The smoke escaped! Twice!

[hermetic]

When voltage drops, current rises ! The reason we transmit power at high voltages like 11kV and up is because the current falls. take a 240 volt supply and put a 1kW load on it. 1000W/240v= 4.16 Amps. Run the same load on 100V and you get 1000w/100V= 10Amps Resistance is measured in ohms, but power dissipated is in WATTS The resistance of any heating element will not give a true current reading when applied to a circuit using alternating current. Hence a 3kW fan heater on 240V draws a current of 3000/240=12.5 amps Pedants be aware that this is a simplified version of the truth!!!
Phil

 

[julianthegypsy]

a 3kw heater plugged in direct without the lead can burn the plug out. the pins in your sockets will be worn. id suggest fit new plug and the socket you plug in to as it will be weakened and the contacts burn and softened .dont run them on any extension lead or adapter

The problem is that the heater has a stupidly short lead and there are no sockets near where we need it. Same applies at home and the workshop. Workshop has more three phase sockets than single and home was built in the late sixties when sockets were a luxury. If I replace the wall socket and make up my own extension from decent components, would that be ok? And the inevitable next question is of course where do I get said components?!

 

[dobbslc]

3kw is about 12 amps constant load! If the extension reel contacts are a bit worn it'll get hot quickly! If its half coiled up it will speed things up.
Add a bit of dust and tarnish and it's all ready to melt. I have a 3kw immersion heater that liked to melt supposedly good quality 13 amp spurs (MK), I couldn't leave that in the loft space next to my sons bedroom! I fitted a 20 amp double pole switch and that was fine. Most things these days are just low quality ****e designed by accountants and made in some sweat shop in China, its always going to be rubbish.

 

[brightspark]

a decent 1,5mm cable extension lead will be ok unwound but will still warm the plug and socket up . just keep an eye on it

 

[julianthegypsy]

a decent 1,5mm cable extension lead will be ok unwound but will still warm the plug and socket up . just keep an eye on it

Thanks, that's really helpful. It's currently running on my lead that I bought from wickes years ago and use at work. There is a birds nest of cable on the floor where I unwound it and I'm checking it!

 

[voipio]

That really doesn't help my understanding, but thanks for trying.

So on an electrical fire, if voltage drops, current drops too?

V = I x R

Yeah, that does make sense.

So maybe the volts went up?

I'll be quiet.

Rearranging, we get::

I = V / R

and

R = V / I

So, for a resistive load, lets take as an example, the good old Tesco kettle in our kitchen. The ratings label on the bottom says 220-240V, 2500-3000W. The resistance measured across the 13A plug pins is 19.7 ohms with cold water in the kettle, and doesn't change much (actually decreased to 19.5 ohms) when the plug was removed from the wall with the kettle close to boiling. This means that at 220V supply, calculated current is 11.17A giving 2456W, and at 240V, current is 12.18A, giving 2924W. In other words, the kettle doesn't dissipate a constant power, irrespective of supply voltage.

On the other hand, devices having switched mode power supplies in them do tend to draw more current as the supply voltage falls, since they are generally designed to produce constant voltage outputs, and if the circuitry drawing from those constant voltage outputs itself imposes an unvarying load, and therefore constant power, then the input power has to remain more or less constant, and that has to come from somewhere i.e. increasing supply current as the supply voltage drops. But this doesn't apply to simple, resistive element fan heaters. They are more like the kettle case above.

 

[Barking Mat]

Rearranging, we get::

I = V / R

and

R = V / I

So, for a resistive load, lets take as an example, the good old Tesco kettle in our kitchen. The ratings label on the bottom says 220-240V, 2500-3000W. The resistance measured across the 13A plug pins is 19.7 ohms with cold water in the kettle, and doesn't change much (actually decreased to 19.5 ohms) when the plug was removed from the wall with the kettle close to boiling. This means that at 220V supply, calculated current is 11.17A giving 2456W, and at 240V, current is 12.18A, giving 2924W. In other words, the kettle doesn't dissipate a constant power, irrespective of supply voltage.

On the other hand, devices having switched mode power supplies in them do tend to draw more current as the supply voltage falls, since they are generally designed to produce constant voltage outputs, and if the circuitry drawing from those constant voltage outputs itself imposes an unvarying load, and therefore constant power, then the input power has to remain more or less constant, and that has to come from somewhere i.e. increasing supply current as the supply voltage drops. But this doesn't apply to simple, resistive element fan heaters. They are more like the kettle case above.

I'll reply when sober tomorrow, but yes, thank you, that makes sensé.

 

[Alan Reynolds]

Were the extension leads fully unrolled?
Using leads not fully unrolled is the most common cause of them melting & its a pet hate of mine!

Good shout. It's ok with a light current draw but a fan heater and you got an induction heater in the reel.

 

[MattF]

You're just an unlucky b*****d, IMHO.

Either that, or like SBF implied, expect Rod Serling popping his head round your door.

 

[Seadog]

but a fan heater and you got an induction heater in the reel.

