Violation
The most likely cause of fracture of the veneering porcelain, that leaves the underlying metal exposed, on a porcelain fused to metal bridge is
contamination of the metal prior to porcelain application.
flexure of the bridge under the occlusal load.
veneering porcelain that is more than 2mm thick.
porosities inside the veneering porcelain
overglazing of the porcelain.