Fuses
 

Hi,

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:

Thanks
David

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?..

Re: Fuses
 

David,

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

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.

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.

Re: Fuses
 

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.

Re: Fuses
 

G8DLG,

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

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. ;)
Thanks.

:wave: Al / Skywave

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?

Thanks
David

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.

Re: Fuses
 

Peter,
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!

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.

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.

Re: Fuses
 

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

http://hometown.aol.co.uk/oldradioparts/oddsnsods.htm

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.

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.

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.

Tony.

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.

Robin

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.

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?

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.)

Regards,

Re: Fuses
 

And transistors are there to protect the fuses;D

I once saw a cascade of blown fuses ranging from a 3A one in the plug to a 30A MCB protecting the subcircuit. I find it difficult to believe that the energy let through by the lowest rated fuse (usually quoted as I*I*t) was enough to trip or blow the bigger ones but that's what happened.

The original fault was a mains filter cap that exploded in a colleague's face. Fortunately he was unhurt. This all happened at BBC Kingswood Warren way back in 1974.

Re: Fuses
 

My experience is that MCBs are much faster than fuses and so spoil the blowing-chain.

My big variac can blow any MCB through the smallest fuse due to the very short but huge amplitude switch-on surge. I actually fitted a smaller MCB to it in the hope that it would beat the ring 30A but in practice they both trip together.

Re: Fuses
 

As I mentioned earlier... don't forget the discrimination.

There are curves published for all types of fuses and MCB's. Basically the curve is a trace of current verses disconnection time and competent designers can look at the curves (normmally using both curves on one graph) and see if there are any overlaps.

If there is an overlap, it's possible for the higher rated fuse/MCB to blow before the lower rated one.

For simple house wiring and small industrial systems there are many computer progs. which do it all for you...you just type in the cable sizes, the known or measured impedances and the size/type of the chosen protection device and low and behold, the print out lets you know the current (ex. pun) situation.

For large systems and those getting close to supply transformers it's often a good idea to do a manual check.

A blown fuse in a plug top is one thing but a wrong setting on a 5000A ACB which could cause pieces of 100mm X 20mm copper bars to vaporise and blow a piece of sheet steel weighing 20Kg over 100 yds is another.

I think I'll try and stick with the mA... it makes sense !

Regards
Tony.

Re: Fuses
 

There are about 4 different types of MCB ranging from the quick blow to ones with operating curves designed to cope with huge in-rush currents. Standard house type are designed to trip quicker than fuses.

Regards


Tony.

Re: Fuses
 

GMB
Is your MCB a type "c" breaker?, you could fit a type "b" to prevent tripping due to inrush of current

Re: Fuses
 

No, it's a type B.

But I don't regard fixing house MCBs as a practical solution as it assumes that I'm in my house. My big variac now has a built-in resistive start-up with a relay added to prevent accidents.

Re: Fuses
 

I think that's the wrong way round. Type C are more resistant to tripping on surges. There's also a type D but that's only for specialist use such as industrial welders.

Re: Fuses
 

Why you don't want a fake fuse:
http://www.era.co.uk/services/devices.asp

I reckon the supply to that socket must have had a really low impedance.

Re: Fuses
 

Fuses and MCBs only protect the wiring 'upstream' of them in order to avoid fires and damage (who wants to rewire everything?) - they are only intended to perform that function. Your equipment can go fry as far as the power distribution cares, that's all the way up your plug-top fuse!

Take a mains radio with a 3 Amp fuse for example, lets imagine that it develops a fault that causes it to draw 2.8A rather than the normal 0.4A. While your radio is busy burning furiously, the wiring up to and including the fuse is happily (and safely) operating normally within all its design limits. The fuse didn't protect your radio and your electric bill is growing larger.

The 'safety' gap between 'normal' operating current and fuse popping current is always difficult to assess absolutely as there are usually surges associated with switch on (e.g. charging capacitors) and even normal running (e.g. spinning up disc drives). The difference between the normal minimum and maximum operating currents is one reason for many fuse and MCB variants.

If you know your equipment will draw 20A for 2ms after switch on, then settle to 0.4A, the only fuse that is guarenteed not to nuisence trip is 20.1A. But the prospect of this fuse supporting a fault current of 18A may be a little unnerving! So you would fit a slow-blo 1A fuse (for example). Better to fit an MCB though since fuse wire heating and cooling (through the current spike) causes the fuse wire metal to expand and contract, thus causing metal fatigue and accelerating failure.

It's a hard job providing really reliable 'protection'.

