Fuses

[Derek Sheldon]

one for you electrickery people out there. Advise on glass fuses sought. I can find plenty of the glass fuses on fleabay, the correct length and amperage but most appear to be 250v. Is it a case of amps is amps or does the current come into it ?.
Also please advise what amp fuses are recommended for a late Opal,  all I can find in the books is “ replace with the same rating as that removed”

[Colin Morgan]

Hi

Yes, amps is amps with fuses, voltage doesn't come into it.

This was posted on the old forum by sqeak in 2013:

"Lucas 6-volt Electrical Equipment as fitted to Austin Seven Cars 1936",
Dynamo field 6 amp
Accessories 25 amp
Dipping reflector 10 amp
Trafficators 25 amp

Colin

[Reckless Rat]

There is a formula for calculating the relation between volts amps and resistance (ohms) which is V=IR where v=volts, I=amps and R= resistance.

Thus a 6volt fuse rated at 6 amps would require a resistance of 1 ohm whereas a 12 volt 6 amp fuse will require a 2 ohm resistance and a 240 volt 6 amp fuse would need a 240/6 = 40 ohm resistor.

I can't see that a mains 6amp fuse would be suitable for a 6v application. Well it might, but it won't blow when you need it to.

[Derek Sheldon]

THANKS

[Mark Dymond]

My understanding is... Colin is closer than Reckless. The Voltage rating is essentially a Maximum voltage, so a 240v rated fuse in a 6 volt Austin is fine. Just chose the right Amps and you are away.
The V=IR will give you the voltage drop across the fuse so in real fuses R is low enough for this to be tiny and is independent of the circuit voltage.

[Graham O]

For a fuse, it is current that counts.  If the fuse will fit and has the right current rating, it will be ok.   For fuses the voltage rating is to do with other things that don't count for low voltages like 6/12V.

The design of a fuse revolves around the current causing the metal link to melt and break the circuit.  So it involves the fuse conductor length, thickness and heat dissipation capability in the fuse to work together to set the rating.

[Dave Mann]

I agree with Colin and Mark and use 240 volt 5 amp fuses for the dynamo field fuse because the short glass 6 amp fuses are like hens teeth. I wouldn't recommend using an automotive low voltage fuse for mains use.

[AustinWood]

As I understand it it is only the current that counts.
The power that melts the fuse is current squared times resistance (of the fuse). In other words the resistance of the fuse decides what current blows it, not the voltage.

[Phil Kingdom]

Many of the old type glass fuses are repairable, simply unsolder the end caps and thread in a new piece of fuse wire though the hole and resolder.

[Dennis Nicholas]

Reckless
You have it a bit back to front.
Voltage is a measurement of electric pressure,  amps is a measurement of electric current flow (flow of electrons from the  atoms/molecules of the material that the current is flowing in) and ohms is the resistance of that material to current flow (Material with lots of easily moved electrons will have a lower "resistance"  whereas an insulator has very few free-to-move electrons).
Use the water analogy.......The battery is the water pump with Volts the unit of pressure, electrons are the flowing water and diameter of pipe/hole in bung in pipe is a resistance to the flow.
Thus for a given pipe/hole/wire size more or less water/current flows depending on pump pressure/voltage - higher voltage = higher pressure so higher current.

There needs to be a little bit of caution over fuse ratings......some ratings are the current (amps) at which the fuse will blow but others are a higher amperage to allow for spikes of higher voltage which would cause a momentary higher current but the fuse stays intact and the current continues back at its NORMAL LOWER rated value after the spike.
e.g. a 25A fuse may be one which melts at 25 amps OR it may be one which normally runs in a circuit which runs at a lower current but it will take 25 amps for a short period. (Yes a subtle difference but important)

There a bit of nuclear physics for the Austin 7 

Denis