Fused spur burn out

bazbazbaz said:

Any thoughts as to why a fused spur connected correctly would burn out and melt the unit? Surely if it is pulling too much current (which is possible as it was feeding a socket used for car charging) the 13amp fuse in the unit would have blown before it got too hot and melted.

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This is a mix of a poor quality spur being subjected to a probable maximum load, poor connection is also a possibility. How much power is the charger pulling.

westward10 said:

This is a mix of a poor quality spur being subjected to a probable maximum load. How much power is the charger pulling.

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Not really sure how much it was pulling. It was feeding a socket used for charging a bmw i3 which was set to draw maximum (as much power as it could). I (stupidly as it turned out) assumed that if it tried to draw too much, the fuse would blow before it caused any issues. But apparently not! I’ve now replaced the spur and changed the i3 to charge on minimum power.

Perhaps it’s time for a proper car charger to be installed!

It looks like the line terminal was not making good contact, as the terminal itself got hotter than the fuse to which it connects. It could have been as simple as a loose screw, but was probably aggravated by the heat conducted from the fuse contacts.

Runaway thermal damage is a known problem with less-than-pefect BS1363 accessories subjected to prolonged low overload. The fuse will do a very good job protecting against an overload of 30A by blowing quickly. Less so with a load of say 18A, which will cause prolonged heating. A good quality accessory installed properly will probably last the course but if there is any weakness about the fuse contacts, or the termination is not 100%, it sets off the progressive increase in resistance that leads to burnout.

Avoidance of prolonged low overload has always been a good design goal as it is hard to protect against without massively over-engineering things.