Re: 'Earth' levels of TV cables.



Forwarded to uk.d-i-y

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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.

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This is ' Bazzer Smith ' don't forget !!!. Words, salt and pinch of, spring to mind.
Dave
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I can think of a mechanism. But its validity depends on the properties of the multimeter the OP used to measure it.
--
Ian G8ILZ

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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.

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Bazzer Smith wrote:

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 ...
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Andy Burns wrote:

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).
--
Cheers,

John.

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John Rumm wrote:

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.
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The Natural Philosopher wrote:

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.
--
Cheers,

John.

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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.

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Bazzer Smith wrote:

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.
--
Cheers,

John.

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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.
--



Graham.

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Just remembered that SIS used B-MAC not C-MAC.
Also intended that to follow-on a different part of this thread. Never mind.
--

Graham.
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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.
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Skipweasel
Pay no attention to that man behind the curtain.
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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.
--

Graham.

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contains these words:

I would imagine its a tad easier these days, they must be using flat panel monitors in new shops.

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Have you ever tried lifting a PDP?
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Graham.
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Graham wrote:

reckon you'd put that down PDQ
NT
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Indeed. I carried an 11/23+ out of the house and into the car, then into the Bletchley Park Computer Museum the weekend before last. It was only a 2U 19" rackmount. No disks, mind.
--
"Other people are not your property."
[email me at huge [at] huge [dot] org [dot] uk]
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wrote:

Doesn't the floating point option make it lighter?
--

Graham.
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