037
Daily Dose of Curious
HOW IT WORKS
The time it takes to open is as much a part of the specification as the current. A motor drawing a heavy surge every time it starts needs a fuse that tolerates a brief overload, which is a different component from one protecting an electronics supply that should never see one at all.
By being the weakest part on purpose. A fuse is a short length of conductor chosen to melt before anything else in the circuit gets hot enough to matter.
What actually happens
Current makes heat
Any conductor carrying current warms up. How much depends on the current squared and the resistance, so doubling the current makes four times the heat.
Normally it sheds it
At or below its rating the element loses heat to the air and its holder as fast as it makes it, and sits at a steady temperature indefinitely.
An overload tips the balance
Above the rating it makes heat faster than it can shed it. The temperature climbs until the element reaches its melting point.
It opens
The element parts and the circuit is broken. A large fault does this in milliseconds; a small overload can take minutes.
The clever bit
A fuse does not protect the appliance. It protects the cable feeding it — which is why the right fuse is chosen from the flex, not from the thing on the end of it, and why fitting a bigger one to stop nuisance blowing moves the weakest point to somewhere you cannot see.
The words
- Time-current curve
- A chart of how long a fuse takes to open at a given overload.
- Rating
- The current a fuse will carry indefinitely without opening — not the current at which it opens.
Where this comes from
- BS 1362 — cartridge fuses for plugs, and their time-current behaviour
- Joule heating: power dissipated is proportional to current squared
This one is not an opinion. It is how the thing works, and these are what that rests on.
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