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I have a vacuum cleaner, which I wish to add a switch to, so that I can leave it plugged in all the time and use the switch to turn it on and off. The vacuum cleaner has a fuse rated 13 Amps.

I purchased a light switch, and noticed when I got home that it said that it was rated for 10A on the packaging.

The vacuum cleaner is rated for 1100W at 230V. I used this electronics calculator, which returned a value of under 5A. Why is this? Is it safe, therefore, to use the 10A switch on the vacuum cleaner?

EDIT: The vacuum cleaner is a ShopVac Micro 4 litre / 1 gallon model. Specs are available here

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4  
Why not get a switchable socket? Or is this an "inline" lamp switch in the cord? I'm surprised the vaccum doesn't have its own power switch. – pjc50 Jan 25 at 14:40
    
I have a switchable socket, and the vacuum has a switch. However, the vacuum and the socket are both in a cupboard. I want the switch to be easily accessible. – Archie Roques Jan 25 at 15:36
1  
I wasn't being lazy, I was on a restricted network where every other site was blocked. Now amended – Archie Roques Jan 25 at 15:38
    
What is the rating on the switch that the manufacturer provided? – Spehro Pefhany Jan 25 at 20:04
    
Just search for Dubai Hotel Fire... As a precautionary tale. – Tyler Jan 26 at 1:48
up vote 6 down vote accepted

There are more things to consider.

First: Note that the vacuum taking only 1100W, i.e., 5A, could still draw a very short peak of much more current when the motor starts; it really depends on how exactly are these 1100W calculated.

Second thing: Is the 13A fuse in the cleaner accessible? Could you exchange it for a 10A fuse? The only thing that can happen then is that the fuse blows up unexpectedly.

Third thing: Is spending couple bucks for a 15A rated switch that much a problem?

While I don't think your house will catch fire because of this (since 1. mostly when something goes wrong, the current drawn is much higher than what it should be and should blow up your 13A fuse and/or your circuit breaker long time before the switch heats up to ignite and 2. the 10A rating of the switch means it should withstand much more in practice, there's a certain safety margin), I wouldn't recommend violating the rules of the game, since in this game, the game over could be quite serious.

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Here in the UK 10A plug-size fuses are available but not common. I would first fit a 10A fuse, then, assuming that works without blowing, fit the 10A-rated switch. After all, this is for an application that's not going to have startup current unattended. Inline switches rated higher than that are rare; when I last needed one I ended up using hardware designed for fixed wiring, on an extension lead clipped to a bench. – Chris H Jan 26 at 9:28
    
I am here in the UK myself :). I went to a good hardware store who were able to provide me with 10A fuses no problem. I've tested the 10A fuse now, and it works fine. I don't aim to ever be out of the room when it's on anyway, so I'll go ahead and fit the 10A switch. Thanks for your help everyone! – Archie Roques Jan 26 at 17:18
    
@ArchieRoques Please, never consider your "I don't aim to ever be out of the room when it's on" as a good reasoning when dealing with electricity. The true reasoning here is that the fuse blowing won't put fire on anything. – yo' Jan 26 at 17:25

The fuse is for your protection, so it's your call. How much do you value your house and the lives of the people in it?

If you want a safe system, then no, don't put a 10 A switch on something fused at 13 A. 10 A is what we call less than 13 A. That means in a overload condition, the switch will break before the fuse. That's bad, since a switch "breaking" can mean various things, like catching fire, emitting a cloud of greasy black smoke, or something else unpleasant.

The fuse, on the other hand, is designed to "break" in a predictable and safe way. That's the point of fuses. As the current increases, you want the fuse to be the first thing that breaks. By it breaking, the current will go to 0, and everything else is saved.

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Fuses are used to protect the wires from bursting into flames because of overloads (and also to protect against electric shocks when coupled to a solid earthing, but that's not the point here). As long as the current rating of the fuse is below the current capacity of the cables (rated currents are standardised such that, for equal ratings, the fuse blows before the cable is damaged), you can use any fuse you want AS LONG AS it can cope with the inrush currents without blowing. Fuses should be provided with a known blowing time - current relationship.

What I'm getting at is that the vacuum cleaner manufacturer may have selected a readily available 13A or 16A power lead fitted with an appropriate fuse even though it is much higher than what it needs, just because it's cheaper. In the end the vacuum cleaner might use only 5A. I doubt this is a peak value though, pump motors usually draw much higher currents when starting, but obviously the cables and the fuse are okay with that. Your switch might not, but it is much likely to be okay with that as well.

You need to be aware though that in case the vacuum cleaner draws between 10 and 13A for any reason (power spec is off, motor has worn out, partial short circuit or other kind of failure), your switch may fail and cause a fire or an electric shock.

In a nutshell: you can, but don't. Or insert a 10A fuse in series before the switch. If 10A fuses can handle the peak current of the vacuum cleaner without blowing (I recommend slow blow fuses), you should be fine.

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OK. If I switch the fuse in the plug for a 10A one, and it doesn't break, then that would be safe? – Archie Roques Jan 25 at 10:57
2  
Reasonably safe. Not as safe as it could be, since you could have a partial short circuit line-neutral between the fuse and the switch which pulls between 10 and whatever rated current your circuit protector is upstream (certainly 16A). The rule is to use overload protectors UPSTREAM of the cables or devices they are protecting. – Mister Mystère Jan 25 at 12:46

The main problem here is that this switch that you intend to add is going to be taking the surge current of the appliance every time that you turn it on. A 10A switch will have lesser capable contacts than a switch rated for 15A or 20A and thus it is highly likely that it is the contacts in the switch that will give out and fail before anything else. The failure will be due to arcing when the switch first closes. This arcing is also one of the most dangerous elements here as it can lead to a fire. I would definitely return the switch and purchase a more robust one.

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1  
AC electric motors draw up to 10* their rating on startup. This is why many devices don't have a switch - they are expensive and fail. – ChrisR Jan 25 at 13:30
    
ChrisR's comment is spot on. In fact, when diagnosing appliances the first thing that I go for is the switch. – dotancohen Jan 26 at 8:03

If it's a standard BS1362 fuse (as fitted to BS1363 plugs), then it's designed to protect the power cord against major overloads, such as a short circuit in the appliance. There are only two "standard" values - 3A and 13A, though 5A is also common, and other values exist. The appliance itself only draws about 5A, so 13A is the next standard value above.

Since the appliance only requires about 5A, a 10A rated switch should be fine. The aim of the fuse is not to protect against a sustained overload - which should not be possible with a single known appliance on the end of the lead. It's to cut the power before the flex bursts into flames if there is a dead short. The chances are that the flex isn't rated at 13A either - it just has to last longer than the fuse.

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The short answer is that it will work until it fails, but the failure could be potentially dangerous. Don't go it. Buy a larger switch. Electric motors have inrush currents that are momentarily very high, so you end up having to make amperage allowances for that inrush even though it will use much less current while running. Thus you may end up going beyond the rating of the switch every single time you use it, although just briefly.

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Switches often have different ratings for ohmic and for reactive loads (see datasheets). With a vacuum cleaner motor, the rating for a reactive load counts. With a garden variety light switch, unless it is also meant as a switch for a socket (unlikely with a 10A rating!), assume it is not designed for a reactive load even close to 10A, since no one will assume anyone putting 2000W+ of reactive lighting gear (Striplights, CFLs, electronic transformers, halogen on conventional transformers would match that description... but 2000W of such lighting is .... bright) behind a single home-grade light switch.

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Normal practice for a switch feeding a socket in the UK would be to use either a 20A DP switch or a 13A FCU.

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