While commissioning a new nanoscale research facility it was discovered that the electrical magnetic fields in the labs were to high. Further testing indicated that the source of the high magnetic field was the CATV system. The lab being part of a major university is interconnected with the rest of campus for communications. When the CATV feeder was disconnected and disassembled the interference was determined to be traveling on the foil ground shield. The research in this new facility is cutting edge and very sensitive. Is there any FPN that allows the CATV system to carry its own ground not bonded to the building system or does anyone know of a device that can essentially clean a dirty field on an existing wire to reduce EMF's?
Electrical Magnetic Field issues on CATV cable
- Thread starter tbab930
- Start date
- Status
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Rather that mask a problem why not take the correct approach and locate the source of the problem.
Solve the issue do not hide it. If it is an issue in one building you may just be relocating the issue to another building.
Solve the issue do not hide it. If it is an issue in one building you may just be relocating the issue to another building.
ultramegabob
Senior Member
- Location
- Indiana
how exactly is the catv tied into the grounding system now? at the main or in a sub-panel?
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
brian john said:
Without a doubt, the best solution to your problem is to locate it. Grounding issues sometimes are a pain/time consuming to locate. I would be willing to bet that somewhere in your system, the grounding/bonding is not correct.
I am curious as to how you/your company solves the issue, please keep us informed.
- Location
- Illinois
- Occupation
- retired electrician
I don't see how you will get away from the net current on the shield of the coax as the NEC rules require that the coax shield be in parallel with the grounded conductor.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080603-0920 EST
tbab930:
What do you mean the "electrical magnetic fields" were too high? What was measured? What instrumentation? Are you talking about radiate energy, electric fields only, magnetic fields only, ground currents, or ground potential differences?
If this CATV cable is brought into the lab in question and is not terminated to anything, then is the problem present?
If you connect a 10 ft wire to the center conductor of the wire and lay this on the floor, and still no connections, then is there a problem?
If you can find a fiber optic interface that will transfer the CATV signal, then you can provide ground isolation. From your description a ground loop may be your problem.
Pierre:
I would not necessarily agree that "correct grounding/bonding" is a solution to this or other noise related problems. What does "correct grounding/bonding" mean. This is sort of like saying a "good ground" will solve your problem. Is 25 ohms to earth a good ground, is 1 ohm, or something else?
Suppose I have two CNC machines fed from a bus to the main panel. Each has an EGC to the main panel. The main panel has an earth ground. The machines are spaced about 2 feet apart. Each machine has its own supplemental earth ground (may be good or bad but experimentally is not important). All electronics in these machines are referenced from the machine chassis. This means connected thru the EGC back to the main panel. There is no neutral wire to the machines because they are a delta load. There is no 120 at the machines, only delta 240. RS232 common is referenced to the machine chassis.
Direct RS232 communication is by separated cables to a single computer. The computer gets its chassis grounded via the EGC of its outlet back to the main panel. Its RS232 is referenced to its chassis. Direct RS232 connection means that there is a moderately low resistance connection of all used leads between the the computer and the CNC. Maybe #22 wire for these connections. Twisted pairs individually Beldfoil shielded are used for the signal lines.
Both CNCs are the same model but built about 2 years apart. With servos on communication can exist with only one machine, the older one. With servos off it is possible to communicate with both machines. The newer machine has brushless servos, and the older DC servos.
The cable distance betwen computer and CNCs is about 75 to 100 feet.
Optical isolation of the RS232 communication path eliminates the problem with the newer machine.
The problem is the high impedance in the ground path to the high frequency noise generated by the brushless servo system. A 0000 cable from CNC common (chassis) to computer common might solve this problem, but optical isolation eliminates the problem and provides a much greater noise margin than would ever be realistically obtained with a large conductor.
Should the CNC manufacturer try to reduce this noise problem? Probably not because it causes no other problem than with direct RS232 communication.
