Arc Flash Analysis vs NFPA-70E Table 130.7.c.a

[Eleceng01]

I am looking for a kind of poll on this. My question arises out of a conversation about whether to perform an arc flash analysis or to use the NFPA table. The electrical system is a small facility with a single utility feed.

The 480V system is my main concern. Are people just assuming the NFPA table or are you having an arc flash analysis done?

I am arguing that the notes in the NFPA table make it difficult (or even impossible) to use, but others (where I work) disagree.

What do you believe?

 

[Zogzog]

You have to do some type of analysis either way. If you use the tables you have to ensure you can use the tables and know your fault current and clearing times.

Make sure you have the proper notes for the tables, they were revised after the 70E were printed. The newest printing has the changes in the back of the 70E, or they can be found on the NFPA website.

Plus the tables make alot of assumptions about body position, working distances, etc. By doing a study you will have closer to realistic values and most hazards can be mitigated with breaker settings, current limiting fuses, chicken switches, maintenance switches, etc.

Some people view the tables as an easy way out, I say do it right the first time, by the time you buy the PPE, develop your ESWP's, and train your workers, you will be glad you did the analysis.

When an analysis is done right you also end up with an up to date one line and proper protective device coordination.

 

[dpc]

I don't see a big problem using the table, provided it is verified that the system is within the boundary conditions stated in the footnotes to the table. And I'm saying this as someone who does arc-flash studies.

One of the biggest advantages of doing the study is that it will identify the portions of the system that have a high potential arc-flash energy. For example, if you have a 480 V MCC with a main breaker, the sections downstream of the main breaker probably have a relatively low arc-flash energy because of the instantaneous trip element in the main breaker. But what about the main breaker itself? A fault within the main breaker compartment could be much slower to clear and result in extremely high arc-flash levels. The table does not help with this.

Either approach will be much better than doing nothing.

 

[JFLovvorn]

I agree with dpc, especially where he mentions the advantage of identifying high-risk areas such as main breaker compartments. I've seen older facilities with fuse-protected 13.8kV/480V transformers in the 1500kVA range where a fault in the main breaker compartment of the MCC would take several minutes to clear. This results in a ridiculously high arc flash hazard value, and it's something you're going to want to know. In some cases it's a problem that can be addressed through procedural changes - not allowing workers to rack out an MCC main unless the transformer's primary switch is open and locked out, for example. But in other cases, such as a double-sided MCC lineup with a main on each side where work must be performed on one side while the other is still in service, it can't be addressed prodecurally and you must re-evaluate your protective devices.