POWER SYSTEMS STUDIES - NEVER ENOUGH INFORMATION

[PE (always learning)]

Hey everyone,

This is a question to those working on power systems studies. I've been doing a lot of power systems studies for low voltage systems and I keep running into issues where I never have enough information from the contractor / manufacturer to provide a fully complete study. I provide short circuit analysis, equipment evaluations, protective device coordination analysis and arc flash hazard risk assessment. Usually I provide a preliminary study that tells the engineer and manufacturer what the proper ratings of their equipment should be. My issue is that towards the end of my projects when I send out a final study that includes arc flash hazard risk assessments, I am kind of in limbo waiting for the manufacturer, engineer, and contractor to finalize everything on their end. I know that I can make assumptions on some things, but it just drives me crazy because I never quite have all the information that I need. For those of you that do these studies, do you just make assumptions and then provide the statements in your studies? Here's an example of an issue that I'm dealing with: I have a study with emergency systems and the contractor has not provided any information about the generator breaker or automatic transfer switches and I can't really make assumptions on this equipment if I'm providing an arc flash risk assessment or ratings for that matter. I don't want to hold up the entire study, but the contractors are impossible to get any information out of sometimes. Let me know what you all think.

Best Regards

 

[charlie b]

It depends on what your role is for the project. I have always been on the side of the Designer of Record. We never supplied the "final" calculations. At most, we would select a set of components and the breaker settings, and prove that a solution exists (e.g., that selective coordination would work with that assumed set of components). We included in the plans and specifications a requirement that if the contractor chose components other than those us included in our analyses, then the contractor was responsible for obtaining the services of a PE who would re-perform the analyses using the actual components and breaker settings.

 

[mayanees]

be the closer

be the closer

You have to learn to become a closer when it comes to Power Studies. I tell trainees that they should keep a list of any unknowns and the associated assumptions used to finish the Report.

In the case of your emergency system breaker information, that's pretty critical to the Study, so that should appear on your list as needing the information. But if pushed you could use a typical breaker used by the generator manufacturer, like Kohler uses Merlin-Gerin, and clarify that in the Report.

But that is the challenge of the Power Study, to make immediate contact upon project approval with the electric Utility provider for their requested information, build the model using building layouts and submittals, run the scenarios, and summarize everything in the report.

Worst-yet is when existing devices fail the short-circuit evaluation, that have been in service for 30 years without incident, and now you're going to report that they need to be replaced. That requires extreme scrutiny to make sure things aren't series rated, or that a current limiting fuse might not be listed with the breaker but does in fact reduce the let-through to an acceptable level.

That's why doing Studies is a profitable venture, and that's also why there's lots of incompetence out there doing them. Anyone can buy a package and perform the analysis.

So my advice is to become the closer.

Good luck. Because of arc flash, Power Studies are an area of electrical engineering that will always be there for the picking.

John M

 

[PE (always learning)]

Worst-yet is when existing devices fail the short-circuit evaluation, that have been in service for 30 years without incident, and now you're going to report that they need to be replaced. That requires extreme scrutiny to make sure things aren't series rated, or that a current limiting fuse might not be listed with the breaker but does in fact reduce the let-through to an acceptable level.

John M

I totally agree with you, this happens all the time and it's very frustrating. It is a nightmare dealing with existing facilities.

 

[PE (always learning)]

Thank you all for the solid advice. I agree with you John, the key to these studies is being able to close. I think that being an engineer, I sometimes tend to over analyze and think to much about trying to make the "perfect" study. I often get caught up with one detail and I need to make more educated assumptions. I have made those lists of unknowns and assumptions that you were mentioning earlier, but it can be aggravating sometimes to produce a study that is 75% complete when I know I will have to go back and update it when I get all final information. I'm learning and growing as I do more studies and appreciate your advice.

 

[gadfly56]

Thank you all for the solid advice. I agree with you John, the key to these studies is being able to close. I think that being an engineer, I sometimes tend to over analyze and think to much about trying to make the "perfect" study. I often get caught up with one detail and I need to make more educated assumptions. I have made those lists of unknowns and assumptions that you were mentioning earlier, but it can be aggravating sometimes to produce a study that is 75% complete when I know I will have to go back and update it when I get all final information. I'm learning and growing as I do more studies and appreciate your advice.

Don't be afraid to throw the offending parties under the bus. If I send an e-mail asking for information of a critical nature, the next time I ask I just forward the same e-mail so the recipient gets the idea that it's a second request and here's proof of the first request. The third request is another forward including the first two and gets cc'd to my boss, the project manager/GC, and maybe the owner. Usually there doesn't have to be a fourth request.

 

[smoothops10]

We do a power study in design mostly with conservative estimates for SCA from utility as it is not available yet and assumed xd’’ for generator of 0.15 pu. We generally didn’t do arc flash calcs in design but more and more clients recently are requesting less than 8 cal/cm2 at all busses so we do what we can.

We also require contractor to hire someone like you to do final study and arc flash labels. I’ve seen many of these guys use the same assumed 0.15 xd’’ for gen even when actual information is available. They also ignore things like the Cummins amp sentry. I’ve even seen utility values assumed simply cause they did not contact utility. Frustrating to me and I don’t accept these submittals. Others might but why would you want to take the liability.