What is an automatic transfer switch (ATS)?


An automatic transfer switch is designed for switching of electrical loads between primary power and a standby generator. The switch monitors both power sources for stability. If the utility power becomes unstable, the transfer switch signals the generator to startup and then transfers power until the utility power source returns to normal.

Romtec Utilities uses ATS when a permanent generator is included in the design of the lift station. The ATS is sized for the required load of the system and can be service entrance rated to include a circuit breaker if necessary. The service entrance disconnect will interrupt excessive over-current from entering the system. This option can be expensive and generally it is more cost effective to locate a separate circuit breaker upstream from the ATS. Sometimes it is also necessary to design for a 4-pole system that switches the neutral. If the generator and utility are each required to be grounded neutral, then the transfer switch must have enough poles to switch the neutral. If the neutral is allowed to ground in both places, multiple undesirable current paths are created for the ground current to flow through. This situation can be dangerous if service work is being done on a line that is thought to be disconnected. If the generator system is completely enclosed within the rest of the system and is not separately grounded, a 3-pole system with a single solid ground can be used. This is cheaper than the 4-pole system.

The ATS is located in its own enclosed control panel that is sized depending on the required load. Service entrance rated ATS requires a much larger enclosure which must be accounted for in the layout of the control panels. Cummins offers a transfer switch with a 1200 amp rating that has an enclosure with dimensions of 90” tall x 39” wide, with a door closed depth of 27”. This enclosure weighs roughly 730 pounds and would most likely need to be floor mounted.

The ATS systems are designed to prevent generator startups from momentary power system losses or variations. They also maintain the generator availability for immediate reconnection after the switch back to utility power, in case of a reoccurring failure. This availability time is adjustable, along with many other time dependent options.

An automatic transfer switch is not normally used for a lift station design with a portable generator. A manual breaker is added so that the generator can be connected to the system and the power switched over.