Branch Circuit Protection in PDU’s

June 7, 2012 By

Corporate data centers account for some of the most expensive and sought after real estate in the world today. Enterprises invest huge amounts of capital to design, construct and secure their investment in IT equipment in the data center.

In a rack Power Distribution Unit (PDU), the power input is often broken into multiple segments. A division of the circuit with current limited by a fuse or circuit breaker is referred to as a branch circuit.

Branch circuit protection can be accomplished by either branch circuit breakers or fuses. To identify a UL489 circuit breaker,   there are two general visual indicators. The first indicator is the size of the externally showing rocker switch. The UL489 breaker will range in dimension from 1” wide to 1.5” long. The lower grade rocker switch or reset type are typically very small (less than ½”). The second indicator is that the chassis will be expanded to accommodate the higher performing breaker. If the vertical 30A/208V power strip is perfectly symmetrical (2” x 2” or less) across the full length of the power strip, it is highly probable that it does not comply with the current standards.

It is now common practice for consultants to open power units to identify comply with to code. Eaton® uses the highest quality hydraulic-magnetic circuit breakers to ensure maximum overload protection as well as comply with to mandated code requirements. The “lower grade” thermal circuit breaker is in fact approved to a different UL standard, UL1077. A UL1077 device is a Supplementary Over-current Protective device. Its marking will show a UR symbol (i.e., it is UL Recognized). A UL489 device is a Branch Circuit breaker, and the marking on it is “UL listed”.

Compared to higher capability circuit breakers, thermal breakers have several subtle issues and capability limitations:

(a)   No Disconnect Capability – Many thermal breakers can only be reset after a fault and cannot be used to manually disconnect the power supply. The only methods to disconnect a power unit is to either pull the power cord out from the power source or open the upstream circuit breaker at the power panel.

(b)  Heat Susceptibility – Thermal breakers are by their nature susceptible to enclosure heat. The typical derating factor for a thermal circuit breaker for a temperature increase from 73°F to 104°F is 8%.

(c)   Lower Overload / Overcurrent Protection – a thermal breaker is often a UL1077 device, and it can break prospective fault currents up to 1 or 2kA. This is far less than the capability of a UL489 branch breaker which must be able to break fault currents of 5kA. The use of inappropriate devices is not only against the regulations but can be hazardous and will cause extended down time due to device failure.

While fuses are generally accepted for certain products, they are not considered a ‘best practice’ for mission critical facilities:

(a)   Increased Points of Failure – current market specifications employ as many as twelve fuses. For a data center with 100 enclosures, that would equal 2400 additional points of failure.

(b)  Reduced Safety to Personnel – for double pole applications (208V) it is possible for only one fuse to blow and the second leg to remain ‘hot.’

(c)   Higher MTTR (Mean Time to Repair) – while a branch circuit breaker can be quickly reset, the replacement of a fuse can take as long as one hour or more depending on the specific model.

(d)  Higher Repair and Replacement Costs – unlike a circuit breaker which can be quickly reset, it costs significantly more to replace a fuse. Depending on the marketplace, both an electrician and apprentice may be required. A full remediation plan may also be required in the maintenance budget to facilitate ‘off-hour’ replacement.

(e)   Voided Warranty and/or Product Certification - Any time a power unit is physically opened; it presents a number of issues. Not only is there concern as to whether the correct fuse is replaced, but whether the product requires re-qualification for safety performance (e.g. hipot, ground continuity, and functional tests).

The fundamental issue is how much risk the end-user is willing to assume, including financial, functional and safety. The proper application of UL60590-1 Edition and the employment of UL489 branch circuit breakers are best suited to mitigate issues of PDU protection.

Data Center Talk updates its resources every day. Visit us to know of the latest technology and standards from the data center world.Please leave your views and comments on DCT Forum

Share on TwitterSubmit to StumbleUpon
Filed Under: Data Center Equipments, Data Center Power, Data Center Solutions, Networking / Security, Storage / Server Tagged With: , , ,

Intelligent PDUs are Ideal Power Monitors!

January 6, 2012 By

A power distribution unit (PDUs) is a single input multiple output device that distributes power to the computers and networking equipment in a data center. Minus all the fancy terminology, a PDU in a data center is analogous to a spike buster in your house or your office; with a few additional protection features, of course.

Types

There are two types of PDUs, they are:

  1. Floor- mounted PDUs – Slightly on the expensive side but they are capable of drawing in a larger capacity of raw power and distributing it to any number of lower capacity equipments. They consist of transformers, circuit breakers and some power level monitoring devices. Each PDU feeds power to a number of racks in the data center.
  2. Rack- mounted PDUs -  They are smaller in size and comparatively cheaper than the floor- mounted PDU. Their capacity pduis much limited and they distribute power only within a rack. These are also called Smart PDUs or Intelligent PDUs.

By themselves, rack- mounted PDUs serve the purpose of a spike buster distributing power to individual racks. But when they are built with power monitoring devices, they are called intelligent PDUs. Some of these units come with not only power monitoring equipment, but also come with power control devices right down individual plug level. Some units also include environmental monitoring devices.

Intelligent PDUs

Intelligent PDUs help companies monitor the actual power usage and capacity in their data centers, they also keep track of rack temperature and humidity and power sequence. They also permit remote IT equipment management. The PDUs can be controlled in real time with the help of a web browser or a command line interface. Alerts can be triggered based on user defined threshold values in order to ensure that all the parameters are in the safe limit.

The advantage of these intelligent PDUs is that they provide detailed individual server power consumption reading. That way, you know which server is guzzling more power. Companies can save millions by monitoring the power consumption of the servers; they can also identify unused capacity and channel excess power to equipments that are in need for more power in the data centers.

The Future

To take energy monitoring one step further, some companies offer PDUs fully equipped with energy management software that helps you keep tabs on whether your energy conservation efforts are working or not, which rack is developing a hot spot and which is being overcooled, and carbon footprint values of servers and other IT equipment.

With the skyrocketing energy consumption and costs of the data center, companies can no longer afford to squander away precious IT capital on badly managed infrastructure. The industry is well on its way to make performance per watt its new measurement metric; with this, every watt is to be accounted for. In spite of all the steps taken to improve data center cooling, the electricity bills still show gigantic figures. Energy conservation must be a joint effort from all sectors of the data center. It makes no sense to add ten different devices to measure ten different parameters when you can simply adopt one device to measure all the parameters. Smart racks are, most definitely, the need of the hour.

 

For more updates on data centers, visit Data Center Talk.

Share on TwitterSubmit to StumbleUpon
Filed Under: Today's Article Tagged With: , , , ,