'Normal' fuses, and fuses for electronic devices...

[Andy Scholes]

Hi All,

I am new to this forum so I apologise if I've put it in the wrong section or if I'm repeating a previously asked question (I did look but couldn't find).

I want to know, is their any real difference between a 'normal' fuse, say, in a standard plug for an appliance at home, or a fuse to protect a motor, and a fuse in an electronic circuit, containing various circuitry..?

Many thanks for any help on this subject , Andy

 

[Nigel Goodwin]

There are various kinds of fuses - essentially down to the speed at which they blow. Normal plug fuses (as used in the UK) are fast blow, whereas most fuses in electronic equipment (TV's etc.) are anti-surge (or slow blow).

This is because TV's etc. have large switch-on surges, and fast blows fuses will fail on a fairly regular basis because of that.

Going back to TV's again, and specifically old CRT sets, you needed to have a 13A fuse in the plug - as a 5A would fail pretty quickly, even though the set normally had a 2.5A or 3.15A anti-surge mains fuse internally.

 

[Andy Scholes]

Thanks for the reply Nigel!

So the only basic difference between them is that the fuses for electronic devices need to be anti-surge?

Would i be right in assuming thyristor controlled circuits that have large transients when switching would require these anti-surge fuses? even though they could still have something like a freewheel diode to stop sudden surges of voltage?
thanks again! Andy

 

[Reloadron]

Selection Explanations and Recommendations is a good read about fuses. Beyond the types of time delay as Nigel mentions we also have voltage and temperature ratings as well as other features. Also today's newer automotive fuses open a whole new area of fuse design and selection. The link I posted is a pretty good read about selecting a fuse for a given application. Anti-surge really isn't a fuse type, rather a time delay fuse is used so the fuse won't open during a short current surge. Even time delay fuses have additional ratings like how long the delay is for currents that exceed the fuse ratings. Pretty interesting stuff surrounding the lowly little fuse, or big fuse.

Edited to say:

or a fuse to protect a motor

Which gets into things like thermal overload protection and things like motor protection vary greatly depending on the size and types of motors.

Ron

 

[4pyros]

Basically you have fast blow, normal and slow blow.
Fast for some electronics, normal like in cars and slow blow for motors and sutch.

 

[audioguru]

We do not use fuse plugs and we do not use fuses anymore in North America. Instead a 120V line has a 15A or 20A circuit breaker at the panel where the electricity enters the home.

 

[BobW]

Unfortunately, there is no simple answer to the fuse question. At least, not if you want an accurate answer. There are as many different types of fuses as there are devices for them to protect. I've spent a considerable part of my career doing fuse coordination design. That is, selecting fuses or a series of fuses to protect circuits and their subsequent subcircuits so that the fuse opens before the device is damaged. People who should know better, tend to oversimplify things into the fast blow and slow blow categories. In fact, there are hundreds of categories. The ideal fuse for a particular device is one whose time-current characteristic curve is identical in shape to the device's time-current damage curve, but shifted down slightly on the current axis.

I'd be happy to provide more info, but usually when I start getting into the technicalities of fusing design, people start to look scared and then back away slowly.

 

[schmitt trigger]

It is called the "I-squared T" characteristic of a fuse.
Then you have the clearing capacity, not to be confused with the rated current.

And there are additional subtleties.

 

[cowboybob]

And don't discount/ignore the circuit component that occasionally sacrifices itself to save the fuse (of whatever sort)...

 

[BobW]

I-squared T is a useful parameter, but still not adequate for a thorough fuse design. Plotted on a log-log graph it would be a straight line. However, neither fuses nor the devices that they protect have a straight line time current curve (hence the term "curve").

Cowboybob brings up a valid point. Sometimes, it's simpler and cheaper just to let the component destroy itself. Why put in a $1.00 fuse to protect a $0.50 component as long as there's no safety issue?

 

[Ramussons]

You need to use "Semiconductor Fuses" to protect Thyristors used, especially, in Invertors.
https://www.littelfuse.com/products/fuses/specialty-power-fuses/semiconductor-fuses.aspx

Of course, FET's and IGBT's are replacing the Thyristors.

 

[crutschow]

Way back when, learning about protecting semiconductor devices from over-current, the saying was "the transistor is faster than the fuse" meaning that it was difficult to protect them from over-current with a fuse, since the thermal time-constant of small transistors was often shorter than that of even fast-blow fuses.

 

[Nigel Goodwin]

We do not use fuse plugs and we do not use fuses anymore in North America.

We have a superior system in the UK, with ring mains and individually fused plugs on all pluggable appliances

MCB's have been common in place of fuses for many years of course, in all new wiring installations.

 

[schmitt trigger]

I only mentioned the I-squared-T curves as a subtlety that better describes the fusing time rather than the old fashioned "slow-blow" or "fast-blow" denominations.

In modern commercial electronic equipment (I'm not talking about "mains" fuses or industrial fuses), the fuses are not intended to protect individual components.
Rather, a fuse is installed to meet safety agency regulations. Namely, that the equipment will not provoke a fire.
That is the reason fuses are now considered "non-user replaceable" components, and are internal to the equipment.

 

[Andy Scholes]

Unfortunately, there is no simple answer to the fuse question. At least, not if you want an accurate answer. There are as many different types of fuses as there are devices for them to protect. I've spent a considerable part of my career doing fuse coordination design. That is, selecting fuses or a series of fuses to protect circuits and their subsequent subcircuits so that the fuse opens before the device is damaged. People who should know better, tend to oversimplify things into the fast blow and slow blow categories. In fact, there are hundreds of categories. The ideal fuse for a particular device is one whose time-current characteristic curve is identical in shape to the device's time-current damage curve, but shifted down slightly on the current axis.

I'd be happy to provide more info, but usually when I start getting into the technicalities of fusing design, people start to look scared and then back away slowly.

Thanks BobW, any more info would be greatly appreciated! I know there is a lot to fuse design and selection, and can become quite complex.

My main problem is trying to understand the difference between the spec for a 'normal' fuse and one to protect electronic devices generally.

thanks Again to everyone who has replied!!!

Andy

 

[audioguru]

We have a superior system in the UK, with ring mains and individually fused plugs on all pluggable appliances

What happens in the UK if somebody puts a nail in a wall to hang a picture or something and the nail shorts the "ring mains" wiring in the wall? Does the ring mains catch on fire or is all electricity disconnected in the ring or in the home?
In North America then the circuit breaker only for that cable disconnects it. My home has 30 circuit breakers. Most are 15A, a few are 30A and 40A and two at the input are 100A.

 

[Nigel Goodwin]

What happens in the UK if somebody puts a nail in a wall to hang a picture or something and the nail shorts the "ring mains" wiring in the wall? Does the ring mains catch on fire or is all electricity disconnected in the ring or in the home?

The MCB or fuse feeding that ring would blow, isolating the ring - normally you would have separate rings for each floor.

The ring system , and fused plugs, means you don't need individual circuits for each socket, and because each ring is fed via two pieces of 2.5mm cable it's effectively 5.0mm wiring - yet uses much less cable (and copper) than radial wiring.

 

[audioguru]

How does an electrician tap into the ring mains for a socket? Here a socket has screws for input and an output for the next socket or light switch.

 

[Nigel Goodwin]

How does an electrician tap into the ring mains for a socket? Here a socket has screws for input and an output for the next socket or light switch.

The ring goes into one socket, and out again to the next one - the holes for the wires are sized to accept at least two pieces of 2.5mm cable.

 

[audioguru]

Our sockets have one hole per wire which is grabbed by the edge of a piece of metal and it has one parallel screw that you wrap the wire around then tighten.