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 Post subject: Excluding Main on QMB style panels
PostPosted: Thu Aug 25, 2011 7:25 am 
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IEEE-1584 requirements for excluding main on QMB style panels?

I'm completing an arc flash analysis using EasyPower and much of the electrical distribution uses Square D QMB style panels. Someone mentioned that IEEE-1584 requires me to do the calculations excluding the main switch on the power panel. I believe the explanation was that the proximity of the main fuses to the downstream arc would cause the fuses to not function properly. What are the external factors that would cause a fuse to become unreliable when it is simply a link? Can anyone shed some light on this?
Most of the panels are on open wire feeders, and relying on the substation fuse would cause very high IE on the QMB branch circuits.


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PostPosted: Thu Aug 25, 2011 8:05 am 
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It's not really a problem with the fuse itself. Properly applied fuses can greatly reduce the incident energy.

It is more of a concern that the arc flash creates a plasma cloud that is conducting. If the cloud reaches the line side of the fuses, the arc flash could continue on the line side even though the fuses already opened.

Because of that concern, the industry tends towards ignoring the effect of the "local" main as being the device that limits the arc flash duration and instead looks towards the next device in a separate enclosure.


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PostPosted: Thu Aug 25, 2011 8:17 am 
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Thanks. I figured it would be something like that, not so much the fuse malfunctioning, but something happening that causes the arc to 'bypass' the fuse.
Is this a requirement, or a recommendation? If it is a requirement, the company will require electricians to wear Level 4 PPE to operate a 30A branch circuit in a QMB panel as opposed to the level 0 PPE that would be required if we include the main.


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PostPosted: Thu Aug 25, 2011 8:48 am 
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A separate fused disconnect from the panel should be able to fix part of this.
Since this seems already built, it's additional costs.


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PostPosted: Thu Aug 25, 2011 11:04 am 
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That may be the best short term fix, but they really need to get rid of their open wire feeders.
Is the risk of the Main fuse 'bypass' so significant that this has been written into the rules, or is it just that the risk is there and we are better safe than sorry? Did this happen during testing, and if so, what percentage of the time? Or is it just that this could 'theoretically' happen?


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PostPosted: Thu Aug 25, 2011 12:49 pm 
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From my understanding, I don't think happens everytime. It's a matter of how bit the arc flash is, how small the enclosure is and I'm sure a few other factors. I don't know percentages but some arc flashes can be pretty nasty. There was a video of someone racking in a breaker (might have been posted here too) and the fireball went everywhere. So, I believe it is just taking the prudent approach.


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PostPosted: Thu Aug 25, 2011 12:58 pm 
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That is pretty much what I thought.
Final Question on the subject (maybe): Is excluding the main fuse something that MUST be done when doing calculations on QMBs, or is it something that can be part of the companies Hazard Risk Analysis? For example, can a company say "If the main fuse limits the downstream fault to a level 0 (<1.2 cal/cm2 at 18") it can be used as the upstream trip device, but if the downstream fault is > level 0 (>1.2cal/cm2 at 18") it must be excluded"?


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PostPosted: Fri Aug 26, 2011 6:16 am 
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I don't believe including or excluding the main is based on what the incident energy is downstream. It is more a matter of the design of the equipment. If equipment is built so it would difficult for an arc flash on a feeder to flash to the line side of the main (i.e. switchgear) then you can exclude it. If it is a panel where everything is located close together, then you include it. I think it is more a judgement call and most are following the pack for now.

You brought up a good point however, if the incident energy is low anyway, do you have to exclude the main? It would seem that a low incident energy means the arc flash / plasma would be small and may not be able to hit the main device.

Geez, the more questions that we all try to answer, the more questions come up.


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PostPosted: Fri Aug 26, 2011 8:04 am 
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Too much overthinking on this simple issue. We don't need to consider multiple continguencies and plasma travel. If you open an energized panel, you need to consider the possibility of an arc forming on any of the exposed live parts. The errant screw driver does not know it's supposed to land only on the load side.


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PostPosted: Fri Aug 26, 2011 8:17 am 
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Yeah, but I'm thinking more about operation of a disconnect switch on the load side of the main fuse. The only way that this can create a 'large' hazard is if it arcs and the plasma cloud causes a large arc across the main disconnect (bypassing the fuse). I know that this is theoretically possible, but I'm trying to find out the probability of it happening and, thus, the risk involved. The plant I am working with has about 200 QMB panels fed off of Open Wire Feeders. Requiring workers to operate a 30A disconnect while wearing level 4 PPE (excluding main) when the hazard should be controlled by the main fuse and would only require level 0 PPE (including main) is just asking for trouble. How can a plant operate like that without spending large capital dollars to fix the problem?


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