NFPA 70E Tables Maximum Clearing Time

[slushin]

So, I know that the tables for NFPA 70E cannot be used if the exact parameters are not met (ie working distance, fault clearing time, max short ckt current, etc). However, I'm confused as to what the meaning of maximum fault clearing time means. Does it mean:

1) the actual maximum time that the upstream devices could take to open. This would include the overload regions of the device curves (which can take hours to trip/blow) and would almost never meet the table requirements.

2) the upstream device's operating time at the maximum short circuit current listed in the table (ie table says max available short circuit can be no greater than 65kA, so use 65kA)?

3) the upstream device's operating time at the actual maximum available short circuit current of the equipment being worked on (ie short circuit study shows that max short circuit current is 37kA, so find the operating time for 37kA)?

4) something else?

As always, thanks.

 

[wbd]

As I posted on Mike Holt' Forum:

The tables represent at best a compromise to give people something to provide the proper PPE. The problem is that the majority of the people do not know what the short circuit levels are in their facility. In order to determine that a short circuit study needs to be done and if that is the case, it is minimal additional effort to do an arc flash evaluation.

All that being said, the table parameters mean that it is the fault current at the equipment being worked on that the protecting device would see. For example: A panel cover needs to be removed to perform IR scan. The breaker feeding this panel would have to trip in less than the maximum time and the fault current would have to be less than the maximum listed fault current. If the panel has a main breaker, this is not the protecting device as it could be compromised in an arc flash. The maximum current parameter is also not the maximum possible fault current but the actual fault current. The tables also ignore that the arcing fault current is less than the bolted fault current. So, how do you know that the parameters of the table are being met?

Therefore using the tables is a compromise at best. There was a study done by DuPont on arc flash events at their facilities and they concluded:

having no PPE resulted in injuries 100% of the time
using the NFPA 70E tables resulted in less than adequate PPE resulting in injuries 50% of the time
performing a study to determine actual incident energy levels resulted in proper PPE and no injuries



So, I guess the obvious question is how much do you want to compromise the safety of your employees?

 

[Skythian]

The modified NFPA 70E 2015 table method is not fit to be used in the first place. Why? The fault clearing time is required in order for the method to be used. The time cannot be determined unless the arcing fault current is known. Although the NFPA 70E incident method provides formula from the IEEE 1584 Guide for calculating arcing current from the IEEE 1584 Guide, the formula is an integral part of incident energy method and the new standard now specifically prohibits mixing both incident energy and table methods. Hence, NFPA has rendered unusable its own table method in the new NFPA 70E 2015 edition.

 

[wbd]

The fault time of the tables is bolted fault current not arcing current.

 

[stevenal]

The words used are "maximum clearing time." Not sure the maximum time necessarily aligns with the available fault current.

 

[dpc]

The fault time of the tables is bolted fault current not arcing current.

Just curious as to what are you basing this statement on? I don't see anything in the table or the footnotes that indicate this.

 

[davidbeach]

Dave, you're back, it's been a while.

 

[dpc]

I'm still alive - just taking a break for a little while. I'm only working part-time at this point trying to make a soft landing in retirement. We'll see how it goes

 

[wbd]

Just curious as to what are you basing this statement on? I don't see anything in the table or the footnotes that indicate this.

Based on the term "Maximum of XX kA short-circuit current available" as there is no qualifier stating arcing fault current and that it may be possible that a short circuit study was done at the facility. That is the big problem with the tables is that the fault current is usually unknown but the tables are used anyhow. The better than no protection at all thought process.

 

[alexfox]

Based on the term "Maximum of XX kA short-circuit current available" as there is no qualifier

I didn't think of that. I should look more into what everyone else is doing. I have a feeling this may be covered ad nauseam on other forums, but I'll contribute here.


It also says "fault clearing time", and knowing that we are dealing with an arcing fault, I use that value. At least I would use the arcing fault if I were using the table but once I've gone that far I always finish with incident energy. Thinking about it, I would go nuts and use the 85% of arcing fault recommendation from 1584. I still get disappointed every time that makes a substantial difference in trip time.

The table parameters set upper and lower bounds on the energy values where the PPE recommendations are comfortable. Take PPE Cat 2 for an MCC as an example. Imagine the box drawn on time/current axes with vertical bounds 85% of arcing current up to 65kA and horizontal bounds at a fraction of a cycle (current limiting fuse) up to 2 cycles (molded case breaker instantaneous). If your situation falls anywhere in that range, PPE 2 should be appropriate. MCC PPE Cat 4 does the same thing using 20 cycles (breaker short time segment).

Most of the time you're trying to answer the question "What range will my protective device trip in during an arcing fault?" When the answer is slower than the instantaneous or current limiting region you could have a hot spot or outlier depending on the system. Here is a quick trick I use to start looking for <600V areas that may be a high energy problem when I'm on a site and haven't done a study yet. NEVER use this for anything more than impressing your friends, it is a bad but fast estimate NOT an engineering calculation.

Max Short Circuit kA (based on system xfmr KVA and %Z) * 80% (wild guess depending on how far the site is from the utility and amount of motor load) * 50% (wild conversion from bolted to arcing fault) / 10 (breaker inst setting estimate)

It works like this:
2000KVA/0.0575/480V/1.732 = 41.8kA Max Short Circuit let thru

41.8kA * 80% * 50% / 10 = 1672A
So I know that devices around 1600A and higher will have a greater chance of tripping in the short or long segments depending on how they are set.

 

[wbd]

I do not have the software that's why I am wondering about the equation as OSHA only considers ARCPRO for calculating Arc Flash catagory above 15kV system.

The best forum for arc flash is Jim Phillips forum:

http://www.arcflashforum.com

You will find this discussed there.