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 Post subject: Location on motor control centers
PostPosted: Sun Apr 24, 2016 10:38 am 
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Sorry if this has been asked and answered already.

What are opinions on how an MCC should be labeled. I have seen labels on every starter and labels every two or three vertical sections. Your incident energy is generally going to be the same for everything on the bus (with the possible exception of any main breaker) so it seems like you could get by with one label if you could manage to make it clear to any observer that the label exists and that it applies to the entire MCC.


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 Post subject: Re: Location on motor control centers
PostPosted: Mon Apr 25, 2016 4:28 am 

Joined: Fri Nov 06, 2015 1:05 am
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Location: Evansville IN
Depending on the facility training program i.e, are Team Members trained on your specific procedure for labeling. To be on the safe side we are beginning to label all buckets even though we started using only two labels(depending on MCC segmentation)


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 Post subject: Re: Location on motor control centers
PostPosted: Mon Apr 25, 2016 6:27 am 
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We generally label every other section or every third section. Putting labels on every bucket (if the buckets are smaller) seems to be overkill. If the buckets are large, taking up more than half a section or more, we'll label that size bucket individually. It's my view that more is not necessarily better. A few well placed labels makes more sense to me.


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 Post subject: Re: Location on motor control centers
PostPosted: Mon Apr 25, 2016 9:52 am 
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If you have just a "single" MCC then I prefer to have one on the main disconnect/breaker section and one for the rest of the MCC, usually located on the vertical wiring cover so that it is still visible with the doors open.

If you have a situation where you have multiple MCC's that are physically butted up against each other so that it looks like one huge MCC but is in reality several individual ones then it is often preferable to have a label for every single section so that there is no confusion over which label corresponds to which MCC.


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 Post subject: Re: Location on motor control centers
PostPosted: Mon Apr 25, 2016 11:53 am 

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I install one label in the center of the top of the MCC and if you normally approach the MCC from one end then I place a second label on that end. Many MCC's with a main circuit breaker have one incident energy rating for the line side of the main breaker and a different rating for the load side. In this case I label the main breaker then outline the breaker compartment with black and yellow tape and place the label for the load side at the top near the center of the MCC.


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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 6:09 am 
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john pfeiffer wrote:
Many MCC's with a main circuit breaker have one incident energy rating for the line side of the main breaker and a different rating for the load side. In this case I label the main breaker then outline the breaker compartment with black and yellow tape and place the label for the load side at the top near the center of the MCC.


This is true if the construction on the MCC will prevent an arc from engulfing or propagating to the main breaker line side connections. So you have to know the construction plus maintenance would have to considered that this is not contaminants in the MCC that could conduct the arc.

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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 6:18 am 
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wbd wrote:
john pfeiffer wrote:
Many MCC's with a main circuit breaker have one incident energy rating for the line side of the main breaker and a different rating for the load side. In this case I label the main breaker then outline the breaker compartment with black and yellow tape and place the label for the load side at the top near the center of the MCC.


This is true if the construction on the MCC will prevent an arc from engulfing or propagating to the main breaker line side connections. So you have to know the construction plus maintenance would have to considered that this is not contaminants in the MCC that could conduct the arc.


So you're suggesting that in many cases you would need to use the higher main breaker incident energy as your energy for the entire MCC?


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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 10:25 am 
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Two issues here. First is arc propagation in a direction away from the energy source such as moving down a vertical section in an MCC. This has been documented many times. The patterns left on the bus bars are called "arc tracks" and can be used forensically to find where the arc initiated. An an MCC this means that the arc flash occurs at the ends of the bus bars in the vertical bus, not where it was initiated for bus faults. The practical effect is minor here, mostly just an awareness.

Second is arcs that propagate in the upstream direction. When we have say a single phase arc or phase-to-ground arc the surrounding air gets hot enough that the breakdown voltage of the air is drastically lowered and subsequently arcs appear on the other 2 busses, resulting in virtually all single phase faults turning into bolted arcing faults within the span of 1-2 cycles. Occasionally this effect can even extend as far as to the other side of a circuit breaker.

MCC propagation vertically has been documented several times. It is certainly possible but I've never seen any cases published in the horizontal direction probably because the horizontal bus is not disturbed very often in the first place.

Mersen published a couple papers detailing the effect of phase barriers and basically showed that they have only a modest effect.

There are several published papers showing propagation of arcs within panelboards to the point where the joint NFPA/IEEE study added a "barrier" test to the standard test cases. The most accessible is that Mersen published the initial exploratory data on this type of test.

I haven't seen anyone suggesting propagation from section to section within switchgear and this seems pretty implausible given that even the big "arc flash everywhere in an enclosure" proponents suggest that it doesn't happen in switchgear.

So overall I would conclude that:
1. Vertical propagation in MCC's is real.
2. "Jumping" circuit breakers is also real at least in panelboards and certainly within an enclosure from one phase to another.
3. Although I haven't seen anything published about it, "jumping" circuit breakers within the same enclosure or vertical section within an MCC might not be very common but it certainly seems plausible given the prior two pieces of evidence.
4. With no evidence for "horizontal" propagation within an MCC, the idea that MCC's should be treated like panelboards (arc flash everywhere) seems very implausible.

I'd welcome any documented evidence to the contrary though.


