Short-Circuit and Device Coordination Programs

[markstg]

Sometimes I will run into and installation that I am unsure of if it is suitable. Then I will request the letter from an engineer...passing the responsibility of my approval directly on to the engineer.

And that is very appropriate. We each have our responsibilites, the engineer to design, the EC and Inspector to install. When the engineer/EC/Inspector have concerns, it is appropriate to discuss, ask questions, get answers, and keep for the record, with the responsible party making final judgement, and sometimes that is the Engineer, sometimes its the Inspector.

Markstg,
Just as a note, I've done countless short circuit, overcurrent PD coordination and arc flash studies and have not been asked to sign/stamp/seal any of them. Although it would be easy enough to do so, but I've never been asked.

Normally I would not think that you submit these calculations (as you don't submit the numerous calculations for cable sizing etc.) as part of the "Issued for Contruction" documents. But if you did, just as you Seal the drawings, I would think you would seal your submitted short circuit studies, if you submit them outside your office, say to the owner for record. In my state all documents produced for Contruction or submitted to the owner for record that are produced by the Engineer must be sealed.

I would not approve it, as I mentioned, I would pass it on.
I was just curious as to who is actually looking at the studies? Is there ever a challenge to the study as being improperly performed?

The electrical engineer of record, ie. the one who has responisble design, is looking at, and probably produced the study.

Generally no one challenges the studies, as in almost all cases, the study is done correctly. But when a circuit breaker trips when everyone thinks it shouldn't have, (eg, the main trips when the fault was sownstream on a sub feeder or branch circuit)the engineer may be called upon to review, and he most likely will look at the study prior to making any changes. Worst case, a circuit breaker doesn't trip, say because its instaneous is set to high, causes damage to equipment. Someone is going to be asking some questions.

I have a question.....On circuit breakers that have a high/medium/low instantaneous trip adjustment, who selects what position to put it in. In my experience, its left where the manufacturer set, unless it trips, then it is set to high. Should the EC set it, should the Engineer tell the EC where to set it? Does it matter?

 

[rcwilson]

In the last 20 years we have used ETAP, SKM, EDSA, CYME and Easy Power, Excel spreadsheets using the MVA method, and hand calculations, etc. I personally haven't used anything but Easy Power and my calculator for a few years, so my judgment is tainted.

Easy Power was the quickest to learn to use. It does load calculations, short circuit, voltage drop, motor starting, load flow, arc flash, and other analysis. It quickly draws coordination curves that can be easily adjusted by sliding the curves around the time-current chart. (Other programs can probably also do that. Some programs require getting out of the coord curve module and inputting new values. That is a pain.)

Other programs were more powerful but we were able to get Easy Power to do most of what we need and store the various scenarios for future reference.

Output from any of the programs needs to be checked by a quick hand calculation and a verification of critical coordination curves. It is hard to tell if the program is giving you the correct device characteristic unless you check it.

 

[dahualin]

NEC requires it, OSHA requires it.

SC and protective device coordination studies are the first two elements necessary for an Arc Flash study.

I am wondering how you guys do the studies for the design document/comstruction document. At this stage, no switchboard/panelboard has been ordered or purchased. We don't have any specific circuit breakers to use for the selective coordination study. Am I missing anything?

 

[VUGear]

I am curious about which AHJ requires this kind of studies and what kind of clients require this kind of studies and what code requires this kind of studies. I have heard about OSHA requires Arc Flash study. Thanks.

Coordination and fault studies are also required to be stamped and submitted as part of the construction documents in Florida for healthcare work. The Agency for Health Care Administration guys are well-versed in reviewing them and will definitely reject them for things like lack of coordination.

 

[Mr.Dave]

I've used Easypower and ETAP. Both have their limitations but overall Easypower is easiest.

For coordinations, arc flash and most load flow Easypower is the way to go. Easypower will not model single phase loads however. They also have great support and a digitizer is available to allow you to add fuse curves from factory published data.

One problem we have noticed with Easypower is sometimes its easy to confuse it, especially if you have multiple generators or ties. Sometimes with the first iteration the VAR limits will peg out on a generator causing you to have to fiddle with settings to get things to come out right. I've seen a ring bus or two confuse it in a similar way. Easypower also has the ability to interface with PI, taking actual plant values, and using them in the model. This comes in handy if you're planning on doing switching between ties, generator pairs, etc.


With ETAP, it is harder to use but I've never seen multiple generators or ties confuse it. I've never done a coordination with it though.

ETAP also can calculate cable ampacity in a duct bank with various duct and cable arrangments.



For what its worth Easypower will take care of you 99% of the time and be the easiest and fastest. They do have a trial period so you could talk to them and do a trial.

Hope this helps!