My wife just got an electric kettle, and it has marked off various levels, but the lowest level - .5L - says “MIN”. (The highest level - 1.7L - says “MAX”.)
I can understand a max level, if the heating element isn’t powerful enought to boil that much water. But what’s a minimum level? What happens if you put in less water than that?
It might boil dry. I’m sure there’s an interlock to turn it off when it reaches temperature, but the minimum level gives an additional layer of protection. It’s like a Swiss cheese model.
Well that’s significant, because if that’s true, then I can just put in as much as I need and watch until it boils and take it off. Waste of time and energy to put in more just because someone thinks I might leave it to boil dry.
As the owner of several electric kettles, I suspect that this is exactly it. Similarly, the “max” level is, I imagine, likely set to prevent water from bubbling out of the spout when the kettle reaches maximum temperature, or from easily spilling out when the user picks up the hot kettle.
I know that British kettles are different (they boil twice as fast for one), but the principle is the same.
Assuming that it is the type that turns off when it boils - it is triggered by pressure from the boiling water. Too little water can mean that it boils dry before the off switch is triggered. If this happens, there will be a safety cutoff, but at best you will have to wait for ages for it to reset, and at worst, you will have to replace a fuse of some sort.
Too much water risks boiling over or spillage as kenobi says above.
Well, not following this, at all. I think these things are equipped with a thermal klixon, which senses extended 212 deg F., or higher temps if the kettle boiled dry. If I need more than one kettles-worth of hot water, I merely refill with cold water, the klixon resets itself, and starts the process over again, for as much boiling water as I require, no wait at all for the switch to reset, since the klixon is effectively below water level, and instantly cooled.
My kettle is an Aroma. I’d be grateful for an explanation of the “Swiss cheese model”, and a description of how much pressure boiling water can exert in an open, unpressurized vessel.
Dan
I watched a video on boiling ethanol in an electric kettle a while ago that informed me a lot on how these kettles worked–my assumption without knowing more about them, was boiling ethanol might not automatically shut off an electric kettle, ethanol boils at 78C, so it would never reach the boiling point of water, so if there was a temperature-based switch, in theory it would never trigger.
The video initially tries boiling 80-proof vodka, and it shuts off not long after it starts boiling. The presenter then notes that a 60/40 water/ethanol solution doesn’t actually boil at 78C, so it wasn’t a perfect test, he then tests again with a much more concentrated solution of ethanol, but it still turns off fairly quickly.
What is going on? At least the kettle he is presenting, has a little bimetallic switch in the base, the different metals expand at different rates as the temperature changes, causing the disc in the switch to buckle, which will close/open the switch. This of course does respond to changes in temperature, but it isn’t quite based on 212F/100C–as his testing shows. In fact, it makes perfect sense that it wouldn’t be based on 100C–100C is the boiling temperature at sea level, but not everyone lives at sea level. Instead, the switch is designed to fire at a temperature significantly lower than the boiling point of water. There is a small tube in the kettle, as the liquid in the kettle boils, vapor enters that tube and pressure in the kettle forces it down the tube, where it eventually pushes down around the bimetallic switch, this warm vapor eventually hits whatever the trigger point temperature the switch was designed at, which causes the switch to fire and turns off the heating element in the kettle. Since it is intended to fire at a lower temperature than the boiling point of water (significantly lower), even boiling something like ethanol that has a boiling point of 78C will also trigger it.
Now, why do these kettles have a minimum fill?
That seems to not be clearly answered upon any quick searching, but it is at least a popular answer that it is simply because the heating element is capable of overheating without enough water in the kettle to mediate. This was, supposedly, more of a concern with older style electric kettles that had “exposed” heating elements. The presenter of the YouTube video actually covers a second kettle, which at first he assumes doesn’t have the pressure vapor system because he couldn’t find the small metal pressure tube, but it’s actually built into the wall of the kettle and he eventually discovers it. That kettle does have a thermal switch thermal pasted to the bottom of the kettle’s base, which he speculates may be a sort of emergency kill switch if the heating element exceeds some safe operating temperature.
At least some people suggest that it is possible that if you go too far below the minimum fill line, the pressure required to push the warm vapor across the bimetallic switch won’t build up, and the kettle will never auto-shutoff. I haven’t seen that tested, but would be easy to test.
This is an example of the type of switch:
And if it does, and if the safety shutoff fails and doesn’t switch off the power, bad things will happen very quickly.
Kettles are designed (obviously) to deliver enough power to heat water, and that’s a lot of power. The specific heat capacity of water is about 10 times that of steel. In other words, the amount of energy required to heat 1 gram of water by 1 degree will heat the same mass of metal by about 10 degrees. So if there’s no water there to absorb the heat, the metal components will get melty hot very quickly.
What works in your favor, however, is the fact that the latent heat of vaporization of water is stupendously large. The energy required to turn 1 gram of boiling water at 100 degrees to steam is about 7 times the energy required to heat 1 gram of water from room temperature to boiling point. So if there is some water there it will take longer than you might expect to boil dry.
The swiss cheese model is a strategy of layering imperfect protective measures so that a very unlikely collection of failures must occur before a real catastrophe can happen.
Excellent, I get that - thanks!
Dan
It’s not the kettle, per se. It’s that the UK uses 220 current, not the 110 that is standard here, right?
I believe that is the relevant point.
And that, I think, is wrong. The power is measured in Watt, not in Volt. You can have a lot of Watt with little Volt and a lot of Volt with little Watt, if you modulate you Ampere accordingly, as W = V x A
The circuit can only deliver so much current— let’s say 20A. If the rms voltage is 110V that gives you at most 2200W of available power. That seems to rule out the use of those 3000 W electric kettles.
Fair enough. Electricity is still magic to me. 
As DPRK noted, the maximum current a circuit can safely deliver is limited by the size of the wire. In the US, you’re allowed to use a 20-amp breaker with 12-gauge wire; 20 amps * 110V = 2200 watts. Assuming similar current/wire limitations in the UK, the fact that they operate at twice the voltage of US systems means that they can delivery twice the power: 20 amps * 220V = 4400 watts.
In the US, 220-volt residential wiring is typically reserved for devices with high power requirements: stovetops and ovens are a pretty common use for 220, but it also finds use for some power tools like welders, high-CFM compressors, and woodworking equipment like planer-jointers and cabinet saws.
The answer to this, at least, is “enough”. If you boil an electric kettle and leave the lid open, it will never switch off (until it does boil dry and the temperature switch kicks in, at least). It doesn’t take much - just the restriction of the spout is enough to detect the generation of the steam.
So what I said is correct?
You had the right idea, although the terminology was wrong.
The UK uses 220v, instead of 110v. This allows more power to be delivered for the same current. If you’d said Voltage instead of Current, it would have been exactly right.
Thanks!
I’ve been using kettles for 40 years and not known of the steam / pressure switch until now - thanks @Martin_Hyde !