why does ground test with meter not trip breaker?


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Old 10-21-14, 06:31 PM
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why does ground test with meter not trip breaker?

To test if the receptacle ground is connected you plug a probe into the hot and a probe into ground and get 120v.
Why doesn't this trip the breaker since you are connecting a hot to the ground wire?
 
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Old 10-21-14, 06:37 PM
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Because the meter acts as load.
 
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Old 10-21-14, 08:19 PM
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Witchcraft obviously.

Have a read: Voltmeter impact on measured circuit : Dc Metering Circuits - Electronics Textbook

The science behind electricity and magnetism gets pretty intense the further you dig.
Want a real mind blower? If you take a piece of wire and shove each end into a slot on your receptacle, BANG! But if you take a very very long piece of wire and make a ton of coils in it (like a phone receiver cord) you can shove both ends into a receptacle with no kaboom! (DO NOT TRY THIS). Read up on induction if you get bored someday.
 
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Old 10-22-14, 10:05 AM
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The question could be re-phrased as " Why doesn't the breaker trip when you apply the voltmeter leads to the Black ( "live") Conductor and the White ( Neutral) Conductor?"

Ans; for the same reason the breaker will not trip when you insert a cord-plug of a toaster into the receptacle slots.
 
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Old 10-22-14, 01:13 PM
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The question could be re-phrased as " Why doesn't the breaker trip when you apply the voltmeter leads to the Black ( "live") Conductor and the White ( Neutral) Conductor?"

Ans; for the same reason the breaker will not trip when you insert a cord-plug of a toaster into the receptacle slots.

Read more: http://www.doityourself.com/forum/el...#ixzz3GuGv8wrS
That's why I specified ground.
I am not plugging the voltmeter into the neutral on the receptacle, I am pluggin it between hot and the top(ground) openings.
 
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Old 10-22-14, 01:44 PM
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You obviously know how to check if you have a good ground. You checked hot to bare ground wire and found the approximate same voltage as hot to neutral.
You didn't ask why breaker trips when checking hot to neutral, because you know it won't.
Well, the bare ground wire is an alternate path to ground for safety, if neutral should fail.
And you tested the ground and know it works if ever needed.
 
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Old 10-22-14, 03:49 PM
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Yes, but if you connect hot to neutral you "join" the circuit. Obviously, it doesn't trip because that's what it's meant to do, complete the circuit.
If a hot wire ever touches ground, it trips.
So, by "joining" the circuit with a voltmeter, you are effectively completing a ground fault by touching the hot to ground but it doesn't trip in this case, the meter just reads 120V showing your ground circuit is working.
 
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Old 10-22-14, 04:11 PM
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The meter has impedance. Any impedance that drops the current flow below the amp rating of the breaker won't trip the breaker.
 
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Old 10-22-14, 04:16 PM
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Ah young grasshopper, you forget one key fact:
The internal resistance of the voltmeter is on the order of millions of ohms (around 2-10 Megaohms). A simple Ohm's law calculation: current = volts/resistance [I=V/R] shows why the breaker does not trip

I = 120/2,000,000 = 0.00006 amps (or 60 microamps), far from the ~15+ amps required to trip a breaker or even the 5-6 mA required to trip a GFCI.

A common early lab problem in Introductory Circuits (EE 101) is to determine the actual resistance of a voltmeter and calculate how much this internal resistance (versus an ideal infinite resistance) affects the accuracy of the measurement (not much except when measuring circuits across a very large resistance)

Now don't try this with the meter set and wired for Amps, you will blow the meter internal fuse (usually rated 10A) and/or the circuit breaker. Ammeter resistances tend to be less than 10 ohms (ideally a dead short).
 
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Old 10-22-14, 04:20 PM
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So, if you have a ground fault, it pulls 15A+ ?
ie, let's say the hot touches the metal junction box or something...
 
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Old 10-22-14, 04:22 PM
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I'll let the electricians explain it. Actually, they already did above. I am pretty good at basic wiring but not theory.
Above you said you "join" hot to neutral to complete a circuit.
But if you noticed, a receptacle's hot and neutral slots are not "joined". They are on opposite sides and to join them, you need a device in between. That device, when testing, is your meter.

Like I said, the electricians can explain it better.

I was late typing, you asked, you got it
 
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Old 10-22-14, 04:29 PM
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So, if you have a ground fault, it pulls 15A+ ?
ie, let's say the hot touches the metal junction box or something...
Yes much much more, usually enough to trigger the instantaneous function of the breaker (magnetic trip) vs the overload (thermal) function. They key is that all conduits, boxes, equipment grounding conductors, cable armor, etc. are bonded together and sized appropriately so that the resistance is low enough to cause the instantaneous trip.

This is why the armor of early BX (armored) cable cannot be used as an equipment ground: once the cable armor starts to oxidize the resistance increases to the point where a breaker trip on ground fault is no longer guaranteed. Modern armored cable has an integral, internal aluminum bond strip to maintain low resistance.
 
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Old 10-22-14, 06:26 PM
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To explain your simple question in detail, I do not think there is a short answer. I spend two months a year learning (or attempting to learn) the theory behind a lot of this stuff
I will try to explain this simply, let me know if this helps you to understand why the meter on the bond does not trip the breaker...

Your neutral and bond are both a path back to ground. They are actually connected at your panel.
Your neutral is a path back to ground for normal operation of devices.
Your bond is a path back to ground for faults, like when your hot touches the side of a box.
Electrical devices offer resistance to the flow of current. Things like light bulbs have high enough resistance to prevent a short, but low enough to allow current to flow and operate the bulb.
When you touch the hot to the box which has the bond connected to it, there is almost no resistance to the flow of current, so the current will reach 15A+ and trip the breaker.
When you apply meter probes to hot and bond, the meter itself has a huge level of resistance, as explained by EEsparks. Also as he explained, this does not allow for significant current flow and therefore does not trip the breaker.
 
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Old 10-22-14, 07:45 PM
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Mr.A I kind of skimmed over the replies when I replied to answer the OP's latest post at the time, but the link you posted regarding measurements was really good and illustrated a few of my points.
 
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Old 10-23-14, 07:52 AM
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If a hot wire ever touches ground, it trips.
If the hot wire ever touches the grounded neutral conductor the breaker will also trip.
 
 

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