Rack PDUs: Basic vs. Metered, and Advice on Power Load Balancing

Posted on June 11, 2014 by Gento

Basic rack PDUs are little more than outlet strips. They deliver electrical power to IT devices and that is about all they do.

In addition to delivering electrical power, metered rack PDUs provide information about the power that is being consumed. It may be as simple as the voltage and the power being consumed by the PDU in total or it may provide much more power information and breakdown that information by each individual outlet. There is a wide range of PDUs that could fall under the description of a metered PDU.

Some power information that could be provided includes the following:

• Current (A)
• Voltage (V)
• Actual power (kW)
• Apparent power (kVA)
• Kilowatt-hour (kWh)
• Power factor
• PDU circuit breaker current and status
• Line current (for three-phase PDUs)

Each of these data points could be gathered for the PDU as a whole or, for more sophisticated metered PDUs, gathered for each individual outlet. On some metered PDUs thresholds can be set for some or all of the values and alerts triggered when the thresholds are crossed.

Kilowatt-hour (kWh) is an important measurement because this is what determines actual power usage. Ideally, the accuracy should be +/- 1% which is considered “billing-grade” accuracy. Whether customers are actually changed for the power they consume or their power consumption is just reported, kWh is the unit of measurement to use. Highlighting kWh can drive efficiency efforts.

PDU circuit breaker current and status is useful to avoid overloading a PDU and risking an outage due to a tripped circuit breaker. If the rack has redundant power it is important not to load each metered PDU to near its load capacity. If the power to one PDU fails the full load will then be borne by just one PDU. If both PDUs are near their capacity when the power to one PDU fails the second PDU will be overloaded and its circuit breaker(s) will trip.

Line current indications are important for three-phase PDUs since the load on each of the lines should be close to equal to keep the power balanced. This can rarely be achieved perfectly but maintaining the power load on each phase to within 10% of each other is a good practice. Some metered PDUs may be able to report on differences between phases and send an alert when the differences exceed some predetermined percentage.

If you are considering the purchase of new rack PDUs look into metered PDUs. Be sure that the PDU can deliver the information at the level of granularity and accuracy you want. For example, if the PDU only meters the total power of all the devices plugged into it comparing the efficiency of different pieces of equipment will be difficult.

Test drive a metered PDU right now or schedule a call with a Raritan power expert today!