Arc Flash Category 0 NFPA 70E panel.

[CapinWinky]

I've only ever been on the maintenance/commissioning side of Arc Flash training and I'm being asked to make sure a panel meets Category 0. I know that's the lowest category, so it should be easy to meet, but I'm having trouble gleaning the actual requirements out of NFPA 70E.

The biggest question is if we can have the mains come into the cabinet to a disconnect like normal or if we need a sequester box for the disconnect. It is sounding like we might, but that seems odd that the lowest level arc flash category would require something so abnormal.

 

[Christoff84]

I'm no expert on this, but I believe to control that you need to be able to control the available fault current on the line side coming into the panel. The only way to achieve that I would think is to have your own disconnect/fusing outside the panel, that way you can say for certain what is in the panel falls within the limits of cat 0.

 

[califflash]

Christoff isn't mainly correct. The hazard catagory is determined by the available fault current to the panel. This is found on the plants one line diagram that they are required to keep up to date. In my experience most plants do not have an up to date on line. In short, the hazard category is derived from the supply, not the machine.

There's not much you can do on the control side to lower this. Even if you had a seperate disconnect, nfpa 70e requires you to don the appropriate safety gear for that fault current of that branch while verifying that power is off with a voltmeter. So you would still have to wear the same year as if the disconnect was integral to the panel.

Technically speaking you could have an isolation transformer with its own fusing with a dedicated feed to your control. This could lower the available fault current since it's based off the transformer size. This seems to me to be a shoddy way to do it unless you have some massive transformers feeding the facility. Most places are not going to have this though.

 

[CapinWinky]

Thanks guys, I was thinking of it all backwards that Cat 0 would be the least stringent, but for the panel side of things it is the most stringent to make sure we limit the power. We're doing an external fused disconnect and looking into if we need any special quick blow fuses or anything.

 

[jseavers]

FYI on NFPA 70E, I just ran into this yesturday.

 

[JaxGTO]

Hoffman now makes Sequester boxes for their panels. Includes a mechanical interlock with the main panel. Covers all Arc-Flash ratings. They even have slope top now for wash-down areas. Keep in mind that even 120vac needs to get locked out.

 

[GlenGineer]

Yeah as previously stated it all depends on the upstream equipment. It also depends on the method you use and the voltage that we are talking about. NFPA 70E has 2 methods: charts and the IEEE calculations. The IEEE calculations are much looser because they take into account the actual conditions instead of some standard value that is 10 times the actual value. The time response of your fuses, breakers, and overcurrent devices is, in my opinion, the most important factor. If your control system is totally 24v then you have nothing to worry about. If your control system is 120v you probably do not have to worry. If there are 480 motors involved then you absolutely need to see their arcflash calculations. Last year I had to figure this all out after a visit from the friendly local OSHA people.

By the way if you have to implement OSHAs arcflash program in your facility remember NFPA 70E is NOT a requirement the requirement is that you implement a system.