Arc Flash Study with No Protective Devices?

[Eleceng01]

We received an arc flash study from a contractor & I have been asked to review it. The contractor did not model any of the protective devices (PD), but made these assumptions.

1) That the first PD will operate in 6-cycles.
2) That the backup PD will operate in 1-second if the first fails.

I am ok with the first one since a relay + breaker time can be 6-cycles. But my review shows many of the arc flash values that they provided are at 2 seconds (I assume from IEEE 1584). Our managements issue is that according to the report, everything is >40cal/cm^2.

I know you need to consider 'worse case' at a location, but it seems to me it isn't right.

Is anyone else doing/seeing studies in this fashion?

I believe that you need to model the relays, fuses, etc. - specifically we have a bus differential on our main bus that they claim is >40cal/cm^2 & I have a hard time believing this.

Am I missing something on NFPA 70E?

Thank you in advance for your thoughts on this.

Kevin

 

[dpc]

Of course you need to consider the action of the protective relays. That is the entire point of doing the study.

The contractor made it easy on himself. Without seeing his scope of work and your specifications for the study, I'm not going say the contractor is totally to blame, but this type of study is not really useful for much of anything. Did they also assume the short circuit currents?

If this report was done by an engineering consultant, you should have a talk with them.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[patrick7961]

you mentioned that the clearing time was 6 cycles. what was the inc energy at that clearing time? if it was above 40 its only going to get worse at 2 sec.

 

[JMatt]

That sounds bogus. If they don't analyze realistic expected fault-clearing times, the study is no good. They may as well draw a 1000 ft. circle around every piece of elec. equip and say, "Do not enter."

I'm a bit less forgiving than dpc. If I were you, I'd get a different consultant. That kind of "effort" is unacceptable.

 

[Eleceng01]

Thanks all! I am in total agreement, but I wanted to make sure I wasn't the one that was wrong before I talk to management. I don't believe the report is worth the paper it is written on!

I will go talk to management now & explain my issues with them.

 

[jghrist]

I have not heard of any requirement to consider backup PD times in calculating arc incident energy.

 

[antigfk]

Ask the contractor if he used the NFPA 70E or the IEEE 1584 calculation methods. If he answers that he used either one of those, then he is a liar and a crook.

 

[Electic]

In the early days of arc flash analysis, I saw reports suggesting what your contractor did, assuming breaker operating time as though everything will clear as quickly as possible.

Unfortunately we soon learned that it is not so simple and breaker operating times need to be extended by time current curves for the available ARCING fault current.

I am not going to say your contractor is a liar or a crook; Much of our work has been to turn down instantaneous pickup points to operate on the available arcing current,thereby reducing arc flash exposure to workable levels, and often it seems we are playing an arbitrary game.

 

[EddyWirbelstrom]

PD operate times at arcing current is essential in calculating the arc-flash incident energy to IEEE 1584-2002.

gjhrist - PTW Arc-Flash software can check for mal-grading between the PD immediately upstream from the arcing fault and the next upstream PD. If mal-grading exists, the user can specify the number of upstream PD to consider when determining which PD will trip.

 

[Eleceng01]

Mal-grading is a new one for me. I kind of like the term though I am more familiar with mis-coordinated.

 

[Zogzog]

I bet this contractor used a free online arc flash "caculator" to do the study and agree that it is not worth the paper it is printed on.

This has concerned me for a while, I have seen many contractors use these free on line tools and have no idea what they are doing. I had one job where I bid around $40,000, a rival firm bid about the same, and someone we had never heard of bid $6,000. Guess who got the job?

 

[EddyWirbelstrom]

When quoting for arc-flash projects to IEEE 1584 I include in the deliverables typical examples of :
- one-line diagrams annotated with arc-flash study results and also PD type and settings.
- spreadsheet with arc-flash study results.
- lables with incident energy at the specified working distance and the required PPE.
PTW's Arc-Flash evaluation produces the above documents.

When comparing quotes the client can compare apples with apples.

 

[Eleceng01]

EddyWirblestrom, I agree it is best to compare apples to apples, but sometimes the client (i.e. project managers who might not be electrical) don't know an apple from an orange! That is why I was asked to review the report, it would have been better if they had asked for my assistance earlier.

All, no they actually used ETAP (not knocking ETAP, just the contractor here). They just didn't follow IEEE1584 or anything else for that matter if you ask me!