Re: 'Earth' levels of TV cables.

[Bazzer Smith]

I sort of covered this in a earlier post but it might get 'buried'. If you

>connect two different TV RF cables togeather (via a coupler or whatever >just directly) should > there be a potential difference (voltage) between the two 'earths'? I ask > because there seemed to be a 4-5 volt difference when I tried this, at > least that's what I measured on my multi-meter, maybe I did it wrong?, I > had it on AC but it 'felt' AC when it went through my fingers :O) > > By the way I have not done any DIY work on my houses electrical circuits, > well not yet anyway ;O) I doubt I could make it any worse even if I did, I > think the builders who built it wore spurs!! > > Maybe its a 'false' reading anyway because 5 volts through a path of > minimal resistance would draw a big current which would probably burn the > wiring out, or atleast > explain my huge electricty bills!!

Forwarded to uk.d-i-y

>

[Agamemnon]

Are they both connected to the same earth terminal ?

Even if they are the earth screen will act as short wave antenna so will pick up electromagnetic radiation and that's probably what your meter is measuring if you set it on AC. Even if you hold the metal part of meters prods one in each hand without anything else connected it will still give you a reading.

How can you feel 4 or 5 volts through your fingers. Were they wet ?

Have you tried measuring the current ?

A volt meter has a very high internal resistance which is why it is able to measure even very low voltages between two unconnected wires. Since the resistance between them is huge very little current is flowing. If you use an ammeter to measure the current it I doubt you would get any current flowing at all.

[gort]

This is ' Bazzer Smith ' don't forget !!!. Words, salt and pinch of, spring to mind.

Dave

[Prometheus]

In article , Agamemnon writes

I can think of a mechanism. But its validity depends on the properties of the multimeter the OP used to measure it.

[Stephen]

If it registers 4 or 5 volts but you can feel it, it's probably mains voltage, or half mains voltage connected to your fingers via the supression capacitors across the mains inlet of one of the bits of equipment you are using. There's a small capacitor between Live and Earth, and another equal capacitor between Neutral and Earth. If one of your mains Earth connections has been disconnected somewhere then the equipment's Earth cable, it's case and any aerial sockets plugged into it, will be at half mains voltage but at a very high impedance due to the capacitors.

If you put your voltmeter on a higher range you will probably see a higher reading as the impedance of the meter will be higher. If you connect an oscilloscope in the same place you will probably see over 100 volts at 50 Hz.

[meow2222]

If 2 cables are connected there isnt 4-5 volts between them. If the cables are 4-5v from earth you wont feel anything. If theyre 4-5v from each other and live you would. I've not come across multimeters reading hf rf before; it would need to be very high freq not to be shorted out by a short metallic connection. If you can sort out the facts maybe we can help. Maybe crosspost to sci.electronics.basics.

NT

[Prometheus]

Correct.

Incorrect!

Who said it was RF?

Besides, have you any idea how much RF (how close and how powerful a TX) you would need to measure 5V?

It might be an inductive coupler with no galvanic coupling.

[Bazzer Smith]

To different power points in the same room. The Freeview dongle is connected to my PC via the USB, I don't know what that means about its earth.

They may have been after I washed my hands from being in the attic.

[Andy Burns]

That means they are probably just connected to the PC's signal ground, not to actual earth, especially if a laptop, also equipment could be double insulated and have no earth ...

[John Rumm]

Yet another possibility is that the whole house has a high earth impedance (e.g. as is quite common with a TT earthing system (local earth rod) and an overhead cable power supply to the house).

[The Natural Philosopher]

I missed the first part of this, but here is something that happened once that may be relevant.

One laptop. One PC, one serial cable being used (before the days of USB etc) to transfer data between them.

One teeny spark, and blown serial ports on both..

One voltmeter of high impedance variety conncted between the PC earth and the laptop ground shows 115v AC..

The cause? The laptop was powered via an RFI filter with a capacitor from live and neutral to the laptop ground, with the SUPPLIED 2 WIRE MAINS PLUG.

i.e. there was NO mains earth on the laptop whatsoever..and the RFI filter bled enough current through the capacitors to cause enough of a surge to blow some components.

[meow2222]

What scenario are you proposing? Leaning on a live appliance? Sticking

2 pins through the heart and applying 4-5v? Either is possible but I'd say quite unlikely.

I dont remember who, someone said it may be. Read up if you really want to know

this seems to answer it:

Normally screens are connected in domestic aerial installs, but sure it might be just about anything. You never know.

NT

[John Rumm]

Yup, had exactly that happen in an office once. We were using two PCs eitehr side of the room, hooked up via a serial cable, and got a slight kick from the shield of the cable on plugging it in - nett result dead port on one PC.

The place was wired with singles in metal conduit, and used the conduit as a CPC. It turned out that the gland on one of the the sockets providing connection to the conduit had worked loose over the years.

[Bazzer Smith]

Its not an overhead cable, anyway both units are in the same room so they should share the same earth, however, one unit is a set top box with its own power supply, so it only has two cables going to it, live and 'earth', dunno which of the two earths it uses (earth or neutral) same would go for the freeview dongle. Anyway I tried measuring amps and it read nothing, volts gave 87V on AC!! So I aint to sure what is going on, all I know is that my signal booster is dead.

I would also add that I detected a spark when plugging the aerial in to the TV directly on one occasion!! TV is OK but it was rather disconcerting!! It was audible and visable, the sort of thing which would easilly blow a transistor designed for small signals as in TV signals.

[John Rumm]

That could be the same *bad* earth.

Sounds like capacitive coupling caused by the input filter on the PSU. You ought to be reading about half mains voltage if that were the case.

[Graham]

c. 1990 I was involved with commissioning SIS systems in betting shops.

I was called out to investigate problems at one newly refurbished and enlarged shop.

All the monitors displayed severe hum-bars and there was a defining hum on the

PA system.

The satellite receiver and C-MAC decoder were located on a wallboard on one side of the building and the monitors, text computer video distribution amp and audio amplifier on the other.

The shop had been two separate units, (you guys are ahead of me now yes)?

The installation was using two different supply phases.

[Christian McArdle]

Basically yes. If you have two imperfectly earthed aerial cables, there is likely to be considerable potential difference between the two earths. Indeed, you often get quite a little belt by touching the two earths before the cables are mated. Often the "earth" never gets earthed at all, because the TV is double insulated and the aerial not earthed.

Christian.

[Graham]

Just remembered that SIS used B-MAC not C-MAC.

Also intended that to follow-on a different part of this thread. Never mind.

[Guy King]

The message from "Graham" contains these words:

For Rediffusion/Ladbrokes? I did dozens of them all over the Home Counties, but a few years before that - some time around mid 80s. Getting those 26" tellies into the racks was a bastard.

[Graham]

No, later than that, at the time when Extel gave way to SIS. BT installed the satellite kit, someone else installed the cabling and we, Granada, did the rest. GTVR (I think they were called UKRS then) installed the TV's and I turned up in my Granada Microcare blazer and installed the computer, connected the whole thing up and commissioned it.