Transfer Switch

Open Transition Transfer Switch (OTTS)

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 backfeeding 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 circuit breaker, with 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.

Closed Transition Transfer Switch (CTTS)

A closed transition transfer switch is also called a make before break transfer switch. In a typical emergency system, 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 is inconsequential, particularly if it is less than 1/6 of a second.

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.

When transferring loads in this manner, during a test or when re-transferring to normal after primary power has stabilized, 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 relative phase angle between the sources of 5 electrical degrees. Since the maximum frequency difference is 0.2 Hz, the engine will generally be required to be controlled by an isochronous governor.

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 occurs.

Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads, which occur during traditional open transition transfer.

This type of switch may also be referred to as a Static Transfer Switch (STS), as opposed to an Automatic Transfer Switch .


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. It should be understood that a CTTS in a system is not a substitute for a UPS (uninterruptible power supply). In addition to providing line conditioning, 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.

Utility approval

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.

Soft-loading Transfer Switch (SLTS)

An SLTS essentially uses CTTS technology but actively changes the amount of load accepted by the generator.


^ Safely Installing Your Standby Electric Generator, Flathead Electric Cooperative, Inc.; Jul. 2006; accessed Dec 2006


^ Generac Power Systems Inc., accessed Dec 2006

^ Pybus, Dennis; Finding Surplus Electric Power in Traditional sources; Electricity Today High Voltage Power & Engineering e-Magazine; accessed Dec 2006

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Categories: Switches

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