1. Large ATS - Stand up units in NEMA 1 enclosure for larger applications that use motorized wiping blade type, circuit breaker or contactor type transfer devices. These systems normally have a built-in bypassing system to enable routine maintenance of the switching mechanism. In the US, they are always listed to UL1008 Automatic Transfer Switch Standards as well as comply to NFPA standards for hospital use. Generally referred to as ATSs, these electromechanical transfer switches have been used for decades to transfer loads between utility and stand-by emergency generators at hospitals, industrial plants, commercial buildings, residential and wherever stand-by generators are used. These are stand-by devices where the normal source is the utility and when it fails, the transfer switch automatically starts the generators and once they are up, the automatic transfer switch automatically transfers to them. The time between sensing the loss of utility power until the transfer is complete and power is restored can take between 20 to 45 seconds or more depending on generator start or paralleling times if applicable. These automatic transfer switches range in size from small individual ones (50-200A) at 208V to multiples of large ones at 480V (up to 4000A) or at higher voltages to handle large facilities. Physically these units can each be 3' - 6' (1 - 2 m) wide X 3' - 4' (1-1.5m) deep and are often installed in the electrical switchgear rooms close to the incoming utility line and the generator room.

Speed of transfer is not a concern here since the facility is designed to tolerate a 20-45 second outage. A small portion of facility loads that cannot tolerate such an outage would be on UPS power with batteries to ride through the transfer time as well as in case generators fail to start the first time requiring human intervention. Once utility power is reestablished then the automatic transfer switch re-transfers back to the utility and automatically shuts down the generators after a cool down period. The process to go to and come back from generators is generally break before make but some systems can be set up for make before break for planned transfers if required. Transfers are always done with the sources in phase because of presence of motor loads that cannot tolerate more than a few degrees of out of phase shock.

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2. Point of Use ATSs - Point of use rackmount automatic transfer switches that use relays as transfer devices (Rack-Mount Relay Transfer Switch - RMATS). If you see a rack-mounted “STS” be careful that in this case STS may not mean “Static Transfer Switch”, but rather, may mean “Source Transfer System” as one supplier has named their relay based rack-mounted transfer switch to have users think it is solid state where in fact it is a relay transfer switch. Note also that for marketing purposes few relay type transfer switch suppliers indicate that their transfer switches use relays, so you have to investigate what type of switch is being advertised to match your requirements. The duration of transfer time is not an indication of solid state vs. relays as transfer times are comparable and within the IT curve limits.  Most relay type transfer switches are not supplied with a bypass. Once you have an alarm you have no alternative but to shut down the load as there are no provisions to bypass the unit. If dropping your load upon an alarm is not what you have in mind, then you need to purchase a rack-mounted automatic transfer switch that comes with a built-in bypass isolation for maintenance.

Regardless of listing make sure the product has a published and name-plated fault rating for safety reasons, so your facility fault rating does not exceed the short circuit withstand of the product. These systems can have transfer times between 1/2 cycle to over a cycle depending on the product as well as depending on the phase angle between sources at the time of transfer. Some designs have a long built-in delay for out-of-phase situations so you need to look at both the in-phase and out-of-phase transfer times to be sure your load's requirements will be met. Almost always they are used in transferring between two UPSs. Some cannot tolerate sources being over 15-20 degrees apart. Be careful if this is a limiting factor for proper use at your facility because when UPSs fail the phase angle difference between the sources can far exceed this. Also be careful that there is no ringing or contact bounce.  You can investigate this by scoping the output of the device. Perform a test transfer to the alternate source and then allow the unit to auto-retransfer. This is where most all relay transfer switches exhibit multiples of zero crossing within a cycle caused by the relay contacts bouncing which may not be tolerable by some loads.  TwinSource systems do not have this problem.

These systems are mostly rated between 12A and 30A and are 1U high without a bypassing option.