Fuses
blow if there is a short cicuit. They are connected in the live wire -brown or red- on the way to the unit. Then for example when a cable inside the a electric unit touches a metqal case the case can become live and give a shock to any person touching the case. To stop this the fuse blows as soon as any live cable touches the case then current is not allowed to reach the unit.
They come in various sizes 3A, 13A for example. The fuse should not blow if there is a normal operating current flowing. If a unit demands a 3A current, fit a 5A fuse