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Where is the switch?
February 8, 2008 7:44 PM   Subscribe

Why do some electrical appliances lack an on-off switch?

Since childhood, I've believed that it is unsafe to unplug an electrical appliance that is under load. More recently, I see this popular safety tip given on the net with the reasoning that unplugging an appliance under load can cause a spark.

At work I deal with 480 volt power cords, and it is considered a serious error to unplug a device like this while under load.

But two appliances come to mind that, as far as I know, never have on-off switches; electrical irons for clothes, and the George Foreman grill. I know there are now several models of George's grill, but I can attest that the one I own did not come with an on-off switch.

Why do these appliances lack on-off switches?
posted by Tube to Technology (23 answers total) 1 user marked this as a favorite
 
Gotta be a cost-cutting measure.
posted by rokusan at 7:53 PM on February 8, 2008


Both those items, as well as my popcorn popper (also lacking a switch), make things hot. I think it is more to make sure no one has a possibility to accidentally turn a heat-making appliance on. Or at least cut down on the possibility.
posted by kellyblah at 7:56 PM on February 8, 2008


Not never: my iron can be turned off.
posted by The corpse in the library at 7:58 PM on February 8, 2008


The electric irons I've used can be turned off. The dial for setting the temperature clicks into 'off' position at the very bottom of the scale.
posted by winston at 7:59 PM on February 8, 2008


Usually those units come with a rotary switch or "pot" that's used to adjust the temperature of the heating surface. A position on the dial is the off position.

Other appliances that have a fixed temperature they heat to may lack an on/off switch to prevent foreign matter from getting to the otherwise sealed heating unit.
posted by Kioki-Silver at 7:59 PM on February 8, 2008


In the case of the Foreman grill, my guess is that if there were a switch, people would just switch it off before attacking the clotted grease with a wet dishrag.
posted by Sys Rq at 8:08 PM on February 8, 2008


I'm interested in this too. It would be nice if someone knew the real answer. Is George Foreman a mefite?
posted by proj08 at 8:54 PM on February 8, 2008


Sorry, I wasn't being very helpful. Here's a data point: I have another appliance which is similar to the George Foreman Grill in nature, and it also lacks an on-off switch. This leads me to think the "safety issue" idea is more likely than the "cost cutting" suggestion. Or, maybe an on-off switch would be another place for water to get in, and they're trying to keep it as water-tight as possible.
posted by proj08 at 9:02 PM on February 8, 2008


None of us have had an answer and neither do I, I'm afraid. I just wanted to note that many computer peripherals do not have them either. Think routers. It's always made me a little nervous that to reset the power on the device you have to literally pull the power cord, but it doesn't cause any ill-effects.
posted by tcv at 9:12 PM on February 8, 2008


My panini press (not a George grill) and my rice cooker lack an on-off switch. So the common theme among these is that they make things hot. The answer follows from that... somehow.
posted by ethnomethodologist at 9:28 PM on February 8, 2008


Heating devices are resistive loads. It's hard to get a spark out of them at disconnect, because so much of the voltage is being dropped by the resistor that gets hot. So, unplugging them is safe.

The chances of getting an arc depends directly on the voltage. Current isn't important here -- example, a Van de Graaff generator. Big sparks, because the voltage is very high, in the hundreds of kilovolts, but very low current -- most are in the microamp range.

At 120V, getting an arc is hard. 480V, it starts to be a problem, and when you get into the kilovolt range, you stop using air breaks and start getting fancy about how you cut power.

Switching in or out HV transmission lines is a scary thing, and 50' arcs happen when you get it wrong.

We'll handwave around inductance and reactance here -- when AC over distance is involved, things get *very* complex, but at home, with a resistive load on a 6' long cord, unplugging that is perfectly safe.

The main reason home devices have switches is convenience, not safety.
posted by eriko at 10:11 PM on February 8, 2008 [1 favorite]


Most, if not all, clothing irons can be turned off as per winston's comment. Anyway, unplugging a device is not a big deal, unless a lot of current if flowing in which case an arc can form which might burn the blades or the socket. Low currents are not an issue. For instance, how many of the power bricks for your phones, cameras, computers, etc. have an on/off switch? None. Yet, the brick is live when plugged in. An electric motor on the other hand is a different story, say a pool pump motor. Yes, you can turn it on and off with the plug, but sometimes it will arc.
posted by caddis at 10:13 PM on February 8, 2008


Most irons either have an external off switch or an internal off switch (usually some kind of gravity switch that will turn off the heating element if the iron isn't moved). The only irons I've seen that completely lack an off switch are travel irons.

Unplugging a resistive heating appliance (iron, griddle, curling iron, immersion heater, etc) when it's on is only minimally dangerous, and not for electrical reasons. Most of the danger comes from the heat.
posted by jlkr at 10:31 PM on February 8, 2008


I always thought it was because they wanted to make sure it was off. So rather than include an on/off switch, which you may or may not turn off, but think you turned off, they want you to unplug it because there is no mistaking that. It's off for sure. But tcv's mention of the routers above kind of pops my theory. It's not like anything bad happens if you accidentally leave those on.
posted by Askr at 11:02 PM on February 8, 2008


What about switch less flashlights that make you tighten the battery caps to turn them on?

