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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 23rd Aug 2004, 5:04 pm   #1
avro
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Default Fuses

A little knowledge can be a very dangerous thing. My first blown fuse happened last week. Luckily I had a similar set for spares in the garage and used the fuse from it. I also duly ordered some .400 amp quickblow fuses. Whilst doing some cabinet oiling today I decided to replace the old grotty fuse with a new quickblow and it was very true to its word and so was its replacement. (Unfortunately Mrs Avro in a fit of cleaning had disposed of the grotty fuse in doublequick time. Normally it would have been there for a week.)

Took a look in one of my Grundigs to find that the fuse was very different, rather curly and .300 amps. I also noticed that the amps recommended dropped as the voltage (110, 160, 220) went up and so I now have replaced .400 amp quickblow with .300 amp curly - hopefully decent reasoning for a Vicar - 220v called for .400 amps. No smoke and it seems to be working at the moment.

BTW what is the generic name for the curly fuses required by our radios (mine is a Saba Freudenstadt 16) so I know what to ask for next time - and am I doing any harm by using the .300amp curly on 240v

This crisis has occurred just as my best friend is coming down from Lincoln to listen to the Proms on the Saba.
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Old 23rd Aug 2004, 5:24 pm   #2
Ed_Dinning
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Default Re: Fuses

Hi Avro, your "curly" fuses are time delay or anti-surge types, you may also see them with what appears to be a spot of solder on the wire, and some will be classifies as "T2A" - 2 Amps time delay. The time delay refered to is of the order of milliseconds (depending on the current flowing) and is to prevent fuse blowing when and inductive load such as a transformer or motor is turned on. You have actually increased your protection by fitting the 0.3A surge type as it is a unit sitting at jusy below the rated fusing current for some time that can cause the damge and overheating. Note that a fuse will carry 50% overload for possible 1/2 hour before blowing.

Send me a PM if you want a longer disseeration on fuses.

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Old 23rd Aug 2004, 6:36 pm   #3
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Default Re: Fuses

To comply with current safety regs, and for general good practice, fuses on the mains side should always have ceramic bodies, not glass. The ceramic ones are known as HRC or High Rupturing Capacity.

If you short-circuit the mains supply, the fuse and cables will carry the full current available from the national grid, limited only by the resistance of the cables etc, until the fuse blows. The fuse will blow incredibly quickly, breaking the passage of a very high current, and generating a lot of energy. A glass fuse could easily blow itself to pieces. However a ceramic HRC fuse contains a substance similar to sand to absorb the energy and a much tougher body that will remain intact.

Once you get onto the secondary side of a transformer, the fault current would be restricted by the winding resistance and transformer losses etc., so glass fuses are fine here.

Glass fuses are of course cheaper, and some manufacturers of consumer goods still use them where they shouldn't!
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Old 23rd Aug 2004, 6:39 pm   #4
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Default Re: Fuses

Quote:
Originally Posted by Ed_Dinning
Send me a PM if you want a longer disseeration on fuses.
Please keep it here Ed - it's of relevance and interest to many repairers/restorers. After all, we all replace them (sometimes with different types or values) without giving them a second thought. Perhaps we should have a better understanding of the details.
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Old 23rd Aug 2004, 6:50 pm   #5
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Default Re: Fuses

Talking of fuses blowing themselves to pieces, One far eastern company made BS1362 fuses for appliance plugs.
They were of so poor a quality the ceramic blew apart when ruptured and blew the cover off the plug leaving the 3 pins in the socket in an extremely dangerous state
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Old 23rd Aug 2004, 8:12 pm   #6
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Default Re: Fuses

Another point. Fuses, just like any other component, can fail through old age. After years of withstanding surges the wire becomes fatigued. If the replacement is OK that may be the problem. As a general rule you should always replace a fuse with an identical one, unless as Paul says regulations change to increase safety. Occasionally Manufacturers advise changes (eg if repeated fuse blowing for no reason is a general problem with a particular model.
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Old 24th Aug 2004, 12:46 pm   #7
Ed_Dinning
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Default Re: Fuses

Here goes on some basics.

The power consumption of most sets fitted with a transformer is approximately constant, as watts = V*A you will find the set consumes more current at the lower voltage tansformer tapping, if it is fed at the correct voltage on that tapping. AC/DC sets will consume about the same current on lower taps, but in this case take less power.

Most fuses have an inverse exponential characteristic; that is, the higher the current the quicker they blow. They also have a max power rating or fault clearing capacity AND a max voltage rating. Exceeding either of these may mean the fuse will not clear the fauly and may start a fire. Some fuses are designed for AC only, some DC only, some both (The arc will actually extinguish on an AC system at each half cycle, depending on the load. Low power factor (very inductive of capacitive loads) may require special fuses.
In actual use a fuse will carry (depending on type) up to twice rated currernt for anything up to 30 secs.

Light bulbs (which have a positive temperature coefficient) and appliances with motors or transformers fitted will take a surge current that depends upon the instantaneous value of the mains voltage at switch on (this explains why a fuse will not always blow). These items are normally fitted with anti-surge fuses, which could withstand 10 times rated current for perhaps 1 millisec. This "time delay" effect is created by adding "thermal mass" to the fuse so it heats up more slowly.

