The Case of the Dangerous Electrical Cabinet

Electrical Cabinet with ControllersThe Problem:

Underneath the electrical cabinet, at the lower left-corner, there was evidence failure to 3 fillet welds and 2 flare bevel welds. The welds were ground smooth with the front and rear of the cabinet. The welds attached a Z-leg member support leg at the bottom of the cabinet’s left side. The Z-leg support was to elevate the left side of the cabinet 1½” above the concrete floor in the control room.

As the cabinet was unloaded from the delivery truck at the final destination, the Z-leg support weld failed. The cabinet unexpectedly tilted off the pallet jack and injured the warehouseman’s leg and ankle.


Electrical Cabinet Welds at Z-LegMy Assessment:

The design of the small GMAW welds was analyzed. The weld failures were visually assessed in accordance with the requirements of AWS D1.3 Structural Welding Code – Sheet Steel. The welds depicted on the fabrication drawings were an insufficient throat size and unsuitable for the unintended unloading forces. Unbalanced lifting and misaligned delivery forces were unnecessarily applied and the welds fractured. The repetitive, multi-point loads exceeded the available weld capacity. The moment forces and bearing loads on the Z-leg welds were unintended and not considered in the design or weld sizes. The cabinet was unstable and top-heavy for lifting from the bottom. The welds were defective and did not properly fuse the Z-leg to the corner of the cabinet.

Electrical Cabinet, Top Heavy

The Consequences:

The warehousemen forcefully slid the cabinet to the edge of the truck before the crane slings were attached to lift the cabinet from the truck. The unintended moment forces and point loads applied to the left-side Z-leg welds exceeded the capacity of the small welds. The unsymmetrical loads and point loads exceeded the moment and bearing capacity of the Z-leg welds.

The Lessons Learned:

The root causes of the fractures in the Z-leg welds were defective GMAW welds in sheet steel, and unintended bending moments and stress-concentrations while unloading at the delivery site. The attachment welds were defective from the manufacturing facility. The failed welds were not suitably fused to the bottom Z-leg and corner of the cabinet. Grinding the weld caps smooth with the cabinet edges reduced the available weld size and strength.

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Welding Failures I Have Known, Interesting cautionary tales from Metals & Welding  expert, Dr Jesse A Grantham, PE