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Electric shock off aerial coax

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ZenStalinist

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PostPosted: Sat Aug 07, 2004 10:20 pm Reply with quote

We have an amplifier, that connects the Skydigital box, the SVHS machine and the aerial to a TV upstairs, recently I was changing the VCR up their and I got a small shock off the coax. My thought is that it is somehow causing an induction from nearby mains cables, or somehow carrying a potential as there is no potential at the amplifier. In America, apparently they ground their aerial system. Would this help at all, and may it, and should it, and how would it be done in the UK? It is not something I ever read about in any electrical books icon_redface.gif
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plugwash

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PostPosted: Sat Aug 07, 2004 11:41 pm Reply with quote

it can be a good diea to ground coax to reduce interferance as well as having possible safety advantage

i actually took the ground to a hi-fi amp that was on the arial system when i did it here (metal case but class 2 equipment) just using a bit of flexible black single core i had lying around (green/yellow would be better though) from the earth pin of a plug
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TexMex

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PostPosted: Sun Aug 08, 2004 12:35 am Reply with quote

First of all, to explain the "earth screen" in coax. It doesn't actually screen the signal lead to any great deal. What happens is, any interferance that gets to the central conductor, also gets onto the "screen". At your TV set you now have two signals. One from the "screen" (this is just pure interferance) and one from the central conductor (interferance + signal).

If you earth the screen of the aerial, at any point, remotely from the set, you will remove some of the interferance, and therefore, the set will not be able to compensate.

You see, the TV set actually relies on, any RF interferance on the signal, to be present also on the screen. Otherwise, it cannot subtract it.

Usually antena amplifiers only use voltages of about 9V. (not enough to get a packet from). I would suspect the PSU for the amplifier. If this is a seperate transformer, see if you can try another with suitable suitable spec.
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AdamW

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PostPosted: Sun Aug 08, 2004 12:56 am Reply with quote

You say "Sky box"... there is a possibility that hasn't been explored here:

Sky boxes have the ability to supply 12V dc from one of the RF aerial outputs. This is to power an extra remote control receiver at the (in your case) bedroom TV.

The manual for your digibox will tell you which one, and how to check it is turned off. You must not use the 12V output when you are feeding the signal into an aerial amplifier unless it is designed to handle it.
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AdamW

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PostPosted: Sun Aug 08, 2004 12:59 am Reply with quote

plugwash wrote:
it can be a good diea to ground coax to reduce interferance as well as having possible safety advantage

i actually took the ground to a hi-fi amp that was on the arial system when i did it here (metal case but class 2 equipment) just using a bit of flexible black single core i had lying around (green/yellow would be better though) from the earth pin of a plug


What about using the turntable grounding connection present on many amps? Or would that be a bad thing?
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plugwash

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PostPosted: Sun Aug 08, 2004 1:24 am Reply with quote

thats what i did

plug earth termina to hi-fi grounding post
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Lectrician

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PostPosted: Sun Aug 08, 2004 9:00 am Reply with quote

All aerial systems are meant to have their screens grounded - i have a bskyb spec for new builds somewhere.

All TV's will introduce a potential onto the screen, if you earth it, it vanishes. Put a multimeter from the screen on the TV (with the coax out), and an earthed radiator, socket earth pin etc. You will be shocked by the volts icon_smile.gif

All decent metal boosters have an earth terminal for a 4mm bond - why would they fit one if it was detrimental to the picture??
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pearce_jj

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PostPosted: Sun Aug 08, 2004 9:25 am Reply with quote

You'd need a lot more than 12v to get a shock though. There could be a fault in one of the TVs, which could be dangerous. IIRC wiring regs require multi output systems to use isolated coax sockets for this reason.

Interesting point on the screen. Certainly most systems don't appear to need earthing.

Without the shield earthed, the coax is operating in a balanced arrangement, whereas earthed it is unbalanced (as one side is earthed). As I understand it, earthing the shield will result in current flow in the shield, increasing with frequency, because the aerial is a balanced dipole. This reduces the efficiency of the system and makes it more suseptable to interferrence.

Looking at some PF100 coax it has a 64-strand copper braid plus copper tape. MaxView shielded coax sockets have an earth screw. Many "digital" aerials encorporated a balun (balance to unbalanced) into the cable connector.

All of this seems to imply the sockets should be earthed and the braid/tape is indeed to supress interference, but a balun must be used at the aerial.

Please don't rely on this though as I'm no aerial expert, this is just based on what I've read and observed and a-level physics!
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AdamW

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PostPosted: Sun Aug 08, 2004 11:47 am Reply with quote

Lectrician wrote:
All decent metal boosters have an earth terminal for a 4mm bond - why would they fit one if it was detrimental to the picture??


