Excessive grounding

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john37

Senior Member
I have this project where a CT simulator just got installed. They tested the power to the room and discovered excessive ground current going to the CT equipment. Could anyone suggest a solution and how to figure out where the excessive ground current is coming from?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100706-1408 EST

john37:

I have no idea what you are talking about.

What is a "CT Simulator". Is this something with one or more current transformers? Or is it something that simulates current transformers. Are there actually current transformers in the thing, or some sort of current transducer. A resistor is a current transducer, so is a compass, or a Hall device, etc.

What is this room?

Does power to this room come thru a single entry point, such as a load center cabinet.

How is this simulator thing connected to whatever is being measured in this room?

We need more precise details.

.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
A CT simulator is basically a CT Scanner

http://en.wikipedia.org/wiki/X-ray_computed_tomography

that includes a simulator for planning radiation treatment for cancer.

For the most part, I think we can basically consider it a fancy CT scanner (at least for this question).

But I'm still not sure I understand. A CT scanner is usually works like an Xray machine, and has similar room shielding - lead lined walls provide the radiation shielding around the CT room.

But I've only seen references to "Excesssive leakage current" in MRI rooms where the room shielding includes an isolated metal screen around the room.

Steve
 

haskindm

Senior Member
Location
Maryland
Also have many questions, but the usual suspect for "excessive ground currents" would be improper ground/neutral connections. Check for sub-panels with grounds and neutrals on the same terminal strips and for neutrals being improperly bonded to the case (bonding jumpers installed where they are not supposed to be).
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Ohhhhh, a CAT scanner! :roll: As in medical equipment.

Around these parts, a CT is a Current Transformer.

We were imagining how one might simulate a CT. :confused:
 

john37

Senior Member
sorry for the confusion! Yes, it's a CT x-ray scanner machine. They just installed it and the manufacturer (Philips) did a power test on the equipment and it showed that there is excessive ground current going to the x-ray scanner. The power feeding the x-ray machine is coming off a distribution panel which is fed off the main switchboard.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100706-2248 EST

john37:

You have to be more specific in your description.

You have a "black box (a CT Scanner)". This has some wires going to it from a panel. Define all of the wires. Define how the scanner is mounted.

What do you mean "it showed that there is excessive ground current going to the x-ray scanner"? How is this known?

Is current popping out of the floor and going into the CT Scanner? If so how do you know this?

Is there an EGC (equipment grounding conductor) coming from your said panel to the CT Scanner? How is it known there is a ground current and were is it flowing? Was current measured in this EGC?

How do you know the CT Scanner is not the source of ground current.

You need to do current measurements, and high impedance voltmeter measurements. When you can more accurately describe the location, wiring, and measurements made, then maybe we can provide more help.

Can you tell us what a simulator is relative to this equipment?

.
 

hurk27

Senior Member
I think this is the typical voltage differential between the neutral conductor and the grounding conductors, if this is the case, then go find some super conductors that wont have any voltage drop and replace all the neutrals.


Kidding, I have run into this problem so many times with IT techs, just about any code compliant circuit will have some voltage drop between a neutral and grounding conductors, many of these techs will specify a voltage deferential voltage of .5 volts or less, now I have no way of getting a voltage level this low, unless you run #8 or larger neutral for each circuit, and if these circuits are fed from a sub-panel, and there is a voltage difference between the neutral and grounding at the sub panel, then you would have to up size this neutral.

The second theory that they could be meaning is they are detecting a high electro magnetic field, this can be caused when circuit conductors are not run together in the same raceway, common problem with mutable location 3-ways and 4-way switching, or boot leg wiring where load is using a hot from one circuit and the neutral from another, this would take turning off all breakers and seeing which breaker is using which neutral by taking amp readings on the neutral.

But the original mention theory of a grounded neutral is or seams the likely one, which one would just turn all breakers off take all neutrals off neutral bar, and see which one has continuity to ground trace it out and remove the connection.
 
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john37

Senior Member
Thanks for everyone's opinion. Sorry if I'm not being totally clear on the explantion. I just found out myself and don't have all the information gathered yet.

All they told me was "Phillips normally takes a power and room survey after the equipment has been installed and is operational. They then hire a third party company to analyze the data. The report shows that there is excessive ground current going to the equipment".

The way the CT x-ray scanner is wired is that there is a 3P100A breaker (480V) feeding from the distribution panel that is approximately located 200 feet away from the CT x-ray room. The feeder then goes to an enclosed 3P100A shunt trip breaker located in the CT x-ray control room which is adjacent to the CT x-ray machine. From the shunt trip breaker it is wired directly into the CT x-ray machine. I don't have the one line diagram with me right now otherwise I would attach it to give a better explanation.

I suggested to the electrician to systematically measure the current on the ground conductors going from the CT x-ray feeder back to the panel. See if the currents are the same on both ends of the feeder grounding conductor. If it is, then they will have to systematically go one step up to the apparatus that feeds the panel and do the same. When they come to a point in the distribution where the grounding conductor does not have the same current (approximate), then the source of the ground current would have to be coming from that section of the feed.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
All the CT installation instructions are normally very specific about the feeder and install. They normally limit the length of the feeder, ask for a specific size ground conductor, restrict how the neutral is terminated, ask for an isolated ground wire ran with the feeder.

Is there an insulated ground ran with the feeder, & is it terminated per Phillips installation drawings? Is there a neutral run with the feeder? If so, I think it should stop at the first disconnect.

STeve
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100707-0820 EST

john37:

You titled this thread "excessive grounding". What were you trying to imply with this title.

Then the term "excessive ground current going to the equipment". Just what does this mean, and why is the wording "going to" used instead of "coming from"?

"Ground" is a very loosely used word without qualification by many people.

Is ground the real earth at some point or some other point? Any two points on the real earth are unlikely to be at the same potential and thus there is a current in the earth.

Is ground the floor in your room or is it the steel framework of the building?

Is ground the EGC?

Is ground the neutral of your three phase circuit at the transformer, the distribution panel, the shunt trip breaker, or the machine?

Where is this excessive ground current being measured?

Suppose there is an explicit EGC, meaning a wire and not a conduit, then was the current measured in this wire? And is this the ground current being discussed? Assume it is and there is no leakage current internal to the CT Scanner and no other wires come to the frame of the machine, then we could assume current is flowing thru a path that includes whatever the machine is mounted upon.

Is the CT Scanner a Y or delta load? My guess is that it is a Y load and there should be near zero current on the neutral? But it could have high harmonic content from rectifiers in the machine and would not be zero

If on the other hand the report is referring to current on the EGC, then the question is what is the source?

.
 
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