@stopcrazypp just got a Bryant 9450FR today and plan on swapping it this weekend. Upon close inspection of it, noticed that only the ground terminal has contact point #3. The two hots and the neutral terminals don't have contact point #3, which is fine b/c it seems only the ground wire may need to be clamped between 2 & 3. Do you know it's only the ground that has #3? TIA!
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Mobile connector flashes 2x - “ground loss”
- Thread starter dohmahmigh
- Start date
stopcrazypp
Well-Known Member
Nope had no idea #3 even existed when I installed mine. I installed mine a long while ago and had no issues installing it using the method it was designed to (clamping all of the wires between 1&2). I only looked up and made that picture to try to help that other person diagnose their installation (and they were the one that came up with that idea). I should note in the end they didn't keep the outlet in that configuration (but rather switched to a different one), given that isn't how you are supposed to install it.
SageBrush
REJECT Fascism
If so, then measure resistance rather than just a continuity tone. It is a wild-west out there for the threshold of resistance that causes a tone. I learned this the hard way when I was trying to troubleshoot CFLs
They id that too (resistance and tone)...
Though I'm wary of resistance measurements on live circuits as the meter likely is applying or measuring voltage...
The potential between neutral and ground should be zero. From the meters point of view the circuit isn’t live. The fact that the neutral is carrying current doesn’t matter. If carrying current affects the reading, there is a problem. High resistance path for neutral to source or from egc bond to neutral.
For there to be potential(voltage that would affect the reading) between neutral and ground there would need to be resistance somewhere, which would also show up when you read the meter. It should not affect the reading, especially if the reading is zero resistance.
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The potential between ground and neutral at an outlet is only zero if both conductors have equal R*I. Since ground should have zero current, that means neutral must have zero current also. Not just on that branch circuit, but all the way back to the bonding point. Especially with a subpanel, local neutral will have an AC waveform due to branch circuit loads (unless they are all 240V)
The meter either applies a voltage and measures current or applies a current and measures voltage. Either will be impacted if there is a developed voltage in series. To see this effect, measure the resistance of a large capacitor for a while.
If it were really fancy, it could sweep (or step) the excitation to null any offset, but that would result in error due to capacitive or inductive elements.
If the resistance is zero between ground and zero (which it should be) the potential will be zero. Since zero * anything is zero.
Both conductors don’t have their own r*I because resistance is measured in between two points. Here, ground and neutral. So the r is between ground and neutral. Which again, should be zero.
So in review:
Potential between ground and neutral (v) = resistance between ground and neutral (r) * current flow between ground and neutral (I)
Since r is zero, v is zero. Always and forever. Maths.
Zero resistance wire?
If I put 10Amps through 40 feet of 2 AWG copper (main to subpanel) that creates a 62mV voltage potential.
You want to show me which dmm can measure ohms to the thousandths? Lol. For the purposes of this test and the test equipment we are using, there is zero potential.
Also, in this test the circuit is live, or not, but isn’t drawing any current since the tests are being done with the charger unplugged.
You can grasp at straws to keep yourself afloat if you like, but remember, they got holes on the end of them.
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The outlet is connected to a subpanel. The subpanel has a shared neutral to the service disconnect. If any circuits are drawing current through the neutral, there will be a potential between the neutral and ground paths.
Fluke 87 specifications from the manual:
For a low resistance measurement (400 ohm range), the injected current is 200 microAmps or 0.2 mA. With a 400 ohm resistance, that results in 80mV of sensed signal.
Now, could a meter low pass filter AC out of the measurement? Sure, but they don't because no one says to measure resistance on a live circuit.
Again, the Fluke 87 manual:
https://assets.fluke.com/manuals/87______umeng0800.pdf
As to a miiliOhm DMM, that would normally require a 4 lead setup so you're looking at more of a bench model. However, the Fluke 289 has milliOhm resolution.
400 ohms of resistance between neutral and ground would indicate a serious problem, even with significant current flow though the neutral. The neutral should be sized to adequately handle the current flow.
Your post equates to saying that for a dmm that anyone outside of someone with specialist equipment has, they will read zero ohms between neutral and ground in a correctly wired AC system. And even with specialist equipment they will only read micro ohms, and the potential will still be essentially zero?
Why didn’t you just say that? I did say that.
I wait for you defend including a $1000 meter in your home gamer tool set.
I really don't understand what you are going on about.
My post you originally replied to said that resistance should be checked with the power off. Which is what the references I listed also stated.
As further example of why, I showed that 10Amps on 40 feet of 2 AWG copper wire (such as might exist between a subpanel and the neutral ground bonding point at the service entrance) creates a voltage offset of 60mV or so. This (if rectified or DC) would be read (for example) as 300 Ohms on a Fluke 87 set to the 400 Ohm range (60:80 to 300:400) assuming good connections otherwise.
I included the model number of a milliohm meter because you brought it up. I'm really not sure why you did though. The main issues of live circuit resistance readings are either damage to the meter or inaccurate reading. Nothing to do with trying to measure the resistances that precisely.
If needed, one could measure resistance on a live circuit by using a much higher stimulus current than a DMM uses along with two voltage measurements (assuming everything else is steady state).
You said you had your doubts about the accuracy of a reading resistance between the ground and neutral with the power on. The point is that it will not materially change the result because there is no material current flow between ground and neutral. I = 0, therefore V=0.