Jim

Re: Fuses
 

This is an oversimplification. As indicated by Peter Munro in an earlier post the really important characteristic of a fuse under surge or short circuit conditions is its "let through" energy. Commonly written as "I squared t". This will be larger for a higher rated fuse and also for a slow blow fuse. The fuse should be chosen to not let through hazardous amounts of energy under these conditions yet not blow under normal surges. Long term heating effects are not important here as the fuse should blow pretty fast if there is this kind of fault.

The normal overload characteristics are rather different. It is assumed that thermal equilibrium is attained and the fuse should blow before anything else overheats dangerously. This can give very real problems, since fuses will take a long time to blow on small overloads (up to 50% or sometimes even more) during which time things can get a bit too hot.

Re: Fuses
 

I've seen a similar layout ,with different values though on telecomms transmission racks .The unusual thing we found was that where you'd expect the smallest to go first , at times it would be some way down the chain that one would go .This was with the manufacturers rated fuses in all places .Asked power staff to explain - they couldn't ,but had experienced it themselves .

Re: Fuses
 

Doesn't S*d's Law say that when a number of fuses are connected in series, the one which blows under fault conditions will always be the hardest one to replace?

Re: Fuses
 

would a 1A fuse with a 250V rating still work in a 12V position?
Probably not. It has never worked for me, in fact I have spent much extra time troubleshooting circuits just because I used a 125v or 250v fuse in an automotive circuit.

Re: Fuses
 

that is so tru , its always the one that you cant get to or one that you dont have with you ,

Re: Fuses
 

This is a slightly complex question.

The voltage rating of a fuse is the highest voltage at which it can be used safely. It's also usually an AC rating. The DC voltage rating will be much lower because a DC arc does not self extinguish at the zero crossovers. However a 125V or 250V fuse will be perfectly safe at 12V DC. It will not be safe at 250V DC.

Automotive fuses are rated in a somehwat different way to other types. I'll leave the explanation to those who understand this better but it may be true that a 5A automotive fuse is roughly equivalent to a 10A normal fuse.

Finally there's the surge rating. I suspect that automotive fuses are rated for pretty high surge currents. This suggests you should use "antisurge" or "slo-blo" types.

Re: Fuses
 

If you had for example a table lamp with a 3A fuse in its plug, plugged into an extension lead with a 7A fuse, plugged into another extension lead with a 13A fuse, plugged into a ring main with a 30A fuse. Then you (or some other idiot) cuts through the table lamp's cable close to the plug. The fault current could be several hundred amps, more than enough to blow any of the fuses, and this current will flow until one of the fuses blows.

You might reasonably expect the smallest rated fuse to blow first, its thinner wire will have a smaller thermal mass, but there's no guarantees, and the other fuses may have been weakened. So when you demonstrate what you did wrong, and cut through the rewired, re-fused lamp cable again with your new un-melted cutters, a different fuse may blow this time!

Stuart

Re: Fuses
 

By G Barham: Very true, See:

Extract taken from one of my electrical documents, as I thought this would be useful, and to dispel some myths.
Emphasis mine to highlight the point

This is why for example, a 5 Amp plug/spur (BS1362) fuse may blow and the 3 Amp plug fuse downstream may not, or they may both blow, ie. a 3Amp plug fuse will not discriminate against a 5 amp plug

cheers,
Baz

Re: Fuses
 

I know that what I am going to say is a little bit OT.
My worry is some imports from the far east. For years I have come across CD players, Touch Lamps some small LCD TV sets and other items with NO internal mains fuse. Yes there can be fuses after the bridge rectifier or mains transformer, but so many items sold here are relying on the fuse in the plug, often the rating is far in excess of the current drawn.
Now I have heard some saying here the plug fuse is to protect the mains cable only, this is total rubbish. Irons, Kettles, Toasters, Washing machines, fires and drills just do not have internal fuses so they rely on the plug fuse for shorted elements, seized motors etc, not just knackered cables.

My worry though is the smaller stuff less internal low rated fuses, assume a unit with a mains tranny with a fault that allows it to draw 1.5a from the mains, a good manufacturer would fit a 800ma fuse internally but all there is, is a 3 amp in the plug, so the mains TX overheats and cooks, at the best it blows the thermal fuse on the tranny at worst it causes a fire. So we see that the fuse in the plug is worthless it protects nothing. So please don't say to me that the plug fuse is ONLY to protect the cable it isn't, it all depends what you have at the end of the cable!

Manufacturers need to seriously look at what they make and how they protect the appliance and the end user.

Re: Fuses
 

Hi all

The thing to remember is that a fuse does not LIMIT the magnitude of the current.

It does not protect anything, including other fuses in the circuit, from surge current damage.

All it does is limit the duration of the current to prevent serious damage (eg fires!). Up to the rated current, the fuse should last indefinitely. At twice the rated current, the fuse should blow in less than - depending on the type of fuse (fast or slow-blow etc) - 60 seconds or so. For higher currents, the fuse will blow progressively faster.

