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 Post subject: Crane feed rail – is it a bus?
PostPosted: Tue Jul 25, 2017 9:38 am 

Joined: Tue Jan 04, 2011 9:33 am
Posts: 9
Crane feed rail – is it a bus?

We have several large cranes with the feed rails running hundreds of feet. The parameters for the model below are to simulate an actual situation.

450 foot run of 3 phase copper feed rail around 1.44 square inches ~ 2000 MCM (circular mils).

In discussion, other Power Engineers there is some consensus that the feed rail should be treated as a zero ohm bus.

In an AF analysis using this length with copper conductors, the current drop is large enough to push the short circuit current into the thermal zone of the breaker.

Therefore, how should the feed rail bus be treated as a cable or a zero ohm bus – that is the question?


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 Post subject: Re: Crane feed rail – is it a bus?
PostPosted: Wed Jul 26, 2017 9:23 am 
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Location: Rutland, VT
I would take the conservative approach and model it as a busway or cable with a bus at the end. I would fault the bus and use that value of IE for the entire crane feed rail.

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 Post subject: Re: Crane feed rail – is it a bus?
PostPosted: Wed Jul 26, 2017 1:29 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 523
Location: Wisconsin
sidlangford wrote:
Therefore, how should the feed rail bus be treated as a cable or a zero ohm bus – that is the question?


Can the bus be accessed at more than one location?

I have done projects, where the crane was parked in a maintenance position, so we treated it like any standard panel.
On a different project it was possible to be on the crane as it moved. We ran three different conditions, 0% , 50%, and 100% of conductor length, and 100% in an effort to identify the highest AFIE.


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 Post subject: Re: Crane feed rail – is it a bus?
PostPosted: Thu Jul 27, 2017 12:21 pm 
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Location: North Carolina
Seriously does it even matter? I mean it matters as far as representing the impedance compared to downstream equipment (on board the crane) and in that case since location is a variable you have to look at where you might be doing maintenance or else I guess take a worst case?

But under almost any other circumstance either personnel are nowhere near the crane rails in the first place or even if they are, the spacing is so wide that it is going to be really close to invalidating IEEE 1584 because of bus gap. And even if you get past all that and actually manage to strike an arc, this is identical to an outdoor power line...it is simply going to be magnetically propelled down to the end of the bus so except at the very end, no one is going to be in harms way.

So unless you can construct a scenario where someone would be exposed to an arc flash occurring on the bus and describing how it will remain fixed in place or else just trying to model the conductor impedance it doesn't seem like anything valuable.

I'd imagine the other reason that most practitioners don't run into this is because the upstream feeder breaker for the crane will most likely drop the incident energy down so low that in most cases it is under a threshold such as 1.2 cal/cm2. I could see where a hot metal crane or a magnet crane would definitely exceed this condition but there's not much of that out there and again...you don't work on the rails right next to it. You use the (Code required) disconnect before starting work. And yes I've ridden cranes lots of times and worked in the starters especially with multispeed motors but when it comes to the rails there's no safe way to get close to them short of shutting off the disconnect and even then, the insulation and spacing makes it extremely unlikely to cause an electrocution much less an arc flash.


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 Post subject: Re: Crane feed rail – is it a bus?
PostPosted: Mon Jul 31, 2017 10:39 am 

Joined: Tue Jan 04, 2011 9:33 am
Posts: 9
We have modeled the feed rail as a 2000 MCM cable because of limitations of the program we are using.
We know the physics are not exact because of eddy currents and other factors but it is the best tool we have at this time. The program calculates a 3K drop in ampere that in some cases would push the current into the thermal region of the TCC.
We agree with everything PaulEng has mentioned but the feed rail itself is not the concern – we would never work on it live… the concern is what is the IE at the line side of the main breaker located on the crane itself. Understanding the limitations of the program using a cables with the correct Z and R the AF went from a level 1 to a level 3. In addition, we have already performed mitigation to get the technicians outside the arc flash zone.

As far as a maintenance position we do have one - but what if the failure happened at the furthest end? In these cause the furthest was the worst case.


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