Voltage readings

Discussion in 'Electrical Forum discussion & Blog' started by jimbo, Sep 4, 2007.

  1. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA

    We get lots of questions to the effect that "I read 67 volts at point X and 53 volts at point Y" or something like that. How can that be??

    Here are some simple little drawings that show how!


    Figure A is the normal circuit. For practical purposes of this discussion, we assume Zero ohms at the receptacle, between bare and white. So a meter connected black to bare or black to white reads the same....120 volts.
    Then we get to figure B. A bad connection somewhere in the neutral line between the receptacle and the main panel is represented by a resistance of 100,000 ohms.

    Now, all meters cause some small current to flow in the circuit. That is how they work. Since current is flowing as a result of imposed voltage, then by definition and in actuality, the meter has some internal resistance. For this discussion, I have assumed a meter of 100,000 ohms internal resistance.

    Simple Ohms's Law calculations show that a meter connected between hot and neutral will cause equal volatage....60 volts....to appear if measured directly across the "bad" 100 K ohms, and therefore the meter from black to white only reads 60 volts.

    A little imagination can show you how if you put this " bad " 100K at different points in the circuit, different results can be seen. Add some load resistance....for example a light bulb or motor, and the possibilities for confusing readings are endless.

    I always find it helpful to actually draw a circuit diagram like this ....including all possible loads......and from that infer why a funny reading has happened.

    Hope this makes sense to you guys!
    Last edited: Sep 5, 2007
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