Because you have a big metal box there so it has to be earthed? icon_biggrin.gif

Surely any required earthing takes place at the equipment end of the cable? I doubt very much that the TV manufacturers would overlook this?

There is an old saying, "no earths are equal", meaning that if you ground something at two different places you will get a pd between the two earth points. Despite not being a TV designer or repairman, I would suspect that the voltage you would get between the tv end of the coax shield and the place where you earth it, the voltage would be just leaked to earth. So I can't really see any benefit either way, but I would sooner leave it earthed entirely by the TV.

Of course the mast should be grounded in case of lightning strike...
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ninebob

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PostPosted: Sun Aug 08, 2004 1:25 pm Reply with quote

Lectrician wrote:
All aerial systems are meant to have their screens grounded ............. All TV's will introduce a potential onto the screen, if you earth it, it vanishes. Put a multimeter from the screen on the TV (with the coax out), and an earthed radiator, socket earth pin etc. You will be shocked by the volts icon_smile.gif


I don't understand. Does this mean my TV is less safe without a co-ax input plugged in? I only use the SCART sockets on mine...
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plugwash

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PostPosted: Sun Aug 08, 2004 2:42 pm Reply with quote

a lot of equipment is class 2

and long isolated runs could build quite a voltage from inductance/capacitande

its probablly no more dangerous than a big static shock because it can't hold the voltage for any significant time
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Lectrician

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PostPosted: Sun Aug 08, 2004 6:55 pm Reply with quote

Its more static volts i think, no danger.

I dont know loads on aerials, but have always been told to earth the braids. They even sell marshaling posts to earth several coaxs at once. If i could find a pic i would upload.

All TV's etc and boosters are double insulated, and have no reference, except coax, and other connections (scart, AV etc).




Below taken from an extract on another forum......

Hope it explains......




I don't intend to get into a long discussion here but it is perfectly normal to be able to get a mild tingle from class II insulated products. This is to do with the need to have the product comply with the EMC rules (electromagnetic compatibility).

In a 3 wire (earthed) product you can couple the chassis to earth; on a two wire product the unit is coupled to both live and neutral with capacitors, typically 470pF IIRC. Thus the chassis sits somewhere around half mains potential all other things being equal and you can measure this with a high impedance AC voltmeter whose other end is grounded. Of course there is no current to speak of (470pF has a very high impedance of about 6 MOhms at 50 Hz) but you can sometimes sense the potential as a faint tingling on the chassis when touched, especially if you touch a grounded product with your other hand. Disconcerting but legal!

HTH.

John Dawson (Arcam)


OK - but I am interested to understand how this can meet the class II requirements of two separate insulation barriers which, if one fails, the other still prevents a shock hazard. If there is a a capacitor between the case and live, and it goes S/C, then the case is live - surely this breaches the requirements? Or - looking through the Farnell catalogue and possibly answering my own question - is it just a case of using class Y1 capacitors, which I guess are designed so that they cannot fail S/C?

However, none of this invalidates the comment I made about removing the earth from equipment that is supposed to have one!

One way to avoid hum loops is to use nearby common ground point and disconnect the screen at one (input) end of each signal connection. The grounding point needs to be very close, since the ground conductors are now part of the signal circuit and any noise on them will be imposed on the signal.

Another way is to use transformer isolation.

And the best is to use balanced interconnections, which don't need an earth reference so you need only connect the cable screen at one end. You can get converters for domestic kit that will provide balanced I/O - worth doing for a long run.
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masona

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PostPosted: Sun Aug 08, 2004 6:55 pm Reply with quote

So does anyone know what happen when lightning strike your TV aerial ?

Do you unplug it or leave it in ? I have heard but don't know if it's a myth regarding aerial lead shooting out sparks from lightning causing fire on your curtains icon_confused.gif

I think it's going to thunder tonight icon_sad.gif
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Lectrician

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PostPosted: Sun Aug 08, 2004 8:12 pm Reply with quote

The voltage contained in lightning would blow apart the coax and the TV and anything else in its way.

Have you ever seen lightning hit a tree?? or the results where it has hit a sand dune?? Not pretty!!

Lightning hit a house here last week, hit a rayburns metal chimney, caused a fire on all floors!!!!!!!!!

If it does hit an aerial, i am sure there would be a more than a few sparks icon_sad.gif , Lucky mine is it the Attic icon_smile.gif

had a holiday in Turkey one year, 2 days of thunder storms......thought i was gonna die icon_smile.gif
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masona

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PostPosted: Sun Aug 08, 2004 9:15 pm Reply with quote

Okay, would it make a difference if you take out the aerial lead out of the TV and earth out the aerial lead temporary ?
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