I recently installed a new HP ML350 G6 server at a client site, connected to an APC Back-UPS device. At least once a day the server would randomly switch itself off while their existing ML350 G5 server would continue running without issue. I failed to find any errors or warnings in the event log and the iLO diagnostic log simply reported that power to the server had been lost.
After doing some testing, and with the help of some recent storms in Perth, I soon discovered that the issue was actually with the Back-UPS device. Any cut in power to the UPS would result in the server switching off immediately and any minor fluctuations in the power source would have the same result despite the battery being fully charged. Initially I thought it may have been an issue with the power supply on the server however researching further, I found that the new HP ProLiant G6 servers are now using active PFC (power factor correction) power supplies which require a clean pure sine-wave power source. Back-UPS are a line-interactive design and only provide a stepped or square sine wave output.
We have now installed an APC Smart-UPS at the client and have had no issues since. Smart-UPS use an online/double-conversion design and provide a pure sine-wave output.
To gain a better understanding of how line-interactive/online UPSs work, check out the following Wikipedia article on uninterruptible power supplies.
So if you are looking at deploying HP ProLiant G6 servers (and if you really care about protecting your server and networking hardware, no matter what manufacturer), make sure you purchase an online/double-conversion type UPS to protect them!
Edit 27/05/2010: This issue recently came up again at another client. We contacted APC directly and were pointed to the following very helpful document. Please use this document if you want official confirmation from APC of why issue occurs and APC’s recommendations: Proper UPS selection for servers with active PFC power supplies.
The official line:
All APC UPS’s are designed to protect themselves when there is a severe overload while on battery. Some general server- class UPS’s such as APC’s core Smart-UPS® models will protect itself by actively limiting the overload to a level that it can manage. Other, more economical UPS designs such as Back-UPS® or the Smart-UPS® SC will protect itself by shutting down quickly when it detects a severe overload. This potential for incompatibility should be considered when selecting a UPS – sometimes the most economical choice is not the best one.
It is important to note that not all PFC power supplies will cause the UPS overload. However, the incompatibility is most acute in the one of the following situations:
• A large server class PFC supply (e.g. rated 500W or more) is used with a Back-UPS or Smart-UPS SC.
• The server is equipped with redundant PFC supplies (has two line cords) that are powered by the same UPS.
• More than one PFC supply is plugged into the same UPS, bringing the total power rating (nominal) of the power supplies to 500W or more.
• A workstation class PC (or high-end gaming PC) is equipped with a PFC power supply rated 500W or more.
In any of these situations, APC recommends that a true, pure sine wave, server class UPS be used. Acceptable models include APC’s Smart-UPS®, Smart-UPS® XL and Smart-UPS® RT family of UPSs. However if, a Smart-UPS SC or Back-UPS RS is to be used, the UPS should be sized accordingly.