How Redundancy Can Lead to Failure
Equipping servers with dual power supplies for improved reliability is a common practice in today’s mission critical data center environment. However, if improperly implemented, this practice can increase the likelihood of power failure. A Tier IV data center application will include two completely independent power paths, each including the six basic power components:
- Utility power source(s) and main power panel(s)
- Back-up generator and automatic transfer switch (ATS)
- Uninterruptible power supply (UPS) and maintenance bypass panel
- Power distribution unit (PDU) (or sub-panel from UPS)
- Rack-level PDU
- Server’s internal power supply (PS)
Of course, not every data center is a Tier IV facility. All operators would like to have complete power system redundancy, but cost usually forces some trade-offs. This usually means that although servers have dual power supplies, the rest of the two power paths are not completely independent (see figure 2). The more common scenario is that each of the server’s PS cords are plugged into a different rack-level PDU. This scenario creates a sense of redundancy for most administrators. In reality, this is where the hidden exposure to power problems starts.
This seemingly simple and common practice is the potential cause of power failures in the data center (see figure 3). In most cases, the dual supplies will share the server load at approximately 50 percent each, when both supplies are active. However, if either PS fails or has lost input power, the remaining PS must draw 100 percent of the power required.
This thinking leads to a classic cascade power failure. The same problem results if an additional server or other equipment overloads the PDU load past the tripping point of either PDU.
Since many racks do not have metered PDUs, adding servers can be risky because the administrator has no way of knowing if the next server will overload the PDUs.
In a multi-phase PDU this is even more important, since it has become very common to use a three-phase 208/120-volt PDU populated with three groups of single-phase 120-volt outlets, being fed from a singe three-phase breaker. In this scenario, if any phase exceeds the rated current, the breaker will trip, and all three phases will be dropped, potentially resulting is a loss of power to the entire rack (see figure 6).