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Posted By: Potseal Fluke 1735 and Neutral to Ground Voltages - 01/23/16 11:41 PM
I am currently involved in testing the power quality for a Hitachi TEM (transmission electron microscope). There have been intermittent issues for the past several months that have stumped the techs and as a result a request was made to test the quality of the supply power.

The only test I conducted thus far was a voltage test on the secondary of the Hammond step down transformer (210V:100V). Using a Fluke 1735 Power Logger we connected the meter for Single Phase + Neutral. The meter was left to record for one week. Once the time was up the meter was disconnected and the data was downloaded. There were 5 events recorded where the voltage dropped significantly for a duration of one cycle (17ms). After showing this information to the tech he was wondering if I could capture any signs of voltage on the ground conductor. He gave a brief explanation and it sounded fairly straight forward. Now, I am wondering how to set-up the Fluke Power Logger to test for this potential phenomena.

I started researching the subject of finding voltage on the ground. I am quite certain that he wants me to verify whether or not neutral to ground voltages exist. The Hammond transformer is obviously isolating the secondary from the building's power supply but the method of grounding is one that I am not entirely familiar with and possibly misunderstand. In this installation an isolated ground is brought from the building ground rod to the transformer secondary and connects to one phase. This is very different from the isolation transformers I've seen elsewhere and the one I use at home.

If anyone is familiar with the Fluke 1735 Power Logger and this form of isolation transformer's connections I would appreciate feedback regarding how to use the meter to record Neutral to Ground voltages on the secondary. I do not see any information regarding this that comes with the Fluke. I put together a diagram to give a visual idea of what I am dealing with. Thanks.

[Linked Image from i63.tinypic.com]
You should bond those electrodes together.
IG generally causes more problems than it fixes, particularly if someone thinks that means TWO different grounds. You only want one grounding system.
Greg, is it not possible that the two ground rods could be connected to the rebar in the basement floor making them common? I think it's called a Ufer ground.
I would be inclined to look for earth currents rather than voltages. Use a scope with a current probe and look out for third harmonics caused by switching power supplies in computers.
I do tend to agree that independent grounds can cause problems.
I'm not quite clear on your diagram as the "isolated ground strip" seems to be connected to the supply ground too (?). Earth loops can be bad news.
Posted By: twh Re: Fluke 1735 and Neutral to Ground Voltages - 01/24/16 03:58 PM
What kind of issues?

It has been unusually warm here but dryer winter air can cause static problems. I had a printer that would mess up when it got a shock and I put bonding on a hanging plastic proximity switch because it would quit working while the switch beside it, mounted in metal, never failed.
Typically they tell you to connect the IG to the same spot in the service disconnect enclosure where the MBJ lands.
For the NEC purposes a Ufer is a pretty good electrode but I would not count on 30' of that rusty rebar to give you a single point ground. There are at least 2 wire tied joints in there.
Originally Posted by geoff in UK
I would be inclined to look for earth currents rather than voltages. Use a scope with a current probe and look out for third harmonics caused by switching power supplies in computers.
I do tend to agree that independent grounds can cause problems.
I'm not quite clear on your diagram as the "isolated ground strip" seems to be connected to the supply ground too (?). Earth loops can be bad news.


Yes, the diagram isn't very clear. The solid green line is connected to the transformer's metal housing. This "bonding" conductor is common to all of the bonding conductors in the room. The green/orange line that comes in through the disconnect switch and carries on over to the transformer along with L1, L2, and bond is an isolated ground that goes all the way back to one of the ground rods in the basement. That same IG terminates at an isolated terminal strip inside the transformer. From the terminal strip a bonding conductor is tied to one side of the secondary creating a neutral. From that same isolated terminal strip another IG carries on to the equipment ground for the TEM.

I really need to go over this whole set-up with the tech because I would like to verify that it is what it should be. Nonetheless, my weekend research found this concerning the transformer:

Quote
The most effective tool for control of neutral to ground and common mode disturbances remains the isolation transformer (Figure 4). Isolation transformers allow the bonding of neutral to ground on the transformer secondary. Disturbance current flow now occurs across the impedance of the bonding strap.


[Linked Image from i66.tinypic.com]

Quote
1999 - POWERVAR, Inc.
Originally Posted by twh
What kind of issues?

It has been unusually warm here but dryer winter air can cause static problems. I had a printer that would mess up when it got a shock and I put bonding on a hanging plastic proximity switch because it would quit working while the switch beside it, mounted in metal, never failed.


Well, as always I'm simply the guy checking the power and all other information comes in droplets at a time. Still, I am always asking questions and I found out that the scope has been having imaging issues and start-up issues since Jan. 2015. All the experts have been brought in and nobody can find anything. Right now the TEM is working properly which really makes it difficult. Since I am supposed to be disproving/proving that there may be voltage on the ground conductor of the transformer secondary I really have my work cut out. Somehow I need to record this potential event when it happens.
Originally Posted by gfretwell
Typically they tell you to connect the IG to the same spot in the service disconnect enclosure where the MBJ lands.
For the NEC purposes a Ufer is a pretty good electrode but I would not count on 30' of that rusty rebar to give you a single point ground. There are at least 2 snap tied joints in there.


Do you think a simple resistance test between the two electrodes would be sufficient to verify their quality of being a common point?
I would think so if it came back at a small fraction of an ohm
As a trouble shooting test, I would jumper out that IG right at the place where the IG lands on the equipment end (connected to the EGC coming in with the supply) basically just eliminating the "isolated" part. IBM figured out in the early 80s that IG can cause more problems than it fixes, particularly if it is not connected right to the MBJ.
We ended up removing it from our recommendations.
Thanks for the feedback. I will post back if something comes of this situation.
Update:

This has been a journey. One thing led to another and I'm currently trying to map out the all the grounding for these buildings...in my spare time.

As for the Electron Microscope? It hasn't had any issues as of late. Nonetheless, I did manage to collect some data and send it to the tech from Hitachi. I finally acquired a Fluke Scopemeter which I could use to measure the Neutral to Ground voltage. Despite being able to record this information the original software was no where to be found. At least $1200 worth of equipment and not only was the software missing but some other attachments including the cable for connecting to a computer. Very irritating. I have contacted Fluke to see if I can download a copy of the software from their site. Hopefully I'll have this meter ready to go the next time an issue pops up.
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