basic question
 

I'm helping out a coworker who is overloaded by doing some research. This is not my area of expertise. We are government and looking to let a contract to (as he quickly explained it) maintain (yearly) our 6 or so UPS systems (from 30 kVA to 120) including providing status on the condition of batteries, ability to provide power for 5 minutes, and provide external maintenance bypasses for each UPS.

He did say that some of the UPS's have internal maintenance bypasses but my understanding is that he wants each one to have an external maintenance bypass (input 480 volts and output 208). My question really is that does make sense - if some already have internal bypasses why would we buy (or what is the advantage of adding) external bypasses for those? Was going to ask him that question but he is out of the office for a few days. Apologize for the ignorance but thought I'd throw this out just to see if anybody could help me get a better understanding.

Thanks!

An external bypass is able to completely isolate the UPS from the critical power path. This can allow work on an unpowered UPS cabinet -- even complete UPS replacement -- without endangering service personnel or the critical load. With an internal bypass, critical power is always flowing somewhere inside the UPS cabinet, making uninterrupted operations unsafe or impossible in some situations. People like external bypasses for their safety and flexibility, but dislike them for their cost and complexity, often requiring custom engineering, implementation and service by a non-UPS vendor. Internal bypasses are comparatively inexpensive, have stand-alone switch logic integrated with UPS controls, and are usually included in the scope of UPS warranty and maintenance.

Hope that helps.

Ken