| Open-Transition ATS | Break-before-make: the normal source disconnects before the emergency source connects. | Usually several cycles
Actual time depends on controller settings, source conditions and switching mechanism. | Commercial buildings, residential backup systems, standby generators and general low-voltage loads. | Simple operating sequence;
widely available;
prevents the two sources from being connected together during normal operation. | Produces a short power interruption;
may restart sensitive loads or interrupt processes that require continuous power. | Adjustable time delays;
source-voltage and frequency sensing;
phase-sequence monitoring;
generator start contacts;
manual test function. | Verify continuous current rating, short-circuit withstand or closing rating, voltage, frequency, number of poles and neutral switching requirements. |
| Closed-Transition ATS | Make-before-break: the emergency source briefly parallels with the normal source before the normal source disconnects. | Typically less than 100 ms
Some loads may experience little or no noticeable interruption when source conditions are suitable. | Data centers, healthcare facilities, manufacturing lines and other loads sensitive to momentary power loss. | Minimizes load interruption;
supports periodic generator testing under load without interrupting critical equipment. | Requires source synchronization and compatible protection settings;
usually costs more and requires stricter installation controls. | Synchronism-check logic;
maximum paralleling time;
in-phase monitoring;
utility permission requirements;
reverse-power and overcurrent coordination. | Confirm suitability for the available fault current, utility interconnection rules, generator controls, voltage class and permitted paralleling duration. |
| Delayed-Transition ATS | Break-before-make with a programmed neutral or load-disconnect interval between sources. | Application-dependent
The programmed delay may range from a short interval to several seconds. | Systems with large motors, transformers, capacitors or residual voltage that could cause high inrush current during immediate retransfer. | Allows motor fields and transformer flux to decay;
reduces the risk of out-of-phase transfer and nuisance tripping. | Creates a longer interruption;
unsuitable for loads that cannot tolerate a delayed restoration without UPS or ride-through support. | Adjustable neutral position time;
load-shed controls;
retransfer delay;
motor-load compatibility;
residual-voltage sensing. | Check pole configuration, neutral design, transfer timing range, motor starting requirements and the short-circuit rating of the complete assembly. |
| Bypass-Isolation ATS | A main automatic transfer section is combined with a bypass and isolation path for maintenance. | Depends on operating mode
Normal automatic transfer performance is maintained when the main section is in service. | Hospitals, emergency systems, industrial plants and facilities requiring maintenance without removing the load from an available source. | Enables inspection, testing or replacement of the automatic section while maintaining source availability. | Larger enclosure and higher installation cost;
more complex operating procedures and interlocking requirements. | Mechanical and electrical interlocks;
visible isolation position;
bypass source selection;
safe test position;
maintenance access. | Rate the automatic section and bypass path appropriately; verify withstand rating, grounding, enclosure rating and the facility’s maintenance procedure. |
| Service-Entrance ATS | An ATS designed and marked for installation at the service entrance when permitted by the applicable electrical code. | Based on selected transition type | Buildings where the transfer equipment is installed ahead of downstream distribution equipment and may serve as the main disconnect. | Can integrate service disconnect functions, source protection and emergency-source transfer in one location. | More demanding requirements for accessibility, grounding, overcurrent protection, labeling and utility approval. | Service-entrance marking;
suitable main disconnect arrangement;
neutral bonding configuration;
overcurrent protection coordination;
code-compliant enclosure. | Confirm service voltage, ampere rating, available fault current, short-circuit rating, number of poles and local authority requirements. |
| Contactor-Based ATS | Uses electrically operated contactors to transfer the load between sources. | Fast to moderate
Timing varies with the contactor mechanism and controller. | Light commercial, residential, small industrial and generator applications with moderate switching duty. | Compact construction;
comparatively simple design;
often economical for lower-current applications. | Contact wear can increase with frequent switching;
may provide less fault-closing capability than a circuit-breaker-based design. | Utilization category;
mechanical endurance;
coil voltage;
contact position indication;
short-circuit protective-device coordination. | Compare continuous current, horsepower or motor-load capability, withstand rating, pole arrangement and switching duty rather than current rating alone. |
| Circuit-Breaker-Based ATS | Uses electrically operated circuit breakers or molded-case switching devices to connect and disconnect sources. | Moderate
Transfer time depends on breaker operating time and controller logic. | Higher-current distribution systems, service equipment and installations requiring strong fault protection or integrated overcurrent functions. | Can provide higher interrupting capability, selective protection and more flexible system coordination. | Typically larger and more expensive;
breaker operating mechanisms may require additional maintenance. | Interrupting rating;
short-time withstand rating;
trip-unit settings;
interlocking;
arc-flash coordination;
remote status contacts. | Confirm the available fault current does not exceed the marked rating and coordinate trip settings with upstream and downstream protective devices. |
| Three-Phase ATS | Transfers a three-phase load between two compatible three-phase sources. | Based on transition mode | Commercial HVAC, pumps, elevators, industrial machinery, data-center distribution and three-phase generator systems. | Supports balanced three-phase distribution and large motor loads;
can monitor phase loss, phase reversal and voltage imbalance. | Incorrect phase sequence, imbalance or neutral configuration can prevent transfer or damage equipment. | Phase-loss sensing;
phase-sequence sensing;
voltage imbalance limits;
frequency monitoring;
neutral switching requirements. | Match the system voltage, frequency, phase configuration, pole count, continuous current and motor-starting characteristics. |
