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 Post subject: Short Circuit Study
PostPosted: Mon Mar 04, 2019 2:37 pm 

Joined: Fri Sep 28, 2018 7:32 am
Posts: 11
Location: Evansville, IN
I'm curious what everyone's thoughts are on considering a panel under rated in terms of its SCCR compared to the available fault current. I am analyzing equipment for the installation of a new sub station which will feed a new production line. Panels are mostly rated for 35kA. The available fault current at each of the panels is between 35.5kA and 38.8kA.

The panels have already been mounted on a new mezzanine, wired and the feed from the new switchgear has already been terminated. EasyPower shows that I can add some length to the feeder from the switchgear to the equipment and it will be enough to drop the values so that the highest available fault current is 35.7.

I am hesitant to ask them to add length to this cable run as it will require re-doing the cable tray and re-pulling some rather expensive cable.

Is there a small margin of error I should allow for and let this be energized as is and not consider them underrated? With values this close the addition of other motors and equipment to the power system could easily add to the fault current so I'm not a fan of adding length to the feeder cables.


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 Post subject: Re: Short Circuit Study
PostPosted: Mon Mar 04, 2019 3:50 pm 
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Joined: Fri Jul 08, 2016 10:01 am
Posts: 227
Location: Indiana
1. Check to see if you already have a series combination. What are your upstream and downstream breakers?
2. Check to see if you can add current limiting fuses upstream to get a series rating.
3. Replace the breakers with 42K breakers.
4. Double check the actual installed feeder length.

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 Post subject: Re: Short Circuit Study
PostPosted: Mon Mar 04, 2019 4:47 pm 
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Joined: Fri Jul 08, 2016 10:01 am
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Location: Indiana
Also, X/R ratio affects the calc so even 42K rated breakers may not even get you there. I've never used EasyPower but in SKM it will show you the calculated bolted fault current and with X/R effect. I expect EasyPower will too

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 Post subject: Re: Short Circuit Study
PostPosted: Tue Mar 05, 2019 7:06 am 

Joined: Fri Sep 28, 2018 7:32 am
Posts: 11
Location: Evansville, IN
bbaumer wrote:
1. Check to see if you already have a series combination. What are your upstream and downstream breakers?
2. Check to see if you can add current limiting fuses upstream to get a series rating.
3. Replace the breakers with 42K breakers.
4. Double check the actual installed feeder length.



Unfortunately the circuit breakers that were used do not have a series rating provided by the manufacturer. The main distribution panel (which feeds all others on the mezzanine) has mostly breakers rated at 42k. We are in the process of swapping out the couple rated for 35ka to those which are rated for 42kA. This resolves the issue with the main panel. Just received confirmation on all feeder lengths.

Replacing breakers in the downstream panels is an option but I need to exhaust all other options first. These are brand new panels built over seas that make use of several breakers in each cabinet.


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 Post subject: Re: Short Circuit Study
PostPosted: Mon Mar 11, 2019 9:17 am 

Joined: Mon May 06, 2013 9:28 am
Posts: 11
Location: Oregon
If you haven't already, be sure to run the Equipment Duty Report in EasyPower. It provides a precise determination of breaker duty based on the calculated fault current through each breaker as well as consideration of system X/R ratio compared with test X/R.


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 Post subject: Re: Short Circuit Study
PostPosted: Tue Mar 12, 2019 2:27 pm 
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Location: North Carolina
Generally you don't want to mess around with "close enough". Magnetic force is proportional to the square of the current. So a 110% overduty is equal to 125% of magnetic force. So don't even play around with "margin". If it's overdutied, it's overdutied. However this is based on actual currents and we're dealing with estimates so...

Don't forget the type of short circuit study. The standard approach basically ignores resistance and only looks at inductance. Why? Because back in the days before we had computers to do this, it took 4 times longer to do the entire calculation in complex arithmetic. So instead we largely ignored resistance and just used a lot of assumptions to make the calculation easy to do with pencil and paper. The result was higher than the longer more accurate method...

Enter arc flash where conservative (high) numbers result in lower incident energy results than normal, so the computer models for the arc flash module went back to ohms law and did it without the assumptions. Of course with most software you can now simply select a different (IEC) model and recalculate using the more accurate model. It takes seconds to click vs. hours to redo the calculations by hand.

Just be careful with how this is done. For instance some arc flash models use the fault current without consideration for generator and motor transients under the theory that it doesn't affect arc flash results very much and usually this is true. So you want to simply change the type of study (from ANSI to IEC) for instance, not just use the reported short circuit values from the arc flash module. And be careful that you understand what the assumptions in the different module are if you don't want surprises later.

If it is overdutied, as has been suggested, you can usually install say a box with Class L, J, or T fuses. These don't take up too much space and don't require lots of cable rerouting. In a dry transformer or in an oil filled one with a large air termination compartment often you can make the connections off the side of the enclosure or in the enclosure itself. You don't really even want the fuse to actually trip but merely to current limit the fault enough so that the breaker can do it's job in the event that a (rare) short circuit actually happens.

Often the other non-upgrade suggestion is to use inductors (line reactors) but every time I try this it does reduce fault current but it also causes all kinds of motor starting issues because it affects line voltage during a motor start. It's a great solution for utilities and large commercial projects but not for industrial plants where the majority of the load is motors.

35 kA is usually the "breakpoint" on costs. There are higher rated breakers but you pay a lot more for them.

And next time, split your loads up more. It is cheaper to have one or two huge transformers and it looks good economically on paper but your troubles are just beginning. Wait until you run the arc flash results!


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