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Join Date: May 2012
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### Short Circuit Force for Multiple Conductors

08/17/2012 6:54 AM

I am passing a short circuit current through a 3phase system having single long conductor per phase which is lets say supported by an insulating piece having slots to place conductors. Now instead of 1 conductor I use 2 conductors per phase. Thus, now the current will be shared by the two conductors in some ratio....but will the stress exerted on the insulating support increase, decrease or remain same...since the forces are proportional to the square of the current flowing. which is now divided....the only hint is that the inner conductors will experience higher forces as compared to outer...

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Power-User

Join Date: Nov 2011
Posts: 205
#1

### Re: Short Circuit Force for Multiple Conductors

08/17/2012 9:33 AM

This very much depends on how you layout the cables whether side by side or trefoil. All this can help to reduce the forces.

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#2

### Re: Short Circuit Force for Multiple Conductors

08/17/2012 10:21 AM

so, you are making a stress study for those cables? is it not what were you suposed to find out?

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Join Date: Feb 2012
Location: Grenoble, France
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#3

### Re: Short Circuit Force for Multiple Conductors

08/17/2012 10:30 AM

Not a simple question . This is the basic question any switchboard manufacturer has to answer when designing a multiple conductors busbar .

Two solutions : either design your own system, and the in fairness you have to test it in a lab , or you use pre-designed systems (bars, supports), that have been calculated with more complex softwares and tested , as per this video (in Spanish only, sorry...). I do not know of publicly available sophisticated softwares for such calculations.

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#5

### Re: Short Circuit Force for Multiple Conductors

08/18/2012 12:48 AM

Cool video of the busbars in action. Thanks.

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Join Date: Mar 2011
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#7

### Re: Short Circuit Force for Multiple Conductors

08/18/2012 11:58 AM

I'll second that "cool". Cool.

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#4

### Re: Short Circuit Force for Multiple Conductors

08/17/2012 10:46 PM

While I have no disagreement, with what was said before, I prefer one solid cable per phase, well separated.

Multiple strands per phase develop forces, that try to separate those strands. Since solid metal can resist these forces magnitudes above of an insulator, the solid metal wins.

You can do the Lorentz calculations, but you get the same.

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Join Date: Mar 2011
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#8

### Re: Short Circuit Force for Multiple Conductors

08/18/2012 12:05 PM

Yeah, after seeing that video it's quite amazing how much force there is and that it is easy to see the phase differences pushing and pulling like crazy. I knew there was force but to see it like that is an eye opener. I can see that to lay the bars flat side to flat side makes it easier to connect them to taps, but wouldn't they be stronger laid short edge to short edge? What about encasing them in styrofoam or urathane foam in addition to the supports, or would that cause overheating?

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#13

### Re: Short Circuit Force for Multiple Conductors

08/20/2012 2:26 PM

I can recall that when we calibrate HV DC Circuit breakers with 4000-5000A, the cables from the calibrating set to the breaker contacts becomes "stiff" instantly. It feels like a hosepipe that went from slack to under full pressure. Best not to stand on them when you apply the current! The busbars are quite strong and the pos & neg are well separated (also for the 3kV isolation).

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Associate

Join Date: Jul 2011
Posts: 29
#6

### Re: Short Circuit Force for Multiple Conductors

08/18/2012 3:40 AM

From your description I assume you are using busbars rather than cables ?

Based on he same phase pitch the stress will increase on the inner bars as you say, since they are closer together, if you are using the same cross section as the single bar you will also have a higher mass being moved during short circuit

I would suggest you look for a support from a reputable manufacturer, they will have tested the supports. Google 'busbar supports' however if you are in an IEC country then you will be working to 61439 and that does require you to carry out your own design verification to ensure the system works, you are not able to use the certification from a component manufacturer to prove your design. If you do not want to do this then your option is to buy a fully developed system ( for example as shown in the video ) and you would become the assembly manufacturer, but you cannot modify the tested system without carrying out your own tests

However some locations will not even allow this, ADDC require your name to be on the report, even if it is a tested system you are buying.

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#9

### Re: Short Circuit Force for Multiple Conductors

08/18/2012 1:58 PM

Standards are fine. But, sooner or later you are forced to deal with primary forces and their understanding. A busbar containing cabinet may be certified, no matter how impressive the video of the cabinet was. Then you are confronted with the need to connect. To connect to something, anything else with a set of flexible cables.

What does it do to the forces? Working against no resistance? I bet, it is impressive. There is no end to this, as the national grid is of flexible conductors. There is no escaping of the picture.

Yes there is. By stepping outside the box, that is, not otherwise. By inserting a transformer, needed anyhow. It works as a current limited source, safely limiting the maximum short circuit current, that is allowed to occur in a system.

Not by answering a primitive question straight, not by a country mile. And you will have to do a damn lot of work your very self.

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#10

### Re: Short Circuit Force for Multiple Conductors

08/19/2012 1:03 PM

In general, the 2 conductors sharing 1 phase will attract each other exerting the force on their support proportional to the current flowing.

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#11

### Re: Short Circuit Force for Multiple Conductors

08/19/2012 8:02 PM
Guru

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#12

### Re: Short Circuit Force for Multiple Conductors

08/20/2012 11:28 AM

While calculating the stress on the main conductors, we do not consider the number of bus bars. But, while calculating the sub-conductor stress (stress between the conductors of the same phase), wedo divide the peak shorct circuit current by the number of bus bars per phase.

As regards the stress on the support insulators, if only one conductor per phase, then the total stress on the support insulator would be pertaining ony to the main conductor stress; whereas, if more than one conductor per phase, then the total stress on the support insulators would be the main stress PLUS the sub-conductor stress.