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electrics - how do i check if current is flowing in a cable?

Discussion in 'General DIY' started by tonyl, 9 Mar 2003.

  1. tonyl

    tonyl

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    hi,

    i've got an old black rubber cable which i don't think is live as everything else seems to use white pvc however i need to be sure!

    i was going to cut the cable and expose (first turning off mains!) and then use a multimeter which i've borrowed. then i thought, errrr how do i use a multimeter? it has an AC section and i've rotated the dial to AC/250V. it has two probes (red and black). but where do I put each one to get a reading?

    Thanks!
     
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  3. steptoeandson

    steptoeandson

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    does not matter which way round the red and black probes go.
    as it's ac ( alternating current ) it alternates above and below 0v.
    therefore there is no negative or positive sides.

    you can put the probes on the cable any way round .

    make sure that when handling this black cable that all
    mains supply is off, as it may have it's own seperate
    fuse box ie not connected the the main C.U.
     
  4. breezer

    breezer

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    the above is true, but you should turn the mains on to see if it is there after you connect a multimeter. a safer way is to use one of these
     
  5. tonyl

    tonyl

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    thanks for the info. however, how exactly do i use the multimeter?
    ie one probe touches the live mains wire, where do i put the *other* probe?

    also the MAINS TESTER from screew fix. how does it work?
    how is it safer etc?
    i may buy one if it will give me better/more reliable results

    thanks,
    tony
     
  6. breezer

    breezer

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    It is easy to use a multi meter, however i would suggest that as it is your first time do not use it on the mains, Electricity has no prejudices IT KILLS ANYONE!

    A multi meter when used correctly is a very useful tool, why not buy one and test things like torch batteries and lamps appliances etc (I am not joking that is how i started)

    You can get one for less than a fiver see this

    but for the time being get a proximity tester like the one i first mentioned, after all we would like to see you posting here again
     
  7. tonyl

    tonyl

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    thanks. i'll go and find a battery now!
    i understand where the probes go on the battery so i form a circuit. but where do the probes go? is it live and earth?
    plus why is the proximity method safer?
    thanks
     
  8. breezer

    breezer

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    it depends what you are checking for as to where the probes go, why not unplug your electric kettle set the meter to ohms and test the kettle.

    The proximity tester is safer because you can not get killed with it, since it never touches anything
     
  9. tonyl

    tonyl

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    thanks.

    the cable i am testing prolly goes to 13amp wall plugs or maybe a lighting circuit. that's all i know. a loop of it was sticking out of the wall
     
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  11. tonyl

    tonyl

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    anyone?

    i've got a cable coming out of the floor. i will cut back the insulation sheath to expose the bare conductors. but where do i put the probes?

    can some1 also give me a method statement for using the proximity detector.

    apologies for asking such basic questions but it is my first time after all!

    thanks,
    tony
     
  12. breezer

    breezer

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    the proximity mains tester is very very easy, you hold it near (or you can touch ) the cable in question, if it lights up it is live
     
  13. MANDATE

    MANDATE

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    My proximity tester could not be simpler, switch on and you have "green" LED. place tester probe near to cable,socket or swich and if live the green is replaced with a flashing "red" LED
    Also do NOT use multimeter on a cable to establish if you have a live.
    If you think about it, the red in the cable could well be live whilst the neutral and earth are not connected to the consumer unit
    If you place the voltmeter probes across red/black or red/earth you will have no reading because there is no circuit, but you may think the red is dead when it is live.
    Hence the reason for a posative test like the proximity tester or mains testing screwdriver, be careful not to create any short across wires
    If you want some" hands on" experience, make up a lead with a 13amp plug at one end and at the other place the brown (live) brown(neut) and (green)earth into a block terminal(each one separate)
    Now you can probe red to blue, red to earth and blue to earth and you will get readings on two of them
     
  14. breezer

    breezer

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    all though what MANDATE says is true, i would recomend that you DO NOT take up his advice, as one of my previous posts says try it with a torch battery and check things like domestic appliances (from the plug so it is switched off)
    The reason for me saying this is it will get you familiar with using a multi meter, also it was not mentioned what to set the meter on, if you get it wrong there could be a big blue flash and bang. I AM NOT JOKING

    remember electricity has no prejudices it KILLS anyone.

    You may say i am being over cautious, but better safe than dead, i do not mind giving advice as to testing things and what to do with a meter, but i would never recomend that for a "first time" you test the mains, stick with a proximity tester, (for the mains) untill you are more confident
     
  15. tonyl

    tonyl

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    ok thanks. i'll get a proximity detector this weekend. will also practice on batterys etc as i want to install some wall plug sockets soon and therefore need some practical experience dealing with electricity.

    thanks for all the advice.
     
  16. tonyl

    tonyl

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    my "EURO SPEC MAINS TESTER" from screwfix has just turned up.
    remember you posted me the link earlier on in the thread.

    so, errrr how do i use it???

    it just looks like an electician's screwdriver.

    thanks
     
  17. breezer

    breezer

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    try holding it next to a known live cable (say your kettle lead) the led will light up, assuming you read the instructions on how to put in the batteries
     
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