Not true, you have a concurrent +ve and-ve going current. Nett effect - 0

However, there is resistance, and resistance produces heat, which is why it gets hot.

 

[brightspark]

safety - Why is it dangerous use a coiled extension cord - Electrical Engineering Stack Exchange

 

[brightspark]

i regular get them where the plug has burn so the punter changes it and it does it again they cant see that the plug and the socket both need changing at the same time otherwise one burns the other out by overheating

 

[julianthegypsy]

What I still don't understand is why a, the fuses didn't blow, or b, the breakers didn't trip? That's the part that's really puzzling me. After all, I didn't do anything outside the bounds of what would be considered normal, though obviously I'll be more careful in future, but there must be thousands of people plugging heaters into extension leads every day and I would have thought breakers were supposed to prevent things like this from happening? It's also weird that bith heaters have been used like this for a year, nothing changed, then suddenly, bang two at once!

 

[julianthegypsy]

i regular get them where the plug has burn so the punter changes it and it does it again they cant see that the plug and the socket both need changing at the same time otherwise one burns the other out by overheating

Now that makes perfect sense!

 

[brightspark]

the fuse will take the load on a rolled up extension lead and it wont be overloaded and pop untill the insulation breaks down and melts then the cores short out and blow the fuse or breaker

 

[Turbo]

The one at the workshop was half unrolled, the one at home was a domestic short four socket extension lead so doesn't roll up, it's only six feet long.

An 25m 3kw rated extension lead rolled up reduces its capacity to 1kw! Your lead looks to be a light duty one, probably only rated for 2kw unrolled. So it's likely that the lead overheated due to being rolled up and trying to run a 2-3 kw heater!

Normally what they do is melt the cable on the reel, I have seen leads where the cable has got so hot it has melted itself into the plastic reel!

As for the trailing socket at home - most of them are light duty rated, maybe only 1.25 cable and not up to running a 3kw heater. If you overload them the pins in the plug get hot & the whole lot melts, just what happened to you! You can't rely on the fuse for protection, it has been shown that it takes 20a to blow a 13a fuse!

To run a 3kw heater you need to be using a 2.5mm2 extension lead!

 

[Turbo]

safety - Why is it dangerous use a coiled extension cord - Electrical Engineering Stack Exchange

Great link, I'd advise everyone to look at it!

Once you see the pictures there you will think twice next time you use an extension reel!

I rarely use reels now but instead use trailing leads in either arctic or rubber covered 2.5mm flex with good quality duraplug sockets & plugs fitted. They range in size from 5 to 10m long & I can combine them to make up whatever length of lead I want. I am slowly converting some of them to 16a commando sockets for added safety with a 16a to 13a fly lead at the end.

It only takes a few minutes to coil the leads back up & use a velcro strip to keep them in place.

 

[Robert Weaver]

When voltage drops, current rises ! The reason we transmit power at high voltages like 11kV and up is because the current falls. take a 240 volt supply and put a 1kW load on it. 1000W/240v= 4.16 Amps. Run the same load on 100V and you get 1000w/100V= 10Amps Resistance is measured in ohms, but power dissipated is in WATTS The resistance of any heating element will not give a true current reading when applied to a circuit using alternating current. Hence a 3kW fan heater on 240V draws a current of 3000/240=12.5 amps Pedants be aware that this is a simplified version of the truth!!!
Phil

For a DC supply, the power consumed is a function of the load resistance and supply voltage. (An AC supply includes reactance, which is frequency dependant with inductance and capacitance having an effect.) P = I x V. Assuming the resistance is constant, changing the voltage from 240 to 110 would drop the current drawn which would drop the power consumed. (Why are there no kettles in america)

Power loss is powerline transmission cables is proportional to I^2 x R. As R is a function of cable length, cross sectional area and material, it's 'fixed'. To reduce loss, the current flowing needs to be reduced, which is achieved by bumping up the supply voltage, hence 11kv.

 

[Monkey Harris]

Pre wired plugs, spawn of the devil.

how many millions,
are there working in the world..?

 

[hermetic]

Just to reitterate, coiled extension cables DO NOT get warm because of induction! As has been stated above current flow in live and neutral are equal and opposite, and so cancel each other out, and net result is zero induction. When a cable is unrolled it is exposed to the air equally along its length, and can radiate any heat directly to the air. The caveat to this is that if it is getting warm (to the touch) it is overloaded! If you consider an extension lead partially or fully coiled in an enclosed drum, it cannot dissipate heat to the air, and therefore overheats. For an extension lead to carry a full 3kW load it should be 2.5mm cable, which will not get warm at all when cerrying 3kW. You also have to consider resistance in the plugs and sockets. It is well known that the pins on Chinese made plugs often do not meet the minimum dimensions for a BS 13 amp plug, and they are therefore loose. The components in the socket also fail to meet BS requirements, and have springs which exert insuficient pressure on the pins of the plug, which in turn creates more resistance, which heats the springs and softens them. Add to that the fact that some of the very cheapest extensions have been found to have either copper coated aluminium, or even plain aluminium cables and you can understand why this happens!
Phil