We have a 4000Amp feeder busway duct that runs from the secondary of a 3000kva transformer to some 480V switchgear. This duct is about 100ft long and runs through an electrical room. The calculated arc flash energy on this bus is 95 cal/cm2 @ 18Inches (Very High).

Since this busway is completely sealed at what approach distance would we need to wear PPE? I am more concerned now since we recently had a failure on this busway but were lucky it was in a outside location where there was no personnel.

Thanks

The busway is enclosed bus bar so it is very similar to switchgear, panels, panelboards, etc in that you need no PPE working around it, walking by it etc.
I am assuming that you are not installing bus plugs while it is energized as that is something that should not be done.

I agree that it what I thought based on the 70E codes. We are not installing anything live just may be working or walking near this covered bus. Just more concerned because we had a fault that could have caused injury if someone was walking near the fault. We may look at some ways to reduce traffic in the area around the bus.

Thanks for the help!

You can also have a fault in switchgear and panels. If you concerned about the fault in a busway, could not the same thing be said for any piece of electrical equipment? Seems this might be a slippery slope.

Any time you get above around 1500 kVA for transformers where the secondary side is 480 V, expect to be somewhere above 40 cal/cm2. The best way to reduce this is going to be by placing some sort of overcurrent or other device on the secondary side. Some examples include fuses between the switchgear between the busway and the transformer or you could place bushing CT's on the secondaries of the transformer and connect them to a 50/51 relay which is wired to trip the primary side protection of the transformer if you are using a circuit breaker on the primary side or if it's a fuse, Square D has a very interesting device on the market that is basically a fuse with a remote "trigger" input where you supply the trip device. SEL makes a nice 6 input multifunction relay (the SEL 651) which can be used to trip on primary and secondary sides if you'd rather have an all-in-one solution. That's about it for truly reducing the secondary side hazard.

Outside of this consider that the failure rate for average circuit breakers is around 1 in 100,000 failures per breaker per year. For bus connections it's much less, about 1 in 1 million for busway. Most industry standards target a 1 in 100,000 fatality rates per year for a given incident (1 in 10,000 overall. Consider the fact also that this would be more applicable to someone who has a desk stationed directly in the area of the busway, not an area that is not used 24x7. If for instance it is only accessed once a day for perhaps 2.4 hours (to make the math easy) then the odds of someone being in the wrong place at the time of failure increases by an order of magnitude.

Hence the fact that 70E requires someone to be interacting with the equipment in such a way that it can cause an arc flash. Just walking past or working in the vicinity of a sealed enclosure doesn't count, and thus it is not an arc flash hazard...see the definition in 70E.

The tough part here though is how to do you service is...you have to test for absence of voltage. You can't really test on the secondary side safely so you are forced to test on the primary side. The question is...is this sufficient? Is there any way to get onto the wrong equipment? If so then you need to figure out a way to do this step safely, which probably means finding a way to reduce incident energy to someone reasonable.

Thanks for all the feedback! I agree with your recommendations and will evaluate the possible solutions to reduce the hazard for this bus. I can definitely see the complications of trying to wear PPE if near this equipment. I also will ensure that they are doing the proper maintenance on this equipment and may increase the frequency of the thermography inspections.

p.s. Thanks for all the stats, they were very helpful!!

The situation you describe doesn't sound like any of the situations described in NEC 240.21(C) that would allow for such a long length of unprotected feeder. Per code, there should have been a secondary disconnect at the transformer. Properly protecting this feeder per NEC most likely would have helped with the fault and the high incident energy.

Take a look at 240.91(C)(1). Subsection (1) is a scenario that might pertain to the OP.
there are times where I have used subsection (2), which is effectively your suggestion.

In my experience, failures in bus-ways like that occur at two general locations.
1: Outside usually near where it penetrates into the building.
2: At the bottom of long vertical runs
and in either case it is because that's where the water accumulates.

Be careful with general statements about Article 240. This is one article that has tripped me up many times. There are a truly massive number of special cases within the article. On top of that there are lots of "exceptions" that exist due to the structure of the code but you won't find them by reading Article 240 in isolation. You need to start at the appropriate Article (feeders, service entrance, equipment, or branch circuits) and follow overcurrent protection for those articles, and then and only then do you get into Article 240 because not all sections of Article 240 apply to all equipment despite what it looks like.

As an example a very common situation that I run into with feeders is with lots of taps on branches. For example if the feeder (regardless of whether it is busway or cable) is rated for say 200 A then if there are two downstream panelboaards that have main breakers rated for 100 A each and as long as the transformer primary protection (fuse or circuit breaker) meets the requirements, there is no need for transformer secondary protection. Not saying that this is good practice, just that it's not needed. Since relatively speaking a breaker in this application is typically quite large and can easily run over $10K or more, it is a cost that is quite frequently designed out. And knowing what we know about arc flash today, this is also a breaker/disconnect that deserves a great big "DANGER--DO NOT OPERATE ENERGIZED" sign, which pretty much makes it useless as a disconnect.

HOWEVER don't throw the baby out with the bathwater! I'm a huge proponent of a slightly different design approach. First and foremost, there is very little operational difference between operating a disconnect on the primary vs. secondary sides of the transformer. The only difference is in whether the transformer and the secondary equipment are de-energized or just the secondary equipment. That's not to say that we don't need discoonnects on the distribution panel(s) downstream of the transformer and I'd recommend using a main breaker/disconnect at that location. However from a very practical point of view we still have an arc flash problem downstream of the transformer. So since a disconnect is kind of useless place an overcurrent protection device in that location. And since we already have downstream overcurrent protection at the MCC's/panelboards which should be coordinated to operate first, the only time that this device would ever trigger is in the event of a short circuit or arcing fault. So just palce a set of current limiting fuses (Class J's, RK1's, Class L's, etc.) in a box on the secondary side of the transformer. It knocks down the incident energy dramatically, allows for a small cost reduction of the downstream equipment (since we are limiting the fault current, AIC requirements can be reduced), takes up very little space, and is relatively inexpensive to the point where in many cases it is cost neutral or very nearly so.

This NEC section would apply to this installation since it is an industrial Installation. When installed they may have used 240.92(C)(1)(1) as the reason for not having protection but it looks like it may be about 105FT therefore it would need protection. I am thinking the simplest solution may be to add secondary CT's and trip the primary breaker. This would provide much better protection and should lower the arc fault level to an acceptable level.

Any other thoughts or ideas?

Yes this was outside and in the bottom of a vertical section. I do suspect it was from water penetration but we are not sure yet.

I believe in the concept of a secondary CT/relay tripping a primary side device.