Minimum Contactor Closing Time for Generator Synchronization

If your generators are synchronized then it doesn't matter how long the switch takes to close. The AC waveforms will be the same and the contactor closure won't cause massive transients.

For a 60 Hz system, one full cycle is ~16.7 ms. Thus a difference of ~1.4 ms would put you 30° out of phase, just to pick an example. Surprise!

It depends on the synchroniser and how you operate. Also, the maximum phase error which is tolerable - the smaller the machine, generally, the more error is allowable.

Some synchronisers have a setting for the breaker closing time - but by steps of 20 ms.

The better synchronisers allow selection of maximum slip rate or slip cycle, they will not close if slip cycle is shorter than setting.

If the slip cycle is 20 seconds, the angle error due to 20 ms timing error is 1/10 of that if slip cycle is 2 seconds. For example 20 ms error with 2s slip cycle is 360 x 0.02/2 = 3.6 degrees. Usually, the synchroniser error is about 5 degrees maximum, excluding breaker closing time tolerance.

Often, it is more important that the breaker has a consistent closing time (same every time).

A proper synchronising commissioning will measure the breaker closing time several times to check value and repeatability, before setting ASU or check synch unit (CSU), then doing several timing check closes with breaker "racked-out" or otherwise disabled from an actual close to prove ASU/breaker overall before a real on-circuit close.

So what do you know about ASU/engine governor and how will you operate (full auto, manual preset before auto synchroniser switch-on, etc.

An analogy to your synchronization question, If you are a passenger trying to catch a boat that you saw starting to move away from its mooring, you need to move a bit faster than the moving boat. Your thinking processes + reaction time to jump in order to get on board. Otherwise you will completely miss the boat and end up in the water! 60 hertz gives you 16ms, so a minimum 20ms contact making response is safe and falls within the tolerance of synchronization.