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Swimmer
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I was troubleshooting a short. I put one DMM lead on the wire terminal of the breaker which was turned off. I put the other lead on the neutral / ground bus of the service panel. I measured 10K ohms. I turned on the breaker and it tripped immediately as is typical of a short.

I found a completely burned outlet, fixed it by moving it up about 5" to get fresh cable, and repeated the ohm measurement by shorting BLK and WHT wires at the outlet. The result was less than 1 ohm as expected.

My conclusion is that the charred remains of the outlet conducted current as if it were a dead short if 120V was applied but did not conduct much current at what ever voltage the DMM uses to push current down the wire.

I don't want to troubleshoot shorts by switching on the breaker. Does anyone have any insights into this?
 

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I agree with the first two posts. I learned as you did: the 9v battery in my Fluke doesn't have enough "ummpphh" to detect such a short. Even the rinky-dink off-shore megger I bought thru Amazon is a better tool for this type of diagnosis.
 

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Install, troubleshoot, maintain, and upgrade electrical systems, plant utilities, PLC's, mechanical
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The carbon on the outlet is conductive and may contain trace metal from the first short circuit.

As stated above it will take a higher test voltage than what a simple VOM meter will produce.

My old Extech megger will output 1000V from 9 volts of batteries.

Even at 1000V it didn't detect a short in a 700 horsepower motor.

Got to have the right meter for the job.
 

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Hackenschmidt
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I was troubleshooting a short. I put one DMM lead on the wire terminal of the breaker which was turned off. I put the other lead on the neutral / ground bus of the service panel. I measured 10K ohms. I turned on the breaker and it tripped immediately as is typical of a short.

I found a completely burned outlet, fixed it by moving it up about 5" to get fresh cable, and repeated the ohm measurement by shorting BLK and WHT wires at the outlet. The result was less than 1 ohm as expected.

My conclusion is that the charred remains of the outlet conducted current as if it were a dead short if 120V was applied but did not conduct much current at what ever voltage the DMM uses to push current down the wire.

I don't want to troubleshoot shorts by switching on the breaker. Does anyone have any insights into this?
This in a nutshell is why you megger rather than just test resistance, the megger generates the voltage necessary to get a result more consistent with actual utilization conditions.

IMO it's best to have a megger that you can start low, like 50V, see if you missed any loads on the circuit, then work up to normal voltage, then 2x normal system voltage if you really want to be thorough.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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What does it measure now that you moved the outlet?

Are you saying you eliminated the issue (theoretically) but dont want to switch the power back on?
 

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Chief Flunky
Field Service Engineer
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I was troubleshooting a short. I put one DMM lead on the wire terminal of the breaker which was turned off. I put the other lead on the neutral / ground bus of the service panel. I measured 10K ohms. I turned on the breaker and it tripped immediately as is typical of a short.

I found a completely burned outlet, fixed it by moving it up about 5" to get fresh cable, and repeated the ohm measurement by shorting BLK and WHT wires at the outlet. The result was less than 1 ohm as expected.

My conclusion is that the charred remains of the outlet conducted current as if it were a dead short if 120V was applied but did not conduct much current at what ever voltage the DMM uses to push current down the wire.

I don't want to troubleshoot shorts by switching on the breaker. Does anyone have any insights into this?
Aside from the Megger suggestion if I follow this you measured effectively from hot to neutral and got something other than an open circuit with no load?? There is your result.

What probably happened is that carbon black is a semiconductor or you might have a paper thin gap. Either way that forms a capacitor and blocks DC so predictably you measured open circuit or almost open. Capacitors pass AC so of course that passes through. If you have a capacitance measurement on your meter that can be revealing. But in addition house wiring is mostly rated either 300 or 600 V. It is factory proof tested at either 1600 or 2200 V so as long as no devices are connected, crank up the Megger to 1000 V and see if it just arcs right through or not. Klein sells a Megger for $150-200.
 

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Hackenschmidt
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The issue I see with the 1000v Megger is that you better know everything that is connected to that circuit before using it and that is almost never the case in these situations
I was shopping around online and saw some decent meggers at good prices but they didn't have the lower voltages, I don't think I'd buy one that didn't let you start at about 120V so you can start low.
 

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I was shopping around online and saw some decent meggers at good prices but they didn't have the lower voltages, I don't think I'd buy one that didn't let you start at about 120V so you can start low.
I agree, mainly because of the point @shocksystems made.
 

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I was shopping around online and saw some decent meggers at good prices but they didn't have the lower voltages, I don't think I'd buy one that didn't let you start at about 120V so you can start low.
How do you properly test 300V wiring at 120V?
 

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Didn't know that but yes, your NM-B is rated for 600. Huh.
 