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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 11:13 am 
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PaulEngr wrote:
Two issues here. First is arc propagation in a direction away from the energy source such as moving down a vertical section in an MCC. This has been documented many times. The patterns left on the bus bars are called "arc tracks" and can be used forensically to find where the arc initiated. An an MCC this means that the arc flash occurs at the ends of the bus bars in the vertical bus, not where it was initiated for bus faults. The practical effect is minor here, mostly just an awareness.

Second is arcs that propagate in the upstream direction. When we have say a single phase arc or phase-to-ground arc the surrounding air gets hot enough that the breakdown voltage of the air is drastically lowered and subsequently arcs appear on the other 2 busses, resulting in virtually all single phase faults turning into bolted arcing faults within the span of 1-2 cycles. Occasionally this effect can even extend as far as to the other side of a circuit breaker.

MCC propagation vertically has been documented several times. It is certainly possible but I've never seen any cases published in the horizontal direction probably because the horizontal bus is not disturbed very often in the first place.

Mersen published a couple papers detailing the effect of phase barriers and basically showed that they have only a modest effect.

There are several published papers showing propagation of arcs within panelboards to the point where the joint NFPA/IEEE study added a "barrier" test to the standard test cases. The most accessible is that Mersen published the initial exploratory data on this type of test.

I haven't seen anyone suggesting propagation from section to section within switchgear and this seems pretty implausible given that even the big "arc flash everywhere in an enclosure" proponents suggest that it doesn't happen in switchgear.

So overall I would conclude that:
1. Vertical propagation in MCC's is real.
2. "Jumping" circuit breakers is also real at least in panelboards and certainly within an enclosure from one phase to another.
3. Although I haven't seen anything published about it, "jumping" circuit breakers within the same enclosure or vertical section within an MCC might not be very common but it certainly seems plausible given the prior two pieces of evidence.
4. With no evidence for "horizontal" propagation within an MCC, the idea that MCC's should be treated like panelboards (arc flash everywhere) seems very implausible.

I'd welcome any documented evidence to the contrary though.


So, my practical gleaning from this is, if I have a starter in the same vertical section as my main, that maybe I should treat it as if it has the same incident energy as the main?


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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 12:01 pm 
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Voltrael wrote:
So, my practical gleaning from this is, if I have a starter in the same vertical section as my main, that maybe I should treat it as if it has the same incident energy as the main?


You may glean some ideas about all of this based on some recent testing on an actual MCC:
http://www.neiengineering.com/wp-conten ... esting.pdf

First there's no guidance here so bear in mind that there's no right or wrong answers. It's all guessing.

There are many arguments that a starter bucket in the first section should be treated differently. Among them the fact that the incident energy on the load side of the main breaker is going to be lower anyway, that most main breakers are tied into the horizontal and not the vertical bus due to differences in current (say 300 A vertical section but 800 A horizontal), and that this "arc jumping" problem within an MCC hasn't been widely published.

So a very conservative approach says to treat everything in each section of an MCC as a panelboard and to treat each section separately, as in switchgear. A less conservative approach may suggest that given that load side conditions are much less than in a main breaker compartment and given the extreme condtiions that must exist in the first place for an arc starting in a starter bucket to move through the vertical bus compartment and heat the air within the main breaker high enough to induce an arcing fault on the line side just seems highly improbable. So a less conservative approach may treat the starter buckets in the first section just like buckets in the rest of the MCC.

I didn't originate this idea. It came from a KBR engineer. Ethics of professional engineering are to take the conservative approach...but somewhere in there, judgement must apply too.


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 Post subject: Re: Location on motor control centers
PostPosted: Fri Apr 29, 2016 12:14 pm 
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PaulEngr wrote:
Voltrael wrote:
So, my practical gleaning from this is, if I have a starter in the same vertical section as my main, that maybe I should treat it as if it has the same incident energy as the main?


You may glean some ideas about all of this based on some recent testing on an actual MCC:
http://www.neiengineering.com/wp-conten ... esting.pdf

First there's no guidance here so bear in mind that there's no right or wrong answers. It's all guessing.

There are many arguments that a starter bucket in the first section should be treated differently. Among them the fact that the incident energy on the load side of the main breaker is going to be lower anyway, that most main breakers are tied into the horizontal and not the vertical bus due to differences in current (say 300 A vertical section but 800 A horizontal), and that this "arc jumping" problem within an MCC hasn't been widely published.

So a very conservative approach says to treat everything in each section of an MCC as a panelboard and to treat each section separately, as in switchgear. A less conservative approach may suggest that given that load side conditions are much less than in a main breaker compartment and given the extreme condtiions that must exist in the first place for an arc starting in a starter bucket to move through the vertical bus compartment and heat the air within the main breaker high enough to induce an arcing fault on the line side just seems highly improbable. So a less conservative approach may treat the starter buckets in the first section just like buckets in the rest of the MCC.

I didn't originate this idea. It came from a KBR engineer. Ethics of professional engineering are to take the conservative approach...but somewhere in there, judgement must apply too.


KBR is our engineering firm, and they're the ones that created the model I am working from, so I guess I could ask them about it.

I'm inclined to label the main with one label and all starter buckets with a different one. In practical terms we probably rarely have a situation where that would change the PPE needed from face shield/balaclava to "blast suit".


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