You brought up how their is a minute ammount of resistance and therefore a minute ammount of current flow, which is true, but not relevant to this conversation either, because home-gamer meters are sensitive enough to be affected by milliohms of resistance or microvolts of voltage. The result of the will not change. There is no meaningful power between ground and neutral. I’m not sure what is hard to understand about that honestly.
Your example of 10 amps of current has no bearing on this test, because there isn’t and there should never be 10 amps of current flow between ground and neutral. There is 10 amps of current flow between neutral and hot, not neutral and ground.
That's not actually correct. N and G are bonded at the service disconnect and should not be bonded anywhere else (although they sometimes are by mistake). So obviously N and G have the same voltage at that bond point (it's a single point). Call that bond point the 0 voltage point.
Then if you check the voltage at any other point on G, it should be 0 volts, as there should be no current flowing on G.
But if you check the voltage on any other point on N, it will be non-zero if there is any current flowing on N between there and the bond point. Because the conductors aren't superconductors, so they have some resistance, and V = IR, so there is some voltage change along the length of the conductor.
So say you are measuring voltage N to G on a receptacle, and there's 50 feet of #12 AWG between that N receptacle terminal and our 0V reference point, and upstream of that receptacle loads are drawing 10A, and downstream of that receptacle the current stays 10A, no additional loads. #12 copper has a resistance of about 2 ohms/kft, so that 50 ft #12 neutral conductor has a resistance of 100 milliohms. With 10A of current on it, the N at the receptacle will be at 1V relative to our 0 point. So if you measure the voltage between N and G at that receptacle, you should get 1V on your DMM.
Now why is that relevant to resistance measurements? If the power is off, and you measure the resistance between N and G at that receptacle, you should get 200 milliohms (double because of the two conductors, N and G). That's high enough I bet many cheap DMM will read it, although I don't really know what typical DMM specs are.
So how does the DMM come up with 200 milliohms? Per Mongo's information, it might inject 0.2 mA DC between the probes, and measures the voltage difference, which will be 40 uV DC. Assuming it can distinguish that from 0V, it will report the imputed resistance.
And what happens if you try that with the power on and 10A AC flowing on the neutral conductor? Now the DMM will be seeing a 1V AC signal with a superimposed 40 uV DC offset. I don't know what typical DMMs will do in that situation. 1V (RMS) AC between the probes might be enough to damage the DMM. Or it may not be able to distinguish the 40 uV DC offset from the 1V AC signal and give the wrong answer. Or maybe it can filter out the 1V AC signal and just see the 40 uV DC offset, and still give the correct 200 milliohm reading (or 0 ohms if that's below the DMM measurement resolution).
So unless you can offer us some further insight about how a typical DMM works internally, and how it will react to seeing that 1V AC signal with 40 uV DC offset, I don't believe you are in a position to contradict the typical DMM manual's instructions not to measure resistance while the circuit is powered.
Cheers, Wayne
Yes. I know that. However, the voltage should be very very low. The conductors should be sized so that the voltage never exceeds a few hundred milivolts. From the meter that was being used perspective I= 0, r=0…
Yes, that is precisely my point. Home gamer meters will not read resistance with this precision. So for the test we are doing the result will not change whether power is on or off.
But in any case, we aren’t worried about miliohms of resistance between ground and neutral since the evse reporting the ground fault isn’t close to being that sensitive.
I didn’t say you should test resistance with the power on, I said for this test, there is no difference in the result.
You can see from my example that last part is not correct. 50 ft of #12 is not a long circuit, and 10A is a very plausible current, and that gives you 1V of voltage difference. Double the length (still plausible at 100 ft) and increase the current to 15A (e.g. space heater or EVSE), and now you're at 3V.
So voltage drops/rises in the single volt range are common. Design guidelines for voltage drop typically allow for a combine 5% VD for the feeder and branch circuit. That's 6V on a 120V. That's also the reason that, for example, a motor on a nominal 240V system will have a nameplate rating of 230V--the difference is an allowance for voltage drop.
Cheers, Wayne
The current is not from the outlet with nothing plugged in, it is from other circuits in the subpanel that share the neutral wire back to the service entrance. See posts #1 and #18.
While lower in magnitude, even with only a main panel with shared neutral and ground bars, the resistance of the bars themselves will have an offset depending what is connected where.
IF the measured circuit's ground and neutral are grouped at the end of a bar with no current flow past them, then they would be at zero relative potential relative to each other.
No one other than you is talking about [measuring] milliohm levels of measurement. Milliamps and millivolts do matter because that is what the meter is using to excite and measure the circuit.
1 mA * 1 ohm = 1 mV
A 20mV offset [due to unmeasured milliohm copper resistance times other load current] with 1 mA of injected current would be reported as 20 ohms.
As to grade of DMM, they all work similarly, but Fluke actually listed the test current.
At that point you should in increase the wire size for the circuit. You should keep voltage drop to less than 3 percent. At 120v to ground that is 3.6v since the meter will not be sensitive enough to detect the difference.
Again, none of this will affect the test that was conducted.
That is not why motors have a 230v name plate. The reason they have a 230v nameplate is so they can be used in 208 and 240v.
You stated that you thought the test could be affected by having the power on. It cannot.
Please explain how a meter will correctly determine resistance on a circuit if a portion of said circuit has a voltage offset.
As you state above, a shared neutral will have a voltage drop of some level:
Or is it:
?
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