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Chief Flunky
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You don’t START at 1000 V. That’s what I do with a 4160 or 2300 V motor…

The standard continuous ratings are 300 or 600 V. But the reality is that the voltage isn’t that clean. Every time a motor starts or stops or a breaker trips for instance you get surges. I can guarantee you anything sensitive to even several hundred Volts would quickly be destroyed. In addition when we say “120 VAC” we mean RMS. Peak voltage is 1.732 times the RMS value. So your 600 VAC rating is closer to 1000 VDC. Not saying that AC and DC ratings are like this, just pointing out the real world when it comes to testing. As a result CSA and UL require them to be hipot tested to twice the nominal AC rating plus 1000 V. So that would be 1600 or 2200 V. Don’t worry about hipot. It’s just a high power destructive version of the Megger test. So at 1000 V unless it is defective you are well within its ratings. If it’s defective you can’t make it worse. Your receptacles are perfectly safe. Your typical surge arrester power strips have either a 250 or 150 V surge arrester in them. If I set it to say 500 V and test with a power strip the output voltage will hover right at the surge arrester “knee”. The Megger only puts out about 1 mA or less so when you have any load at all it can’t keep up and the output voltage will collapse. A Megger is sensitive, not powerful. Think about it. If I use 1000 V and measure 10 Megaohms what is the current? About 100 nanoamps. That cheap Klein goes down to about 1 nanoamp. The “bigger” ones just have more power to push to bigger (1000 HP+) motors where charging takes about 30 seconds but this is residential, not testing 5000 HP motors for a gas plant or pipeline pump. So you are in no danger of doing damage or making things worse.

There are two “hazards” with Megger. The first is a lot of equipment cones with warnings not to Megger it. I have never seen an actual case of equipment damaged by a Megger but just like the surge arresters any “semiconductor” is still somewhat conductive so it will mess with your results. These must be disconnected. So these days that means lifting the wire off the nuisance trip device (NTD aka AFCI in some circles) in the distribution panel. Normal breakers are fine. All lightning arresters and capacitors also need to be disconnected from the circuit to be tested.

The second hazard is that although the current is nonlethal, it has been proven to cause spontaneous cussing if applied through your fingers or brushing your arm up against anything under test, use the “clip” on the ground and practice the one hand only rule.

The problem with Megger in residential is the poor way builders do receptacle layouts. I have seen huge 4000 square foot houses with every light fixture and receptacle on one of 3 circuits (for the 3 load bearing walls). Panel schedules always just say “lights and recess” if they were ever filled out. Best practice is to just unplug everything before doing a Megger test. That is probably one of your first TS steps anyway so it should already be done.

As far as test voltages the motor test for under 1000 V motors calls for 500 V whether its 120 V or 995 V. The voltage needs to be high enough to overcome the motor capacitance but that’s all. IEEE 400 isn’t very specific but talks about using 1.7 x V + 1000 where V is the equipment rating not nominal voltage. The 1.7 factor is just under the factory test which is 2 as a maintenance test. NETA is often quoted and says 500 V on 300 V ratings and 1000 V on 600 V ratings. Here is why I don’t recommend NETA. NETA stands for the National Electrical Testing Association. This is an organization of companies that promote and do electrical testing and nothing else. Many run all kinds of useless tests which aren’t based on any actual scientific logic and deliver all kinds of crazy inane results and claims to customers that equipment service shops have to deal with. NETA actually recommends disconnecting all power cables and hi potting it every few years. Aside from the proven problems that DC hipot damages service aged cables while failing to detect anything that all other standard tests do not already detect, the fact is you are more likely to cause more problems with the work itself such as from galled or loose connections or accidentally swapping cables. Not making this up, it’s in their standard!

So I’m comfortable using anything between 100 and 1000 V on house wiring. But 3-5 V is way too little voltage to overcome insulation faults.
 

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The second hazard is that although the current is nonlethal, it has been proven to cause spontaneous cussing if applied through your fingers or brushing your arm up against anything under test, use the “clip” on the ground and practice the one hand only rule.
LMAOOOOO
:ROFLMAO: :ROFLMAO: :ROFLMAO:
 

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Hackenschmidt
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How do you properly test 300V wiring at 120V?
I meant that when shopping they all had the higher voltages, but fewer and more expensive had the lower voltages.

My impression is for this kind of troubleshooting I'd test at 120 or lower, then step up to twice the system voltage, for 240v systems, 500V, for 480V systems, 1000V.
 

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Electrical Contractor
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I was shopping around online and saw some decent meggers at good prices but they didn't have the lower voltages, I don't think I'd buy one that didn't let you start at about 120V so you can start low.
Could get an older hand crank and slowly ramp up the rpm
 

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Could get an older hand crank and slowly ramp up the rpm
I think I got every bozo a second time when the battery operated units became popular.

"What, there's no crank, it's just a resistance tester!"

Yeah, I'm one of those azzholes. :p
 
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Ahem.... cough......... supco500..............bout a hundred. I pull it out like twice a week at least. I mean the supco500....................
 
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