Thread: Fuses
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Old 9th Feb 2011, 12:31 pm   #39
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 438
Default Re: Fuses

Hi all

The thing to remember is that a fuse does not LIMIT the magnitude of the current.

It does not protect anything, including other fuses in the circuit, from surge current damage.

All it does is limit the duration of the current to prevent serious damage (eg fires!). Up to the rated current, the fuse should last indefinitely. At twice the rated current, the fuse should blow in less than - depending on the type of fuse (fast or slow-blow etc) - 60 seconds or so. For higher currents, the fuse will blow progressively faster.

In places where the fault current can be very high, eg on the mains side of equipment, you must have a HRC fuse in the circuit because normal glass fuses may not break very high fault currents. In the UK, this fuse is in the 13A mains plug, and will break the very high currents you would get with a short circuit to the live wire.

On UK mains, a 3A mains fuse will allow a dissipation of 750W continuously or 1.5kV for up to a minute, too high to protect most small appliances. With the HRC fuse in the plug, fuses on the mains input side of appliances do not need to be HRC types, and can be normal glass fuses with a low current rating chosen to protect the equipment.

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