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 Post subject: Energy Reducing Maintenance Switch
PostPosted: Thu Jan 27, 2011 1:56 pm 
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Location: Raleigh, NC
We were discussing NEC Article 240.87 in class the other night; specifically the informational note regarding the "no intentional delay" setting. A member of the class stated that setting the instantaneous trip to off and back to its normal setting must be done while the breaker is de-energized. Refering to Jim Phillips' article in Electrical Contractor, I said that I thought that that would defeat the purpose of being able to temporarily enable a "no intentional time delay" setting in order to reduce the required level of PPE. If the circuit could be de-energized in the first place, there would be no reason to try to reduce the required level of PPE. I don't have any experience with breakers with this setting and I don't want to steer anyone wrong. It just doesn't make sense to me that the breaker would need to be de-energized to change the setting. Anyone with experience with settings on these breakers?


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PostPosted: Thu Jan 27, 2011 2:31 pm 
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chrisrappl wrote:
Refering to Jim Phillips' article in Electrical Contractor...


Guess I better respond :)

This is one that goes back to what does “interaction” mean. It is a bit of an interpretation. Changing a setting could be considered interaction by some but is it an interaction that could cause an arc flash hazard? I believe many of the setting functions do not create a physical operation or change to the breaker itself; it is more of a programming issue - it depends on the design of the breaker. You are correct, if you have to de-energize to lower a setting to perform live work, you defeated the purpose.

I do recall one instance about 15 years ago where I was increasing a setting on an older GE MVT and it tripped. Quite a surprise since I did not imagine increasing the setting would be an issue. There was a design hiccup where it went to zero between settings - oops. :eek: In that case a setting change resulted in a physical operation (and made my heart skip a beat)

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PostPosted: Thu Jan 27, 2011 2:44 pm 
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There are several trip units that will default to thier lowest setting when the settings are changed while the dial is in between positions, but in reality that is not usually an issue. For LTPU if you make a change, and it default to the lowest setting of 0.5x then the LTD starts, so unless it takes you a long time to move that dial there should not be a trip. If you change INST or ST and it defaults to the lowest setting it could trip, but you would need a fault to be present to get to the lowest of either of those.

But I think Chris is refering to maintenence switches, which is really just a temp INST setting more or less and I don;t see how that could cause a trip, or any other condition that could be considered an "interaction" that could cause an arc flash.

And welcome to the forum Chris, get that breaker pulled yet? :)


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PostPosted: Thu Jan 27, 2011 3:09 pm 
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Location: Raleigh, NC
Zog,

As you know, the shutdown did not go well. The generator failed and the UPS shut down. The generator voltage regulator has been replaced but needs to be load tested. The UPS factory is suggesting that the +17 / -15 % incoming voltage operating parameters be tightened to allow more time to open the input contactor when necessary. Supposedly once the UPS voltage sensor detects out-of-operating-range voltage, it opens the input contactor within 4-6 cycles. If the input contactor doesn't open in time, the modular UPS units shut down in an effort to save themselves. We are looking into providing TVSS protection on the incoming side of the UPS to try to add some protection and maybe buy a few cycles of time. We may be looking at voltage regulation, but I think that this would be best accomplished with updated digital controls at the gen-set.

So, to answer your original question - No. We have not been able to shut down in order to change out the lighting main breaker.


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PostPosted: Fri Jan 28, 2011 10:34 am 
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As a general rule, if the settings on the trip unit are rotary switches, or removalable pins, or dip switches, then you need to de-energize before chaning. The problem is that the switch may create a small arc that damages the circuit board. If the unit is programmable digitally then you can generally change settings while the breaker is still energized.


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