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Why is a double pole switch used after the kwh metre in our houses and not a simple s

  1. Feb 5, 2009 #1
    Why is a double pole switch used after the kwh metre in our houses and not a simple s
     
  2. jcsd
  3. Feb 5, 2009 #2

    russ_watters

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    Re: Why is a double pole switch used after the kwh metre in our houses and not a simp

    Houses get two "hot" wires (out of phase with each other) and one neutral, for 240 V phase to phase or 120 V phase to neutral.
     
  4. Feb 5, 2009 #3
    Re: Why is a double pole switch used after the kwh metre in our houses and not a simp

    As russ say, you need both the live and neutral shut off.

    The mains is not ground isolated (floating). It's transmitted down the power lines with respect to the earth, which your house and everything in it is also connected to. So the voltage exists with respect to everything around you, touching a hot wire and something connected to the earth will shock you.

    An isolated floating) voltage doesn't do this. The voltage exists only between the two terminals. Theoretically, you could put an output lead from a power station in your mouth and not be shocked provided it was an isolated (floating) voltage. You'd only be incinerated if you came into conductive contact with the other terminal, the earth wouldn't count.

    A battery is a floating power supply. There are 9v between the terminals, but if you try measuring the voltage from one to the earth... nothing. Do the same with the mains and you'll one of the hot phase voltages.

    You can isolate (float) a voltage by passing it through a transformer or capacitor (in series with the lead), because these both involve conversion to another energy form that has no coupling to the earth (a magnetic bubble in a transformer or an electric field in a capacitor).

    Building sites use transformers not only to run their tools at a lower voltage (to discourage muscles latching up during a shock) but to isolate the voltage produce from the earth. They can hold onto one terminal and not get shocked even if they're standing in a puddle of mud with their shoes off.

    All consumer electronics (bar lightbulbs) feature a transformer partly for this reason.

    You need to be very careful when discussing earth / ground and isolating, as they tend to get used for a few different things. Electronics guys will refer to the negative on their supply as the ground rail for example, but then also call it grounding the case of electronics when they connect it to the earth. The two aren't the same in terms of the safety issues. Isolating can also imply switching off. In this example, it means removing the reference it has to a point, not turning it off.

    Think of it this way. You measure six foot tall from your feet to the top of your head. It doesn't matter if you get into a plane and fly 30,000ft into the air, you're still six foot tall with reference to the distance between your feet and head. That's an isolated (floating) measurement.

    Another way would be to say, I measure six foot from the floor (earth) to the top of my head. Now you get in the plane and, because your measurement is relative to the floor, you're 30,006ft tall. That's a referenced, none isolated, none floating measurement, like the mains. The mains voltage is that 6ft part, it knows the difference between the two hot wires. But it also knows there's a difference between it's head & feet measurement and the earth.

    The national grid use this trick to make earth grounding easier. Rather than connect everyone's house up using a million miles of expensive earthing cable, they use the earth it's self as one of their electrodes. The earth works really well, using lots of cable would add resistance to the earth terminal, which you never want.
     
    Last edited: Feb 5, 2009
  5. Feb 5, 2009 #4

    mgb_phys

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    Re: Why is a double pole switch used after the kwh metre in our houses and not a simp

    Only things you are likely to get a shock from NEED one - that's why shaver sockets in bathrooms in the UK have a transformer. In most of the rest of the world you can have regular mains sockets in a bathroom!

    In the UK you have a double pole switch for extra safety, it's possible because of a fault that the neutral line could become live (it is connected to earth somewhere between your house and the substation). In the US as Russ said there is a slightly odd arrangment where they have a choice of 110v or 220v. Most appliances are on 110v but you can run things like cookers/washers by connecting them between the +- 110V lines to give you 220V.
    (this isn't the same as getting a multiple phase supply in the UK - the 110V are in phase jsut one is high and one is low)
     
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