Why don't lamps come with a 1 amp fuse in the plugs?

[ericmark]

We have plugs and connectors in lead sets rated 2.5A and we use a 3A fuse and also rated 10A and we use a 13A fuse for example the kettle lead is rated 10A.

It is the same with 5A and 6A many lighting products like ceiling rose rated at 5A but we use a 6A MCB.

As you have pointed out fuses will take over the rated current and for it to be spot on the ambient temperature must also be spot on and the cooling must be carefully controlled i.e. all plugs must be in free air.

This clearly will not happen it is just approximate. Even the slight alteration in plug design to include finger guards will reduce the plugs capacity to remove heat from the fuse.

A magnetic trip with oil damper can be set to very fine limits but unless we need those fine limits then far cheaper to use a thermal device and it is a long time since I have seen motor overloads with dash pots.

Be it a fuse or a trip we want it to deliver the rated current for ever and a day without failing so they will always be designed to let through a little more than the rated value.

If we need a more accurate control for example motor overloads then we use a more expensive device able to be set more accurately. I have one job with two extractor fans which removed explosive gases and I was given the task to build a system which would close down the plant if both the extractors failed and I used a current switch to detect they were running and a very finely set overload which was tested to show it would trip with a stalled fan. But look at the price of a motor overload and compare to simple fuse and it is easy to see why we use a simple fuse in most cases.

 

[SimonH2]

I also hope that a 13 amp fuse would blow on a over loaded extension lead before a fire started from the wires melting and thus arcing due to the heat.

Now there's a whole new kettle of fish.

As already pointed out, the equipment designer SHOULD take that into account. Ie, any "good" design should allow for the 13A fuse being capable of passing around 22A indefinitely - ie the extension lead (particularly the socket block) "should" really allow for that. Unfortunately, the fact that it's a very price sensitive market, copper costs, and a lot of stuff comes from China and is clearly designed by people with no concept of our standards or good engineering means that a lot are marginal just for a "normal" 13A load.

But, it's actually not that common to get overload situations without human intervention - eg plugging two fan heaters into one extension. Apart from motor loads which can increase if the motor is overloaded (taking too generous a cut with a circular saw ?), few domestic loads will show anything in between "normal" and "fault" (ie effectively a short circuit). In fact, the only load I can think of would be a heater element that shorts to earth part way along it's length - but these days, such a fault would trip the RCD*, and if not, the element itself is likely to burn out quickly.

* Yes, I know there are still a lot of houses without RCD - but I can't help thinking that's a fairly quickly reducing proportion. Anyone know if there are any "proper" estimates of the number of installations still without RCD protection on the sockets ?

 

[JohnW2]

I really don't see that dropping from a 13A fuse to a 10A one (obviously for a load no greater than 10A) would really achieve much

Lets say you have a 3Kw appliance with no or little inrush current, I would rather the fuse blow at the current required to blow a 10 amp fuse and not a 13 amp fuse. Thus using a 10 amp fuse in a 3kw appliance seems a little safer than using a 3kw.

Sure, but the important word in what I wrote was "much". In the face of a particular overload current, one would obviously expect a 10A fuse to blow a little quicker than a 13A one - but my point was the difference between the ('very high') currents needed to blow them 'fairly quickly' would not be pretty small.

I also hope that a 13 amp fuse would blow on a over loaded extension lead before a fire started from the wires melting and thus arcing due to the heat. ... i.e. 3 core 1.5mm flex may be rated at 16 amps, but I assume a 20 amps load indefinitely, while it may get hot it would not get to hot to cause a fire as it has been designed with this in mind.

As I've said, that should be the case if the 'tabulated CCC' for the cable/OPD combination is at least 13A.

Also if 13 amp fuses don't blow until over 20.8 amps, why are they called and labeled as 13 amps?

'They' obviously could have decided on a different way of expressing the 'ratings'of OPDs and the CCCs of cable. However, I think it would be very confusing, particularly for the general public, if, say, for a circuit supplying a 13A load, the appropriate fuse/MCB/whatever was one with a rating of, say, 20A and the appropriate cable was one with a 'CCC' of 20A, wouldn't it?

I assume fuses are designed like this to allow for high inrush currents, other wise why not start using 10 amp fuses in 3kw appliances.

