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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 7th Dec 2004, 12:09 am   #1
Eric_Harvie
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Default Fuse ratings.

I've protected my vintage radio's with 1amp fuses,but what about modern stuff? I'm thinking along the lines of computers,hi-fi,video,etc. I've just had a couple of power surgesne took out one of the pcb's for the central heating unit (thankfully insured),and ,the other ,damaged the pcb on the video which had a 5amp fuse (internal fuse not damaged). Should I have 3amp fuses overall? I would appreciate any advice.
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Old 7th Dec 2004, 12:39 am   #2
paulsherwin
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Default Re: Fuses. What a to-do, and what to do?

Quote:
I've protected my vintage radio's with 1amp fuses,but what about modern stuff? I'm thinking along the lines of computers,hi-fi,video,etc. I've just had a couple of power surgesne took out one of the pcb's for the central heating unit (thankfully insured),and ,the other ,damaged the pcb on the video which had a 5amp fuse (internal fuse not damaged). Should I have 3amp fuses overall? I would appreciate any advice.
I assume you're talking about plug fuses. These won't do anything to protect against mains surges. Their main function is to blow if there's a really major fault, like a wiring short, and stop the house wiring melting. I've been thinking about this during the last half hour, having acquired yet another old radio with a 13A fuse in the plug. We always change these, of course, but what benefit do we really get? What fault in an old radio will actually blow a 3A plug fuse that won't blow a 13A fuse? I'll fit one anyway, but I think we're waving a dead chicken here

Protection for the *equipment* (as opposed to the external wiring) comes from internal fusing and circuit breaking arrangements, which we as end users can't do much about

Best regards, Paul

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Old 7th Dec 2004, 1:20 am   #3
Tim
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Default Re: Fuses. What a to-do, and what to do?

I tend to agree with Paul.
Think about it. Even for a one amp fuse to blow means 250 Watts has to be going somewhere, and in the case of a fuse blowing fault, probably much more and nearly instantaneously.
I always understood the plug fuse was there to protect the mains flex and house wiring, rather than the appliance.
Fuses are a bit of a black art, as the right value is really a compromise between everyday working current and fault current. Who knows what that will be?
Even then fuses blow at more than their rated current anyway( open fuse wire by quite a margin cartridge fuses somwhat less).
However, don't let all this talk convince you there is no benefit to the correct value of fuse.
A properly rated fuse is better than nothing(especially a nail or peice of copper wire!!) and is the last defence against something REALLY bad!
We can't win really, Murphy loves fuses to demonstrate to doubters that his law really does exist!
How many times have you had the item a fuse is designed to protect " blow " before the fuse does?
Partly,I think I tend to fit a 3 amp fuse to a radio because "it's the proper thing to do" and don't actually think about it much more.
You will find plenty of (hopefully humorous) comments regarding fuses found in plugs in some of my repair reports. Suffice to say I don't think I have had to buy a 13 amp fuse in 15 years!!
There's no need to panic, these sort of faults are pretty rare, and sets tend to be fairly reliable even after a good many years.
Cheers
Tim

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Old 7th Dec 2004, 4:57 am   #4
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Default Re: Fuses. What a to-do, and what to do?

Hi Eric

There are devices which protect from power surges. When voltage is exceeded they short within microseconds. This in turn blows a fuse saving the radio.

You will find these listed under varistors or just surge suppressors.

I have some rated for use on 130 volts. They act as a short around 170. You will need a rating of 240 volts and short around 280.

A fuse will save your radio from burning up but may be too slow to protect it from damage.

Norm

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Old 7th Dec 2004, 9:50 am   #5
ppppenguin
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Default Re: Fuses. What a to-do, and what to do?

The subject of fuses and protection is more complex than it looks. It has been extensively discussed in earlier threads though the search tools in YaBB are not very efficient.

Fuses can protect against overcurrent and short circuit. Without unduly repeating what has gone before, 3A fuses in the mains plug of an old radio will give sensible prtection against short circuit. Overcurrent protection is much harder and will rely primarily on fuses and other measures in the sets themselves.

The new subject here is voltage surges. Fuses do not and cannot protect against these. Norm_Leal's comment on surge protectors (varistors) is correct. You can also buy extension leads with built-in surge suppressors.

The amount of protection you need depends on where you live. Mnay of us are fotunate to live in suburban areas with underground power distribution and little or no local heavy industry. We usually get a pretty clean and consistent mains supply. If you live near a steelworks (do we still have them ) the mains will have all sorts of surges. if you live in the country with overhead power distribution there will be effects from lightning.

A friend who lives in rural Wales had a lightning hit on the power pole outside his house. Mains sockets blown off the wall and damage to all sorts of equipment. A surge suppressor will not protect against that sort of horror.
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Old 7th Dec 2004, 10:16 am   #6
wireless_paul
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Default Re: Fuses. What a to-do, and what to do?

Go for 1A fuses in the plugs of radio's. When doing up a radio with a mains transformer, try to get a 250mA fuse in the HT secondary.
Paul E
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Old 8th Dec 2004, 1:55 am   #7
AC/HL
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Default Re: Fuses. What a to-do, and what to do?

Imperfect as they are, I would get the best available protection by using the smallest possible fuse, consistent with reliable operation. 1A would be adequate for a radio or Hi-fi etc. but a computer may need 2-3A to cope with the surge, especially if the monitor is switched by the base unit. If spikes are a local problem, surge suppressing plugs are available.
As Paul says, additional fuses can be added if the value of a radio warrants it.
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Old 8th Dec 2004, 10:51 am   #8
Andrew_Keith
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Default Re: Fuses. What a to-do, and what to do?

I agree, in my equipment i always fuse at the lowest possible value. Some time ago i went on a P.A.T. testing course, and they informed us that the plug fuse is only there to protect the appliance cable. They showed us an example of a clock radio where the mains cable had developed a short at the appliance end. The plug fuse had been rated at 13amps. And the cable had actualy started to melt before the fuse blew!!. I fuse all small appliances at 1amp the only exeption being the T.V. which has a large start up surge, and blows anything below 5amps. I would personaly rather a fuse nuesance blew every now and then than worse. 1amp fuses are still readily available at electrical wholesalers.

Andy.
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Old 8th Dec 2004, 10:54 pm   #9
Eric_Harvie
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Default Re: Fuses. What a to-do, and what to do?

Thankyou for the helpful advice.I shall invest in some extension leads with surge protection and use between 1 and 3amp fuses.
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