Troubleshooting questions
- Thread starter zappy
- Start date
- Status
- Location
- Simi Valley, CA
Is it a MWBC? You could have be reading the unbalanced load. I don't always trust those plug in testers, sometimes they say something, but it's really something else. Like it will tell you that you have an open neutral, but it's really an open gournd.
Thank you. So between a $8 plug tester or a $200 ampprobe I think I'll trust the ampprobe.
K8MHZ
Senior Member
- Occupation
- Electrician
No matter the price of the tester you need to know the impedance as well.
There are many instances in which a DVOM can read 50 volts where a solenoid tester would read almost zero. An analog meter would read somewhere in between and an LED tester may or may not indicate.
- Location
- Simi Valley, CA
I agree, I wouldn't be to quick to condem the $8 tester, but I also wouldn't spend all of my time looking for something that may not be the problem.
K8MHZ
Senior Member
- Occupation
- Electrician
I agree, I wouldn't be to quick to condem the $8 tester, but I also wouldn't spend all of my time looking for something that may not be the problem.
It's very important for a troubleshooter to know the difference between voltage readings with or without a load.
It's also very important to know how your test equipment works. I literally know mine inside and out.
Have you ever taken a crank megger apart? They are fascinating.
I also build much of my own test equipment and I own some odd pieces as well. How many of you have a mhometer or a Geiger counter in your mess....I mean mass of gear?
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Zap, this is a case where I'd try a voltmeter in parallel with, say, a 100w bulb. Measure again, and between all three pairs.
I'm not sure what kind of meter you used. Is this a single circuit? Check at the panel end to see if the neutral is shared.
I'm not sure what kind of meter you used. Is this a single circuit? Check at the panel end to see if the neutral is shared.
K8MHZ
Senior Member
- Occupation
- Electrician
Wouldn't it be nice to have a solenoid tester with a digital readout?
They may exist, but I haven't seen one.
mivey
Senior Member
- Location
- Simi Valley, CA
It's very important for a troubleshooter to know the difference between voltage readings with or without a load.
It's also very important to know how your test equipment works. I literally know mine inside and out.
Have you ever taken a crank megger apart? They are fascinating.
I also build much of my own test equipment and I own some odd pieces as well. How many of you have a mhometer or a Geiger counter in your mess....I mean mass of gear?
Well I don't know if I'm proud of the fact or afraid to admit it, but I never owned more than a Wiggy with a built in continuity tester, an amprobe, and a plug in tester, and I never walked away from a service call where I didn't solve the problem.:grin:
I think a lot of service guys, who should know better, just go in and start throwing soulutions at a problem that they really don't understand. I've always said that it was like tracking an animal , you've got to understand the signs. Then once you have seen a problem you have to make a real effort to remember everything that went along with it. The next time you see it you should have a pretty good sense of what and maybe even where you should look. I probably learned more doing service calls than I ever did roping houses. Nothing more satisfing than going in and finding right away a problem that two or three other guys already looked at.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
Mivey,
my Fluke T+Pro seems to read ghost voltage. I still have to bust out the Knopp sometimes. That's my only complaint, other than that I love it.
my Fluke T+Pro seems to read ghost voltage. I still have to bust out the Knopp sometimes. That's my only complaint, other than that I love it.
kwired
Electron manager
- Location
- NE Nebraska
are you by chance testing either a GFCI or GFCI protected receptacle?
don't know why but I recently had pretty much exactly what you had (open neutral and around 14 volts on an exterior receptacle. found garage receptacles with same readings the GFCI receptacle protecting them had proper line voltage and would not trip with test button. Replaced GFCI and all was fine.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
100209-2012 EST
zappy:
I believe I mentioned to you before that you need to find some good reference material, books, on electrical circuit theory, and then study and play with different circuits.
Your current problem. String a wire from a screwdriver in the earth outside. From this reference measure the voltage to various places with a high impedance meter. To hot lines anywhere your voltage reading should be within a few volts (on heavily loaded circuits maybe not more than 5 V) difference from the line to neutral voltage in the main panel. Same on any neutral. Light loads much closer to 0 V difference. On EGC the voltage to outside earth should not be very dependent on loads and probably should be less than 1 V as a rough criteria. Also voltage from the earth probe to main panel neutral-EGC point should be a fraction of a volt, if not investigate problems in this area.
Suppose you get an EGC reading of 14 V or maybe 40 or 60 V, then likely the EGC is not connected to neutral at the panel and to earth. Put some load across your meter terminals, maybe the referenced 100 W bulb by Larry (about 10 ohms with little voltage across the bulb) .With this load and with a floating EGC the voltage should drop to near 0.
Now suppose the EGC is good. Based on not reading a phantom voltage. Then use your ohmmeter to measure resistance from the EGC to neutral at the main panel. If near 0 ohms, then connect a test 100 W bulb from a hot line somewhere to your EGC and measure the voltage of the EGC back to your earth reference. If this voltage is low, probably a fraction of a volt, then you have a good preliminary indication of a good EGC.
If you know the EGC is good and you read 14 V to neutral, then there is a neutral problem. This problem voltage should vary substantially with loading. Now you search for the problem.
.
zappy:
I believe I mentioned to you before that you need to find some good reference material, books, on electrical circuit theory, and then study and play with different circuits.
Your current problem. String a wire from a screwdriver in the earth outside. From this reference measure the voltage to various places with a high impedance meter. To hot lines anywhere your voltage reading should be within a few volts (on heavily loaded circuits maybe not more than 5 V) difference from the line to neutral voltage in the main panel. Same on any neutral. Light loads much closer to 0 V difference. On EGC the voltage to outside earth should not be very dependent on loads and probably should be less than 1 V as a rough criteria. Also voltage from the earth probe to main panel neutral-EGC point should be a fraction of a volt, if not investigate problems in this area.
