The heat from a fault current path across the surface of a bushing or insulator causes porcelain to ______.

• A. Grenade
• B. Crack
• C. Warp
• D. Melt

Answer: (A)

When heat from a fault current travels along the surface of a bushing or insulator, the porcelain is heated quickly at the surface while the interior stays relatively cooler. This creates thermal strain because the outer layer wants to expand more than the inner material. Porcelain is brittle, so those rapid thermal stresses cause cracks in the glaze and body. Cracking is a common failure mode because it compromises the insulating surface, allows moisture to seep in, and can lead to tracking or further damage. Melting would require far higher temperatures, warp is less likely from a short, localized heat pulse, and an explosive “grenade” effect isn’t a typical outcome of surface arcing on porcelain. So cracking is the most expected result.