For a start, a '10A fuse' is not designed to normally carry a current greater than 10A (e.g. a 3kW load) - so if one did that, the fuse could well get pretty hot during normal service (maybe damaging the plug) and the life of the fuse could well be reduced. Also, it's not just about 'inrush current'. There are bound to be some 'spread'of manufacturing tolerances and some uncertainties about exact operating currents. Particularly in the case of fuses (which can't be individually tested), if one manufactured fuses that were designed to blow with, say, just over 13A, some would blow with currents of 13A or less - which would be a major inconvenience.

Kind Regards, John

 

[JohnW2]

As already pointed out, the equipment designer SHOULD take that into account. Ie, any "good" design should allow for the 13A fuse being capable of passing around 22A indefinitely - ie the extension lead (particularly the socket block) "should" really allow for that.

As I've been saying, although it's undoubtedly true that some people 'break the rules',if one works with cable 'CCC' figures which are appropriate for the cable/OPD combination concerned, then that should automatically be the case - i.e. the 'tabulated CCC' then takes into account that the characteristics of the OPD are such that it would allow appreciably more current to flow than its nominal rating (In).

But, it's actually not that common to get overload situations without human intervention - eg plugging two fan heaters into one extension. Apart from motor loads which can increase if the motor is overloaded (taking too generous a cut with a circular saw ?), few domestic loads will show anything in between "normal" and "fault" (ie effectively a short circuit).

True - but, as I recently pointd out to EFLI, anything containing a motor can create a substantial overload (rather than fault) situation if the motor gets jammed (or just mechanically overloaded) - as withness my recent experience with a WM pump motor.

In fact, the only load I can think of would be a heater element that shorts to earth part way along it's length - but these days, such a fault would trip the RCD*, and if not, the element itself is likely to burn out quickly.

Indeed, I've used that example myself to indicate thatthere are rare situations in which a 'simple resistive load' can theoretically create an overload (rather than 'fault') situation. It can also theoretically happen with within- or between-windings shorts in wirewound components. One obviously should not rely on 'burning out of the element' as one's overload protection! In situations such as the wirewound components I've just mentioned, there will usually not be a fault to earth, in which case an RCD, if present, obviously would not operate.

* Yes, I know there are still a lot of houses without RCD - but I can't help thinking that's a fairly quickly reducing proportion. Anyone know if there are any "proper" estimates of the number of installations still without RCD protection on the sockets ?

As you imply, the statistics we commonly seen are pretty old, and probably always were pretty suspect (being designed 'to make a point'), and I agree that RCDs must be becoming increasingly common. Practising electricians (e.g. those in this forum) ought to be able to provide a reasonable estimate of what proportion of the many installations they see have, and do not have, RCD protection.

Kind Regards, John

 

[EFLImpudence]

Lets say you have a 3Kw appliance with no or little inrush current, I would rather the fuse blow at the current required to blow a 10 amp fuse and not a 13 amp fuse. Thus using a 10 amp fuse in a 3kw appliance seems a little safer than using a 3kw.

But that is actually against the regulations.
You would have an OPD < design current.

Also, what is this 3kW appliance?
Without being picked up again for 'forgetting' motors and a 3kW motor may need more than 13A protection anyway, it is likely that the 13A fuse is not actually necessary.

I also hope that a 13 amp fuse would blow on a over loaded extension lead before a fire started from the wires melting and thus arcing due to the heat.

As John has explained, this is catered for in the values given.
Compare the csa of 10A (or 100A) fuse wire and 10A cable.

i.e. 3 core 1.5mm flex may be rated at 16 amps, but I assume a 20 amps load indefinitely, while it may get hot it would not get to hot to cause a fire as it has been designed with this in mind.

Not necessarily because a 20A load is still subject to the same proportion of overload (if at all) as the 16A load.

Also if 13 amp fuses don't blow until over 20.8 amps, why are they called and labeled as 13 amps?

Because it will start to overheat at 13A
You cannot have a wire which is not overheating at 12.9A and melt at 13A.

I assume fuses are designed like this to allow for high inrush currents, other wise why not start using 10 amp fuses in 3kw appliances.

