A lot of redundancy needs to be put into place and with that are dual power sources. Each server requires two power jacks on the end so if one goes out, the other remains on while the other one is replaced. 4 servers need two PDU’s. As you can see here, I am installing one of 2 for 4 servers. It also has a handy power outlet on the front so I can plug in any device that I use for monitoring or fixing.