When i overloaded the rcd spur in the conservatory by switching on a
electric fire the 13 amp fuse in the unit did not blow but overheate and melted the fuse holder and the rcd. Is this normal or was the fus defective
-- bemnix
Probably a poor connection or a cheap spur unit. A fuse would get warm under such a load - but not enough to melt the unit (AFAIK)
Well something sounds defective to me, although it might be poor contacts in the fuse holder rather than the fuse itself. After all, the electric fire would not have drawn more than 13A itself and there is not usually much else in the conservatory to tip it over.
Christian.
3kW fire? Even that shouldn't present a problem for a 13A fuse. It should carry that current indefinately.
More likely they fuse contacts a bit slack or dirty. Assuming a 3kW fire and thus 13A of current. A contact resistance of just 1 ohm will dissipate 13W, doesn't sound much but when it has no where to go you do get quite high temperature rises. A small electronics soldering iron is about that rating, tip temp >500C...
Becauses fuses aren't very good at medium sized overloads. A continuous
4.2kW on a 3kW circuit is probably a worse case scenario for protecting against. Much more and the fuse would blow quickly. Much less and the RCD spur would not have overheated.Christian.
We always need the full facts. Doh!
Because a 13A fuse is designed to carry 13A indefinately and needs a significant (>100%) overload for it to generate enough heat to melt the wire in short space of time.
4.2kW is only a 40% overload, TBH I'm suprised the fuse only took 10mins to blow... But it did and did so before the holder caught fire so it worked correctly.
Because fuses don't blow immediately. There's a time/current curve, you can run a small overload through a fuse for a very long time, or a big overload (short circuit) for a short time, before the fuse blew.
For a 13A fuse, a 17.5A current is only about a 30% overload. Cables and fittings should be able to withstand that for a limited period of time without being damaged.
Owain
Thanks for the help. Sorry i did not make myself clearer. There was tumble dryer already running (2.2 kw) when the fire (2 kw) was switche on. (Teenage daughter). I would expect that the fuse should have blow immediately. It carried on for about ten minutes and then blew. When went to change the fuse I discovered that the plastic fuse holder an the surrounding RCD had melted. The whole unit had to be replaced. M question is: Why did the fuse not blow immediately?
Bewildered Bemni
-- bemnix
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