Noise in Test Equipment

Sounds like a ground loop problem.

Good luck. Diagnosing and solving a noise problem can require a skilled engineer/technician to be looking at your setup.

Thanks. U suppose there is a poor grounding on the mains.. not at the test instrument setup? Because when this same test setup is done on the other line, it works fine.! Is it?

No, not a bad ground but a ground loop. This is a different problem. Then again it could be a bad ground, too.

Yes,

Just check Volts between Ground & Neutral terminals of faulty line. should be less then 1.0 Volts.

Check if Phase and Neutral are reversed in the problematic line's Power Supply. This could give a marginal difference (depending on the EMI filter's characteristics of a switching power supply) that is not audible but measurable. S.M.

moving away from the equipment itself..

I would power off as much lighting and other electrical loads as you can while doing a test on the problem side..

Even if it's not on the same circuit a little florescent lamp for example can introduce a surprising amount of noise onto nearby electrical circuits.

When I was doing a lot of work with power line carrier devices I would connect an oscilloscope to the power line at various points around the home or business to look at the 60Hz waveform. I could actually see the signal I was looking for induced onto the waveform. I was also looking for noise in the system that would hinder operation.

At the time, I was usually running a bunch of A/V etc. at the same time so filtering and such was a big priority. The o-scope would clearly show both sides of a single AC cycle. This would usually tell told the whole story.

You can directly plug a scope directly to AC. (some hand held meters have a big screen and a scope function, but not the resolution of a regular scope ...unless you wan to spend huge money)... There are a few simple bits you need in order to plug an o-scope into a wall socket, but when you consider that you can find a simple 20MHz scope for about 200 bucks these days?.. It might be worth you looking into considering your industry.. I'm sure you could google up the bits in seconds..

If you don't already look at your power line.. It's a great 1st place to start looking for noise and wiring issues.. Plus it will give you peace of mind in a couple of seconds..

In the short term or long term.. If you have decent UPS from a computer. Try using that as the power source at the problem location. Some of these have a good power filer section.

...more stuff to think about and rule out..

Since you have noticed a distinct difference between two loads in your facility, you could investigate the differences between the two. Break out the drawings and determine if they fed from the same feeder, different branches, etc., on down the line. If you can, isolate one line (power it down) while testing the other. Ensure everything that is intentionally grounded still has a good connection, mechanically and electrically.

A handheld power quality analyzer such as the Fluke 43 is a one-stop shopping method for most technicians to determine where your noise is coming from. Monitor both production lines over a 24-48 hour period of normal operations in your facility.

As mentioned by another poster; does the problem begin when workers arrive and the lights are turned on or does it happen when the coffee makers start up because they happen to be on the same branch? Microwaves? Refrigerators? Line motors? PC control center? Television in the lounge? Any number/combination of items can be the root cause.

Using the scope is a good method if you can get one. A PQ analyzer has a scope function built in. 3-phase PQ analyzers can be had for not much more money.

An in depth analysis can usually be performed by a master electrician or engineer-for-hire.

Personally, I only plug my test equipment into the same power source as the gear under test. This should eliminate any ground loops if you are wired correctly, or help you isolate them to a specific location if not.

I am assuming you are already averaging the signal being measured with the scope(s). However, you probably are not triggering the scope(s) from any syncronous signal. Try triggering off of the test signal when using a pure tone, and averaging in the time domain. This will reduce the level of any non-syncronous noise, including the power supply components (make sure your test signal is non-synchronous with the power line). The signal-to-noise ratio would be the figure of merit for the go-no/go value.

Excellent suggestion. A spectrum analyzer using han averaging may also help weed out any distracting artifacts.

Exactly right. Most scopes have built-in FFT magnitude, which would do the job fine after averaging the data in the time domain (cheaper and simpler than adding an SA).