I paid $1.50 for it, and shipping cost me $4.50... What else can I say?
posted by BeaverTerror at 2:19 AM on February 9, 2008


There's no on-off switch on, say, a Tivo. I suppose it's assumed to be on all the time, similar to routers.
posted by chengjih at 5:54 AM on February 9, 2008


On computer devices like routers and external harddrives it's undoubtably a cost savings measure with the rationalization that it's a user convenience to always be "on."

Speculation: For heat generating appliances maybe there is an extra complication/aesthetic consideration (solved with $$) in that the switch would have to be designed to handle the high current and mounted so as to neither melt nor risk burning the users finger.
posted by Kevin S at 7:29 AM on February 9, 2008


eriko writes "Heating devices are resistive loads. It's hard to get a spark out of them at disconnect, because so much of the voltage is being dropped by the resistor that gets hot. So, unplugging them is safe."

You can get a fine spark out of a 15A resistive load at 120V. The arcing at disconnect is why better quality switches have silver alloy contacts. The silver is better able to handle the arcing at disconnect/connect.

One can see this sparking easily by observing a plug in a dark room when disconnecting a resistive load like a hair dryer or space heater. I used to use this method to check for defrost element heating when I was caught without my tools (being a appliance technician is almost as bad as being a doctor or singer sometimes).

eriko writes "The chances of getting an arc depends directly on the voltage. Current isn't important here -- example, a Van de Graaff generator. Big sparks, because the voltage is very high, in the hundreds of kilovolts, but very low current -- most are in the microamp range."

How _long_ of a spark one gets is determined by voltage, but you can get high strength sparks at very low voltage. You can easily stick weld at the 24V provided by a pair of car batteries for example.
posted by Mitheral at 8:22 AM on February 9, 2008 [1 favorite]


Mitheral is right, unplug any appliance in a very dark room and you will see the spark, this doesnt nessecarily equate to "danger" however. There are many, many more appliance that either have no off switch OR take power no matter what condition the switch is set to than you think. Many of your on/off appliances are "on" no matter what, your TV, Microwave, Stereo, nearly all computer equipment, are taking power at all times. There could be some chance of an unusually large arc from a running washer or drier but south of that I wouldn't worry at all, I unplug my old fridge regularily when it acts up, it takes a ton of power and has no off switch.

I say, practically no danger.
posted by parkerama at 11:32 AM on February 9, 2008


When you mechanically separate a connection, the space starts at zero and grows as you separate the contacts. There is no magical switch which instantly creates "off", there will always be some arching between the contacts when a mechanical switch is flipped. The issue is only damage and/or visibility. Some arching is deadly, some arching is below the threshold of detection, sometimes otherwise minor arching can start a fire. In your home, the reason to use switches instead of just unplugging is almost always the fire hazard. Mechanical switches have all the same arching problems as unplugging, but being enclosed, nothing flammable will be nearby when it happens. More generally, some devices require a shut down procedure, or special switch, to prevent the bad stuff from happening. If you just unplug that kind of device, you will be bypassing the extra steps that have been taken to make turn off easier, which might be bad for reliability, or it might be deadly.

When thinking about the amount of arching caused in the mechanical switching process, there are two important considerations, inductance of the load, and ionization in the air. High inductance circuits have a lot of inertia - current which is flowing through an inductor wants to keep on flowing (inductance is like mass). If you try to interrupt current, voltage increases - so here you are with a mechanical switch, gently increasing the distance between contacts, but the voltage is growing as you separate the contacts, "fighting" - at some point other characteristics factor in, eventually the voltage can't get any higher, and the circuit is broken. It is also a reason why mechanical switches are often spring loaded, so that the separation distance increases more decisively.

This is further complicated by ionization of air. Ionized air is a conductor, not an insulator, and air ionizes in the presence of electrical arching. Since there is always some arching, there is always a little ionized air around when you flip a switch - combine ionized air and inductance in the load, and you can have an electrical contact that is really hard to break!
posted by Chuckles at 12:29 PM on February 9, 2008


When it comes to your specific examples, I think someone already mentioned that they are both heating devices, and they will both have heat settings and thermostats. The user can turn them off just fine, it just doesn't look like a switch.

Some very gentle heating devices don't have any switch or control of any kind. Some glue guns and soldering irons, for example. Presumably, these devices draw so little power that the arch isn't a safety issue, and putting a switch would increase cost.

And one final issue.. Unplugging by pulling the cord can cause damage, but telling kids to never pull the chord probably gets tiring, so parents telling kids to avoid unplugging might be expedient :)
posted by Chuckles at 12:52 PM on February 9, 2008


You can easily stick weld at the 24V provided by a pair of car batteries for example.

Point taken, but now we're getting into current questions. You could weld at 1V if you have enough current and the electrode have a low enough resistance. And having welded with a couple of batteries, getting the arc right is a pain.

The trick is to use six car batteries. :-)
posted by eriko at 7:46 PM on February 9, 2008


Presumably, these devices draw so little power that the arch isn't a safety issue, and putting a switch would increase cost.

To me, "drawing an arc" means "arcing in free air." As Mitheral notes, all connections will arc at a close enough distance in air, but if it arcs .1mm inside the socket, it's not a safety issue.

And, there's what a friend said. "It's not the volts, or the amps. It's the power. Don't fuck with 1KA at 1KV."
posted by eriko at 7:55 PM on February 9, 2008


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