.
tbab930:
What do you mean the "electrical magnetic fields" were too high? What was measured? What instrumentation? Are you talking about radiate energy, electric fields only, magnetic fields only, ground currents, or ground potential differences?
If this CATV cable is brought into the lab in question and is not terminated to anything, then is the problem present?
If you connect a 10 ft wire to the center conductor of the wire and lay this on the floor, and still no connections, then is there a problem?
If you can find a fiber optic interface that will transfer the CATV signal, then you can provide ground isolation. From your description a ground loop may be your problem.
Pierre:
I would not necessarily agree that "correct grounding/bonding" is a solution to this or other noise related problems. What does "correct grounding/bonding" mean. This is sort of like saying a "good ground" will solve your problem. Is 25 ohms to earth a good ground, is 1 ohm, or something else?
Suppose I have two CNC machines fed from a bus to the main panel. Each has an EGC to the main panel. The main panel has an earth ground. The machines are spaced about 2 feet apart. Each machine has its own supplemental earth ground (may be good or bad but experimentally is not important). All electronics in these machines are referenced from the machine chassis. This means connected thru the EGC back to the main panel. There is no neutral wire to the machines because they are a delta load. There is no 120 at the machines, only delta 240. RS232 common is referenced to the machine chassis.
Direct RS232 communication is by separated cables to a single computer. The computer gets its chassis grounded via the EGC of its outlet back to the main panel. Its RS232 is referenced to its chassis. Direct RS232 connection means that there is a moderately low resistance connection of all used leads between the the computer and the CNC. Maybe #22 wire for these connections. Twisted pairs individually Beldfoil shielded are used for the signal lines.
Both CNCs are the same model but built about 2 years apart. With servos on communication can exist with only one machine, the older one. With servos off it is possible to communicate with both machines. The newer machine has brushless servos, and the older DC servos.
The cable distance betwen computer and CNCs is about 75 to 100 feet.
Optical isolation of the RS232 communication path eliminates the problem with the newer machine.
The problem is the high impedance in the ground path to the high frequency noise generated by the brushless servo system. A 0000 cable from CNC common (chassis) to computer common might solve this problem, but optical isolation eliminates the problem and provides a much greater noise margin than would ever be realistically obtained with a large conductor.
Should the CNC manufacturer try to reduce this noise problem? Probably not because it causes no other problem than with direct RS232 communication.
.
DanZ
Senior Member
Given the nature of the work to be done in the lab, shouldn't they (try to) completely isolate every potential form of interference if they're that worried about it?
As to how, well, I have no clue. But it sounds like they are worried about every little tiny bit of interference interfering with their work.
As to how, well, I have no clue. But it sounds like they are worried about every little tiny bit of interference interfering with their work.
quogueelectric
Senior Member
- Location
- new york
Cable shields can act in funny ways depending on proper an or improper instalation.
I have found that the older cordless phones would travel down the shield of the cable cos and would act as a kind of repeater to carry a signal that normally wouldnt propagate that far in the air on the shield instead.
The first thing I would try is a ground loop isolator that goes in line with the cable. It is a cheap and easy fix I use the Holland ccl-1 they are about 10 bucks and eliminate all of the horrors on cable that you didnt install.
This would never happen on one of my installs but I cant be everywhere and this is a quick easy fix that works for catv in the home depot dropout houses.
I have found that the older cordless phones would travel down the shield of the cable cos and would act as a kind of repeater to carry a signal that normally wouldnt propagate that far in the air on the shield instead.
The first thing I would try is a ground loop isolator that goes in line with the cable. It is a cheap and easy fix I use the Holland ccl-1 they are about 10 bucks and eliminate all of the horrors on cable that you didnt install.
This would never happen on one of my installs but I cant be everywhere and this is a quick easy fix that works for catv in the home depot dropout houses.
ghostbuster
Senior Member
- Location
- Great White North
quogueelectric said:
Jensen also has an inline isolator.
Note: there is a real good CATV cable mag thread and case study in M/H newsletters Jan/Feb 2008 .Hope this helps- Status