Note that fuse manufacturers publish curves of the operating times of their products. The fuse holder also plays an important part in the fuse performance as it helps to dissapate heat generated at normal operating currents and will have a maximum curent rating.

A fuse that is running constantly warm will often fail due to thermal ageing over a period of months. Fuse selection is complicated; as well as surge ratings to be taken into account. The main factor has to be safety. The fuse is thus not necessiarily selected on the current consumed by the appliance, but by the strength of the wiring and items it must flow through. The fuse must blow if it lets sufficient current flow, for a long enough time that dangerous overheating could occur if the item is used in the max ambient temp for which it was designed. It should not however overheat itself or be subject to nuisence blowing.

Not such a simple thing after all.

Ed

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Old 24th Aug 2004, 1:16 pm   #8
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Default Re: Fuses

Ed is quite right to say that fuses are not at all simple.

They actually offer protection against 2 different types of fault.

SHORT CIRCUIT
Exactly what it says. A very high current flows and the fuse must rupture quickly and safely before anything else is destroyed. The fuse must not itself cause a hazard while doing this. There is no danger of wiring or components overheating because the fault won't exist for long before the fuse blows.

OVERCURRENT
Too much current flowing for a longer period. At twice normal current a fuse may take 30 minutes to blow. Plenty long enough for other stuff to overheat. The fuse must be rated so that it blows quickly enough for any likely over current before anything else overheats too much.

No fuse can give useful protection against small over currents. Long term overload of 10% can overheat wiring and cause it to deteriorate. The fuse will take at least hours and maybe weeks or months to blow.

Here's an example of a 6A fuse feeding a lighting circuit in your house. If some clod drives a nail through a wire the resulting short circuit should blow the fuse in a tiny fraction of a second. The wire won't overheat in that time. If the fuse was 32A the wire might blow somewhere instead of the fuse. This fuse can supply 230V x 6A = 1380W of load. So it's fine for 13 x 100W bulbs. If you somehow plug a 3kW heater into that circuit (maybe some idiot's wired a 13A socket on it) the fuse should blow in a few minutes. The wiring will get warm, but not for long enough to do any damage.

If you have 15 x 100W bulbs on the circuit the wiring will get warmer than it should and the fuse will take a very long time to blow. Bad news. This is the marginal case I mentioned before. This case should be made unlikley by correct design. The guidance notes for the wiring regs suggest allowing 100W per lighting point and no diversity. That 6A fuse should feed no more than 13 lighting points.
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Old 24th Aug 2004, 4:54 pm   #9
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Default .....and circuit breakers.

Hi Gents, following on from Jeff's example, which shows what an imprecise device a fuse is (it's difficult to test them without blowing them!). Households and some electronic equipment now use MCB's (minature circuit breakers). These give what is called "close excess current protection", which allows much more precise control over the current that is allowed to flow. When fitted these allow the house lighting circuits to be wired on 1mmsq wire instead of the 1.5mm sq usually fitted.

They are not easily obtained in ratings low enough for radios unfortunately, although some high end audio equipment now fits them in place of fuses as a welcome safety improvement.

Ed

MCBs can certainly give closer over current protection than fuses. They still won't give much protection against small prolonged overloads.

Up to the 14th edition of the wiring regs they distinguished between close and coarse excess current protection. MCBs and cartridge fuses give close protection, rewireables give coarse. In the 15th edition everything changed. The cable was rated according to a number of factors** and you then chose a suitable fuse or MCB. If you chose a rewireable then you were penalised by about 30%. This often meant going up to the next size cable. This gave a clear warning that rewireables were a bad idea.

I'm not sure if the 16th edition outlaws rewireables for new installations but nobody should really be selling or using them now.

**The original 15th edition rules for de-rating bunched cables were far too restrictive. If taken very literally you would probably have to up-size all the cables where they entered a fuse box! The rules have now been changed to something sensible.

Hi

I agree that on paper breakers are a good idea. I also suspect that “punters” re-wiring with incorrect gauge wire or fitting “stronger” wire if a circuit persistently “blows” cause a high proportion of domestic electrical fires.

However, a reactionary part of me asks if in principle whether the replacement of a bit of wire with a much more complex electromechanical device adds safety in a situation where the " punter " knows what they are doing. This outrageous statement follows from my experience of “sticky” breakers in an industrial installation. They “clicked” all right, but seemed to “stick” and not move down. Sometimes the current would still flow unimpeded and sometimes the contacts of the breaker would arc. This may be an isolated incident, but it has left me suspicious of breakers as the only form of protection!

I have to confess to still having a re-wireable fuse box here, although I did fit an earth leakage breaker as a priority when I moved in. If I were installing a new installation, I would consider fitting both, the re-wireables acting as a backup in the event of “sticky” breakers. Normally of course the breaker would “fire” long before the fuse goes open.



Chris


Please remember that an earth leakage breaker does not provide any protection against overcurrent or short circuit. It can only detect faults that involve a current flowing to earth.

It is not a substitute for a fuse or MCB.

Last edited by Darren-UK; 26th May 2007 at 12:32 pm. Reason: Slightly OT, but partially relevant, posts merged.
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