In places where the fault current can be very high, eg on the mains side of equipment, you must have a HRC fuse in the circuit because normal glass fuses may not break very high fault currents. In the UK, this fuse is in the 13A mains plug, and will break the very high currents you would get with a short circuit to the live wire.

On UK mains, a 3A mains fuse will allow a dissipation of 750W continuously or 1.5kV for up to a minute, too high to protect most small appliances. With the HRC fuse in the plug, fuses on the mains input side of appliances do not need to be HRC types, and can be normal glass fuses with a low current rating chosen to protect the equipment.

Stuart

Re: Fuses
 

I had the voltage rating on fuses explained to me by the local supplier thusly:
The voltage rating on the fuse is the highest voltage that the fuse will take after it opens and not arc over.
So its fine to use a higher voltage rated fuse on lower voltage but not the reverse.

That being said, one must keep in mind that the $35.00 semiconductor will blow to protect the $.25 fuse.

Here in the States, the ceramic sand filled fuses are generally used in high heat applications, according to my distributor. Typical use for them here is in microwave ovens.

The voltage rating for a fuse is the max amount of voltage it will take before arcing across the opened fuse metal ends.
IE: A 32V car fuse, can be used on the mains voltage, but if it fails, the voltage can arc over the ends of the broken fuse ends.
If a fuse of a 250V rating is used, it is supposed to not arc till the voltage exceeds 250 V.

I have some big industrial fuses here that one can unscrew and one can replace the fusing wire element inside them when they blow. It can take a pretty good load to open a 1000 A fuse.
(Hopefully its not a "body resistance test" type load.:o I saw way too many examples of that working for the state labor & industries safety division in my youth.)

Re: Fuses
 

I haven't noticed this in the thread,

Looking at voltage ratings, think about HT+ on valve circuits etc.
The disconnection time is linked to the arc time within the fuse after the element breaks.
So things to think about are.

At fault current how much current can flow before the fuse disconnects.
What is the Arc distance and at what voltage of operation.

Then how is the arc quenched to increase the speed of operation.

So at fault, the fault draws as much current as the supply can deliver until initial element break. Then fault current will continue to flow until the gap (+ ionised gas) is big enough for the arc to stop.

So if you think about sand filled fuses as the element breaks the sand falls into the gap. If the element is under tension from a spring it will pull apart faster (arc distance) There is always more (if you have a DC fuse in a circuit you can use a cap)

Just a thought. If you use surge suppression for transformer magnetisation current you can fuse closer to the operation current of the circuit.

Just for fun.
Tubeglow.

Re: Fuses
 

One more quick thought,

the type of voltage across the fuse is obviously different with AC and DC.
So the AC voltage is following a sine wave + whatever transients are on it.
The fuse element is heated with the combination of V & A so this is varying on an AC supply. The clearance time is dependant upon the arc quench with a drop in voltage this is helped (depending where on the cycle the fault occurs).
Fault current is dependant upon impedance of supply and VD.<<how much current can you draw from a fuse board..a fuse is a piece of wire a CB is two contacts.

On DC the fault has a clearance time dependant upon Arc and distance gap of the fuse element. There is no change in supply voltage except possible fast release of current from supply caps.

The fuse rating is different on different fuse types cartridge fuse 1.5 times rated value. The clearance time is dependant upon fault resistance path.
If you have a "Bad" earth then the clearance time will be slower. You need enough current to flow as fast as possible to make the fuse element operate.
This is known as ADS(automatic disconnection of supply)
A fuse not exceeding its operation point could allow extreme damage. heating of both earth cables and supply cables possible shock from elevated voltage on a chassis. Waiting for the fuse to blow..

Slow blow for things like magnetisation current of a transformer. (Surge at turn on..)
Quick blow for electronics or supplies with low current rating that will not stand high fault current..

Over rating of fuses to allow for surge like magnetisation current is a bad idea..when its value would exceed well above operational current.

Just for fun..

Tubeglow.

Re: Fuses
 

General rules I adopt:

Use current rating 25-50% above the maximum steady-state current the equipment can draw.

Ensure voltage rating is at least equal to the circuit voltage.

Use anti-surge (T) types if there is a switch-on surge, otherwise normal (F).

If the supply impedance is low (ie operates from mains) use ceramic, sand-filled high-breaking-capacity (HBC) fuse. Otherwise, glass types so you can see if they've failed.

Use 1.25" fuse rather than 20mm whenever possible.

If in doubt, arm yourself with a dozen spare fuses and replicate/simulate the possible faults. Ensure the fuse does its job each time. (Makes a mess of the equipment, but great fun).

