arc flash - goofy results - thoughts?

[malachi constant]

Not sure whether this is a good forum to discuss arc flash studies/results, but here goes. I can post the SKM file if it helps.

Is it normal for available arc flash energy (cal/cm^2) to increase the further down the system you get?
For instance, at the main bus I've got 6.6 cal/cm^2.
15' of copper later, at the next bus I've got 14.4 cal/cm^2.

Out of curiousity I cloned a couple more buses and got...
15' later, at the 3rd bus, I've got 20.6 cal/cm^2.
15' later, at the 4th bus, I've got 22.4 cal/cm^2.

FWIW, the main is protected by a 1600A Littelfuse KLLU. System voltage is 208V 3P. This is the first round of studies I've done myself, and is typical for most of the facilities. This does not strike me as normal and I wonder if I'm doing something wrong...or maybe it is just normal.

Thanks!

 

[jim dungar]

Is it normal for available arc flash energy (cal/cm^2) to increase the further down the system you get?

Absolutely yes.

simply, the further downstream you go the less current flows due to the increased conductor impedance. The less current, the longer it takes for a protective device to operate.

 

[texie]

Absolutely yes.

simply, the further downstream you go the less current flows due to the increased conductor impedance. The less current, the longer it takes for a protective device to operate.

The OP question and your answer is very timely for me as, just today, this came up in practical way on a job today. I had a group of electricians applying available fault current and incident energy labels to panels that were provided by the engineer of record. One came up to me and said there must be some mistake as the label for the supplying panel had a higher available fault current but yet a lower incident energy than the panel it was feeding downstream. So I had to have a little class on the difference between available fault current and incident energy with the crew. Now mind you, I can only go so far in explaining this as I am an electrician, not an engineer, but from many miles and reading I have a basic grasp.
My point is, we have a long way to go in this area as a trade. I know you engineers fully understand this, but the average journeyman has not a clue. For example, the few journeyman in the group who even new of available fault current thought it related to the PPE required.

 

[charlie b]

The thing to keep in mind is that the fault will continue to throw sparks about until some upstream OCPD does its thing, and thereby terminates the event. The longer it takes for the OCPD to actuate, the more energy will be released at the fault point. But the time it takes to actuate a breaker or fuse is very dependent on the characteristics of the breaker or fuse. Altering a breaker's setpoints will significantly alter the available arc flash energy at every point downstream of that breaker. That is why you cannot perform an arc flash study until the exact make and model of every breaker and fuse is known, and until any decisions regarding breaker settings have been made. I am having a hard time getting several of my clients to understand this basic concept. They want me to submit a series of calculations, including selective coordination and arc flash, at multiple stages throughout the design process. Please note that the design is not final, it has not been put out for bids, the EC has not been selected, and the EC's choice of equipment vendor has not been made. So an arc flash study performed before all these things have happened would be completely meaningless.

 

[Sahib]

charlie:
one way to convince your clients about the variability of arc flash hazard is to tell them that arc flash warning label on equipment is the responsibility of the owner and not that of the equipment manufacturer.

 

[abhishekbt]

Thanks for sharing the results, I would like to request you for SKM file and I am sure it will help many including me as well.

 

[malachi constant]

Absolutely yes.

simply, the further downstream you go the less current flows due to the increased conductor impedance. The less current, the longer it takes for a protective device to operate.

Bumping this back up as I have a related follow-up question. But before I get to that: your answers to my initial question made perfect sense - thank you!

So here's the next question.
* Bus1 is protected by a breaker 80' upstream. At Bus1 there is a 15,411 fault current, and the energy is 1.9 cal/cm^2.
* Bus2 is 70' downstream of Bus1. Bus2 has a main fuse just ahead of it. At Bus2 there is a 14,917 fault current, and the energy is a whopping 60.9 cal/cm^2.

I assume the SKM program is doing simple steps here. (Q: What is fault current? A: 14,917. Q: How much energy is released before the nearest overcurrent protection devices clears? A: the nearest OCPD is the fuse, so 60.9.) But wouldn't the upstream breaker likely trip before the fuse opened? When I remove the fuse from the system I get the energy down to 1.8 cal/cm^2.

My guess is SKM uses the logic per my assumptions above, and that it does so because they assume the OCPDs are coordinated with each other. In this particular instance the OCPDs are not coordinated. (That wouldn't otherwise be a big deal as there is nothing else in between the breaker and fuse.) Should I remove the fuse from the model and use 1.8 cal/cm^2?

Or I could be wrong - maybe the fuse is actually limiting current, causing the breaker to not trip as quickly. I doubt SKM is that detailed though. To test this a little more I took the Bus1 breaker out of the model, put the Bus2 fuse back in, and again got 60.9 cal/cm^2 on Bus2. So the Bus1 breaker has no affect on Bus2. SKM is almost certainly only looking at the first upstream OCPD. I understand why, but think it is giving me results that defy common sense.

I appreciate any of your thoughts. I do not want to be the engineer that plugs stuff in the software without trying to understand what's going on under the hood. Thanks!

 

[mayanees]

SKM

SKM

A couple tips with SKM:
In order to address the miss in coordination that you refer to, when in the Arc Flash Study module, Report Options, check the "check for miss in coordination" box, and look up a couple levels.
Also, an excellent AF troubleshooting tool is to right-click on the bus you're assessing, go to arc flash with TCC, and it will lay the arcing fault current on a TCC along with the ocpd, and show you exactly what's happening.
Peace

 

[malachi constant]

A couple tips with SKM:
In order to address the miss in coordination that you refer to, when in the Arc Flash Study module, Report Options, check the "check for miss in coordination" box, and look up a couple levels.
Also, an excellent AF troubleshooting tool is to right-click on the bus you're assessing, go to arc flash with TCC, and it will lay the arcing fault current on a TCC along with the ocpd, and show you exactly what's happening.
Peace

"Check for miss in coordination" - that is a brilliant suggestion. That is exactly what I was looking for. Thank you.
Regarding #2, when I right-click on the bus it does not have a "go-to-arc-flash-with-TCC" option. I can go to arc flash, and go to TCC, but not seeing anything on those that lays the arc fault on a TCC with the OCPD. That would be very helpful, if I am missing a step please advise.

Thanks!

 

[mayanees]

"Check for miss in coordination" - that is a brilliant suggestion. That is exactly what I was looking for. Thank you.
Regarding #2, when I right-click on the bus it does not have a "go-to-arc-flash-with-TCC" option. I can go to arc flash, and go to TCC, but not seeing anything on those that lays the arc fault on a TCC with the OCPD. That would be very helpful, if I am missing a step please advise.

Thanks!

There's a step Go to Arc Flash + TCC drawing for the version we're on (7.0.2.9), and I'm not sure how long that's been available.