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What is the difference between in-sync and in-phase?

There is often confusion between the in-phase and the In-sync features of ASCO transfer switches. It is important to understand the difference between these features especially during the process of selecting transfer switches for different applications.


The in-phase monitor is a feature available only in open transition units. It is set to OFF by default and can be turned ON at any time. When enabled, the controller will disallow the transfer of loads from one live source to another live source until the in-phase conditions are met. For ASCO units, the in-phase parameters that need to be met are:
(1) phase angle difference must be not more than +/- 60 electrical degrees,
(2) the frequency difference should not be more than +/- 3Hz

The in-phase monitor is useful for the transfer of motor loads since the sudden application of another source that has a large phase angle difference from the source previously applied will cause the loads to draw a large inrush current which can be large enough to trip breakers and may cause damage due to the excessive production of torque.


The in-sync monitor is a feature available only in closed transition units and it has more stringent requirements than the in-phase monitor since the live sources are momentarily paralleled during closed transition transfers. For ASCO closed transition units, the following are the parameters that need to be met to allow transfer from one live source to another live source:
(1) phase angle difference between the sources must be less than +/- 5 electrical degrees,
(2) the frequency difference must be less than +/- 0.2 Hz, and
(3) voltage difference must be less than 5%

There is zero interruption of power during a closed transition transfer and since the transfer happens when the sources are almost identical, the loads will not sense the transfer from one source to the other source.

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