In some telco facilities that do have AC generator backup, there are a few seconds of down time while the generator kicks in. In these spaces, AC equipment will in all likelihood re-boot, while DC equipment stays up through the transition.
That's why the PAIX is wired thusly: +--------+ Utility----->|Transfer|--+->Rectifier---->Battery---->Cage DC Generator--->| Switch | \ +--------+ \ +======+======+ UPS UPS UPS | | | Transfer Switches \ \ Cage AC The batteries are sort of the capacitor in a huge DC power supply. The triple AC UPS is so that the electricians can do maintainance on one of the UPSes while still leaving two others online. Stephen was talking about dual rectifiers and a DC transfer switch but I havn't been in that room for a while so I don't know if that happened. It costs a lot of money -- almost as much as connectorizing the cross connects -- but the goal is 100% uptime on both the AC and DC.