what is by-pass switch

You can do this simple search on www.google.com. I just entered the words "transfer switch" and Google came up with the following: Results 1 - 10 of about 1,400,000 for transfer switch. (0.15 seconds).

Of those options, here is the link to what Wikipedia states about a Transfer Switch. Here is another link, to what is in your subject: bypass switch.

As answered in a post you already made, a transfer switch will switch between two sources of power. There are automatic transfer switches (ATS); bypass-isolation transfer switches (BP-ATS); manual (both mechanical and electrically operated) manual transfer switches (MTS); delay transition transfer (DTTS); and closed transition transfer switches (CTTS). Automatically operated transfer switches include adjustable sensing for source dropout and pickup, as well as settings for various associated time delays. Most modern transfer switches include passive "in-phase" monitoring to minimize inrush currents.

A bypass switch is simply two switches in one. The bypass switch is a mechanically operated unit which allows servicing the automatic switch. This configuration allows one to close the bypass in parallel with the source that the ATS is connected to, and then isolate the load from the ATS. The ATS can be de-energized and removed for maintenance and transfer tested without load for minimal impact to the site. I have seen many schemes for accomplishing this.

Have a look around at the information provided in Automatic Switch Co's (ASCO) website. You will find good information there and on other switch manufacturers sites. Good luck in your learning.

http://www.ascopower.com/cgi-bin/redirect.pl?/main.htm

thanks a lot ,sir.