Talking back between our readers

When David Loucks of Eaton wrote an article on selective coordination, he didn't have to wait a month for a reaction from Plant Engineering readers. The reaction came the next day, via the Talkback feature at www.PlantEngineering.com “In his article,” the reader wrote, “Mr. Loucks indicates that even when fault current falls within the instantaneous trip range of two circuit b...

11/15/2007


When David Loucks of Eaton wrote an article on selective coordination, he didn't have to wait a month for a reaction from Plant Engineering readers. The reaction came the next day, via the Talkback feature at www.PlantEngineering.com

“In his article,” the reader wrote, “Mr. Loucks indicates that even when fault current falls within the instantaneous trip range of two circuit breakers exposed to the same magnitude of fault current, the breaker closest to the fault could be the first breaker to open, as the breaker closest to the fault would likely be the smaller (lower amp frame and trip) of the two breakers and would have lower mass to overcome allowing it to start opening first and introducing arc impedance to the faulted circuit, thus reducing the fault current to a level lower than the instantaneous pickup of the upstream (and slower) breaker.

“My first question is once the instantaneous trip has been triggered on any circuit breaker, is this not the point of 'no return,' meaning that the breaker will trip, regardless of what happens to the fault current over time?”

And the reader added this thought: “Relying on the idea of a downstream breaker acting faster than a larger upstream breaker in the instantaneous trip range of both breakers to prevent the upstream breaker from tripping instantaneously sounds like something that would need testing to confirm such operation and not something that can be guaranteed.”

At PlantEngineering.com, Loucks was able to respond with a complete explanation, and a conversation had begun.

“Regarding the instantaneous tripping issue, yes, if in fact both breakers were in the process of opening, then you would have a point of no return; and while the smaller breaker might open first, the upstream breaker would still trip eventually,” wrote Loucks. “But what I was talking about was not the breaker contacts opening, but rather the tripping mechanism within the breaker that tells the breaker when to trip.

“The larger breaker will have a larger, heavier tripping mechanism that must be accelerated with a force and/or distance greater than what would be required on a smaller breaker. When that smaller breaker tripping mechanism operates a few milliseconds before the big one, you add arc impedance to the circuit.

“And I agree about your testing comment. You couldn't guarantee that this would work in all cases. Another interesting reason that could explain why we don't hear about selective coordination issues, at least from properly sized and set breakers, is this %%MDASSML%% the 2005 code mandates selective coordination on emergency systems.”

The Talkback feature at www.PlantEngineering.com is a great way to contact story authors and editors. It's also a great way to build a network of plant managers. That kind of dialogue is important today as plant managers around the world work to solve common problems on the plant floor.





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.