How is it that there is continuity and a voltage reading yet no short circuit?

Discussion in 'General Electronics Chat' started by foolios, Feb 4, 2009.

  1. foolios

    foolios Thread Starter Active Member

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    Excuse me if I don't explain this correctly. I am trying to get my head around how this works. Please ask for clarifications and I will do my best to do so.

    My question is about how is it that when I do a voltage check from line side to load side at a meter box with a main breaker switch closed that I will get 120V and that I will get a beep when doing a continuity test yet there isn't a short in the system to allow a complete circuit?

    If there is nothing plugged into the walls of a residence, how is it that there is a circuit for the voltage reading and continuity test to occur?

    See my diagram below. At the wall outlet, there is an open break in the circuit until a load is plugged in. How then am I getting these results when I do my test before the main breaker when nothing is plugged into the wall outlets in the entire residence?
    I was told it was because of the ground wire at the outlets. But that makes me think that that would actually be a short circuit if that was the case and we'd have continuous electrical usage...

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    Thanks in advance for any explanations.
    Last edited: Jan 23, 2010
  2. Audioguru

    Audioguru New Member

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    A breaker is not "opened or closed". It is turned on or it is turned off.
    If the breaker is turned on then of course voltage is at the wall outlet even if nothing is plugged in to draw current.

    A continuity tester tests wires that are not powered, the breaker must be turned off.
  3. SgtWookie

    SgtWookie Expert

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    Ground is connected to Neutral at the main breaker panel. In most normal circumstances, this is the only place that the two meet.

    Outside the residence, usually right near the electrical service entrance, there are one or two 8' long copper-clad stakes driven into the ground, with an AWG 4 or larger conductor clamped to it. This usually gives a reliable earth ground for many years.

    This conductor is run to the main service panel, and attached to the ground buss. There is a separate Neutral buss, however the two are tied together. This keeps the line voltages from rising to very high levels, should a fault occur in the power company's transformer; the transformer's fuses will blow instead of creating a very dangerous situation.

    Although Ground is connected to Neutral at the service panel, the ground wire should never be used to carry return current to the service panel. The ground wire is there to provide safety for the human users of the electrical service; the chassis of appliances are connected to the ground wires to protect them from an internal fault.

    GFCI (Ground Fault Circuit Interrupt) breakers such as used in kitchen, bathroom and outdoor recepticles detect current flow in the ground circuit, and will open the circuit (trip the breaker) if the current in the ground wire exceeds around 15mA.
  4. studiot

    studiot E-book Developer

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    Please remember that you need a circuit for current to flow, but not for voltage to be present.

    Think about an ordinary battery there is always voltage between the terminals (for a good one), whether it is connected into a circuit or not.
  5. mik3

    mik3 Senior Member

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    Where do you place the meter probes when you perform the voltage measurement and continuity test?
    Is the main braker turned off during tests?
  6. foolios

    foolios Thread Starter Active Member

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    I am placing the meter probe from the line side of the meter box(the jaws in it) to the load side of the meter box.

    What I test for is to make sure that the main circuitbreaker is off(open).
    What I want to understand is:
    If the main switch is closed(on) and there is nothing plugged into any outlet in the residence and there is nothing on that would draw current, would I get a voltage reading? If so, why?
    How can there be a voltage reading if there is nothing plugged in or on to be powered up/complete the circuit if that's so?

    Also, in this same scenario, would a continuity test be possible? As I read above, continuity tests can only be done with the power off, so in this case of a main closed, would that mean a continuity test can't be done even though there is nothing plugged in to complete the circuit?

    THanks for the replies.
  7. mik3

    mik3 Senior Member

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    If the main breaker is turned on there will be a voltage between neutral and live wires on the outlet. The voltage on the outlet does not depend on the appliance connected to it but it always there and waits to power an appliance. When an appliance is connected it will draw some current and this current depends on the power of the device. Don't confuse voltage with current.

    As for the continuity test don't make it with power on, you may get shocked or blow the meter.
  8. beenthere

    beenthere AAC Fanatic!

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    If you are looking for continuity under the stated conditions -
    then you have to read continuity. Having the breaker closed means it is acting like a switch that is turned on. Checking from one side of it to the other will naturally show continuity.

