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i work for a cable company and we have this phenomenom called voltage feedback. it breaks down into two parts. first part is we read an AC voltage between the coax and house ground, which we say is "floating voltage". second part is where we grab the coax and touch the ground and get a "tingle". sometimes we can attribute the tingle to a bad electrical plug, bad t.v., etc. but sometimes, none of these apply and we are baffled. any more knowledge would be extremely heplful.
Touch the coax you mean - the shield side, correct?
The first thing your supposed to do, is create a ground before the cable enters the house, right
Guess where that ground is SUPPOSED to be?
The same place the electrical system is grounded, the telco system is grounded and the CABLE system is grounded. If it's the other side of the house, all bets are off.
You didn't say what the range of the size of the voltage was nor what the current is. Do you have any idea?
There is no question that there is a difference of ground potentials.
Let's think about what can cause that?
1) hot and neutral reversed on an outlet - big time problems
2) Leakage current in an attached device
3) Your ground is bad
4) The house electrical ground is bad
5) Your not grounded at the same point as the house.
6) If a computer and TV shared a surge supressor and the outlet has an open or intermittant ground, your in trouble.
7) A distribution amp
8) a cable modem especially ones connected by USB
I'll have to think some more?
I would:
Install your ground block (you know where it's supposed o go)
Install the system's cable
Measure V between house outer conductor and your conductor
If it's substantial then you have to ask?
Test #1: Is your ground at the house the same potential as the house ground with no cables connected to your ground block at the house?
Your not going to like what I say, but it will work. Go to the electrical service and run a wire from an exposed point to the location of your ground connection. Measure V between this wire and the outer shield of the house's connection.
That's the hard one if the voltage is large.
If it is small but you still have the voltage problem, then disconnect half of the coax cables inside. exist/not exist, then pick the other half and use the binary search technique to find the culpret.
With my long wire test, you could use the the voltage between your properly grounded ground block with nothing attached to the customer's side and and the ground from an extension cord from the nearest electrical outlet with nothing plugged in.
i work for a cable company and we have this phenomenom called voltage feedback. it breaks down into two parts. first part is we read an AC voltage between the coax and house ground, which we say is "floating voltage". second part is where we grab the coax and touch the ground and get a "tingle". sometimes we can attribute the tingle to a bad electrical plug, bad t.v., etc. but sometimes, none of these apply and we are baffled. any more knowledge would be extremely heplful.
i have no answer - i have the problem. Our cable tech said that our TV is sending voltage back into our cable box - 50+V on each of the video cables! I would love to know that there is something out there that would absorb the voltage between the video cables and the cable box.,...or some way to ground the TV - it only has a 2 prong cord. Any thoughts?
The correct solution is to call the tech's manager or corporate manager and as for an on-site visit by a local engineer to find and correct their problem.
Unless I've forgotten how TVs work, they are receivers and it is very unlikely that they would emit a backwards emf (static) let alone a 50V+/- charge.
What happens when you disconnect the cable that supplies the TV from the cable receiver, apart from the loss of picture.
I suspect that there is a dirty frame ground or a floating frame ground with the cable box. Ask the tech to change it out ans see if the voltage is still present on his end of the cable.
That floating neutral problem is caused from having your power source ungrounded. Whenever you have a reading like this, (Voltage between ground and neutral). What happens when loads are applied to an "ungrounded system", it causes the "phase voltage" to try and balance itself, since it has no reference to ground. Remember the circuit will try to balance itself if you do not give it a reference point to ground. One way of checking this is to slowly take loads off of the power source and note the voltage readings between neutral and ground. As the loads are taken off, the voltage slowly should read closer to zero between ground and neutral or in this case, the coax and ground. Whenever you have this problem, always check to make sure your source has a "grounded neutral". This will keep the "zero" point from shifting as you apply load.
Another way, which I have seen is a defective ground in an outlet and devices with switching power supplies and RFI filters attached. The RFI filter dumps energy onto the ungrounded ground raising the ground potential of everything connected.
That can cause no problem until you connect to a real ground which could be the coax or could be the TV. Open grounds and neutrals can even occur with defective outlets. I saw one outlet which would loose it's ground when both plugs were used.
Computer had a grounded outlet and used a switching powersupply with an RFI filter.
The computer was plugged into an outlet strip with other devices.
The ground was defective in the outlet.
There was about a 70 VAC potential between the ground of the outlet strip and a nearby properly grounded outlet.
The computer (was a micro-computer at the time) died because it communicated with a properly grounded device.
When the outlet was removed and disassembled, the ground bar had sparks marks and it was a pressure fit with no fasteners. BAD GROUND, not a BAD NEUTRAL in this case!!!
Computer had a grounded outlet and used a switching powersupply with an RFI filter.
The computer was plugged into an outlet strip with other devices.
The ground was defective in the outlet.
There was about a 70 VAC potential between the ground of the outlet strip and a nearby properly grounded outlet.
The computer (was a micro-computer at the time) died because it communicated with a properly grounded device.
When the outlet was removed and disassembled, the ground bar had sparks marks and it was a pressure fit with no fasteners. BAD GROUND, not a BAD NEUTRAL in this case!!!
There is no doubt grounding can do weird things...Thanks for the details. I myself have seen things similar and it has come down to soil testing to assure "zero potential" existed. I was involved with the relocation of a service ground to a different area because of a high resistivity problem. Improper ground plays havic in electronic equipment because it is so reliant on its reference point.
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