arc flash risk

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Volta

Senior Member
Location
Columbus, Ohio
the blur said:
So it seems there's really nothing you can do to prevent it. besides not dropping any tools...... or de-energizing.
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Deenergizing. And that carries some risk too.

Otherwise, designing, or redesigning the system to limit the current available, and/or faster acting fuses for shorter arc durations.
 
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the blur

Senior Member
Location
cyberspace
Considering most of my work is existing equipment, and I'm on the servicing end, I just have to deal with the inherent risk. I do notice the newer panels have cardboard seperators between the legs, and are better shielded on the inside than a 20 year old panel.

also, with all this AF information, I am not as comfortable anymore opening a panel. which is good and bad in a sense.
 
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Volta

Senior Member
Location
Columbus, Ohio
the blur said:
Considering most of my work is existing equipment, and I'm on the servicing end, I just have to deal with the inherent risk. I do notice the newer panels have cardboard seperators between the legs, and are better shielded on the inside than a 20 year old panel.

also, with all this AF information, I am not as comfortable anymore opening a panel. which is good and bad in a sense.
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Yeah, I'm with you on that. Heck, when I was eighteen, I was invincible!

That was a while ago though, and now the more I know, the more I know can go wrong.
 
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Andy Sherrill

Member
With existing equipment, we often have to adjust our upstream feeder breakers to trip sooner to limit the arc flash energy. As others have stated, this does nothing to prevent the flash, it only minimizes the energy. These breakers are normally set for coordination. Coordination wants the feeder breaker to an MCC to give enough time to trip fuses from a given compartment so the outage is minimized. This often drastically increases the arc flash energy due to the increased time. In the rare instances we have to do something without de-energizing, we sometimes have to sacrifice coordination and set the feeder breaker to quickly trip to clear a fault IF it does occur while we are in the panel. The one drawback to this effort is it is often very difficult to get the breaker settings back to exactly the same settings.

One other word of caution concerning PPE. The ratings on PPE are not what we normally think of for ratings. PPE that is rated for 20 cal/cm2 does not completely protect you from a 20 cal/cm2 arc flash. I don't remember the exact details, but I think this rating means something like you should not receive more than 2nd degree burns? Also, be careful how all of this is applied. You can get arc flash PPE that is rated at very high energy levels. However, when the energy levels get around 40 cal/cm2, the arc blast becomes a major issue. The higher rated suit may keep you from getting burned, but it will not protect you from the blast that sends you flying into the closest wall or other objects.
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
the blur said:
Is the arc flash risk less if a service entrance panel is unloaded ? by unloaded I mean turning off all of the loads, HVAC's, sub panels, etc, etc. or would I presume currently the arc flash risk is higher with a few hundred amps flowing through the panel ?????

I'm not talking about OSHA rules, or PPE ratings, just wondering the science behind arc flash.
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To directly answer your question: the available arc flash energy will be less, but the amount will be ranging from miniscule to less than significant.:lol: In other words it will not be chaning from one category to another. It is not as dependent of the amount of current than the connected loads 'type/nature' to 'store' energy and release it back onto the fault.
 
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