Why would it be dangerous to replace a blown fuse with a copper penny?

Answer 1

The purpose of a fuse is to break a circuit in the event of some form of power surge within the circuit. This surge could be a momentary occurrence, such as those experienced immediate before and after power cuts, or it could be a result of a fault within the circuit, such as a broken transformer.

Replacing a fuse with anything other than an appropriate fuse, would put anybody using the circuit at risk of electrocution, which could prove fatal depending upon the nature of the circuit and its use.

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Replacing a fuse with a penny is old technology that was used in the 40's and 50's. Distribution fuse boards are not sold anymore. An update to the old fuse boards was brought about in the 60's, where an insert was placed in the fuse socket that had different internal threads for different rated fuses. This move made it impossible to use the old penny trick. A lot of the old fuse panels did not have a main disconnect so if a penny was used instead of a fuse in the circuit on a fault a very high current could occur. The only fuse in the circuit would be the primary side of the feed transformer. These fuses are sometimes rated 300% higher than what the Transformers need. This is to stop needless tripping if a branch accidentally falls across the primary line. A small branch would burn off without interruption to the power service. Over fusing would allow the current draw on the secondary to rise to 10,000 amps before the circuit would disconnect itself. In most cases the service feed wires would just melt and the circuit would open. So in effect one penny could cost the home owner thousands of dollars in repair work.

New installations installed now are all circuit breaker distribution panels with main disconnects. That being said, even with new technology like using re-closers on the primary lines, one thing that has not changed is that the utility companies today still over fuse their transformers.