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Radio_Dave 8th Jan 2007 11:12 am


I want to stock up on some 20mm fuses but what's the difference between 'glass' and 'ceramic' and then 'quick blow', 'slow blow' and 'time delay'... etc :shrug:


Merlin 8th Jan 2007 11:33 am

Re: Fuses
Ceramic ones are usually sand-filled. This prevents the wire vapourising onto the outside of the tube and continuoing to conduct after blowing- this is pretty much overkill for things like radios.

In theory, a quick blow fuse will blow in a matter of milliseconds if current exceeds the ratied value. A time delay or slow-blow fuse won't, so that if the current is only a brief transient the circuit will keep on working, it will only blow if the surge is sustained for more than a couple of seconds. This is handy for things like inrush current where you don't actually want the thing to blow. You'd probably use time delay type fuses in the HT line.
In practice I find there to be little difference between quick and slow blow types, they seem to be much of a muchness, at least when working with radios/amplifiers. I guess it makes a difference with more sensitive circuits. Maybe others disagree?..

pmmunro 8th Jan 2007 2:09 pm

Re: Fuses

Ceramic bodied fuses are usually High Breaking Capacity (HBC) designs which are constructed to withstand the very considerable energies which can be released when a fault occurs in a circuit connected to the mains supply.

A typical domestic 13A socket can have an earth loop impedance of less than 1 ohm, giving a prospective fault current measured in thousands of amps.

A glass fuse SHOULD NOT be used as a mains fuse as it can break open with very considerable force, certainly enough to damage a fuseholder and potentially be dangerous. Despite this some equipment manufacturers did use glass fuses as mains fuses in the past.

The time characteristics depend on the application and what is to be protected. The common types are:

FF - semiconductor fuses; very fast characteristics, intended to protect devices such as thyristors, triacs and some rectifiers. VERY expensive and only justifiable where the cost of replacing the device is greater than the fuse which can be as much as 20.

F - general purpose fuse with "Fast" characteristic. Used where high surge or inrush currents are not likely to occur.

M - medium fast characteristic able to withstand moderate surges. Unusual in small fuses as used in electronic equipment.

T - Also known as "Slow-blow" or "Anti-surge" fuses. These use one of a number of techniques to give the fuse the ability to withstand currents in excess of the rated value for a short period. Typically used with loads such as transformers, smoothing capacitors and incandescent lamps.

Miniature fuses, as used in electronic equipment, normally conform to standards which specify an operating current of 1.6 times the rated current. Manufacturers publish sets of curves which show how long it will take for a fuse to blow for a range of currents. Times range from several hours to "instantaneous" - a few milliseconds.

I would suggest that you select a range of values typical of the type(s) of equipment that you intend to work on - for mains radios this might be 100mA, 250mA, 500mA, 1A, 2A and 2.5A in glass, type (F), 1A, 2A and 2.5A in HBC (these generally have some surge resisting capacity) and 250mA, 500mA and 1A in type (T).

There are two sizes in common use; 20 x 5mm and 1.25" x 0.25".

Then there are fuses to BS 1362 (Plug top fuses) only available as HRC ceramic types. For these 3A and 13A are the essentials, but 5A is useful for equipment with switch-mode power supplies such as computers and monitors. Other values are available, but not at all common.

"Selection box" kits are available and probably represent a good range if bought from a reputable supplier. You will probably be surprised at the cost of these but find that this works out to be no more expensive than buying the fuses separately.

There is much more to fuse technology then "just a wire that melts" and there are good books on the subject - see also manufacturers' web sites.

Peter M. Munro

ppppenguin 8th Jan 2007 2:45 pm

Re: Fuses
Peter Munro has written an excellent introduction to the subject. His mention of the rather specialised FF fuses reminds me of the definition of a semiconductor:

A device that blows in order to protect the fuse:)

If you have to replace a blown glass fuse it's worth a careful look at the old one. If it's badly blackened or even shattered then there was a short circuit. If most of the wire is intact with no evidence of violence it may have just died of old age or possibly a mild long term overload. It's pretty obvious that you would use different methods to diagnose these different faults.

Paul Stenning 8th Jan 2007 7:11 pm

Re: Fuses
I have made this excellent thread sticky for now, so it doesn't get lost. This is definitely one to move to the archive in due course.

G8DLG 8th Jan 2007 8:18 pm

Re: Fuses

Originally Posted by pmmunro (Post 96613)
A typical domestic 13A socket can have an earth loop impedance of less than 1 ohm, giving a prospective fault current measured in thousands of amps.

I think you nean several hundred amps,at least at the sub circuit feeding the socket,(theres not a lot of current to be got from mains,especially AC),but a fuse has to break considerable energy at short circuit.
Fitting the correct fuse is of utmost importance,it should be rated equal to, or less than the smallest wire in the circuit,for most radio sets a 2A HRC fitted in the plug gives good protection.

pmmunro 8th Jan 2007 9:20 pm

Re: Fuses

I'm a bit surprised by your statement "there's not a lot of current to be got from the mains, especially AC". Maybe I'm misunderstanding your meaning.

