Why do people like 13 amp fuses?

[StormShadow]

Hi I've collected quite a few second hand Ham power supplies over the years and almost every power supply that I've had, I've had to remove the 13 amp fuse that has been put in the plug and replace it with a 3 amp fuse? Why do people put 13 amp fuses in power supplies? the whole point of a fuse is that its supposed to protect the appliance encase something were to go wrong. 13 amp fuses are supposed to be for high current appliances like kettles/toasters extension leads etc etc not power supplies. Surly Hams must know this right?

[M6UCW]

Where is comes to power people should not mess with stuff unless they know what they are doing (or use a qualified electrician) especially parts meant to protect you and the equipment like fuses.

There would be nothing worse than having to explain what you did to the insurance company when your house turns into a bonfire whilst you are making a cup of tea.

This should help anyone that may be confused - http://www.diy.com/diy/jsp/content/know ... a_plug.pdf

[cjay]

Hi I've collected quite a few second hand Ham power supplies over the years and almost every power supply that I've had, I've had to remove the 13 amp fuse that has been put in the plug and replace it with a 3 amp fuse? Why do people put 13 amp fuses in power supplies? the whole point of a fuse is that its supposed to protect the appliance encase something were to go wrong. 13 amp fuses are supposed to be for high current appliances like kettles/toasters extension leads etc etc not power supplies. Surly Hams must know this right?

Because they don't get that 10 amps at 12volt doesn't need ten amps at 240.

As a telly engineer I used to see all sorts, when questioned the answer was usually 'it was the only one I had' or 'well the 3 amp one kept blowing'. Worse still were the tin foil brigade and the time we repaired an arcade machine monitor for a travelling fairground, yeah, *shudder* cut down nails used as fuses is not an urban legend.

[Maccers]

Sometimes even qualified electricians can't be trusted. My company called me once to try and find a fault in a job that one of our other engineers had been at only to find thay had swapped the fuse out for a nail!!! When I asked why he done this the reply was "so the fuse didn't blow before I found the fault"!

[M6UCW]

[RogerD]

They probably think that if a 3A fuse would blow, that kind of fault would blow a 13A too
And they probably re-used a plug from something else heavy duty and didn't even consider the fuse rating.

[crusty]

The fuses used in plugs won't protect sensitive equipment or the operator from being shocked. They stop fires. Internal quick-blow fuses are the ones to scrutinise in our hobby.

[Darkstar1]

Another way to protect your radio is strap an idiot diode across the power in, when it comes to choosing fuses Ohms law comes to mind all hams should know this durr.

[cjay]

Another way to protect your radio is strap an idiot diode across the power in, when it comes to choosing fuses Ohms law comes to mind all hams should know this durr.

A better way is to use a decent relay or if the radio isn't particularly high powered, a MOSFET. Saves blowing fuses

[Darkstar1]

Another way to protect your radio is strap an idiot diode across the power in, when it comes to choosing fuses Ohms law comes to mind all hams should know this durr.

A better way is to use a decent relay or if the radio isn't particularly high powered, a MOSFET. Saves blowing fuses

Agreed but a zener diode is just a second line of defence in case some joker uses a fuse too large, also it is cheaper than a mosfet.

[ChrisMagpie]

Why do people put 13 amp fuses in power supplies? the whole point of a fuse is that its supposed to protect the appliance encase something were to go wrong. 13 amp fuses are supposed to be for high current appliances like kettles/toasters extension leads etc etc not power supplies.

The fuse is only there to protect the flex. It's not there to protect the appliance. If the appliance needs a fuse for protection, then such a fuse *must* be built into the appliance itself.
Mains flex with a cross sectional area of 0.5mm² needs a 3 amp fuse to protect it, but short runs of 0.75mm² flex can indeed be protected by a 13 amp fuse.
We bought a cordless screwdriver from Argos. It sits on a charging cradle. There's no wall wart - the mains flex goes straight into the charging cradle. The charger is rated at *eight watts* and there's a 13 amp fuse in the plug! They can do this, because the mains flex has a cross sectional area of 0.75mm². The same appliance will be sold on the continent with an unfused europlug moulded onto the flex . . . in which case, the protection for that flex will be the 16 amp breaker on their radial circuit.

The manufacturer of the cordless screwdriver could save copper / money by fitting 0.5mm² flex and a 3 amp fuse. Cost of a special production run for the UK market versus making a standard product for all markets, and then achieving the savings through mass production / economies of scale.

Hmmm! I'm not saying it's best practice, just that they're allowed to do it. I did swap the 13 amp for a 3 amp in the 8 watt charger, though. Old habits die hard!

[Warden]

I bought a new power supply last year that came from a shop with a 13 amp fuse fitted.