how to measure nuetral

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hilltop

Member
Hoe does one measure millivolt spikes on a nuetral??

I have sensitive equipment that is seeing consistant millivot spikes [recorded by instrument mapping] which occurs approx. every 2 minutes.

How do I test my service nuetral to see if it is generated at the source or the load???
 

Smart $

Esteemed Member
Location
Ohio
Hoe does one measure millivolt spikes on a nuetral??

I have sensitive equipment that is seeing consistant millivot spikes [recorded by instrument mapping] which occurs approx. every 2 minutes.

How do I test my service nuetral to see if it is generated at the source or the load???
Measure with equipment on and with equipment off (i.e. completely disconnected), for starters. If the spikes are generated by the equipment, they will obviously not be present when equipment is completely disconnected.

PS: Millivolt level fluctuations can easily be generated by load equipment simply by passing varying amounts of current through the neutral conductor.
 
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charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
At what voltage level does your equipment operate? From the perspective of the service, a millivolt is nothing, and second-by-second variations on the order of volts are also nothing.
 

mivey

Senior Member
Compare the voltage and current waveform. If the voltage goes down and the current goes up, the problem is downstream from your meter. If they move in the same direction, the disturbance is upstream from your meter.
 

gar

Senior Member
100206-0914 EST

hilltop:

You need to provide far more information for anyone to provide useful information.

What is the sensitive equipment? What does it measure? How is it connected to whatever it is that you are measuring? Is this equipment powered from the AC line with your said neutral, or is it battery powered and you are trying measure something about your AC line?

Suppose you have an AC powered straingage instrument. Maybe a force gage. If you remove the cable to the loadcell and make an input connector with a short across the signal input terminals and plug this in, then are the noise spikes still present? If so, then if you use a car battery and an inverter to power your instrument do you get the noise spikes? You need to do some isolation of different components to try to determine the noise source.

If I connect a probe, in this case a 10x on 5 MV scale, to my scope input and pick the most sensitive range, then with the probe connected to nothing I see 60 Hz and some high frequency stuff.

Probe near an 8 ft Slimline and I get 60 Hz, noise spikes synced to the line, some higher frequency oscillation somewhat correlated to the 60 Hz, and some general uncorrelated high frequency noise (sometimes it is an AM broadcast station).

Next connecting the short ground lead on the probe to the probe tip and little noise is observed. Note, with a 10x probe and 5 MV/CM my scale is really 50 MV/CM. Using an external ground wire from the scope chassis to the probe tip and the noise reduction is not as great as with the short ground wire.

.
 

brian john

Senior Member
Location
Leesburg, VA
Hoe does one measure millivolt spikes on a nuetral??

I have sensitive equipment that is seeing consistant millivot spikes [recorded by instrument mapping] which occurs approx. every 2 minutes.

How do I test my service nuetral to see if it is generated at the source or the load???

My first question is why do you feel this is necessary?

Are there equipment operational issues?

Do you feel this is a consistent issue or every so often.

Check Fluke Hyoki, and Dranetz web site's
 

hilltop

Member
This is an 120V HPLC. Tjhe panel source is 120/208 served by a 20KW UPS. The load on the UPS is 74%.
They use the mapping of millivolts to measure instrument data..the unusal spikes they are seeing are causing them not only to loose data, or just make it too hard too read but apparently the equipment itself sometimes shuts down.

The HPLC is plugged in to an ordinary 120v duplex receptacle..actually they are using power strips for each unit as each component plugs in individually. We have tried isolating pumps on the equipment with no change in spikes.
We have removed any other source from the panel that may generate line noise.
We have many different services in the building...they had someone run a test on a like instrument which would be served by a different feed and the test results were fine.
Some of the instruments even have small line conditioners installed between the HPLC and the power strip....and still seeing the same millivoltage spikes. If you look at the equipment data the spikes are continous and occurr between every 2 to 3 minutes

I am reasonably sure we have a nuetral or ground issue as this area is served by a service that was installed in the late '50s to early '60's.
My question is how do I test for this small of a spike....is there a specific fluke model someone can direct me too?

Thank you!
 

gar

Senior Member
100208-0956 EST

What is an HPLC and what measurements does it make?

HP might mean Hewlett-Packard, LC might mean Inductance-Capacitance something.

They use the mapping of millivolts to measure instrument data..the unusual spikes they are seeing are causing them not only to loose data, or just make it too hard too read but apparently the equipment itself sometimes shuts down.
Is this a recording millivoltmeter of some sort. What instruments? Where are the instruments? How are the instruments connected? What impedance levels? What kind of input wiring from the instruments to the HPLC? Are there balanced signal lines or is the ground path part of the signal circuit?

.
 

wptski

Senior Member
Location
Warren, MI
This a "Dips and Sweels" example of the VR1710 logging but a somewhat poor one. It shows L-N and N-G voltages down to mv. All the tabs are for the VR1710 except the "Voltage and Amps" which is for some other product that uses the same Power Log software.

Test_8.jpg
 

gar

Senior Member
100208-1550 EST

hilltop:

Returning to your original post.
Hoe does one measure millivolt spikes on a nuetral??
Measure relative to what? A ground rod somewhere? The main water pipe? The chassis of you device that is being bothered? The voltage drop along the neutral from the main panel to the instrument?

Why do you care about millivolt spikes on the neutral?

I have sensitive equipment that is seeing consistant millivot spikes [recorded by instrument mapping] which occurs approx. every 2 minutes.
Why would millivolt spikes on the neutral have anything to do with what your equipment sees?

How do I test my service nuetral to see if it is generated at the source or the load???
Again the question arises --- what is the reference point to use for the measurement?

Consider this: the power company has a transformer that supplies your main panel. There is a neutral wire from the transformer to the main panel. This wire was connected to earth at the transformer. It is also connected to earth at the main panel.

If no current flows in the neutral wire there is no voltage drop along the neutral wire. If the neutral wire resistance is 0.01 ohm and there is an on-off cycling of 100 A thru the neutral, then there will be an on-off voltage drop along the neutral of 0 to 1 volt.

If I run a test probe wire from the transformer neutral to a recording instrument at the main panel and the other signal input to the recorder to the neutral at the main panel, then I will see a 0 to 1 V changing signal.

If instead I put the reference lead on the neutral at the main panel and the signal input on the neutral at the panel. then I see 0 volts change even though there is a 1 volt change between the transformer and the main panel.

You have to define what you want to look at and for what reason.

--- to see if it is generated at the source or the load???
You need to think about this part of your question and try to define what you really want to know.

.
 
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