Arc Flash 70E Calculations(simple Explaination)
- Thread starter yoopersup
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Arc Flash 70E Calculations(simple Explaination)
The is no truly accurate way to explain arc flash calculations in a "single sentence".
Basically, arc flash is concerned with the amount of energy released during an arc. It is made up of two components the size of the arc and the length of time it exists. But these two factors are not directly related to the normal maximum short circuit amps used for selecting AIC/SCCR ratings of devices.
The is no truly accurate way to explain arc flash calculations in a "single sentence".
Basically, arc flash is concerned with the amount of energy released during an arc. It is made up of two components the size of the arc and the length of time it exists. But these two factors are not directly related to the normal maximum short circuit amps used for selecting AIC/SCCR ratings of devices.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Arc Flash 70E Calculations(simple Explaination)
You start with the three phase bolted fault current available at the piece of equipment (not the infinite primary maximum), convert to arc flash three phase fault current, using the time current coordination curve of the OCPD's upstream, see how long the device will take to open (clear the fault), the calculate incident energy and the associated PPE required.
You start with the three phase bolted fault current available at the piece of equipment (not the infinite primary maximum), convert to arc flash three phase fault current, using the time current coordination curve of the OCPD's upstream, see how long the device will take to open (clear the fault), the calculate incident energy and the associated PPE required.
zog
Senior Member
- Location
- Charlotte, NC
Re: Arc Flash 70E Calculations(simple Explaination)
www.epowerplus.com has some arc flash calculators for free download.
www.epowerplus.com has some arc flash calculators for free download.
Re: Arc Flash 70E Calculations(simple Explaination)
Guys
I got everything I need books ,Computer programs , Vidios ect. I;m looking for a Down to earh way to explain the the Journeyman in the field what its all about.So far no ones come up with a simple approch ,When you explain it to them there mouths hang open like well yaaaaaa????
Guys
I got everything I need books ,Computer programs , Vidios ect. I;m looking for a Down to earh way to explain the the Journeyman in the field what its all about.So far no ones come up with a simple approch ,When you explain it to them there mouths hang open like well yaaaaaa????
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Arc Flash 70E Calculations(simple Explaination)
Yooper,
I don't think that your question can be answered, as it is a very complicated process. I think it is silly to expect that every journeyman (or even every engineer) will be capable of the calculation. Many still don't understand the OSHA rules regarding energized work.
My suggestion is to walk them through some situations and perform the calcs, then show what the results are and what they mean. Then change the OCPD upstream and redo the calc, and show why the result is different.
Yooper,
I don't think that your question can be answered, as it is a very complicated process. I think it is silly to expect that every journeyman (or even every engineer) will be capable of the calculation. Many still don't understand the OSHA rules regarding energized work.
My suggestion is to walk them through some situations and perform the calcs, then show what the results are and what they mean. Then change the OCPD upstream and redo the calc, and show why the result is different.
charlie
Senior Member
- Location
- Indianapolis
Re: Arc Flash 70E Calculations(simple Explaination)
Also, keep in mind that you can't determine what amount of incident energy is at the service equipment unless the serving electric utility can run the calculations from the substation, through the overcurrent protection devices, and through the transformers feeding the service. They also have to be able to guarantee that there will be no switching to a different substation for the calculations to remain accurate.
In other words, you can't really do the calculations without making some wild assumptions.
Also, keep in mind that you can't determine what amount of incident energy is at the service equipment unless the serving electric utility can run the calculations from the substation, through the overcurrent protection devices, and through the transformers feeding the service. They also have to be able to guarantee that there will be no switching to a different substation for the calculations to remain accurate.
In other words, you can't really do the calculations without making some wild assumptions.
- Location
- Illinois
- Occupation
- retired electrician
Re: Arc Flash 70E Calculations(simple Explaination)
Don
And if you can't do the calculations, you can't select the correct PPE, so therefore, you are not permitted to work on energized equipment. I don't see any way that work on energized equipment can be permitted under the OHSA rules given the impossibility of selecting the correct PPE.
Don
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Arc Flash 70E Calculations(simple Explaination)
You are technically correct. An exact calculation cannot be made due to variables. Unfortunately, there is a lot of energized work going on. So we might as well follow the guidelines and standards to achieve some level of protection vs. no protection. JMHO
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Arc Flash 70E Calculations(simple Explaination)
It seems strange to me to do the calculations based on a bolted fault. I would think one of the only times a bolted fault would truly occur would be upon the initial energizing of a circuit, and then only if there was a rather extreme wiring error. Other than that application, I would think that 99% of faults would be arcing faults, which provide a slower fault-clearance time.