    One verifies the state of a breaker by looking for voltage between the hot lead and its associated neutral. If you still see 120 VAC, the breaker is closed. If open, the voltage will not be present.

    Checking continuity is done on circuits which have been disconnected from the power source. If you suspect an open, the check may be done with a voltmeter and power on. You place a lead at the source and probe with the other. Finding a spot where the source voltage is read means there is an open between the metered points.
  9. foolios

    foolios Thread Starter Active Member

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    I need to break this down a little. Lemme isolate just the voltage concerns.
    When I test for voltage at the meter box and the breaker is open, I get no reading. Now, isn't there potential for voltage right here at the box just like if the breaker was closed and there was nothing plugged into the wall outlet; which makes this the same test when I'm also testing for voltage at the wall outlet? There is a break isn't there? Whether it's at the open main or whether it's at the open wall outlet. I don't understand how it is any different between the two when it comes to reading potential voltage.

    As far as if it's about completing a circuit, of which is where I believe my misunderstanding lies.
    I can see if there was an appliance that was plugged in and on. This completes a circuit. I should get a voltage reading. Voltage wants to push current.
    But how is this occuring when there isn't an appliance plugged in and on to complete the circuit? Yes, there is a neutral connected to each outlet but it's not directly connected, otherwise there would be a short.

    Sorry if I'm beating a dead horse, I really am curious to understand at least this part of it.
  10. beenthere

    beenthere AAC Fanatic!

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    The situation is very like an earlier example, where you can see voltage across a battery in the absence of any circuit than the meter. That is like reading 120 VAC at an outlet that is otherwise open. The voltage is just the difference in potential between the hot wire and the neutral.

    If you open the breaker feeding that branch, the voltage goes away at the outlet, and at every other part of that branch.

    If I understand this -
    - you have read across an open breaker and see voltage. With a high impedance voltmeter and a bit of leakage in the branch, that's not too surprising. Plug in any load, though, and that voltage will go away.
  11. foolios

    foolios Thread Starter Active Member

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    But unlike the battery where we are directly connecting the two ends, the circuit we are testing isn't directly connected; or so I don't think.
    See, I can understand that we can get a voltage reading at the wall outlet when the breaker is closed and there is nothing plugged in. We are jumping from one side of the outlet to the other. Which is a complete path from line to neutral.
    But the problem is when we take a voltage read from the meter box. Even though I am jumping the meter across the line side to load side in the meter box, there is a break at the wall outlet. It is open. Therefore if we look at the battery analogy, I would guess that this would be similar to us adding a wire to the positive post and a wire to the negative post of the battery. We place the multimeter onto both ends of the wire, we then break a small opening in one of the wires, similar to the unplugged wall outlet. There is no voltage read. This is exactly how I imagine it would be if we read for voltage at the meter box while there is nothing to close the circuit at the wall outlet.
    Why is this not the case?



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    Last edited: Jan 23, 2010
  12. beenthere

    beenthere AAC Fanatic!

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    Actually, the reverse is the case. Take that wire with the meter across both ends. Place it so both ends contact the battery terminals. Result - no voltage is seen. The wire conducts, but has no potential difference from the one end to the other. Cut the wire with both ends still in contact, and the battery voltage will be present. An open circuit always reads the source voltage.

    I confess that I don't follow your example diagram at all.

    I might also suggest that you use a battery and wires to experiment with, as slips tend to be much less fatal. I do not recommend poking under the cover of a distribution box. What is true for AC voltage can be as well demonstrated with low voltage DC. Use a small switch in place of the circuit breaker.
  13. foolios

    foolios Thread Starter Active Member

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    Sorry, I will identify each part in an edit of this post.

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    Last edited: Jan 23, 2010
  14. beenthere

    beenthere AAC Fanatic!

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    Of course you will read voltage out at the meter. Unless the utility supply has failed, you always will. Remember that voltage is the potential that can cause current. And that open circuits always show the source voltage.
  15. italo

    italo New Member

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    The sargent gave you the only plausible answer the rest are guessing or argue about off-open names to signify the same action and condition. If you measure across THE BREAKER when off there is 120V present but only to the meter when you close it there is no voltage because it is a virtual short. MEASURING from neutral it is a different story
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