I do confess to using a 250V-rated fuse (1.25" type) on a 400V 10A DC rail, but it was to hopefully save the PCB in case of a downstream fault, not as a primary means of safety protection. As it happened, it was never put to the test.

Re: Fuses
 

What type of circuit was it?

Its always been a bit difficult to protect HT+ in valve amps. You can use microwave fuses but they are a bit big.
I know in the past any fuse is better than no fuse..I do cap quench on B+.
The HT windings are often mA rated, so its quick blow. Then you have the cap charge to deal with at power up.
Some people put fuses in the cathode of OP valves because its low voltage.
But if you get a supply cap short or OP Tx fault its another story.

Tubeglow.

Re: Fuses
 

Earlier in this thread mention was made of lower current draw equipment with no mains fuse. In my test equipment collection I have a number of 50s and 60s instruments that fall into the same category. I realise I can modify them and fit an internal fuse but a simple and elegant solution would be plug fuses of less than 1Amp. So far internet searching has drawn a blank so I presume such things do not exist. Does anyone know of a source?

Al

Re: Fuses
 

I assume that it's taken as read that the BS1362 fuse is there to protect the cable, rather than the appliance internals. The appliance is expected to have further pertinent protection for itself, so to speak. Thus, there's no theoretical need for less than 3A rated fuses in the plug, though 2A and even 1A ceramic 1" x 0.25" are available.

I respect the philosophy of not hacking vintage kit around, but I always place primary side safety as an exception to this- safety always comes first, even if PVC insulation, heatshrink sleeving, insulating covers, and currently-compliant fuse-holders wouldn't have been original fitment. It's unusual that some sort of entirely safe fuse-holder can't be fitted to a piece of kit.

Re: Fuses
 

I suppose if you want to make no internal modifications to the vintage device you could fit a fuseholder in a plastic box and wire the unit's mains cable into that (fuse in the live wire of course). That shouldn't be any more dangerous electrically than fitting a fuseholder in series with the live wire inside the set.

In the event of a catastrophic failure the plug fuse should still handle the high current available from the mains, and blow safely. For a more minor overcurrent that wouldn't blow the plug fuse, the added fuse will fail.

Re: Fuses
 

For the application of general valve equipment such as radios and amplifiers, the general trend is to add an AC fuse(s) on the power transformer secondary for B+ supply. That B+ usually supplies all circuitry, so is often the next best extension beyond just an AC mains side fuse. AC winding protection, compared to B+ DC fusing has benefits.

For most equipment, I can't see a benefit in heater supply fusing, as the heater winding is usually operating at close to rating, and so it would be difficult to generate a substantial multiplier in current above a fuse value that would typically have to be at least 20% above winding rating. In addition, the heater supply does provide quite a substantial turn-on overload that needs to be considered for the AC main side fuse, as well as any secondary side fusing.

PSUD2 is a good tool to simulate the turn-on current surge levels in a B+ supply. For a valve rectifier, that surge is delayed from mains turn-on surge (power transformer magentisation and heater surge current) unless hot-switched. But if an ss rectifier is used, then that turn-on surge is additive to the mains turn on surge.

A NTC part added to the mains supply input circuit is a very common modern addition to equipment. It is easy to retrofit in most vintage equipment, and reliable if properly selected. The NTC is commonly sized to manage the ramp up of a certain capacitor filter supply - so can be very applicable to alleviating turn-on surge current through the mains side fuse relating to PT magnetising current and ss diode supply current, but not for heater supply suppression as that is a much longer time-frame. As such, the NTC can allow a standard fuse, compared to a slow-blow, or a fuse value increment reduction.

Re: Fuses
 

Fusing heater supplies can be problematic - remember that the heaters have a significantly lower resistance when cold - providing a fuse which offers significant protection while not being vulnerable to the fatigue-failures caused by regularly being run close to fusing-point... OK, you can get slow-blow fuses but I still feel that fusing heaters is likely to compromise the overall reliability of the equipment.

[in equipment with valve rectifiers the first couple of seconds after power-on is likely to see the highest heater-current: not only are the heaters cold/low-resistance, but also until the rectifier(s) come up to emission and start drawing power the power-transformer has no other load on it so can potentially feed quite a bit more than its continuous-rating current to the heaters...]

Fuses in the B+ side I accept (not just one fuse in the centre-tap-to-earth; that won't save your transformer if a diode fails s/c!).

Re: Fuses
 

Practicality tends to rule nowadays, with a minimal update of old equipment including a PT secondary side CT fuse, and either new ss diodes, or inserting ss diodes in series with valve diodes.

The addition of an ss diode in series with a valve diode provides protection against valve diode s/c.

Some equipment needs fusing in each leg of a full-wave CT rectifier circuit, such as when a bias supply is derived from one of the legs.