Fuse Choice

[OceanSprint]

My VHF draws 5.5 A max when transmitting on full power (according to the ICOM manual).

It is fed power from a line direct from the domestic battery, but is locally provided with a 20mm glass fuse at a junction box.

I was going to put in a 10A quick blow glass fuse from Maplin, till I read that these fuses blow / break at "35A or 10 times the rated current which ever is bigger".

So a 10A fuse will blow at 100A - seems excessive and doesn't protect the power feed cable which is not a heavy one.

So, do I fit a 1A fuse, expecting it to take the 5.5A no bother, and blow at 10A ?

If so, what does the "rating" of a fuse mean? Surely the "blow" current is the critical bit?

Thanks

Bill

 

[CreakyDecks]

Fuse ratings aren't as simple as you might think. A given size wire doesn't have a current at which it will "blow". It will blow in a certain time depending on the current flowing through it. The rated current is the highest current that can flow continuously without the fuse breaking the circuit. If your radio wants 5.5A when transmitting on full power then a 5A fuse is plenty big enough, your battery would probably go flat before the fuse blew. If you use a 5A rated fuse then you should really be using wiring that will take 10A. Remember the fuse is to protect the wire, not the radio.

 

[nigelmercier]

... If your radio wants 5.5A when transmitting on full power then a 5A fuse is plenty big enough...

The longest call you are likely to make is a Mayday, not a good time for the fuse to decide it has had enough. I'd be inclined to fit a 7.5A blade fuse in a fuse box. I'm surprised Icom don't specify the fuse size.

 

[David2452]

As has been said, a 7.5a should suit (but do check either the manual or ask Icom, they are most helpful), however, and equally importantly, you mentioned a direct connection to the battery, if this is not already fused you would be well advised to fit one as close to the battery as possible, the rating can be higher for that is it is to protect the cable not the load.