Suppose you get an EGC reading of 14 V or maybe 40 or 60 V, then likely the EGC is not connected to neutral at the panel and to earth. Put some load across your meter terminals, maybe the referenced 100 W bulb by Larry (about 10 ohms with little voltage across the bulb) .With this load and with a floating EGC the voltage should drop to near 0.
Now suppose the EGC is good. Based on not reading a phantom voltage. Then use your ohmmeter to measure resistance from the EGC to neutral at the main panel. If near 0 ohms, then connect a test 100 W bulb from a hot line somewhere to your EGC and measure the voltage of the EGC back to your earth reference. If this voltage is low, probably a fraction of a volt, then you have a good preliminary indication of a good EGC.
If you know the EGC is good and you read 14 V to neutral, then there is a neutral problem. This problem voltage should vary substantially with loading. Now you search for the problem.
.
Sounds like you really know your stuff. I try and read what your saying, But you lose me. I guess I'm just a hand-on person. I would need to see this performed to really understand. I figured out the problem at my customer's house. I noticed the lite on my plug tester was dim on some outlets, and some it was normal. So I used my "wiggy" on the outlets in question, they had no ground. Checked the panel and thought I found the problem. There was 2 ground wires under one lug, and one just pulled right out. That wasn't it. So I figured the room right above the sub-panel in the garage was probably where the homerun came in. Opened up the switch box and what a mess. Long story short, loose ground, shotty work, by from what the homeowner told me was a licensed electrician. I couldn't believe it. I told him, I'll write down what I found, and you can send him the bill! He didn't want to bother. I just want to say thanks for everyone's advice! Even if it goes over my head!
Last edited:
mivey
Senior Member
He probably did not want to bill himself. :grin:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
100210-0814 EST
Zappy:
I am hoping to give you some concepts that can assist your troublehooting.
I differ from others in that I suggest using a high impedance high resolution voltmeter, and when necessary adding in parallel a much lower impedance for some tests. This may be little more awkward than just using a Wiggy, but with an accurate high resolution meter I can get information that you can not get with a Wiggy.
Small voltage drop measurements can be a very important troubleshooting tool. For normal 120 to 240 volt circuits this means resolving 0.1 V (100 MV) when on a range that reads 240 V. A #12 copper wire has a 20 deg C resistance of about 1.6 ohms/1000 ft. When a wire is not heated from a heavy load, then the 20 deg C (68 deg F) is a valid temperature to use. So 1 foot of #12 is 0.0016 ohms. A 10 A current thru this 1 ft length produces 16 MV drop, and 10 ft 160 MV. A typical DVM (digital voltmeter) that can resolve 0.1 V at 240 will also have a millivolt range that can resolve 0.1 MV.
Let's start at a center tapped pole transformer. If we can get a meter lead connected to that center tap point we can get a lot of troubleshooting information. How can we do this? This center tap will always be connected to a ground rod at the transformer. If we assume that the connections from the center tap to the ground rod are good and very little current flows thru this ground rod, then a connection of our meter lead to the wire to the ground rod is very close to being directly on the center tap point.
Using similar assumptions the neutral wire from the transformer to the main panel can also be considered to be equivalent to the transformer center tap if no current is flowing in the neutral. Or if current is flowing in the neutral, then the assumption is modified by the phase and voltage drop along the neutral.
For the present I will stop here and ask if this is clear to you?
.
Zappy:
I am hoping to give you some concepts that can assist your troublehooting.
I differ from others in that I suggest using a high impedance high resolution voltmeter, and when necessary adding in parallel a much lower impedance for some tests. This may be little more awkward than just using a Wiggy, but with an accurate high resolution meter I can get information that you can not get with a Wiggy.
Small voltage drop measurements can be a very important troubleshooting tool. For normal 120 to 240 volt circuits this means resolving 0.1 V (100 MV) when on a range that reads 240 V. A #12 copper wire has a 20 deg C resistance of about 1.6 ohms/1000 ft. When a wire is not heated from a heavy load, then the 20 deg C (68 deg F) is a valid temperature to use. So 1 foot of #12 is 0.0016 ohms. A 10 A current thru this 1 ft length produces 16 MV drop, and 10 ft 160 MV. A typical DVM (digital voltmeter) that can resolve 0.1 V at 240 will also have a millivolt range that can resolve 0.1 MV.
Let's start at a center tapped pole transformer. If we can get a meter lead connected to that center tap point we can get a lot of troubleshooting information. How can we do this? This center tap will always be connected to a ground rod at the transformer. If we assume that the connections from the center tap to the ground rod are good and very little current flows thru this ground rod, then a connection of our meter lead to the wire to the ground rod is very close to being directly on the center tap point.
Using similar assumptions the neutral wire from the transformer to the main panel can also be considered to be equivalent to the transformer center tap if no current is flowing in the neutral. Or if current is flowing in the neutral, then the assumption is modified by the phase and voltage drop along the neutral.
For the present I will stop here and ask if this is clear to you?
.
Good one. I didn't think of that.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
Your most important troubleshooting tool is between your ears.
Just do not connect directly across a live circuit to play "radio". :grin:
Just do not connect directly across a live circuit to play "radio". :grin:
Strahan
Senior Member
- Location
- Watsontown, PA
Can't remember all the details from my problem (it is on a post here somewhere) but it was very similar to yours. Found the problem to be a broken EGC from my receptacle in question to the panel. Not easy to find and involved cutting drywall. If you can and it has probably been mentioned string a run of romex from the panel to the recep in question and take readings.
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