Understanding time current curves: Part 2

Jan 30, 2014 3:17:29 AM | Posted by David Paul

The second installment of a three-part series about time current curves (TCCs) covers short and long time settings, including their purpose and examples of such overcurrents.

Continued from Part 1

The light blue curve represents the circuit breaker settings for the feeder circuit breaker. The lower portion of the curve (below 0.05 sec or three cycles on the time axis) is the instantaneous trip function. The purpose of the instantaneous trip is to trip the circuit breaker quickly with no intentional delay (no more than a few cycles) on high magnitude fault currents. This quick trip protects electrical distribution equipment from damage and keeps arc flash hazard categories low. Clearly these type faults must be interrupted quickly and do not allow the system to wait and see if the fault will self clear. The minimum instantaneous setting determines the minimum trip setting for the circuit breaker. In the case shown in Part 1, the instantaneous setting is 2,400 A and the maximum value displayed is available fault current at the circuit breaker. Small changes in the instantaneous setting can result in significant changes in the Arc Flash Hazard category, so this is a setting which must be carefully selected according to sound engineering principles.

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Understanding time current curves: Part 1

Jan 28, 2014 8:36:30 AM | Posted by David Paul

The first installment of a three-part series about time current curves (TCCs) provides a quick overview of item identification and how to read TCC plots.

A time current curve (TCC) plots the interrupting time of an overcurrent device based on a given current level. These curves are provided by the manufacturers of electrical overcurrent interrupting devices, such as fuses and circuit breakers. These curves are part of the product acceptance testing required by Underwriters Laboratories (UL) and other rating agencies. The shape of the curves is dictated by both the physical construction of the device as well as the settings selected in the case of adjustable circuit breakers. The time current curves of a device are important for engineers to understand because they graphically show the response of the device to various levels overcurrent. The curves allow the power systems engineer to graphically represent the selective coordination of overcurrent devices in an electrical system. Modern power system design software packages, such as EasyPower, SKM Power Tools, and Etap, contain graphical libraries of curves to allow the power system engineer the ability to plot, analyze, and print the curves with minimal effort compared to the previous methods used when coordinating a power system.

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Configuring control systems through wireless devices, remote I/O: With great power comes great responsibility

Jan 23, 2014 10:18:02 AM | Posted by Bruce Brandt

In the new world of wireless devices and remote I/O we have the power to create control configurations that use I/O and that are not physically connected to the controller running the control loops.

In the new world of wireless devices and remote I/O we have the power to create control configurations that use I/O and that are not physically connected to the controller running the control loops. In the case of at least one distributed control system (DCS), the I/O attached to its gateway can be assigned to different controllers on a per I/O point basis. Within the confines of the configuration application there are pointers to where these live, but in the context of project documentation the world becomes a bit blurry.

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Microsoft Windows: The evolution and where MS operating systems stand today

Jan 10, 2014 4:22:41 AM | Posted by Art Howell

Since the 1980s Microsoft has been a leader in operating systems, but these days the competition with Android and Apple devices has them struggling to keep up.

My recent posts have entertained the subjects of PCs in the workplace and virtual machines and how they fit into the workplace. Following suit, I’d like to touch on some Microsoft (MS) Windows history and what possible role the operating system (OS) might play in the future of computing.

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Good vs. poor documentation: Don’t be ‘that guy’

Dec 17, 2013 8:20:23 AM | Posted by Jeff Monforton

A well organized and well documented system, complete with commentary within your code, can only help you and your fellow developers and programmers.

Over the years we have all had to modify, repair, debug, and otherwise live with someone else’s code. The platforms vary, but the challenges remain the same—the biggest of which is, “What in #@$! was this guy thinking?!” Looking at that single—sometimes painful and often confusing—question leaves us wondering how it happened in the first place.

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Some perspectives on MES implementations: Part 1

Dec 17, 2013 8:19:13 AM | Posted by John Clemons

A lack of understanding or definition of MES within a company can lead to the same mistakes being made on project after project.

I think everyone knows by now that I’m a manufacturing execution systems (MES) guy from way back. I’ve been doing MES since way before they even called it MES. In fact, when I started doing it we didn’t even have a name for it. MES is one of the names for a class of computer-based systems that are focused on the execution side of manufacturing. Another common name for these systems is manufacturing operations management (MOM).

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Penny wise and pound foolish

Dec 17, 2013 8:18:01 AM | Posted by Bruce Brandt

A few appropriate costs early in a project can create major savings down the road and avoid a whole lot of aggravation in the process.

I’ve just finished helping support a project startup that has gone rather well in spite of some decisions to save cost at the start. I continue to be amazed time and time again that companies underestimate the impact of cutting the wrong costs, simply because they don’t know what the project really should cost before they go for funding. The project in this particular case is actually the follow-on to the first phase of a system migration from one DCS platform to another. The customer’s old platform had been in service for a very long time and was becoming unsupportable when things failed. Since any migration was going to be a rip-and-replace, they decided to investigate what was now available in the market place and chose not to stay with the original installed platform vendor for technical reasons. Phase one was done by the vendor of the new DCS, but to save cost the configuration was off-shored. This might have worked if the customer had developed a really good scope, but for reasons I do not know, they did not.

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Classified-area control panels

Dec 17, 2013 8:16:12 AM | Posted by Andy Crossman

4 basic design concepts that could help avoid costly errors or worse.

Designing a control panel rated for a classified area can be a daunting task for individuals with limited or no prior experience. These four basic steps should get you started in the right direction:

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