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Differences Between Manual, Nonautomatic, and Automatic Transfer Switches

Transfer Switches are used to transfer electrical loads between two sources of power, most commonly a utility service and a backup generator system. Manufacturers offer transfer switches for manual, nonautomatic, and automatic operation. This article clarifies their modes of operation and discusses characteristics and common applications, starting with the simplest type.

Manual Transfer Switches

Manual Transfer Switches (MTS) operate just as their name implies – a person actuates mechanical controls to transfer loads to an alternate source. Because some models utilize simple controls and basic features, they can offer reliable load switching at relatively low cost. Nevertheless, they can be capable of carrying and switching large loads. For example, ASCO Power Technologies SERIES 300 MTS are available with ratings up to 3000 Amperes.

Manual transfer switches offer versatile solutions for applications where switching events are planned. Examples include businesses that need backup power but can tolerate very short outages as well as facilities that do not have a permanent generator on-site, but rely on a temporary source that is connected when utility outages occur.

Planned switching events also include maintenance scenarios, such as when a facility’s sole generator is taken offline for service. Because this could leave a regulated facility without backup power for life safety loads, Article 700.3(F) of the National Electrical Code® (NEC®) requires that such facilities be equipped with a transfer switch and connecting means for a temporary or portable generator. This solution enables facilities to maintain access to backup power even when their primary generator cannot be used. In ASCO’s own product lineup, MTS are available with integrated connection panels. Note that temporary power sources can be added using an MTS, but the NEC requires automatic switches transfer of life-safety and legally required loads.

Manual transfer switches can be operated to switch between power sources at any time. While this simplifies design, it is important to recognize that MTS typically are not equipped with devices to monitor the acceptability of voltage, frequency, or phase angle differences on the alternating current circuits they connect. Consequently, they work best in applications where transfers between two live sources are unlikely or unnecessary, or where systems can tolerate differences in electrical characteristics of power source and load circuits. Other considerations include that MTS must be located where they can be accessed easily, and that they cannot be controlled from a remote location.

For additional information, view ASCO’s MTS product webpage. For more information on their versatility and compliance advantages, read ASCO’s Tech Brief entitled Temporary Connections for Generators and Load Banks.

Nonautomatic Transfer Switches

The term Nonautomatic Transfer Switch (NTS) is often applied specifically to electrically operated switches. These are equipped with electronic controls and electrically powered switching mechanisms. This type requires a person to initiate a transfer sequence, which is then supervised automatically by the equipment’s controller. To be clear, NTS are incapable of self-initiating transfers when utility outages occur.

Because NTS controllers monitor source voltage, frequency, and (were applicable) phase angle differences, loads can be transferred only when an alternate power source is available and acceptable. This makes NTS suitable for applications that cannot tolerate forced connection to unacceptable power sources. Nonautomatic transfer switches can be operated remotely through a remote annunciator or dedicated controls, and are thus suited for locations where personnel access to a transfer switch will be limited, or for remote control implemented to protect personnel from a hazard near a transfer switch.

For additional information about NTS, view ASCO’s white paper entitled Nonautomatic and Manual Transfer Switches for Backup Power Applications.

Automatic Transfer Switches

Automatic Transfer Switches (ATS) sense power outages and transfer loads between power sources without human intervention. Consequently, of the three transfer switch types, they offer the least potential for operational disruption. These switches automatically provide backup power by executing the following functions:
  • carry current continuously
  • detect power failures
  • start the emergency source
  • transfer load to the emergency source
  • sense restoration of power to the normal source
  • retransfer load to the normal source.
Importantly, as with MTS and NTS, an ATS must be able to carry any fault currents that could reach the transfer switch location so that faults can be cleared by overcurrent protection devices designed for that purpose.

Safety-related loads warrant the highest levels of power availability. Consequently, Articles 700 and 701 of the National Electrical Code® (NEC®) require ATS for life-safety and legally required loads. They are also the transfer switch type of choice for mission-critical and business critical optional loads, like those found in data centers.

When compared to MTS and NTS, automatic transfer switches are available with the greatest ranges of control options, switching configurations, transfer sequences, communication accessories, and custom engineered capabilities. For more information about available configurations, review Part 1 and Part 2 of the ASCO Power Technologies white paper entitled Application and Design Factors for Automatic Transfer and Bypass-Isolation Switches. For more information about essential ATS functions, view the ASCO technical brief entitled Basic Automatic Transfer Switch Functions. For information about advanced ATS configurations for specific applications, review the ASCO document entitled Benefits of Custom-Engineered Transfer Switches.

Summary

Manual Transfer Switches require a person to mechanically operate the switch to transfer loads. They are used where short power disruptions are not critical to facility objectives, and offer flexibility for providing secondary backup power when needed. Nonautomatic transfer switches require a person to initiate a transfer through its controls, which then supervise the transfer sequence. Without human intervention, Automatic Transfer Switches transfer load when a power source becomes unacceptable and retransfer back when the normal source is restored. For more information, review the resources linked above or contact an ASCO Power Technologies representative.

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