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Thread: Arc Flash Labelling for Service Equipment

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    Arc Flash Labelling for Service Equipment

    OK, it seems like this would be a common issue, but I searched and reviewed the previous threads, and didn't see this questions asked or answered anywhere, so hopefully this hasn't already been posted.

    What is proper practice for arc-flash labeling of service entrance equipment (e.g., main service disconnect switch)? I can calculate the available fault current at a service disconnect (based on fault current provided by the utility), but since there is no overcurrent device ahead of the main fuse/breaker, I cannot establish a clearing time, and therefore the flash boundary, etc, ahead of the main. I can calculate this for the load side of the main, but to label the equipment with this info would be somewhat misleading without noting that it applies to the load side only.

    I may have overlooked something, but I couldn't find where this is addressed in 70E...

    Thanks in advance for your assistance.

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    If you calculate the aic rating at the first disconnect then it should be realized that the rating is just up to that panel. If other panels are added then the rating would obviously change. The rating you post at the panel would be the basis for what size aic rating one would need in that panel.
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    If I can get the utility transformer %Z and primary fuse info, I'll model from the primary side and calculate the actual IE. Since I usually can't get that info, I just assume a clearing time of 2 seconds.

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    Arc flash labelling requirements are listed in the NEC in 110.16 and in 70E it's 130.5(H)

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    Quote Originally Posted by fsu-eng View Post
    I may have overlooked something, but I couldn't find where this is addressed in 70E...
    It is not specifically addressed in NFPA70E. Are using using a commercial software package or the formulas in the appendixes? Most software is based on IEEE 1584 equations, so you might look there for guidance.
    It is not uncommon to use 1000 Sec as the fault clearing time for the utility. It is likely that service entrances are always in excess of 40 cal/cm^2.
    Just because you can, doesn't mean you should.

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    In order to calculate the incident energy at the service equipment, you need to model back to the utility protective device. Typically this would start with:
    1. Utility available fault current and X/R at the riser fuse
    2. Riser fuse make and model, speed
    3. Riser cable, length, insulation, material, size, conduit
    4. Utility transformer (assuming utility owned) kVA, %Z winding connection
    5. Secondary cable length, no per phase, size, material, insulation, conduit


    You then determine the incident energy based on the clearing time of the utility protective device. Depending on the system, it may be possible to use a 2 second maximum time. Most likely it will be a high value of incident energy.

    Note that IEEE 1584-2002 has been superseded by IEEE 1584-2018 so there are new equations. The equations in NFPA 70E-2018 are based on the superseded IEEE 1584-2002 and should not be used.

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