MCC dual ended

[cornbread]

To tie or not to tie is the question. I have a critical MCC that I have to add new sections to for an upcoming project. Since the MCC is critical, I?m proposing we make it a double ended MCC (tow mains phase properly fed from two different transformers). Note: both transformers are 1000 KVA with the same impedance. The intent is to be able to back feed from two different sources. Typically the MCC will be fed from only one source, if we need to transfer the load to the other xfmr we will briefly parallel thru the MCC bus to have seamless switching. Two discussions on site are; do we need a tie breaker to connect the new section of MCC? I realize paralleling the xfmrs will double the fault current for the period of time, but if the arc flash hazard is acceptable are there any problems with this approach to keeping critical load on line?

 

[ron]

Higher fault current will likely reduce the arc flash hazard at the MCC.
Be sure you have adequate AIC ratings for all of the contribution from both sources.

 

[Cold Fusion]

Plenty of systems are put in this way. Most of this is design decisions.

do we need a tie breaker to connect the new section of MCC?

That would be a design issue. You would only need a tie CB is if a possible operating lineup is isolated sections, each running from it's own source.

if the arc flash hazard is acceptable are there any problems with this approach to keeping critical load on line?

In addition to ron's comments:
Are the feederCBs electricaly operated?
Is there any chance the two xfm could be out-of-sync?

If electrically operated, I would wire such that closing one would automatically open the other.

If any possibility the two xfm could be out-of-sync, then I would definitely want electrically operated CBs, and a sync check relay.

cf

 

[weressl]

To tie or not to tie is the question. I have a critical MCC that I have to add new sections to for an upcoming project. Since the MCC is critical, I?m proposing we make it a double ended MCC (tow mains phase properly fed from two different transformers). Note: both transformers are 1000 KVA with the same impedance. The intent is to be able to back feed from two different sources. Typically the MCC will be fed from only one source, if we need to transfer the load to the other xfmr we will briefly parallel thru the MCC bus to have seamless switching. Two discussions on site are; do we need a tie breaker to connect the new section of MCC? I realize paralleling the xfmrs will double the fault current for the period of time, but if the arc flash hazard is acceptable are there any problems with this approach to keeping critical load on line?

The normal way to do this to have a closed transition Main-Tie-Main switchgear. There is an ongoing argument about the fault withstand requirement of a gear when it is assured that it will only be fed from multiple sources for a transitional time only. According to anecdotal information this is an accepted assumption in EU.

 

[weressl]

Higher fault current will likely reduce the arc flash hazard at the MCC.

How so? The higher fault current comes through TWO breakers so each will see ~half of the current, so even if you are not in the short/instantanous range, the time would not change. If you are in the instantaneous range, the circuit breaker let-through is constant, unless the CB has current limiting feature.

 

[ron]

Arc flash will remain the same for in incident on the main horizontal bus, but the feeder breakers may activate in the instantaneous region with higher fault current. Depends on the point of the TCC where the arcing fault current falls.
That is why I said may, and not for sure, because it is application specific..

 

[cornbread]

Appreciate the relpies.

 

[rbalex]

... There is an ongoing argument about the fault withstand requirement of a gear when it is assured that it will only be fed from multiple sources for a transitional time only. According to anecdotal information this is an accepted assumption in EU.

It isn't anecdotal in IEEE 666, Design Guide for Electric Power Service Systems for Generating Stations. Section 4.6.1 specifically allows this arrangement and equpiment rating.

I don’t know how the battle currently rages but I was in the thick of it for four Code cycles (‘96-‘05). I gave up personally trying to do anything in 2002.(See Proposal below) I was a member of API’s NEC Task Force when they tried their final shot in 2005.

The issue arises from the literal text in the first sentence of Section 110.9. While none of the principal switching devices in a basic closed-transition scheme will actually be called on to interrupt “…the current that is available at the line terminals of the equipment,” the computed available current still includes the contribution of both sources.

In theory though, down steam overcurrent devices would potentially need to be capable of interrupting the full available fault current. However, when I said that the potential was “infinitesimal” in my Proposal substantiation, I meant it. Statistically, the generally accepted age of the earth is not long enough for the required fault event to have happened once – even with a 2-second closed-transition.

CMP 1 knows this; API submitted the calculations in the 99 Code cycle. The Panel didn’t dispute the calculations; they just said that 2-seconds was unsubstantiated.

I only knew two of the Panel members at the time. One of them actually agreed with us in ‘99; the other was somewhat ambivalent. With the recent addition of a few more “Users” to argue the case within the Panel, the tide may change - but I don’t see any feet in the door this time around.

For some reason, CMP 1 just wants to keep throwing the responsibility to the AHJ. (Maybe they were frightened by the IEEE Standard's number) The basic problem with leaving it to the AHJ’s plan checkers or inspectors is that most of them don’t understand the issue. When it is explained to them they instantly opt for the more conservative interpretation, since it is “safer.”

By intuition, it may be. However, while I am usually willing to draw a very clear line between “SAFE” and “UNSAFE” without considering the cost, “SAFER” should also be cost effective since Section 90.1 says the NEC is to be “practical.”

 

[weressl]

Arc flash will remain the same for in incident on the main horizontal bus, but the feeder breakers may activate in the instantaneous region with higher fault current. Depends on the point of the TCC where the arcing fault current falls.
That is why I said may, and not for sure, because it is application specific..

Just the OPPOSITE. Arc flash energy WILL increase, but the breakers will only see one half - roughly - of the total contribution as it will flow through two identical impedance path.