Thread: Fuses
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Old 29th Jan 2011, 9:51 pm   #36
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 448
Default Re: Fuses

Originally Posted by DAVEHALL View Post
I've seen a similar layout, with different values though on telecomms transmission racks .The unusual thing we found was that where you'd expect the smallest to go first , at times it would be some way down the chain that one would go .This was with the manufacturers rated fuses in all places .Asked power staff to explain - they couldn't ,but had experienced it themselves .
If you had for example a table lamp with a 3A fuse in its plug, plugged into an extension lead with a 7A fuse, plugged into another extension lead with a 13A fuse, plugged into a ring main with a 30A fuse. Then you (or some other idiot) cuts through the table lamp's cable close to the plug. The fault current could be several hundred amps, more than enough to blow any of the fuses, and this current will flow until one of the fuses blows.

You might reasonably expect the smallest rated fuse to blow first, its thinner wire will have a smaller thermal mass, but there's no guarantees, and the other fuses may have been weakened. So when you demonstrate what you did wrong, and cut through the rewired, re-fused lamp cable again with your new un-melted cutters, a different fuse may blow this time!

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