utility. Types Open transition 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 switch or 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 A closed transition transfer switch (CTTS) is also called a make-before-break transfer switch. 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 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. 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. 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 occurs during traditional open transition transfer. 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.