Cable Gnd. VS. Utility Gnd.-Failure

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ghostbuster

ghostbuster

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Great White North
18 amps of ground current flow from the utility ground system to the cable ground system was measured.

Cable box caught fire.(new one installed above failure location)

Full investigation still being conducted.
MVC-016S.jpg
 
e57 said:
Looks like it had more than 18A at some point from all the smoke marking.
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That current measurement was taken yesterday.The two grounds were temporarily (10 seconds) tied together to get that reading.This problem still exists at this site.

The cable co-ax braid would have been the limiting factor for the max. current before that melt down occurred.


Note:Approx. 10 -15 houses on this street have the same problem to varying degrees.At present, all these cable grounds have been seperated from the utility grounds to prevent any further cable meltdowns.:smile:
 
This sure sounds like a loose utility neutral somewhere, particularly if all 10-15 homes were on the same transformer or the same lateral. I'd call the POCO to get them looking at it.

Jim T
 
Bad Neutral

Bad Neutral

I assume CATV bonds the coax shield to the utility neutral at the service through the ground. Somehow a voltage difference is being created. I agree that a bad neutral at the POCO transformer would cause the neutral current to flow through the earth or any other convenient path. The Coax is probably a better ground path than earth. Figure that the coax is earthed near a splitter which is near the transformer and might be bonded to it. Have the utility test, with a CATV person on site, all of the neutral connections at the transformer. Guaranteed that if you are not there to watch, the problem will go away and they will say "we did nothing!" I think they will find a loose neutral or a bad neutral-transformer connection.
 
jtester said:
This sure sounds like a loose utility neutral somewhere, particularly if all 10-15 homes were on the same transformer or the same lateral. I'd call the POCO to get them looking at it.

Jim T
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Jim
The utility has been involved already.They say their system is a-ok.

All these houses back onto a 230 kv. transmission line corridor(15-20 feet away).The magnetic field within the house areas is 100-200 milligauss.We suspect their may be an induced 60 hz. onto the cable ground.:smile:
 
The cable ground is one of the code required parallel paths for the service neutral current. Any time the impedance of the service grounded conductor goes up, more current will flow in the cable ground. When this current is excessive, you get the result shown above.
Don
 
ghost,
At present, all these cable grounds have been seperated from the utility grounds to prevent any further cable meltdowns.
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That could move the problem inside making the problem worse.
Don
 
Unwanted ground currents

Unwanted ground currents

Oh, 230kV line. Was there a fault that passed along this line about the time of the CATV failures? If there was a local fault of the 230kV line, the CATV cable may have tried to pass some of the fault current back to the utility causing the damage. If the fault passed along the 230kV cables, mutual coupling could have created significant voltage/current on the CATV shield.

The NESC and NEC allow floating of grounds if unwanted current may occur.

Best to install a surge device between the CATV shield and local ground for safety reasons.

Also, high voltage isolator between the customer's CATV equipment and the customers inside wiring may be needed or you will create a safety hazard. You have moved the high voltage difference inside to blow out a TV or CATV box. That may lead to a fire. Not good.
 
ghostbuster said:
That current measurement was taken yesterday.The two grounds were temporarily (10 seconds) tied together to get that reading.This problem still exists at this site. Wouldn't do that again...

The cable co-ax braid would have been the limiting factor for the max. current before that melt down occurred.


Note:Approx. 10 -15 houses on this street have the same problem to varying degrees.At present, all these cable grounds have been seperated from the utility grounds to prevent any further cable meltdowns.:smile:
Click to expand...

This 18a is created by some sort of voltage. What was the difference between the two? (Doubting 230kv as the neighborhood would have other more serious problems) Either way it soulds like the CATV Co's problem to solve.
 
don_resqcapt19 said:
The cable ground is one of the code required parallel paths for the service neutral current. Any time the impedance of the service grounded conductor goes up, more current will flow in the cable ground. When this current is excessive, you get the result shown above.
Don
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DON

According to the CATV tech:he physically disconnected the output cable feed at the amplifier (approx. 150-200' upstream from these houses).There was still current flow between the utility and cable ground at this house.(Therefore,this current apparently is not returning through this powered-60 volt ac. amplifier, back to the utility neutral point)

Note:This tech has been scratching his head for over a year on this problem!:smile:
 
Ghost,
According to the CATV tech:he physically disconnected the output cable feed at the amplifier (approx. 150-200' upstream from these houses).
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Was the cable still connected to other houses? If so there is still the parallel path with the service grounded conductors as the cable is bonded to the grounded conductor at each house.
Don
 
don_resqcapt19 said:
Ghost,

Was the cable still connected to other houses? If so there is still the parallel path with the service grounded conductors as the cable is bonded to the grounded conductor at each house.
Don
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Don

According to the tech,all the houses in this area have had the cable ground seperated from the utility ground.

Next week,I will continue this site investigation with catv and utility personnel.:smile:
 
I am a utility person and have had this problem several times. It doesn't have to be at a single transformer but could very well be. I doubt 1 pot is feeding that many houses. We have found this problem from anywhere from a loose neutral connection at the house to a loose neutral jumper on a pole a half mile away. I have opened the primary disconnect going into a subdivision and isolated the neutral on a few jobs just to help isolate what section the fault was in. Latest one was about a year ago with a pool. The customer had a number 12 wire tied to the ladder and thrown off into the pool to get at the same potential. They had about 5 to 6 volts differential. We tried everything we could think of and came to the conclusion that the ground system around the pool must have been bad. It was an older house and pool. I order a "Ronk" Blocker, google it, and installed it and have had no problems since.
 
chris kennedy said:
Is there a meter base behind that meter? I don't see any connectors on the conduit.
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that's an OLD school meter base. we have a lot of them in my area, usually two per house w/ one of them jumpered to the second. the second one was used to meter water heaters separately (at a lower rate) before they started installing power consumption limiters on them.
 
UPDATE

1 Utility visit:
A. Visually examined all neutral connections ,said they look A-OK
B. Agreed when CATV ground attached to their ground,there was neutral
current flow into the CATV ground system.

C. Trouble truck then left site-didn't know how to solve the problem.



2.Magnetic field affects from nearby high voltage transmission line:

Our group measured at site, over 1 amp of current flow induced into a 6' dia loop of #4 cable (located at ground level).Ambient magnetic field 150 milligauss.:smile:
 
Latest Update:

Measured 30 volts -60 hz. between utility and cable ground at house.Utility said they measured up to 45 volts approx. 6 months ago.

Disconnected all cable power and rf signal on all cable systems in this area approx. 500 houses with no signal .Still have 30 volts and 18 amps of current flow between grounds.Opening catv connections at all splitters ,still draws an arc on the ground fittings.

Safety groups have been contacted.
 
UPDATE:

Utility turned off all power to this area(7 transformers turned off )

17 amps of current flow still measured between CATV ground and utility ground.

It would appear, as originally suspected ,to be an induced current problem
 
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