Appliance fuses

[PBC_1966]

There is no reason to strive to get it as close to Ib as possible.

And in some cases reasons not to make it too close due to inrush currents at switch on. Although more common for internal use in equipment, sometimes anti-surge fuses are employed in order to provide the closer protection but permit short-term surges of considerably higher current which would blow a regular fuse of similar rating.

 

[Ray Tay]

ericmark, as suspected, the EU to make things a little more compatible, rather than a sound old fashioned electrical approach.

 

[ban-all-sheds]

And I'm sure that manufacturers who need only make one design for a single market of 500M people regard that a "Brussels red tape".


Not.

 

[bhm1712]

ericmark, as suspected, the EU to make things a little more compatible, rather than a sound old fashioned electrical approach.

Surely making things more compatible is the right thing to do? Nothing is compromised safety wise. As has been said already, you have a very poor understanding of electrics in general and fuses in particular, so your reasoning and logic is simply flawed. You are in no position to comment as you have already admitted yourself "I'm not a qualified electrician"

 

[ban-all-sheds]

rather than a sound old fashioned electrical approach.

Where you see a "sound old fashioned electrical approach" others see ignorance and superstition.

Either put forward a reasoned explanation, with authoritative supporting references, of why Ib ≤ In ≤ Iz is not sufficient, or stop making a fool of yourself.

 

[bernardgreen]

Ray Tay

Did you know that some types of fuse have a service life time quoted, for some this can be as short as 500 hours carrying rated current before the fuse should be routinely replaced by a new fuse. ( or it blows due to tireness )

The same fuse supplying a load of 75% of the fuses rating at an ambient temperature of 25°C would have a much longer service life, it could be as long as 100,000 hours of carrying the load current. At higher ambient temperatures the service life would be shorter.

Generally for a reasonable service life a fuse should be rated at 150% of normal load current and the cable be rated according to the current and time to blow parameters of the fuse that is protecting the cable.

Load 7.5 amps. fuse rating 10 amps.

The fuse in a 13 amp plug is not to protect the appliance, it is for when a metal capped stilleto heel cuts through the cable and there is a need to prevent the cable glowing red hot.

 

[bernardgreen]

http://www1.cooperbussmann.com/pdf/e117a412-79c0-436e-943e-969a1f80ad6e.pdf

where page 17 has this

Cyclic loading which will lead to premature fuse fatigue can be defined as regular or irregular variations of the load current, each of a sufficient size and duration large enough to change the temperature of the elements inside the fuse in such a way that the very sensitive weak spots will fatigue. In order to avoid this when selecting a fuse, certain calculations can be made to ensure that there is an appropriate safety margin.

 

[kai]

...and Inductive loads containing Motors or Discharge Lighting need a slightly higher rating of fuse (or indeed MCB) than the equivalent Resistive (heating) load, as Inductive loads have a brief start up power surge, which places stress on the fuse or MCB, whilst a Resistive one won't place any undue starting stress on a correctly rated fuse or MCB.

 

[ericmark]

I would still use fuse protection when using semiconductors to control as the semiconductor fuse will protect the device, where using a overload or MCB they are not fast enough acting to save the semiconductor. However that is a special case. With a 16A supply to 13A sockets I have not seen a fuse rupture due to a cable fault. Only with a 32A supply to 13A sockets has the fuse ruptured with a cable fault, and often then often the MCB has tripped as well.
I have had fuses rupture due to equipment faults, vacuum cleaner motor burning out for example, however it has not saved the motor is still needed to be renewed, so in real terms the fuse in the plug is to reduce the fire risk.

So if we consider domestic equipment and fire risk, then for a vacuum cleaner limited to 1.6kW or 7 amp clearly using a 7 amp fuse is too small, but using a 10 amp fuse it will possibly rupture that little bit quicker than a 13 amp fuse which could mean it ruptures before it goes on fire. However my vacuum cleaner is very much smaller in size, and when the motor is running then there is no connection to any low voltage supply. One would hope if it eats it's own lead then the switch mode power supply would auto shut down. But it also has a habit of eating any other lead left lying around. It has really tidied up the house. No longer are there phone charging leads left every where, if they are it eats them.