The earth loop impedance at the origin of a comsumer's installation may be determined, by inquiry (asking the supply authority), by measurement, or by calculation (usually only practical for a system using a local generator).

For a new installation none of these may be practical and a guide figure of 0.35 ohms is used. That would give a prospective short circuit current (rms) of 230/0.35 = 657A. So in this case my statement of "thousands" is perhaps a little high. However, I have measured systems, close to a substation where the ELI has been less than this, so maybe it's not too great an exaggeration.

The normal consumer unit MCB is rated for 6 KA, even for 6A types.

Even 657A is a very high current for a 5 x 20mm fuse and the wiring of consumer electronic equipment.

Many electricians have had unfortunate experiences involving flying molten copper and steel and burnt pliers or screwdrivers. Such experiences are very convincing concerning the effects of releasing significant amounts of energy in a short time.

The important point is the suitability of fuse construction techniques for a given application.

Quoting "Fuses for Electronic Applications", Littlefuse, 1991,

Breaking capacity is defined as - "A characteristic of a fuse is the maximum value which can be interuppted by that fuse under specified conditions. A simple non-filled glass fuse, as used in electronic circuitry for example, has limited interrupting capacity.

If such a fuse is used for mains protection it may be subjected to an excessive current in the event of a fault. The fuse COULD EVEN EXPLODE and become a FIRE HAZARD. For such an application, fuses with a higher breaking capacity MUST be used, which normally contain a filler material".

(My capitals for emphasis).

For a 20 x 5mm fuse to IEC 60127, the breaking capacity (not the current at which the fuse opens) is ten times the rated curent or 35A, whichever is the lesser for glass and 1500A for the HBC type.

When you state "especially AC", do you have in mind large storage batteries?

Some of our correspondants with telephone exchange experience will have many stories to tell about the current available from "the battery".

If you really want to know, consult the title quoted above or "Electric Fuses", A. Wright & P.G. Newbery, IEE, 1995.

Peter M. Munro

Skywave 8th Jan 2007 10:25 pm

Re: Fuses
My contribution to this important topic. :zzz:

Mains "plug type" fuses.
As we all know these are commonly fitted in multi-way mains distribution "blocks" - typically 4-way, although 6-way and larger sizes are available. Not wishing to condone the practice of cascading multi-way dist. blocks, but where there are, in effect, several fuses in series - between the originating "13A plug" and the ultimate load, it is advisable to "fuse down" as the final load is approached. Example - 13A fuse in the primary plug, 10A fuse in the 4-way distribution block, 5A (or possibly 7A) fuses in the appliances that connect to this distribution block. Apart from the convenience of knowing where to (most likely) look first in the event of tracing a blown fuse, this also provides a safety margin for the distributed currents. But make sure that the inter-connector wiring is rated accordingly - these fuses so fitted are primarily there to protect the cabling and the inter-connectors, not the ultimate loads.

Well, that's what I was taught (in Gov't. Service).

I expect someone will make an "amendment" to my understanding - which is fine, :thumbsup: since selecting fuses, cabling and connectors for a particular application / distribution is not a straightforward affair - as some maintain - and if nothing else, I'm a member of this Forum to learn as well as to contribute.

BTW If anyone wants to expand this Thread into "cables & connectors, mains distributions systems", please start a new Thread. ;)

:wave: Al / Skywave

Radio_Dave 9th Jan 2007 10:45 am

Re: Fuses
Thanks for the very informative and comprehensive replies :).

What about voltage rating? For example, would a 1A fuse with a 250V rating still work in a 12V position?


ppppenguin 9th Jan 2007 10:50 am

Re: Fuses
The voltage rating is maximum so a 250V rated fues will be fine at 12V. I don't know if there is any practical difference between AC and DC. Switches for AC typically use a smaller gap because the arc extinguishes at the zero point of each cycle.

G8DLG 9th Jan 2007 12:24 pm

Re: Fuses

Originally Posted by pmmunro (Post 96689)
I'm a bit surprised by your statement "there's not a lot of current to be got from the mains, especially AC". Maybe I'm misunderstanding your meaning.

