Main Knife Fuse Frequently Blows in Concealed Wiring—Possible Causes and Fixes?

The main knife fuse in a concealed wiring installation frequently blows despite no apparent overload. Possible causes include poor insulation or insulation failure, especially at cable joints, leading to leakage currents or short circuits between line-to-line, line-to-neutral, or line-to-ground. Testing insulation resistance with a 500V insulation tester is recommended to detect leakage currents. Fuse rating and type should be verified: the fuse must match the circuit load, calculated as Voltage × Fuse Current Rating (e.g., 230V × 16A = 3620W maximum load). Using a fuse with an inappropriate rating or type (fast blow vs. slow blow) can cause premature blowing, especially with equipment having high startup currents like compressors or air conditioners. Signs of insulation failure include burning smells, indicating fire risk and necessitating wire replacement and system shutdown until repaired. It is also advised to analyze the fuse blow timing (instant or delayed) and check if any new equipment was added before the issue started.
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