TUYEN LAM ELECTROMECHANICAL ENGINEERING CO., LTD. (TLC)

BUSBAR FABRICATION STANDARDS AND THE ART OF BOLT TORQUE CONTROL FOR PREVENTING OVERHEATING

In Main Distribution Boards (MSBs) or transformer systems, copper busbars serve as the "highways" transmitting massive amounts of energy. However, more than 80% of electrical switchboard fires do not stem from system-wide overloads, but rather from localized hotspots (Hotspots) at busbar connection points.

This failure is the direct consequence of improper bending fabrication and substandard bolt torque. Below is a detailed analysis from a physical and industrial standards perspective to help engineers master this technique.

1. Busbar Punching Standards

Punching is not merely creating a hole to thread a bolt through. The quality of the punched hole's surface directly dictates the actual contact area of the joint. Mechanical Clearance (Clearance): the punch hole diameter (D) must be larger than the nominal bolt diameter (D) from 1mm to 2mm to compensate for installation tolerances and thermal expansion.

M8 Bolt: Punch hole diameter is 9mm

M10 Bolt: Punch hole diameter is 11mm

M12 Bolt: Punch hole diameter is 14mm

M16 Bolt: Punch hole diameter is 18mm

Burr Limit: during punching, if the clearance between the punch and die is too large, the edge of the hole will be dragged downward, creating a large burr. These burrs will act as "spacers" that prevent the two busbars from lying completely flat against each other, reducing the actual contact area by up to 70%. The maximum allowable burr must not exceed 10% of the busbar thickness.

Edge Distance: the distance from the center of the hole to the outer edge of the busbar must be at least 1.5 times the hole diameter (ideally 2 times) to ensure mechanical strength and prevent the copper from tearing under high torque.

2. Busbar Bending Standards

Copper is a ductile metal, but when bent, the outer fibers are stretched (under tension), while the inner fibers are compressed. If poorly bent, the busbar will crack or thin out at the bend, increasing localized resistance. Minimum Bend Radius: to prevent outer copper fibers from cracking, the inner bend radius must strictly comply with the busbar thickness (t):

Annealed Copper: Minimum radius = 1 x t

Half-hard Copper (the most common type in switchboards): Minimum radius = 1.5 to 2 times t 

Example: For a 10mm thick busbar, the minimum punch radius must be 15mm. Using a sharp V-shaped punch will cause dangerous micro-cracks in the outer copper fibers. Springback: after releasing the bending press, the copper busbar tends to spring back by about 2-5 degrees. Brake press operators must over-bend the material so that it settles at the desired 90 degrees angle after springback.

3. The Secrets of Bolt Torque & Causes of Localized Overheating

Why do both over-tightening and under-tightening cause switchboards to catch fire? Let us break down the physics at the contact surface of the joint.

Fault 1 - Under-torque -> Extremely High Contact Resistance

To the naked eye, a copper surface looks flat, but under a microscope, it resembles mountain ranges. When two busbars are pressed together, electrical contact occurs only at these microscopic peaks (called A-spots). If the tightening force is too light, the pressure is insufficient to flatten these peaks, leaving the actual contact area extremely small. Result: contact resistance skyrockets.

According to Joule's Law (P=I^2 x R), the generated heat power will be immense, causing the joint to glow red-hot.

Fault 2 - Over-torque -> Plastic Deformation and Mechanical Loosening

Many electricians have a habit of using impact wrenches or long leverage handles to "pull as hard as possible" to tighten as much as possible. This is a fatal mistake. When over-torqued, the steel bolt is stretched beyond its elastic limit into the region of plastic deformation (damaging the threads and causing permanent elongation). Meanwhile, copper is softer than steel and will be crushed, undergoing permanent plastic flow under excessive pressure.

When current flows, the copper busbar heats up and thermally expands. Because the bolt has lost its elastic recovery properties, when the system cools down (load decreases), the joint cannot spring back to its original clamping state -> the joint becomes permanently loose. In the next load cycle, contact resistance surges, triggering a fire.

Fault 3 — Omitting or Installing Belleville Washers Upside Down

Copper busbars have a very high coefficient of thermal expansion. Steel bolts have a lower coefficient of thermal expansion.

Solution: It is mandatory to use Belleville (conical spring) washers. These washers act as ultra-high-stiffness springs, continuously maintaining a constant clamping force on the joint regardless of the thermal expansion and contraction of copper and steel.

Incorrect Installation: Belleville washers are cone-shaped. If installed upside down (with the cone base facing upward instead of nesting against the copper bar), flattening them during tightening will instantly destroy their spring function.

4. Real-World Practical Example: Calculating Heat Generation at a 1600A Joint

Comparing a properly torqued joint versus a poorly torqued (insufficient force) joint on a busbar run carrying I= 1600A (using M12 bolts).

Scenario A — Proper Torque (M12 torque reaches $85\text{ N}\cdot\text{m}$, contact grease applied)

Thanks to proper clamping force and conductive grease filling micro-gaps, the joint's contact resistance is extremely low: R≈3 μΩ.

Heat dissipation loss at the joint:

Evaluation: The heat generated is extremely small 7.68W (equivalent to a tiny LED bulb), easily dissipating into the surrounding environment via the busbar surface. The joint remains cool (temperature around 45-55 C degrees).

Scenario B — Poor/Loose Torque (Hand-tightened to $\approx 30 N.m copper oxide uncleaned)

Due to weak clamping force and unruptured insulating oxide layers, contact resistance surges to: R≈80 μΩ.

Heat dissipation loss at the joint:

Evaluation: The heat dissipation power reaches 204.8W (equivalent to a heavy-duty soldering iron plugged continuously into the joint). Because the joint area is small, this heat accumulates rapidly, driving the temperature at the contact point past 150 - 200 C degrees after just a few dozen minutes under load. The tin plating melts, the copper oxidizes black, and resistance escalates further until the joint burns out or melts completely.