which runs the genset for a short period of time every month or every couple weeks.
An ats for commercial and industrial gensets larger than 20kW normally uses 2-wire start control to interface with the genset. These larger gensets are liquid cooled with radiators. 2-wire start means that there is a single contact with a pair of control wires connected to the genset. Some ats controllers need 12Vdc control power, which requires another 1 or 2 wires to the genset battery
An ats for smaller air cooled gensets 20kW and less, often use a different control scheme. Sometimes there are more control wires. Sometimes more of the controls are in the genset or in the ats.
When purchasing an ats one must first know if the genset uses 2-wire start control. If it does then there are a wide range of choices for the ats. If the genset uses something other than 2-wire start control then you pretty much have to use the genset manufacturer's ats.
The next factor to consider is the current rating of the ats. It must be rated to carry the larger of either the genset full load current, or the panel main breaker rating it is backing up. For example, if the genset is 12kW single phase 120/240V with a full load current of 50 Amps, and it is backing up a typical residential service of 200 Amps, then the ats has to be rated for 200 Amps. However, if the genset is only backing up a sub-panel fed by a 100 Amp breaker, then the ats only needs to be 100 Amps.
The third choice concerns the service disconnect. If the ats will be installed as the first device downstream of the power company meter then most codes will require a service rated disconnect between the ats and the meter. This can be a new separate disconnect installed when the ats is installed, or it can be purchased factory installed in the ats. Space, and the ats cost, and installation cost are factors in making this choice.
Finally there are some other choices to make for selecting the ats such as the following:
* Indoor or outdoor enclosure
* System Voltage, phases and frequency (For residential applications in North America these will normally be 120/240V single phase 60 Hertz)
* Number of switch poles (Again for the same type applications this will be 2 for switching the 2 hot legs. However, there are a few jurisdictions in New England where local inspectors insist that the neutral also be switched, which means 3 poles would be needed.)
* Optional battery charger.
* Load shedding relay or auxiliary contacts for preventing large loads from operating.
For larger commercial or industrial applications there will be additional considerations such as adjustable settings, delayed transfer if there are large motor loads, special instrumentation, surge protection, special short circuit withstand ratings, elevator relay, extra pole for neutral switching at hospitals, 3 source transfer systems, etc.
All this may sound complicated. But for most applications it is very straight forward. And we are happy to assist with the selection process.

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