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150:(SCRs) to transfer a load between two sources. Because there are no mechanical moving parts, the transfer can be completed rapidly, perhaps within a quarter-cycle of the power frequency. Static transfer switches can be used where reliable and independent sources of power are available, and it is necessary to protect the load from even a few power frequency cycles interruption time, or from any surges or sags in the prime power source.
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The switch will operate in a make-before-break mode provided both sources are acceptable and synchronized. Typical parameters determining synchronization are: voltage difference less than 5%, frequency difference less than 0.2 Hz, and maximum phase angle between the sources of 5 degrees. This means the engine driving the generator supplying one of the sources generally must be controlled by an isochronous governor.
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connects the generator to the home's main electrical panel. The generator supplies power to the home's electric load, but is not connected to the electric utility lines. It is necessary to isolate the generator from the distribution system to protect the generator from overload in powering loads in the house and for safety, as utility workers expect the lines to be dead.
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Typical load switching applications for which closed transition transfer is desirable include data processing and electronic loads, certain motor and transformer loads, load curtailment systems, or anywhere load interruptions of even the shortest duration are objectionable. A CTTS is not a substitute
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A typical emergency system uses open transition, so there is an inherent momentary interruption of power to the load when it is transferred from one available source to another (keeping in mind that the transfer may be occurring for reasons other than a total loss of power). In most cases this outage
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There are some loads, however, that are affected by even the slightest loss of power. There are also operational conditions where it may be desirable to transfer loads with zero interruption of power when conditions permit. For these applications, closed transition transfer switches can be provided.
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with an external operating linkage as the switching mechanism. The linkage operates two circuit breakers in tandem, closing one while opening the other. Manufacturers of transfer switches can provide installation guides to select the size of switch and provide recommended installation procedures.
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As well as transferring the load to the backup generator, an ATS may also command the backup generator to start, based on the voltage monitored on the primary supply. The transfer switch isolates the backup generator from the electric utility when the generator is on and providing temporary power.
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It is generally required that the closed transition, or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable (such as when normal power fails) the switch must operate in a break-before-make mode (standard open transition operation) to ensure no backfeeding
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from an emergency generator back into the utility line, for example. One example is an open transition automatic transfer switch (ATS). During the split second of the power transfer the flow of electricity is interrupted. Another example is a manual three position switch or circuit breaker, with
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For example, in a home equipped with a backup generator and an ATS, when an electric utility outage occurs, the ATS will tell the backup generator to start. Once the ATS sees that the generator is ready to provide electric power, the ATS breaks the home's connection to the electric utility and
129:(uninterruptible power supply); a UPS has a built-in stored energy that provides power for a prescribed period of time in the event of a power failure. A CTTS by itself simply assures there will be no momentary loss of power when the load is transferred from one live power source to another.
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A transfer switch can be set up to provide power only to critical circuits or to entire electrical (sub)panels. Some transfer switches allow for load shedding or prioritization of optional circuits, such as heating and cooling equipment. More complex emergency switchgear used in large backup
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A soft-loading transfer switch (SLTS) makes use of a CTTS, and is commonly used to synchronize and operate onsite generation in parallel with utility power, and to transfer loads between the two sources while minimizing voltage or frequency transients.
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The control capability of a transfer switch may be manual only, or a combination of automatic and manual. The switch transition mode (see below) of a transfer switch may be Open
Transition (OT) (the usual type), or Closed Transition (CT)).
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that switches a load between two sources. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic and trigger when they sense one of the sources has lost or gained power.
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When utility power returns for a minimum time, the transfer switch will transfer the house back to utility power and command the generator to turn off, after another specified amount of "cool down" time with no load on the generator.
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generator installations permits soft loading, allowing load to be smoothly transferred from the utility to the synchronized generators, and back; such installations are useful for reducing peak load demand from a utility.
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An open transition transfer switch is also called a break-before-make transfer switch. A break-before-make transfer switch breaks contact with one source of power before it makes contact with another. It prevents
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Homes with standby generators may use a transfer switch for a few circuits or the whole home. Different models are available, with both manual and automatic transfer. Often small transfer switch systems use
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Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads which occurs during traditional open transition transfer.
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utility power on one side, the generator on the other, and "off" in the middle, which requires the user to switch through the full disconnect "off" position before making the next connection.
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With closed transition transfer, the on-site engine generator set is momentarily connected in parallel with the utility source. This requires getting approval from the local utility company.
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Pybus, Dennis; Finding
Surplus Electric Power in Traditional sources; Electricity Today, Issue 5, 2001, p. 36 et seq. Accessed Apr. 2014
51:(ATS) is often installed where a backup generator is located, so that the generator may provide temporary electrical power if the
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Magnum Closed
Transition Soft Load Transfer Switches (product literature). Eaton / Cuttler-Hammer. Accessed Apr. 2014
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Safely
Installing Your Standby Electric Generator, Flathead Electric Cooperative, Inc.; Jul. 2006; accessed Dec 2006
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A closed transition transfer switch (CTTS) is also called a make-before-break transfer switch.
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ASCO Series 7000 Soft Load Power
Transfer Switches (product literature). Accessed May. 2017
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is inconsequential, particularly if it is less than 1/6 of a second.
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A static transfer switch uses power semiconductors such as
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253:Electrical Power Systems Quality Second Edition
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683:"Socomec Transfer switches from 25 to 6300 A"
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209:Generac Power Systems Inc., accessed Dec 2006
23:3-phase transfer switch single-line diagram
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142:Static transfer switch (STS)
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31:Intelligent transfer switch
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49:Automatic Transfer Switch
16:Type of electrical switch
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644:Vandal-resistant switch
489:Magnetic proximity fuze
251:Roger C. Dugan et al.,
589:Silicone rubber keypad
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649:Wireless light switch
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424:Electric switchboard
255:,Mc Graw Hill, 2002
394:Contact protection
384:Centrifugal switch
303:has generic name (
205:2008-06-17 at the
187:2007-01-12 at the
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687:www.socomec.co.in
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654:Zero speed switch
279:www.layerzero.com
99:Closed transition
39:is an electrical
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559:Railroad switch
519:Nintendo Switch
499:Magnetic switch
474:Latching switch
449:Inertial switch
444:Infinite switch
399:Crossbar switch
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429:Float switch
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282:. Retrieved
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624:Time switch
574:Sail switch
564:Reed switch
554:Push-button
549:Push switch
544:Pull switch
529:Photoswitch
454:Kill switch
419:Dry contact
301:|last=
92:backfeeding
619:Thermostat
594:Softswitch
579:Sea switch
459:Key switch
439:Humidistat
414:DIP switch
379:Cam switch
284:2016-10-18
263:, page 72
171:References
599:Spark gap
706:Switches
700:Category
409:Cryotron
293:cite web
203:Archived
185:Archived
154:Home use
115:occurs.
53:utility
352:Switch
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125:for a
41:switch
354:types
80:Types
305:help
257:ISBN
127:UPS
47:An
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