They are not designed like this; that is physics.
As above for using 10A; it will be overheated all the time.


Replace all your appliance flexes with 4mm² and hard wire them and you won't need any fuses.

 

[JohnW2]

Replace all your appliance flexes with 4mm² and hard wire them and you won't need any fuses.

Doesn't that take us back to the OP? Whilst what you postulate would obviously be fine as far as cable protection was concerned (assuming a 32A OPD and Method C installation of the cable), there will still be some of us who (because of often absent/inadequate internal protection) would be less than totally happy with, say, a very-small-load electronic something being 'protected' only by a 32A OPD.

Kind Regards, John

 

[SimonH2]

I'm thinking that it might help the OP to explain how a fuse works.

Basically, it's a thin piece of wire. As current flows through it it gets hot. If it gets hot enough then it melts and breaks the circuit.
That's the very simplified version !

If you look at some of the small glass fuses, especially the "F" types, you'll see that the wire is just that - a thin bit of wire surrounded by just air. It has little thermal mass, so when subjected to overload it will heat up fast and melt quickly.
These are often specified for protecting semiconductors which are similarly unable to take sustained overloads. They are not suitable for loads with high inrush currents (particularly directly starting motors, and even more particularly motors driving high inertia loads) since they tend to blow too quickly.

Look at other fuses and you'll see different designs. In the small glass fuse style you'll find some where the wire is tightly would round a thin (non-conductive) core. This increases the thermal mass and makes the fuse slower to heat up and blow.

The fuses you'll find in a 13A plug have the wire embedded in sand. Again, this increases the thermal mass (thus slowing down the fuse and increasing it's surge capacity), but it also has a safety function when the fuse does blow (in controlling the arc that forms in the time between the wire breaking and current being stopped).

If you've seen consumer units with cartridge fuses, or industrial fuses, you'll also see that (as a general rule of thumb) the higher the capacity of a fuse (or range of fuses) the bigger they are - to dissipate the heat produced, both during normal operation and when "blowing". Fuses used in high voltage circuits, and DC circuits, have to be physically longer to allow for the length of arc that can form.

But I digress.
ALL the time current is flowing through the fuse, it will generate heat - the higher the current, the more the heat. Power=volts x amps, volts = amps x resistance, so power = amps^2/resistance. Double the current through the fuse and the heat produced is quadrupled.

That's why it's far from uncommon to find a 13A plug getting warm (or even "very warm") when a high load device (eg heater) has been on for a while. The current through the fuse is making it get hot - hence the comment above about not using a 10A fuse for a 3kW load as it'll get hotter than it and the plug are designed for.

Clearly, when carrying it's full rated current, the fuse must not get so hot that it : damages the holder (plug in this case); causes a fire hazard; or starts to degrade the wire (remember that light bulb filaments have to be in an inert gas to prevent oxidation at high temperatures). So there will always be a significant difference between the "will carry indefinitely and without problem" and "guaranteed to blow" currents.
If you refer back to the graphs previously posted, the difference between the rating and "will blow given long enough" currents is about 1.66 - so the difference in power generated in the fuse is about a factor of 2.8.

Taking the example of running a 10A fuse at 13A, the fuse will be generating about 70% more heat that it (or the plug) is designed for.

 

[EFLImpudence]

Doesn't that take us back to the OP?

In a way, it does - or it removes the reason for the question were they wired like that in the first place.

Whilst what you postulate would obviously be fine as far as cable protection was concerned (assuming a 32A OPD and Method C installation of the cable),

It would be the flex to the appliance instead of plug lead.

there will still be some of us who (because of often absent/inadequate internal protection) would be less than totally happy with, say, a very-small-load electronic something being 'protected' only by a 32A OPD.

Yes, that is the problem - unnecessary worrying.
Appliances that need it (fans apart) have integral fuses - boilers, tvs etc.

Would you be happy with your tv wired without a plug?



No one worries (or can do anything) about the small internals of an oven - clock, timer lights, not to mention all the quite small internal wiring.

 

[JohnW2]

ALL the time current is flowing through the fuse, it will generate heat - the higher the current, the more the heat. Power=volts x amps, volts = amps x resistance, so power = amps^2/resistance. Double the current through the fuse and the heat produced is quadrupled.