However robotic cleaners can damage leads although in the main it simple stalls and switches off and waits for human intervention to remove the lead. So specially with thin lead sets there is a need for correct fusing. So in respect to lead sets the IEC 60320 stipulates the maximum current permitted. So electric shavers have C1/C2 connector so shaver adaptors should have a 0.2A fuse or other over current limiter. Next is 2.5A and I have not seen a 2.5A fuse for a plug but I would say 3 amp is close enough for C3 to C8. The C9/10 is rated 6A not seen a 6A fuse so as to if a 5 or 7 amp is used I am unsure. Not a HiFi fan so not used one. The C11 to C18 does however raise a question they are rated at 10A and clearly we do have 10A fuses. However the C15, C15A, C16 and C16A are the typical plugs used for kettles. But the kettles are often rated above 2.3kW limit of the plug. So should one use a 10A or a 13A fuse where the connector is rated at 10A?

The C13/C14 is used a lot on computers and is often supplied with a 3A fuse in the plug. However where the cable is not marked with anything which shows less than 1mm² should it have a 10A fuse. Remember when we do inspection and testing of in service electrical equipment the lead set is a separate item to the computer.

 

[ericmark]

...and Inductive loads containing Motors or Discharge Lighting need a slightly higher rating of fuse (or indeed MCB) than the equivalent Resistive (heating) load, as Inductive loads have a brief start up power surge, which places stress on the fuse or MCB, whilst a Resistive one won't place any undue starting stress on a correctly rated fuse or MCB.

Untrue, most tungsten lights which are clearly a resistive load will give a higher load until the element has warmed up. This is specially true with quartz lamps. As to the load when charging up the capacitors on a switched mode or pulse width modulated power supply I would expect there to be quite an inrush.

Even a simple halogen or inferred heater will have a higher start to run load, as to the more complex heaters like the induction hob I really don't know but I expect there is still an inrush.

The reverse is also true with motors those with inverter control must have some in rush as the capacitors charge but putting an energy meter on my inverter controlled fridge/freezer and freezer there is no measurable inrush and the power used varies I would assume depending on if cooling the fridge or freezer part or both.

It will depend on the design, most inverters are powered up before the motor is started so should be very little in-rush, the ramp up parameters will decide the current used.

Today we can no longer make general statements as to start and run current. The electric world has changed.

 

[bernardgreen]

Untrue, most tungsten lights which are clearly a resistive load will give a higher load until the element has warmed up.

For example Barthelme 00211502 lamps, rated at 12/15 volt 2 watt have a calculated current of 166 mA and a calculated HOT resistance of 72 Ω

When cold the resistance is 10.2 Ω so at switch on the initial current is 1.2 amps, more than 7 times the "normal" current.

The lighting installation I am setting up has 372 lamps in it. it is not simply a case of switch them on and hope the breakers do not trip.

 

[Iggifer]

The lighting installation I am setting up has 372 lamps in it. it is not simply a case of switch them on and hope the breakers do not trip.

Why ever not?

That is a MASSIVE amount of current if you switch more than a handful at a time

 

[endecotp]

Untrue, most tungsten lights which are clearly a resistive load will give a higher load until the element has warmed up.

For example Barthelme 00211502 lamps, rated at 12/15 volt 2 watt have a calculated current of 166 mA and a calculated HOT resistance of 72 Ω

When cold the resistance is 10.2 Ω so at switch on the initial current is 1.2 amps, more than 7 times the "normal" current.

For tungsten bulbs like that one, the inrush is of very short duration.
I reckon the filament in that bulb might be 3mm long and 0.05mm diameter (to within a couple or orders of magnitude!). So volume = 10^-12 m^3, density of tungsten = 20 g/cm3 = 2 x 10^4 kg/m3, mass of filament = 2 x 10^-8 kg. Specific heat capacity of tungsten = 135 J/kgK; say temperature increase is 1000 K, energy required to heat filament = 2x10^-8 x 135 x 1000 = 2.7 x 10^-3 J. At power = 2W, that much energy is delivered in 1.35 ms. Much less than one mains cycle.

I'd be surprised if an inrush of that duration would cause any real-world problems.

Inrush is more significant with things like motors, power supplies with large capacitors, heaters, etc. where it can last for several seconds.

 

[ericmark]

It is enough to cause dimming switches and PIR switches to fail due to inrush so I would say that significant!

 

[endecotp]

Triacs? Interesting.
I would expect them to survive 10X normal max current for a 50Hz cycle.