Fault Current and Arc Flash

Status
Not open for further replies.
T

trsmith

Member
Location
North Carolina
We are establishing an Arc Flash program to comply with NFPA 70E. I have one line diagrams, conductor lengths, conduct size, number of conductors, size race way. Our utility company will be submitting the fault current on the secondary side of their transformer. Could you guide me in making the calculations, or, offer some software and/or "on line calculator"? Thanks in advance for your time.
 
Be aware that the "estimates" provided by your local utility will likely be higher than actual available fault currents which will in return skew your calculations. This may result in lower levels of protection than you may really need.

Most information on fault currents provided by the local utility is not sufficient for calculating PPE. They are only accurate enough for calculating SCCR and AIC ratings of equipment and panelboards.
 
trsmith said:
We are establishing an Arc Flash program to comply with NFPA 70E. I have one line diagrams, conductor lengths, conduct size, number of conductors, size race way. Our utility company will be submitting the fault current on the secondary side of their transformer. Could you guide me in making the calculations, or, offer some software and/or "on line calculator"? Thanks in advance for your time.
Click to expand...

Programs: ETAP or Power Tools for Windows
Calc Methods: IEEE 1584

Because your asking this question, you obviously need help in understanding arc flash, therefore hopefully you are going to have someone trained in doing these calculations guide you through them and review the results.
 
Getting fault current from the Utility for arc Flash

Getting fault current from the Utility for arc Flash

I have been doing a bunch of arc flash studies lately. It can be challenging to get the information from the utility!
What I request is the actual three phase symmetrical fault current at the utility transformer primary, the fuse type and size and the impedence and size of the utilty transformer.
I think they are afraid of giving the actual versus the "infinite bus" value because service equipment does need to be protected from a future higher fault current.

So I request "both" numbers. Often it takes MULTIPLE emails & phone calls. Usually they have not even heard of arc flash, but that is changing fast.

It is important to look at the infinite bus number to size for AIC, and the lower fault number to look at arc flash. And remember the intention of NFPA 70E is to promote workers to work on deenergized equipment where possible.

Good Luck
 
utility numbers

utility numbers

I have found on projects also requiring arc-flash and coordination studies that the utility (in my experience) is also hesitant to provide this information because from my understanding it is not a static number it is a fluctuating value that is always changing, and at the rate the utility changes out it?s equipment it is harder to tell, in the case if you primary is coming from a utility sub located somewhere off site if that utility sub is fed from two different transformers and you service could be from either or depending on maintenance schedules of the utility, load available, or an emergency situation. So from that aspect you will not be able to pinpoint this vale all the time, there are a lot of factors involved. On one project I had the utility inform me that the available fault current to the site was 40 but depending on the situation could be as much as 75-80 after some utility maintence that was going to happen that next summer at their sub.
 
When we did our arc flash study, our utility account rep and one of their engineers came to visit us. They brought all the information we requested and more. We told them exactly what we wanted from them and they had no problem with supplying it. In fact, they were surprised that more companies, in the area, were not doing the analysis, to the same level that we were.

This facility can be / is fed from two different directions. We ran the numbers for both, the higher of the two is what we use.

We had always heard that getting what we want, from the utility was going to be difficult. In our case it was not. Bottom line, you must put forth whatever effort is required, to get the information you need. In the long run, it may save someone?s life.
 
I also had good luck getting numbers from the local utility for the one study I have done. I just told them that I was doing an arc flash study, and what I needed.

That was probably over 40 different services, and I know it took the utility a lot of time to compile all the info. and to do the calculations to provide the numbers I needed.
 
???

???

One question when you have multiple sources what is the best way to handle it if you design for the larger of the two you could throw off you protective coordination study, that results in the breaker settings, that could increase the arc-flash if your numbers are to high. I guess the only way to validate it is through the testing section of the specifications at the end of the project.
 
"I guess the only way to validate it is through the testing section of the specifications at the end of the project."

You nailed it right on the head there! All of these studies are worthless if you do not test the protective devices. The study is just based on a piece of paper (Curve) that tells you when it should trip.

The IEEE gold book says that air circuit breakers that have not been maintained within the last 24 months has a 77.8% of failing. That dosent mean they wont trip but it means they will fail in some respect, maybe take too long to trip, maybe too quick, maybe bad contact resistance or insulation resistance.
 
The IEEE gold book says that air circuit breakers that have not been maintained within the last 24 months has a 77.8% of failing. That dosent mean they wont trip but it means they will fail in some respect, maybe take too long to trip, maybe too quick, maybe bad contact resistance or insulation resistance.
Click to expand...
So 70E needs to say, "no hot work of any kind unless the breaker has been tested within the last 24 months".
Don
 
SKM is also a very good software program for arc flash computations, although a friend of mine is the main IEEE 1584 consulant for Power Tools, which I would also recommend.

And I'd like to add that I've found that all circuit breakers derogate over time, with that time being dependant mainly on environmental conditions. Maybe I've been totally sold by Bussmann, but I prefer to use fuses as the secondary protection because they will not derogate and thus will trip in the "expected" time and multitude.

Looking at this I guess that I am biased toward Bussmann.
 
Status
Not open for further replies.
Top