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).
- Sizes of the contact areas
- Contact pressure
- Condition of the contact areas
- Machining copper busbars
- Bending bars
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.
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.
TIP! Connection on extension rod, adaptor or spreader:
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.
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)
Recommended screws and minimum characteristics
|I [A]||Bar width|
|1 bar||2+ bars|
|≤ 250||–||≤ 25||1||M8||8-8||15/20|
|≤ 400||–||≤ 32||1||M10||6-8||30/35|
|≤ 630||–||≤ 50||1||M12||6-8||50/60|
Tightening torques that are too high lead to the limit of elasticity of the bolts being exceeded and creeping of the copper.
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.
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).
The hydraulic punch is used to make precision holes easily … and with no chips.
It is strongly recommended that a full-scale drawing is made of the bars, in particular for bends and stacking of bars.
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 to any angle α
l = (2r + e) × π × (180 – α) / 360
- 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
Creating a twist
The length L of the twist is at least twice the width l of the bar:
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 V-block: r min. = e
Reference // Busbars and distribution by Legrand