As you have discovered,675A indeed.We are talking about short circuit current in a SUB CIRCUIT ie at the 13A socket.A lot more current would be had further towards the sub station transformer.Another point with AC is the reactive impedance of cables and transformer,all this will limit the current.
As for storage batteries,you are really into danger,short circuits are catastrophic,unless its extra low voltage.
DC is not something to play with.
I used to perform mains short circuit testing on MCBs,I know mains current can be surprisingly small,some MCBs would not trip fast enough to pass the tests!

plumbweiss 4th May 2007 10:44 pm

Re: Fuses
If you're after a very small radio fuse, you may find it difficult to get a 20mm or Belling-Lee midget fast HRC ceramic / sand filled fuse below about 2A, you can get them but not from the usual sources. Very few valve radios will have used HRC fuses from new anyhow. Some telecomms lightning arrestors have 125mA HRC ceramic fuses in but you won't need to pay the high price for this sort of thing for a mains fuse.

enthalpy 17th May 2007 8:02 am

Re: Fuses
Those little 13 amp plug top fuse links have a prospective short circuit current rating of 6kA. I always think it is very clever that a component so small can break such a current with no outward sign! That is if they are built to BS1362.

Mersey_Swimmer 1st Jun 2007 10:51 pm

Re: Fuses

Originally Posted by plumbweiss (Post 115195)
If you're after a very small radio fuse, you may find it difficult to get a 20mm or Belling-Lee midget fast HRC ceramic / sand filled fuse below about 2A,

Could I be cheeky and point out that I stock them?

I used to do industrial electronics, and much of that involved troubleshooting big DC drives up to 900kW. It's a frightening experience having your test gear wired up to power semiconductors connected to a 1.5MVA distribution transformer via a set of 800A fuses! Most of the work is done live, one slip........!

While the shock risk is no greater than working on the domestic mains as phase to ground is still only 230v, the main danger from high-energy 415v circuits is explosion. Most casualties resulting from accidents in sub-1kV installations seem to be the result of blast injuries.

So it's best never to treat the mains supply with anything other than the greatest respect. Enormous fault currents CAN flow before safety devices (fuses, circuit breakers etc.) actually operate.

Steve Anderson 7th Jul 2007 10:47 am

Re: Fuses
Interesting thread this. Now it's apparent that this is primarily UK based board with all the mention of BS1363 plugs, fuses and other protection devices. I live a country where there is no fusing between the substation transformer and the appliances. Really!

The main feed comes overhead down the road on 25mm squared cable, then 10mm squared into the house. The meter is only rated at 15A, yet we reguarly pull over 40A through it due to all the air-con. I have asked for it to be uprated, but after six years I've given up and I'll just wait for it to catch fire. It's not in the house but on the pole outside.

We use US style plugs and sockets of either 2 or 3 pins at 220V which have no fuse in them. It's only been in the last few years that legislation has been passed that all new installations actually have a ground/earth! i.e. the sockets might have had three pins, but it didn't mean that the third one was connected!

Steve A.

G Barham 13th Jan 2008 4:46 pm

Re: Fuses
Electricity can be dangerous, in the wrong hands, and we mustn't forget the reactive effect and discrimination. All good things when used correctly.

Even the 6" nail and piece of tin foil have their place in the sun.


RobinBirch 14th Jan 2008 6:24 pm

Re: Fuses
And having seen the end caps of a 13A domestic ceramic fuse blown out with molten metal splattered all over the inside of a plug when a direct live to earth and neutral short was created by my dad putting a pair of heavy duty cable cutters through a mains lead, don't ever think that the containment given by the body of the fuse is 100% reliable.


GMB 14th Jan 2008 11:56 pm

Re: Fuses
I believe that the standard mains fuse values were carefully chosen so that the smallest one in a chain usually blows first.

It's about a factor of 3 to get this effect, hence 100A master with 30A ring and 13A on the plug and even 3A or 5A plugged into an extension.

Ray Cooper 15th Jan 2008 12:36 am

Re: Fuses
This business of tandemed fuses is a very interesting one. Let's just take an imaginary example:-

You have a load, protected by (say) a 7Amp fuse. It's fed from a plug-top containing a 13Amp fuse. The load develops a fault, and the 7Amp fuse blows. The 13Amp fuse will be protected - right?

Answer: not necessarily. After you've fixed the fault and replaced the 7Amp fuse, you may still find that the 13Amp fuse has blown too. Why?

The answer turns out to be rather interesting (well, I think so, anyway...)

When a fuse blows, it absorbs energy. It requires a finite amount of energy to cause the fuse wire to melt: but on top of this, when the fuse opens an arc is started which dissipates a further amount of energy until the arc is extinguished.

Now here's the interesting bit. If the total amount of energy expended by the 7Amp fuse (blowing + arc) is greater than the amount of energy needed to melt the wire in the 13Amp fuse, then that latter fuse will melt and go open-circuit. There probably won't be an arc in that fuse, but at the end of the day, it'll be open-circuit anyway. Interesting, or what?

Brian R Pateman 15th Jan 2008 1:25 pm

Re: Fuses
The fuse in the appliance is to protect the appliance.

The fuse in the plug is to protect the cable between the plug and the appliance.

The fuse in the distribution board is to protect the sub circuit wiring.....and so on.

They are all there to protect the user.

(This is the simple answer - unless of course you want t ogo into the intricacies of discrimination in power supply system design.)


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