Actually a fair bit more than that. In view of the potentially substantial temperature rises we are talking about, there will be a consequential very appreciable increase in resistance as current increases. The power dissipated, hence heat produced, when current is doubled is therefore likely to increase by a factor appreciably greater than 4.

Kind Regards, John

 

[JohnW2]

Yes, that is the problem - unnecessary worrying. ... Appliances that need it (fans apart) have integral fuses - boilers, tvs etc. ... Would you be happy with your tv wired without a plug?

If I were sure that there was adequate internal fusing then, yes, I would obviously be happy for a TV (or anything else) to be externally 'protected' only by a 32A (or any other) OPD. However, with many of the 'small electronic devices' (and some larger things) one often really doesn't know whether there is any/adequate internal fusing. Whatever, returning to the (usual) situation in which these things are connected via a plug, rather than hard-wired, since 1A fuses exist, why not use one in these cases?

Kind Regards, John

 

[winston1]

Yes, that is the problem - unnecessary worrying. ... Appliances that need it (fans apart) have integral fuses - boilers, tvs etc. ... Would you be happy with your tv wired without a plug?

If I were sure that there was adequate internal fusing then, yes, I would obviously be happy for a TV (or anything else) to be externally 'protected' only by a 32A (or any other) OPD. However, with many of the 'small electronic devices' (and some larger things) one often really doesn't know whether there is any/adequate internal fusing. Whatever, returning to the (usual) situation in which these things are connected via a plug, rather than hard-wired, since 1A fuses exist, why not use one in these cases?

Kind Regards, John

These devices are designed to be used Europe wide, even world wide, where the only protection is the CU 16A MCB. There does not seem to be any problems.

 

[JohnW2]

These devices are designed to be used Europe wide, even world wide, where the only protection is the CU 16A MCB.

Indeed.

There does not seem to be any problems.

You must have information which is not available to me. I'm sure that 'problems' are rare, but am equally sure that they will not be non-existent. As I said, if one is connecting via a BS1363 plug, why not use the smallest adequate fuse, even if it won't make any difference 99%+ of the time?

Kind Regards, John

 

[winston1]

These devices are designed to be used Europe wide, even world wide, where the only protection is the CU 16A MCB.

Indeed.

There does not seem to be any problems.

You must have information which is not available to me. I'm sure that 'problems' are rare, but am equally sure that they will not be non-existent. As I said, if one is connecting via a BS1363 plug, why not use the smallest adequate fuse, even if it won't make any difference 99%+ of the time?

Kind Regards, John

In the past I used 1A fuses for lamps and found whenever a bulb blew the fuse did too, doubling the cost, so I went back to 3A. With today's CFL and LED lamps one could I suppose reconsider.

 

[eveares]

I really don't see that dropping from a 13A fuse to a 10A one (obviously for a load no greater than 10A) would really achieve much

Lets say you have a 3Kw appliance with no or little inrush current, I would rather the fuse blow at the current required to blow a 10 amp fuse and not a 13 amp fuse. Thus using a 10 amp fuse in a 3kw appliance seems a little safer than using a 13 amp fuse.

I also hope that a 13 amp fuse would blow on a over loaded extension lead before a fire started from the wires melting and thus arcing due to the heat.

i.e. 3 core 1.5mm flex may be rated at 16 amps, but I assume a 20 amps load indefinitely, while it may get hot it would not get to hot to cause a fire as it has been designed with this in mind.

Also if 13 amp fuses don't blow until over 20.8 amps, why are they called and labeled as 13 amps?

I assume fuses are designed like this to allow for high inrush currents, other wise why not start using 10 amp fuses in 3kw appliances.

 

[333rocky333]

for example the kettle lead is rated 10A.

Why is this when the cable is often 1.5

Reason I ask is we often get 13 amp fuse rated appliances, where the lead gets damaged.
rather than fit a seperate cable and plug, it would be nice to simply cut off the socket and fit the new lead with a nice moulded plug.

I can get 10 amp rated leads in reasonable lengths but 13 amp rated ones are short and sparse, however they generally both are 1.5 cable.

Am i missing something or can anyone see a problem using the 10 amp 1.5 leads with moulded plug, cutting the socket off and using a 13 amp fuse