It seems strange to me to do the calculations based on a bolted fault. I would think one of the only times a bolted fault would truly occur would be upon the initial energizing of a circuit, and then only if there was a rather extreme wiring error. Other than that application, I would think that 99% of faults would be arcing faults, which provide a slower fault-clearance time.
charlie
Senior Member
- Location
- Indianapolis
Re: Arc Flash 70E Calculations(simple Explaination)
You guys are right. The normal fault will be arcing with its inherent dynamic impedance. That means you don't have a clue about how many cycles the overcurrent device will hold in. The fault current you get from most (all?) electric utilities is the maximum under any conditions. In our case, it is the lowest impedance transformer we have, an infinite bus, and a bolted fault. None of those conditions will likely exist.
The information is given to permit you to choose the proper gear and overcurrent protection, not for incident energy calculations. If the overcurrent device takes longer to clear, the incident energy could be much higher than if you had a higher amount of fault current available.
You guys are right. The normal fault will be arcing with its inherent dynamic impedance. That means you don't have a clue about how many cycles the overcurrent device will hold in. The fault current you get from most (all?) electric utilities is the maximum under any conditions. In our case, it is the lowest impedance transformer we have, an infinite bus, and a bolted fault. None of those conditions will likely exist.
The information is given to permit you to choose the proper gear and overcurrent protection, not for incident energy calculations. If the overcurrent device takes longer to clear, the incident energy could be much higher than if you had a higher amount of fault current available.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Arc Flash 70E Calculations(simple Explaination)
Ryan,
The procedure begins with bolted fault current, then utilizing formula, you get an arc flash fault current value, which is less than a bolted fault current.
The formula was developed after performing some test faults and normalizing the formula to those results.
More testing done, will result in modification of the formula over time to even further give closer results to reality (never actually achieving reality because there are too many variables IMHO)
[ May 05, 2005, 04:49 PM: Message edited by: ron ]
Ryan,
The procedure begins with bolted fault current, then utilizing formula, you get an arc flash fault current value, which is less than a bolted fault current.
The formula was developed after performing some test faults and normalizing the formula to those results.
More testing done, will result in modification of the formula over time to even further give closer results to reality (never actually achieving reality because there are too many variables IMHO)
[ May 05, 2005, 04:49 PM: Message edited by: ron ]
zog
Senior Member
- Location
- Charlotte, NC
Re: Arc Flash 70E Calculations(simple Explaination)
I guess I am confused on why a journeyman in the field would need to know how to do the calculations. If it is select the proper PPE then use the PPE selection tables in the 70E, thats what they are there for. (Just be sure they understand the limitations of the tables and you have the revision to the 2004 70E tables)
If they need to know a FHB, then tell them to use the 4 ft rule, make sure this is only good up to 300kA-Cycles. Anything higher, use the basic calc (sqrt of(MVA*53*t)) using transformer nameplate data and some basic clearing times.
Anything above that, send them to a good NFPA 70E training class.
I guess I am confused on why a journeyman in the field would need to know how to do the calculations. If it is select the proper PPE then use the PPE selection tables in the 70E, thats what they are there for. (Just be sure they understand the limitations of the tables and you have the revision to the 2004 70E tables)
If they need to know a FHB, then tell them to use the 4 ft rule, make sure this is only good up to 300kA-Cycles. Anything higher, use the basic calc (sqrt of(MVA*53*t)) using transformer nameplate data and some basic clearing times.
Anything above that, send them to a good NFPA 70E training class.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Arc Flash 70E Calculations(simple Explaination)
Thanks Ron, I wasn't aware of that.
Thanks Ron, I wasn't aware of that.
ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Re: Arc Flash 70E Calculations(simple Explaination)
Zog,
Unfortunately, the notes to those tables make the tables essentially unusable, since the conditions are almost impossible to meet.
Zog,
Unfortunately, the notes to those tables make the tables essentially unusable, since the conditions are almost impossible to meet.
kencybart
Member
- Location
- Des Plains, IL
Re: Arc Flash 70E Calculations(simple Explaination)
To yoopersup and company,
Most "jobs" can be found in table 130.7(C)(9)(a) and table 130.7(C)(11) of NFPA 70E that tell the journeyman what to wear when working on energized equipment. If it's not listed in the tables, then they have to do the math.
To yoopersup and company,
Most "jobs" can be found in table 130.7(C)(9)(a) and table 130.7(C)(11) of NFPA 70E that tell the journeyman what to wear when working on energized equipment. If it's not listed in the tables, then they have to do the math.
charlie
Senior Member
- Location
- Indianapolis
Re: Arc Flash 70E Calculations(simple Explaination)
Bob, the problem is that you can't determine what that number is with any certainty.
Re: Arc Flash 70E Calculations(simple Explaination)
You may be right, Journeyman may not NEED to know, but many like myself are always trying to learn the intricacies of all aspects of the electrical field. That is why these question and answer forums are open to all for observation and personal betterment may come from it. Absolutely no disrespect meant by my statement
Jay
Journeyman Electrician
Keene NH
You may be right, Journeyman may not NEED to know, but many like myself are always trying to learn the intricacies of all aspects of the electrical field. That is why these question and answer forums are open to all for observation and personal betterment may come from it. Absolutely no disrespect meant by my statement
Jay
Journeyman Electrician
Keene NH
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