| Automatic transfer switch (ATS), open transition | Disconnects the normal source before connecting the alternate source (“break before make”). | Brief interruption during transfer; duration depends on the system and equipment. | Can be used in a system designed to meet a specified NFPA 110 Type. Type 10 systems are designed to supply the load within 10 seconds after loss of normal power. | Emergency lighting, fire alarm systems, life-safety circuits and essential building services. | Common and straightforward; connected equipment must tolerate the transfer interruption. |
| Automatic transfer switch (ATS), delayed transition | Disconnects the normal source, waits for a programmed interval, then connects the alternate source. | Includes a deliberate neutral interval in addition to the source-transfer time. | The delay does not itself define the NFPA 110 Type. The complete emergency power supply system must meet its required response time. | Motors, transformers and loads that may need time to dissipate residual voltage before reconnection. | Can help reduce electrical stress or out-of-phase reconnection; the intentional delay must be compatible with the load and required response. |
| Automatic transfer switch (ATS), closed transition | Momentarily connects the alternate source before disconnecting the normal source, with both sources briefly in parallel. | Typically no-break or near-no-break transfer when synchronization and operating conditions permit. | NFPA 110 response classification remains a system-level requirement; closed-transition operation does not automatically establish a Type 10 rating. | Data processing, process controls and other loads sensitive to short interruptions. | Requires suitable source synchronization and utility approval where applicable; paralleling is brief and controlled. |
| Bypass-isolation ATS | Combines automatic transfer with a bypass path that can maintain the load while the ATS is isolated for service. | Depends on the transfer mode and system arrangement; the bypass can support continuity during maintenance. | The bypass feature is a maintainability function, not an NFPA 110 response-time category. Verify the complete system’s required Type. | Hospitals, emergency power systems and facilities where transfer equipment maintenance must not interrupt critical loads. | Provides a maintenance option but adds equipment, space and operating procedures. |
| Manual transfer switch | An operator selects and connects the available source manually. | Depends on operator response and switching procedure; not inherently automatic. | Manual operation alone does not demonstrate compliance with a required automatic response time such as Type 10. | Non-life-safety backup circuits, portable generator connections and selected low-priority loads. | Requires trained personnel and a clear procedure; generally unsuitable as the sole means of automatic transfer for loads requiring rapid restoration. |
| Service-entrance-rated ATS | An ATS arrangement designed and listed for use at the service entrance, subject to applicable electrical-code requirements. | Determined by its transfer mechanism, such as open or closed transition. | Service-entrance rating does not specify the NFPA 110 Type; response performance depends on the overall emergency power system. | Facilities where the transfer equipment is installed at or near the utility service entrance. | Confirm the equipment listing, overcurrent protection arrangement and installation requirements for the specific application. |