ALWAYS FIT PROPER FUSES
BEFORE THE ADVENT OF RCDS (residual current devices), all houses used fuse wire in their mains consumer units.
The 12V systems still largely use fuses and they serve a very simple purpose – protecting the wiring from catching fire. Every item that is connected to a 12V battery on your vehicle – directly or indirectly – must be protected by its own fuse.
A fuse is a strip of metal that is designed to melt at a certain level of current. How fast the fuse blows will depend on the over-current. So a 10A fuse exposed to 11A may take a while to fail, while it will fail instantly if 30A flows down it.
If you try to draw too much current down a wire (for example, plugging a large compressor into a 12V socket), the fuse will melt and sever the connection in the wire, preventing further current from flowing. They will also sometimes make a popping sound when they fail.
Fuses must always be replaced by one of an identical current rating, usually clearly labelled on the fusebox (for example, water pump 10A). Never fit a higher-rated fuse.
But if you don’t fit a fuse, what would happen? Well, assuming the wire was correctly sized and you didn’t overload the circuit, it would be fine. However, if you plugged or wired in something that exceeded the wire’s capacity, it would get hot – like a kettle’s heating element – and eventually the insulation would melt and ignite nearby inflammables, such as fabric trim, plastic, rubber or wood. It would carry on heating up until the current was removed (relighting even if you sprayed it with a fire extinguisher). Electrical fires are incredibly dangerous – so always fit fuses! Peter Rosenthal