DAILY NEWS Dec 14, 2012 7:06 AM - 0 comments

Considerations for specifying your next power distribution unit

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By: Dave Proli, director of engineering, Marway Power Solutions Inc.

There are many considerations when evaluating how to efficiently and reliably distribute power to connected IT servers, industrial instrumentation and/or military systems that demand the highest level of performance. Power conditioning, conversion, control and monitoring make up the four pillars of a strong power distribution foundation. Since the PDU is instrumental to power management, it is the most logical place to integrate these important functions.  Power conversion provides the appropriate power configurations needed by downstream equipment, while conditioning that power is vital to assure the highest level of system uptime.  Control features can automatically manage outputs in response to a variety of conditions, or allow a user to make manual adjustments.  And lastly, the ability to monitor power status information, available capacity, and various parametric measurements is equally important to ensure optimal performance.  These four pillars of power distribution are key attributes to consider when specifying your next PDU.

Power conditioning

The fundamental purpose of a PDU is to distribute power from one or more inputs to one or more outputs. They are very commonly used when a single facility feed must power multiple load devices. But often the quality of the power at the facility feed is less than desirable for many types load devices. Understanding the quality of the available power supplying the PDU is the first step in identifying any power conditioning needs. For some applications, the PDU and load devices are installed in a fixed location (ex: IT server rack, manufacturing facility, etc.). While for other applications, the PDU is integrated as part of an end product that is deployed in the field (ex: portable test system, portable communications system, industrial equipment, etc.). If it is in a fixed location, the quality of the available power drop will usually be more predictable. The utility feed and other devices connected to that feed are usually fixed. Thus it is easier to identify any specific power quality issues that must be addressed. If the product will be fielded in a variety of locations, power quality can be more unpredictable. Problems may come from the utility feed itself or from other nearby equipment injecting noise or other problems into the power line. The second step in identifying power conditioning needs is to understand the sensitivity of the load equipment. Some loads do not require much in the way of conditioning. Their internal design allows them to ride through power quality events (ex: noise, spikes, sags) without significant impact to their performance. However, many types of load equipment are very sensitive to power quality problems. This is especially true of equipment incorporating digital electronics (ex: computers, programmable logic controllers, digital electronic devices, etc.). These devices are more prone to malfunction and failure when operating under the burden of power quality problems. Implementing a good power conditioning scheme is important to prevent power problems from affecting load equipment.

Severe power quality problems such as momentary dropouts, brownouts, and blackouts are best handled by a UPS which can compensate by using its own battery power. These types of problems are less common in a well-established and maintained power grid. But they still can happen, and understanding their effects on your load equipment should be part of a cost-benefits analysis on whether a UPS is warranted. More prevalent power quality problems include voltage transients (spikes) and waveform distortion (noise, notching, harmonics). These types of problems are most often induced by various phenomena near the point of use, rather than at the power plant. In older designs, devices for managing these problems were separate “bolt-on” products added as an afterthought to the overall power distribution system. However, the mitigation techniques for eliminating these types of problems are very well suited for integration within a PDU. A surge protective device (SPD) provides an effective means to prevent transients from reaching load equipment (See figure 1). There are multiple technologies available, with metal oxide varistors (MOVs) being the most common. EMI filters reduce the amplitude of noise, notching, and harmonics on the power line, thus protecting downstream equipment from their damaging effects. And transformers are useful for providing isolation, as well as providing protection against common mode noise.

Power conversion

In many types of equipment, the loads require different voltages or phase configurations from each other, or from the facility feed. In this situation, some type of power conversion is needed to satisfy the discrepancy. This can be as simple as a step-down transformer to reduce the voltage provided from the facility so that it is compatible with the load. Sometimes a delta source is available, but the load(s) require a wye supply. Or the conversion may involve changing an AC source to a DC feed as needed by the load equipment. In legacy equipment, power conversion products were installed and wired as separate devices. But it is often convenient and cost effective to implement the power conversion requirements inside the power distribution unit, especially when the requirements of the various loads differ from each other. Transformers can be integrated into the PDU to provide voltage scaling or phase reconfiguration. Power supplies can also be integrated to provide DC outputs when needed. And for situations where only a DC power source is available, DC to DC converters can be integrated to change the voltage as needed, and in some cases inverters can even be added to provide AC outputs. With this philosophy, the PDU itself manages the task of accepting the available facility power, and then providing the specific power configuration needed by each load device without the need for external components. And of course PDUs can be specified to provide the specific connector type that is most convenient for each device in the system.

Power control

Most power distribution units have at least one main circuit breaker with a manual handle which can be used to turn off power to the whole system when needed. This type of device provides both a manual means (the handle) and automatic means (tripping due to an overcurrent condition) of controlling power. In many applications it is beneficial to support additional power control features in the system, either manually or automatically. One obvious way is to provide branch breakers or control switches for individual outlets or groups of outlets. Less intuitive solutions involve integration of interlocks into the PDU (See Figure 2). If the desired response to a certain condition is to remove power from one or more circuits, then efficiencies can be gained by incorporating that interlock within the PDU, rather than implementing it with external components. Some common examples include emergency power off (EPO) circuits, over-temperature or cooling interlocks, and power quality interlocks which shut off critical circuits during undesirable conditions such as over/under-voltage or improper phase rotation. Sequencing is an automated control feature often implemented to power up circuits in a staggered order. This helps avoid excessive inrush current load on the PDU input which could be caused if all of the output circuits were to turn on at the same time. Remote control of power switching can also benefit the overall design. Whether the application periodically requires a user to remotely cycle power to one or more load devices, or to provide a remote means of starting/stopping the overall system, integrating remote control into the PDU can be helpful. Typical remote control interfaces include dry contact, command line via serial or TCP/IP, and webpage.

Power monitoring

Obtaining information about the status, power consumption, and measured characteristics of various circuits in a system can not only be useful for troubleshooting problems, but can help predict and prevent undesirable conditions from affecting critical loads. In some cases it is beneficial to provide this type of information locally for the operator to see. This can include features as simple as indicator lamps to show which circuits are powered on or which interlocks are triggered. Or it may involve local displays showing voltage, amperage, or power consumed (figure 3). In other cases it is more useful to review that information remotely via a communications interface, or to have the PDU itself send an e-mail or text message indicating a parameter has exceeded a pre-configured threshold. In all cases, if the parameter being monitored involves power, the most logical place to implement that monitoring capability is within the PDU.


When designing the power distribution scheme for a system or product, power conditioning, conversion, control, and monitoring are important features to consider. Whether the application requires one or all of these power distribution pillars, specifying them within the PDU can be of great benefit. A PDU package which integrates these features can save packaging space and overall equipment cost while increasing the simplicity of the entire system.



Fig. 1 A voltage spike may have a very high voltage value, up to several thousand volts, but the duration is on the order of microseconds.
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Caption: Fig. 1 A voltage spike may have a very high voltage val...
Fig. 2 This partial view of an application specific PDU control panel shows an EPO button, three-phase circuit breakers and manual circuit switches.
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Fig. 3 From top to bottom: indicator lamps, a digital meter, an analog meter, and a multifunction meter.
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Caption: Fig. 3 From top to bottom: indicator lamps, a digital m...

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