Rack Power Strips - Managing Overload Conditions
The first thing that I need to clarify is what the power strip in the rack or cabinet is called? There is no single industry standard term, and the term used the most by IT technicians ("PDU") is not the best term to use. The reason PDU was not a great term to begin using for the power strip is because the electrical distribution system already had a PDU defined. The electrical distribution in a data center will sometimes have a transformer downstream from the UPS, either a voltage step-down transformer or simply an isolation transformer, with output breakers feeding multiple power panels or overhead busways further downstream. The component with a transformer and output breakers is referred to as a Power Distribution Unit (PDU). Some other terms that are more appropriate include:
Power Outlet Unit (POU)
Rack Power Distribution Unit (rPDU)
or simply Power Strip
The other day I had a client ask me what they should be paying attention too when monitoring the load on the 208V/3Φ 30A POU within their server cabinets. The client had been instructed by the POU vendor that as long as the local meter amperage reading on the POU did not exceed 80% of the supply circuit breaker rating feeding the POU all would be fine. If all the ITE loads are equally balanced across all three phases then this would be true. However, loads that are un-balanced across the phases may overload one of the receptacle bank’s protection integrated into the POU, while the overall POU load is still below 80% of the circuit breaker rating feeding the POU. To demonstrate this, let’s use an example of 9 servers connected to a POU. These loads would typically have dual power supplies and be connected to an “A” and a “B” POU, but for the purpose of this example we will assume all the ITE is powered from one POU source (which is a failure mode that should be planned for anyways). The POU and 9 servers consist of:
(3) servers with an actual load of 1000VA & C19 power connections
(6) servers with an actual load of 530VA & C13 power connections
The POU is fed from a 208V/3Φ 30A circuit (80% rated)
The POU has integral 20A breakers or fuses, 100% rated, for all 208V/1Φ line-to-line circuits
Total POU load is 6180VA, well below 80% of the rating of the circuit breaker rating feeding the POU (208V x 30A x √3 x 0.8 = 8646VA)
All of the ITE loads are evenly distributed across all the source phases in the configuration shown above. No problems with the integral 20A breakers/fuses or the 30A supply circuit breaker.
The ITE loads are distributed across the top two receptacle banks in the configuration shown above. No problems with the integral 20A breakers/fuses, but one phase of the 30A supply circuit breaker is in an overload condition (greater than the 80% rating) and is susceptible to tripping the supply circuit.
The 1000VA ITE loads are distributed across all the source phases, but the 530VA ITE loads are connected from the top down all within the top receptacle bank in the configuration shown above. No problems with the 30A supply circuit breaker, but the integral 20A breaker/fuse for the top receptacle bank is in an overload condition and is susceptible to tripping the top receptacle bank integral breaker/fuse. The local meter amperage reading for the POU this particular client was using was measuring the Phase A, Phase B and Phase C supply current. As you can see it is also important to pay attention to the amperage of the combined ITE loads connected to each of the three receptacle banks on each POU. The client was able to easily manage these loads by monitoring each receptacle bank amperage using the power management tool they integrated with their POUs.
