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Designmen
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Join Date: Jun 2020
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Spacing between same phase busbars

03/07/2023 9:57 AM

Hello everyone,
I'm watching a various switchboard busbars systems and I noticed that when we have two busbars per phase, in some switcboards they are spaced at some distance, while in other they just lay side by side. I attached picture for better understanding.
Is it correct to put two busbar of same phase without spacing? I know that when we connect two busbars it must be connected with appropriate number of bolts (depending on busbar size) so I just wondering is this correct or there should be some space between them?

I'm concern because if we have some cable which is not properly connected to circuit breaker terminal, that terminal and cable lug will be buried and become "black". Because I'm fearing that two bars placed side by side, without spacing will also have some "burning" between them...If you understand my concern..
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Brave Sir Robin
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#1

Re: Spacing between same phase busbars

03/07/2023 6:45 PM

Generally speaking, it doesn't make any difference either way unless you are trying to dissipate heat.

I would speculate that mechanical issues, e.g. connecting bus bars to equipment drive the spacing over any electrical issue. The reason you wouldn't bolt them together as a monolithic block (without an insulating layer or coating), has to do with skin depth of copper at 60 Hz (~8.5 mm). Hmmmmm . . . . ever wonder why most bus bars are manufactured out of 1/4 inch copper bar/plate?

https://en.wikipedia.org/wiki/Skin_effect
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Stef
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#2
In reply to #1

Re: Spacing between same phase busbars

03/08/2023 2:06 AM

I haven't really wondered on the 1/4 bar plate because from my work life it can be any size from 1" square, 3x1" 4x1/2" it just depends on what the designer specified or what his mate had an excess of at the time.

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Stef
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#3

Re: Spacing between same phase busbars

03/08/2023 2:11 AM

From the busbars I have worked the gap is often there for the take offs and direction changes to interleave said connections between the busbars. Outdoor ones have the gap to allow the rain to evaporate and not cause corrosion from airborne contaminants sitting wet between the contacting faces when bolted together.

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Designmen
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#5
In reply to #3

Re: Spacing between same phase busbars

03/08/2023 10:22 AM

Have you ever seen busbar placed side by side without gap? Should this be problem?
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PWSlack
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#4

Re: Spacing between same phase busbars

03/08/2023 3:20 AM

It would be a matter of concern if the <...we...> did not have design procedures that cover this matter.

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raghun
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#6

Re: Spacing between same phase busbars

03/08/2023 12:10 PM

Having two busbars without gap seems illogical as it could as well have been one single busbar of larger cross section in such a case.

Two smaller cross section busbars instead of one larger one are preferred to reduce the loss of current carrying capacity due to skin effect at large current ratings.

When the two busbars are provided, I have always seen a small gap between them with a positioning insulator holding the two busbars firmly and the busbars supported at regular intervals.

These are important for two reasons - busbar positioning with insulators to avoid clashing of busbars and regular supports to prevent busbars getting damaged due to dynamic forces when exposed to large currents during faults.

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Designmen
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#7
In reply to #6

Re: Spacing between same phase busbars

03/08/2023 2:04 PM

If you have 2500A, you need two busbars 100x10, you haven't 100x20 busbar, you can't buy it, because they are not produced.
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rogerggbr
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#8

Re: Spacing between same phase busbars

03/10/2023 7:48 AM

Spacing between busbars is used for various reasons, e.g. more cooling (more surface area), ease of interleaving (fishplates), other connections. Downside is that bars of the same phase will "pinch" when subject to high fault levels (e.g. test conditions) and may need additional packers between the normal supports.

Not having a space is OK, subject to the rating required (and tested) and joint overlap etc. Thicker bars are also stronger of course.

Search out "publication 22" from copper development association for a lot of helpful design data (available online).

If you have not subjected the design to high fault level and temperature rise tests then please leave it to those who have - strange things can happen ;-)

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Brave Sir Robin (1); Designmen (2); PWSlack (1); raghun (1); rogerggbr (1); Stef (2)

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