Arc Flash Hazard Assessment on SWGR CB

[eeprom]

Hello. I am trying to assess whether or not it is safe to operate switchgear with possibly very high incident energies. NFPA 70e table 130.7(c)(15)(A)(a) says that normal operation of a circuit breaker or switch does not require arc flash PPE. My question is, what if the calculated incident energy of the switch or CB is very high, like >100 cal/cm2? Is there a value of IE above which it is NOT safe to operate the CB?

Thanks
EE

 

[ZeroSeq]

Any value of energy which exceeds the PPE you are wearing is unsafe.

 

[wbd]

First off, if you did an actual study so that the incident energy is known, then you cannot use that table but must perform your own risk assessment. (See Article 130.1 NFPA 70E-2015). Based on this risk analysis, PPE may not be needed for the task regardless of the incident energy level. Now if the available fault current exceeds the rating of the breaker, that is a whole different hazard.

One possibility is remote operation of the breaker.

With an incident energy level that high, did the study engineer look at ways of reducing that IE?

 

[eeprom]

I did the study on this, and yes I am using a remote operation of the breaker. I am asking a more general question. NFPA 70E considers the normal opening and closing of a circuit breaker as not requiring arc flash PPE. Does that assessments change with very high IE?

EE

 

[wbd]

Ok, then you should be able to calculate the incident energy at the distance the remote operator is being used.

The table is not concerned with the level of IE as operating a breaker is considered normal, i.e. it is what the breaker is designed to do. As long as it has been maintained, is installed IAW manufacturer's directions, has all the cover screws in place or latched and there is no indication of impending failure such as noise from arcing or any overheating, then you not doing anything that would cause an arc flash by operating the breaker.

Just curious what software you used for the study?

 

[ScottyUK]

wbd,

That's interesting. I have had a few discussions about this clause and I generally agree with your interpretation. Others insist on full flash suits for doing routine switching operations, on the basis that an arcing event theoretically could occur during routine operations.

 

[Zogzog]

The properly maintained part is the catch, that's more than just annual testing like some have been assuming, you need to refer to the OEM's recommended maintenance intervals and often you will find that involves a complete overhaul after a certain duration (10-15 years typically) or a certain number of operations (2,000 is a common one)

 

[eeprom]

I use ETAP for the study. I also agree with the assessment.

 

[wbd]

eeprom,
I curious on the very high IE as I have not seen many that high. Did you use the utility's available fault current and if this is the first main bus on the transformer secondary side, did you model back to the utility primary protective device?

 

[eeprom]

WBD,
Yes, this is the first bus downstream of a 2MVA transformer with Z of 4.6%. I do many of these studies and I find that nearly always the main disconnect has a dangerously high IE. I often require that the main disconnect is remotely operable. FYI, the IE on this particular project is ~78. I mis-stated earlier it being > 100.

The utility has not provided the available fault current (nor do they ever). What I typically do is find out what utility transformer feeds our station transformer. I put the utility's transformer in my model and then use infinite bus in front of that transformer. Sometimes I can even find out the feeder length and size.

EE

 

[wbd]

ok, I have never had a problem with any utility providing the available fault current, X/R, riser cable data, protective device data at the riser and transformer data. I believe that your incident energies are unrealistically high due to using an infinite bus.
You should be able to find out the actual values.

I have reviewed arc flash studies performed by major companies and found that many used unrealistically high utility fault currents. For example one report had the utility fault current at 40,000A at 34.5kV when I got the fault current from the utility it was actually 5,200A. Big difference. I also found that the study did not take into account the recloser 2 line sections away.

So bottom line is I do not believe your incident energy values represent the actual incident energy values present.

 

[eeprom]

I know, I've heard that before. Perhaps it depends where you work, whether or not the utility will give you (or even know) the fault current. I have gotten that data on a handful of occasions. But I should add that I do a lot of these fairly simple studies (disclaimer: I am not a major company, but I am currently correcting studies done by one), and the lower fault currents often result in the highest IE. High fault currents are relatively safe because of their fast clear times. A small station transformer with a long feeder will sometimes have a >2 second clearing time.

You can't expect the utility to call you every time they make a change to their grid. There are so many variables that can change over time that the best approach is to be cautious. The goal is to prevent putting personnel in a position where they can be hurt.

 

[wbd]

eeprom - I also perform studies and have been doing so for over 10 years. I worked with many utilities and some you have to be persistent in your request but I have always ended up with the actual values. Early on it was more difficult to get these values but the utilities have gotten better as they learn why the actual values are needed.

Most grids are fairly stable and changes are one reason why IEEE 1584-2002 has a 5 yr time frame on reviewing and updating the study.