Load ramping is an advanced feature of switched power distribution units (PDUs) and is used to protect data center power systems from inrush-related overloads by customizing the turn-on sequence of data center equipment. To better understand load ramping, a basic understanding of what causes inrush overloads is useful. Let me begin with a question: What are inrush overloads?
Inrush overloads are short-term overloads of a power system caused by the peak or inrush currents drawn by an electrical load at the moment it is energized. A simple example of an inrush current would be the turn-on of an incandescent light bulb. Before the light is turned on its filament is cold and has a low resistance to electrical current flow. When voltage is applied to the bulb, its low resistance allows a very high current to flow instantaneously. But as the filament heats up its electrical resistance increases. With an increased resistance, less current flows until it stabilizes to a steady state value. Inrush currents in data center equipment occur in a similar manner but on a much larger scale.
Some of the primary sources of inrush currents in data centers are power transformers, internal power supply units and electric motors. Electric motors can pull between 5 to 10 times of their rated full load amps (FLA) at turn-on. Depending on a variety of factors, the initial inrush of power transformers can be 8 times FLA. Additionally, the charging currents of input capacitors of power converters can cause inrush currents up to 20 times their rated current until they reach their fully charged, steady state value.
Inrush currents need to be addressed in the design phase of a data center’s power system because, if unaddressed, performance and reliability problems are likely to occur. Nuisance tripping or contact welding of circuit breakers are two possible outcomes. Voltage dips caused during transformer magnetization could affect the start-up of critical network equipment, which underscores the importance of line conditioning. Inrush currents can also thermally stress power converter components such as rectifiers, leading to the failure of the power supply unit itself.
There are different ways to address inrush-related overloads in data center power systems. Choosing the right type of circuit breaker and sizing it to handle peak currents is one way. Selecting equipment with internal current limiting is another. Load ramping addresses the problem of inrush overloads in a smart, convenient and cost-effective way.
Load ramping uses the capability of individual outlet control featured in switched PDUs and the programmability offered through power management software to customize the turn-on sequence of data center electrical loads so when they are energized, the cumulative inrush current of equipment in a common power system is reduced thus eliminating the risk of inrush overloads that can affect the availability of network equipment.
