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Power Quality & UPS

Power Usage Effectiveness in Data Centers

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Guidance for calculation of efficiency in data centers

BY VICTOR AVELAR, Schneider Electric

The benefits of determining data center infrastructure efficiency as part of an effective energy management plan are widely recognized. The standard metrics of power usage effectiveness (PUE) and its reciprocal data center infrastructure efficiency (DCIE) have emerged as recognized standards. In this feature, PUE will be used as the efficiency metric. All discussions equally apply to DCIE.

In the course of providing energy audits for data centers, Schneider Electric has identified a number of practical problems with establishing the PUE of specific data centers.

One or more of the practical problems are frequently present in typical data centers, and are almost always present in data centers that exist in a shared-use facility like an office building. Since most data center operators who attempt to determine PUE will encounter one or more of the aforementioned problems, a standard way to deal with them should be defined. That is the purpose of this feature in Electrical Source Magazine.

Power Quality & UPS

The Art of Ground Testing


Correcting the misconceptions of ground testing


Ground resistance testing is sometimes described as an “art”, and indeed something can be said for that description. Familiarity with the basic issues and common problems, however, can take much of the “art” out of it and put your testing program on a firm, repeatable basis.

Use a dedicated ground tester, an instrument that has separate current and potential circuits that the operator can manipulate at will by placing test probes in the soil. This is the critical difference between a ground tester and other common testers with a resistance range.

When testing a discrete item, like a motor or circuit, two test leads are usually sufficient. The small amount of resistance that they contribute may be negligible, or it can be nulled out or measured and treated as a tare.

Power Quality & UPS

Go with the Power Flow


The importance of load studies


If you’re running an industrial power system—whether for an airport, a mill, a factory or a smelter—you’re focused on more than the traditional utility concept of keeping the lights on. You definitely need that power to be available, no doubt, but what you’d really like to know about are the details: quality of power, flow and reliability—items that impact your machinery, your schedule, your production and, often, your bottom line.

In the first edition of Power Quality Essentials, we’ll examine one of the most important power aspects to analyze: tracking changes in power flow and profiling the load. While it’s true that load flow discussions in the power industry may be on overload at this point, it’s talked about so often because it’s vitally important.

What does studying the load flow do for you? It helps you ensure that your power is connecting in the best ways, that it’s stable for delicate equipment, that it’s reliable for a production line, and that it’s working to your utmost advantage.

Power Quality & UPS

Electrical Grounding and the Weather

Photo credit: Megger

How resistance testing is affected by the elements


Ground resistance testing is affected by the weather as much as any common electrical test—and sometimes even more so. Both the instrument and the test item are affected. With ground resistance testing, the test item is not so readily managed as a motor or electrical circuit which can be dried, cleaned, placed in a controlled environment or some other adaptation to accommodate easy or standardized testing. Here, the “test item” is actually a complex interplay between the grounding electrode and its soil environment. Both of these elements will be examined in turn.

The soil component can be a relatively tight hemisphere around a ground rod or a rather vast expanse surrounding a large grid. The composition of the soil itself constitutes a baseline for its resistance with types such as loam and clay exhibiting relatively low resistivity (good electrical grounding) while others like sand and rock are typically quite high. Superimposed on basic soil structure are the effects of weather in the form of moisture and temperature. These factors also have a broad range of influence on electrical measurement.

It should come as no surprise that a paramount consideration is moisture. A “good” ground is a low resistance ground and the reciprocal of low resistance is high conductivity. Whether or not soil is a “good” conductor is a point that must be considered in perspective. Compared to copper, it is certainly not. Soil would hardly function in any effective manner if constricted into a long strand, as copper is in a wire. But the advantage of the earth (that is, the planet itself or the immediate portion of it) is simply that there is so much of it.

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