Transfer switch

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.

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.

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).

The switch will operate in a make-before-break mode provided both sources are acceptable and synchronized.

This means the engine driving the generator supplying one of the sources generally must be controlled by an isochronous governor.

With closed transition transfer, the on-site engine generator set is momentarily connected in parallel with the utility source.

[2] 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.

Because there are no mechanical moving parts, the transfer can be completed rapidly, perhaps within a quarter-cycle of the power frequency.