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aguywithfeet

Post subject: Electrode gap question Posted: Wed May 05, 2021 11:28 am 

Joined: Mon Mar 09, 2020 9:27 am Posts: 13

Can someone help me understand what the significance of the electrode gap is in arc flash studies? I understand it in the IEEE tests and I believe i am correct in saying the smaller the gap the higher the IE, but does it matter in a panel how far the bus bars are if the only way they will short is via something that contacts one electrode then the other? Which i would think would be even smaller of a gap than the 25mm i typically see in 600v and less systems. Are we assuming the current/ voltage will go so high in the bus bars that it will spontaneously arc across?
Hopefully this makes sense. I expect to need to clarify. Thanks in advance.


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Jim Phillips (brainfiller)

Post subject: Re: Electrode gap question Posted: Wed May 05, 2021 1:16 pm 

Plasma Level 

Joined: Mon Sep 17, 2007 5:00 pm Posts: 1620 Location: Scottsdale, Arizona

The gap is used in the arcing current, incident energy and arc flash boundary equations.
Physically, it has the largest affect on the arcing short circuit current which can also affect how long it takes an upstream device to operate. The greater the gap, the greater the arc impedance and lower the current, the small the gap, less arc impedance etc, greater current.
IEEE 1584 is based on the gaps used in the test set up which is reflected in Table 8 of IEEE 1584. If you wish to measure the actual gap, you can use that value as long as it is within the range of the model: 208 to 600 Volts  0.25 inches to 3 inches 601 to 15,000 Volts  0.75 inches to 10 inches.
Most will use the gaps in Table 8 since it is difficult to obtain this information and there may be different gaps within the same equipment. It is actually meant to be the distance between bus bars/conductors of electrical equipment. To initiate an arc flash, it is assumed something like an uninsulated tool or other object makes initial contact and then either melts away or is blown away leaving the arcing current to jump across the conductors through the gap.
As usual since I'm IEEE 1584 ViceChair, I have to say this just my personal opinion and may may not represent an official view of IEEE.


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Jeff S

Post subject: Re: Electrode gap question Posted: Mon May 10, 2021 7:56 am 

Joined: Tue Dec 31, 2013 12:19 pm Posts: 37

Also, if you look at the equations in IEEE 15842018, you will see that incident energy increases as the gap increases with all else being equal. I found that to be fascinating and counterintuitive, but that's what the equations say. And if you created a plot of "Gap Versus Incident Energy", you'll be able to see that increase. So taking the shortest gap is not the most conservative approach.


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aguywithfeet

Post subject: Re: Electrode gap question Posted: Mon May 10, 2021 11:01 am 

Joined: Mon Mar 09, 2020 9:27 am Posts: 13


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