Notices
Results 1 to 9 of 9

Thread: measure voltage + current travelling through negative wire

  1. #1 measure voltage + current travelling through negative wire 
    Forum Bachelors Degree
    Join Date
    Jul 2009
    Posts
    404
    Can anyone tell me how to measure the voltage and current travelling through the negative wire after it has been through an appliance?

    Is it as simple as attaching the positive of the voltmeter to the negative wire (after the appliance) and the negative of the voltmeter to an earth? If so could I use the earth found on a regular UK socket or would this be bad practice?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Note that this is AC and so "positive" and "negative" don't really apply. If you are measuring voltage, make sure you have your meter on AC. And BE CAREFUL.

    As to measuring voltage, it depends what you are measuring the voltage relative to! If that is not too obvious.

    Normally, in a mains electricity supply, it is measured relative to the neutral ("negative"). But as it is the neutral you want to measure, then I suppose the obvious thing is to measure it relative to earth. And, yes, you can use the earth pin in a socket but be very careful how you access it; all exposed metal work on the appliance should be earthed (but as you may be looking for a fault...)

    Neutral should be at or near zero volts. I don't know how much it is allowed to vary from this. A significant voltage suggests a fault somewhere.

    As for measuring current, the easiest way is one of those clamps that goes round the cable. Otherwise you need to disconnect or cut the wire to put the meter in series (and, if you are asking a question like this, I'm not sure you should be doing that. ).


    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  4. #3  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    A clamp-on ammeter would be the way to measure the current.

    The grounding conductor (usually a bare copper wire in the US) does not normally carry current, and is connected to ground at the service entrance. (Using American terminology here). Therefore since this conductor does not carry current, it should be at the same potential as the ground bus at the service entrance. However, if you have a poor connection to ground, then the voltage at the service entrance could vary quite a bit and might not be exactly at earth ground potential. If there is a concern about that, you would have to find some other reference to earth ground, like a metal stake in the ground, a metal water pipe, or something like that.

    Likewise, if there is a short circuit in your appliance, the neutral could be shorted to the chassis of the appliance, and would therefore be at the same potential as the ground pin. This condition should trip your circuit breaker if you have a ground fault circuit interrupter.
    Reply With Quote  
     

  5. #4  
    Forum Bachelors Degree
    Join Date
    Jul 2009
    Posts
    404
    thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?
    Reply With Quote  
     

  6. #5  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    Quote Originally Posted by fatman57 View Post
    thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?
    As Strange said, you have to connect the meter in series with the load to measure the current. Generally, the voltmeters I've seen are not designed to measure a-c current, and then only very small d-c currents. The skinny leads on your meter would never be able to take it.
    Reply With Quote  
     

  7. #6  
    Brassica oleracea Strange's Avatar
    Join Date
    Oct 2011
    Location
    喫茶店
    Posts
    16,965
    Quote Originally Posted by fatman57 View Post
    thanks all! Can I not use the amps selector on my voltage meter to read the current? I presume a clamp-on is used to measure insulated wires and very high current...is this correct?
    The thing is, you can measure voltage between wires but you have to measure current along the wire. In other words, to measure current you need to break the circuit (e.g. cut or disconnect the wire) and then insert the meter in the circuit so the current flows through it.

    If you put your meter on the current setting and connect it across the mains you will probably destroy it (or blow a fuse).

    Harold mentioned a ground fault circuit interrupter - this is a residual current breaker (RCB) or earth-leakage current breaker (ELCB) in the UK. If you don't have one in your consumer unit ("fuse box") - they have only been mandatory in the UK relatively recently - then it might be worth buying one to put in the socket where the appliance is plugged in. This will tell you immediately if there are certain types of fault (the breaker will blow) and will give you a little extra protection if you do something silly.
    ei incumbit probatio qui dicit, non qui negat
    Reply With Quote  
     

  8. #7  
    Cooking Something Good MacGyver1968's Avatar
    Join Date
    Aug 2006
    Location
    Dallas, Texas
    Posts
    2,051
    Just be sure if you use a clamp on meter to only measure one of the conductors. If you put both the hot and neutral in the clamp, you won't get a reading.

    It's far more difficult to use a multimeter, as Strange and Harold pointed out, you have to open up the circuit somewhere and place the meter in series:

    Fixin' shit that ain't broke.
    Reply With Quote  
     

  9. #8  
    Forum Bachelors Degree
    Join Date
    Jul 2009
    Posts
    404
    thanks all and thanks for the safety tips!
    Reply With Quote  
     

  10. #9  
    Forum Professor
    Join Date
    Jan 2013
    Posts
    1,771
    Quote Originally Posted by fatman57 View Post
    Can anyone tell me how to measure the voltage and current travelling through the negative wire after it has been through an appliance?

    Is it as simple as attaching the positive of the voltmeter to the negative wire (after the appliance) and the negative of the voltmeter to an earth? If so could I use the earth found on a regular UK socket or would this be bad practice?
    To be entirely successful, and at ease, with the wonders of electricity, it is IMPERATIVE that one recognize that VOLTAGE does not travel around, through conductors, etc.

    Voltage can exist only between two places, or points, such as points A and B, or point A and Earth Ground. Voltage may be present for an eternity, without any further effect taking place, until the opportunity presents itself for the voltage to cause current flow. Current flow is the INTENSITY or Amount of electricity flowing. Voltage does not "flow".
    Reply With Quote  
     

Similar Threads

  1. Negative Magnetic Polorization To Increase Negative Polorized Projectile Velocity
    By bryan in forum Personal Theories & Alternative Ideas
    Replies: 11
    Last Post: January 10th, 2012, 03:17 AM
  2. Need some help with turbines, voltage, amps and wire
    By Fmp2491 in forum Electrical and Electronics
    Replies: 6
    Last Post: August 26th, 2011, 01:29 AM
  3. Current-voltage in electrical breakdown
    By thyristor in forum Physics
    Replies: 0
    Last Post: May 2nd, 2011, 11:36 AM
  4. Travelling hotspots or travelling plates?
    By Pong in forum Earth Sciences
    Replies: 32
    Last Post: December 26th, 2009, 03:28 PM
  5. Voltage and Amps having negative correltion
    By oversteve in forum Electrical and Electronics
    Replies: 1
    Last Post: July 5th, 2007, 08:22 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •