Menu
Search
Shaping and connecting rigid busbars in LV switchgear
Shaping and connecting rigid busbars in LV switchgear (photo credit: Edvard Csanyi)

Busbars – machining, bending and shaping

The busbars constitute the real “backbone” of every low voltage switchgear. The main busbar and branch busbars supply and distribute the energy.

Creating busbars generally involves machining, bending and shaping which require a high degree of expertise to avoid weakening the bars or creating stray stresses.

The same applies to connections between bars, whose quality depends on the sizes and conditions of the contact areas, and the pressure of this contact (number of screws and effectiveness of tightening).

  1. Sizes of the contact areas
  2. Contact pressure
  3. Condition of the contact areas
  4. Machining copper busbars
  5. Bending bars

1. Sizes of the contact areas

The contact area (sc) must be at least 5 times the cross-section of the bar (Sb). Sc > 5 × Sb for main busbar continuity links, it is advisable to establish contacts along the entire length of the bar in order to ensure optimum heat transfer.

Busbar contact area and cross-section
Figure 1 – Busbar contact area and cross-section

For branch busbars, the contact area can be smaller, complying with the condition Sc > 5 × Sb. For equipment connection plates, contact must be made over the whole surface of the plate for use at nominal current.

Busbars contact area
Figure 2 – Busbars contact area

Connection aplate surface
Figure 3 – Connection aplate surface

TIP! Connection on extension rod, adaptor or spreader:

Connection on extension rod, adaptor or spreader
Figure 4 – Connection on extension rod, adaptor or spreader

Go back to creating busbars actions ↑


2. Contact Pressure

The contact pressure between bars is provided using screws whose size, quality, number and tightening torque are selected according to the current and the sizes of the bars.

Too high a tightening torque or not enough screws can lead to distortions which reduce the contact area!! It is therefore advisable to distribute the pressure by increasing the number of tightening points and using wide washers or back-plates.
Busbars contact pressure
Figure 5 – Busbars contact pressure

Devices to prevent loosening:

Devices to prevent loosening
Figure 6 – Devices to prevent loosening

Applying a mark (paint, brittle coating) will show any loosening and can also be used to check that tightening has been carried out correctly (tell-tale)

Applying a mark (paint, brittle coating)
Figure 7 – Applying a mark (paint, brittle coating)

Left: Connection on 120 x 10 bars (4000 A); Right: Double connection: 100 x 10 bars (3200 A) and 80 x 10 bars (2500 A) on common 120 x 10 bars
Figure 8 – Left: Connection on 120 x 10 bars (4000 A); Right: Double connection: 100 x 10 bars (3200 A) and 80 x 10 bars (2500 A) on common 120 x 10 bars

Recommended screws and minimum characteristics

I [A]Bar width
[mm]
Number
of screws
∅ Screw
[mm]
Minimum
quantity
Tightening
torque
[Nm]
1 bar2+ bars
≤ 250≤ 251M88-815/20
2M68-810/15
≤ 400≤ 321M106-830/35
≤ 630≤ 501M126-850/60
2M106-830/35
2M88-815/20
8001250≤ 804M88-815/20
4M106-830/35
10001600≤ 1004M106-830/35
2M126-850/60
16002500≤ 1254M126-8 50/60

Tightening torques that are too high lead to the limit of elasticity of the bolts being exceeded and creeping of the copper.

Go back to creating busbars actions ↑


3. Condition of the contact areas

Apart from pronounced oxidation (significant blackening or presence of copper carbonate or “verdigris”), bars do not require any special preparation.

Cleaning with acidified water is prohibited, as, apart from the risks, it requires neutralisation and rinsing. Surface sandin (240/400 grain) can be carried out, complying with the direction of sanding so that the “scratches” on bars that are in contact are perpendicular.

Busbars contact area condition
Figure 9 – Busbars contact area condition

Go back to creating busbars actions ↑


4. Machining copper bars

Copper is a soft, “greasy” or “sticky” metal in terms used in the trade. shaping is generally carried out dry, but lubrication is necessary for high-speed cutting or drilling operations (up to 50 m/mn).

Sawing (8D medium tooth) in a clamping vice
Figure 10 – Sawing (8D medium tooth) in a clamping vice

It is possible to make holes with drills for steel, but it is preferable to use special drills (with elongated flutes for easy detachment of chips)
Figure 11 – It is possible to make holes with drills for steel, but it is preferable to use special drills (with elongated flutes for easy detachment of chips)

The hydraulic punch is used to make precision holes easily … and with no chips.

Go back to creating busbars actions ↑


5. Bending busbars

It is strongly recommended that a full-scale drawing is made of the bars, in particular for bends and stacking of bars.

Bending busbars
Figure 12 – Bending busbars

The bars are separated by their thickness “e”. The total centre line length before bending is the sum of the straight parts (L1 + L2) that are not subject to any distortion and the length of the curved elements on the neutral line (in theory at the centre of the thickness of the metal).


Bending to 90°

l = (2πR) / 4 = (2r + e) × π / 4

Useful formula: l = R × 1.57

Bending busbars to 90°
Figure 13 – Bending busbars to 90°

Bending to any angle α

l = (2r + e) × π × (180 – α) / 360

Where:

  • r is bending radius (or radius of the tool)
  • R is radius to the neutral line R = r + e/2
  • l is length to the neutral line
Bending to any angle α
Figure 14 – Bending to any angle α

Creating a twist

The length L of the twist is at least twice the width l of the bar:

Creating a twist
Figure 15 – Creating a twist

Calculation note!

The calculation must be carried out based on the tool used and its actual bending radius r.

Bending on bending machine: r = 1 to 2e
Bending on bending machine: r = 1 to 2e
Figure 16 – Bending on bending machine: r = 1 to 2e
Bending on V-block: r min. = e
Bending on V-block: r min. = e
Figure 17 – Bending on V-block: r min. = e

Go back to creating busbars actions ↑

Reference // Busbars and distribution by Legrand

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

4 Comments


  1. Jeremy
    Dec 09, 2016

    Thank you for the information. It was very helpful


  2. David P
    Dec 07, 2016

    I would like to have use of Belleville washers addressed. Our physical plant specs require used of these washers to connect sections of switchgear together.


  3. Khakis Khan
    Dec 07, 2016

    Thanks, your article very informative and contains professional information. Which is useful for the engineers working in the field.


  4. Kondal
    Dec 06, 2016

    Thank you for the articles being posted.
    I am an electronics person working in Maintenance of UPS systems.
    Thanks once gain.

Leave a Comment

Tell us what you're thinking... we care about your opinion!


Get PDF