Three Places Rack PDUs Capture Power Data From and Why You Should Care

Scott Fier
July 10, 2014

One aspect of intelligent rack PDUs that’s often poorly understood is where the devices are actually capable of capturing power data from;  And more importantly, the distinct advantages to monitoring the power drawn from IT devices, PDU breakers, and branch circuits that can help you to develop energy efficiency standards and lower the risk of unplanned downtime.  So when monitoring power via rack PDU, consider the advantages of monitoring at the outlet-level, PDU breaker, and PDU branch circuit. 

1. Why monitor at the outlet?

• Monitoring at the outlet allows for trending load to the IT device. By trending at the IT device you can use this data for capacity planning.  For instance, you will know what a new IT device, of the same type, can expect to draw.
• If you monitor at the outlet, you can set thresholds below the breaker trip threshold, such that, if a single power supply is approaching the outlet threshold, you can remediate it by powering off the outlet automatically or manually.  This may eliminate a cascading failure.

2. Why monitor at the PDU breaker?

• Monitoring at the breaker allows the PDU to know when a breaker trips.  And not just when a breaker trips, but which breaker so that you’re able to quickly reset.

3. Why monitor at the branch circuit of the PDU?  

• When you have a 3-phase system, you can more easily monitor for balancing across the branches.
• Keep in mind that branch circuit load is different than line load because of the 3-phase line pairs.  Line load rating is usually going to much higher than branch load rating.  Each branch will typically have a 20A breaker, where each phase of a 60A 3PH circuit will have a 48A rating.  So, you need to know, not only the load of the phases, but also each branch.
• In high power PDUs (40A or higher), there are 6 branches (6 breakers), 2 per phase.  In this case you can be balanced among the phases but not balanced along the branches.   So, monitoring at the branch allows you to check for balance among the branches as well as the phases.
• Besides knowing about balancing, monitoring branches will also allow you to calculate whether a failover can be accommodated by the other branches.
• Monitoring branch circuits allows you to set thresholds to prevent breakers from tripping.

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