Charger fusing – what margin?

[BabaYaga]

I have a had a 25A mains charger for many years, with no problems. In the summer of 2013 I noticed a new, odd behavior. Despite the battery bank (3x80Ah) being well discharged, the charger never seemed to put out more than 12, max 15A. Apart from this, the charger worked as normal, current falling and voltage rising towards 14,4V and staying there until the bank was fully charged.
It was not until several weeks later that I discovered the likely explanation, which can be seen in the pictures.
The 30A blade fuse protecting the charging circuit had become hot, melting the plastic housing of the fuse but without actually blowing it. My guess is that it was a gradual process, where the heat probably restricted the current flow even more, producing more heat etc. But obviously not enough heat for the fuse to blow (I guess it would have, in the end).
Does this sound like a good explanation?
Can I avoid this happening again? Is a 30A fuse too close to the max output of the charger, 25A?
I'm not sure of the wire gauge, something like 6mm2 I think.
But I chose the fuse more based on the charger, thinking 5A was a safe margin. Maybe it is not?

Any views much appreciated!

http://www.ybw.com/forums/images/attach/jpg.gifhttp://www.ybw.com/forums/images/attach/jpg.gif

 

[pvb]

A 25A charger won't often deliver a full 25A, so a 30A fuse in the wiring would be OK. The problem with blade fuses is slight corrosion can increase contact resistance, leading to heating. This can happen regardless of the current rating of the fuse.

 

[William_H]

A 25A charger won't often deliver a full 25A, so a 30A fuse in the wiring would be OK. The problem with blade fuses is slight corrosion can increase contact resistance, leading to heating. This can happen regardless of the current rating of the fuse.

It looks to me like the blade contacts are the same piece of metal as the actual fuseable link. So the only contacts to have resistance, the blades them selves are quite clean.
It is very unusual to see a fuse partially blown. I can only suspect that something happened momentarily to overload the fuse but the current ceased before the fuse opened.
I would suggest replace with the same 30 amp fuse and see how it goes. good luck olewill

 

[Davegriff]

I've had 2 or 3 fuses melt the same way, funnily enough all on battery chargers. As william_h points out, the blades appeared to be the same piece of metal, and I too had assumed contact resistance of the blades or maybe the fact that all mine were the cheapo variety, and their ratings not up to scratch. If I can find one in the workshop bin while home will examine more closely.

 

[rosssavage]

I have suffered this same phenomenon, but not on a battery charger.

My lazerette lighting circuit is protected by a 10A blade fuse (nom current draw 5A) but I found the fuse, and its plastic holder, melted yet still serviceable.

I came to the same conclusion about high contact resistance within the fuse holder itself.

Fuse and holder replaced, all fine now.

 

[BabaYaga]

Many thanks for the replies, so far.
I also thought about the possibility of resistance in the interface between the fuse and the fuse holder. But if so, should there not be signs of heating in that area of the fuse? The "legs" look fine to me (one obscured by my thumb), signs of heating are instead obvious on the "bridge", the actual fuse part.
Perhaps the fuse was faulty from start, like a tiny crack?

 

[rosssavage]

Probably. I didn't look too carefully at mine, but there was obvious heat damage to the fuse holder too, so maybe that is was a different failure mode to yours?

Unless the legs are acting as heat sinks and conducting the heat to the thin fusible link, which has to dissipate the heat somewhere?

Either way, a 30A fuse is ample for a 25A supply - don't increase the rating to hide a possible failure mode, the fuse is there to protect the wiring and ultimately your boat!!

 

[pvb]

I think it's important to buy good quality blade fuses and holders. There are lots of very cheap ones about which may not handle larger currents successfully.