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#139525 - 11/23/03 02:18 PM Grounding shields of Hifi/Video cables  
:andy:  Offline
Member
Joined: Oct 2003
Posts: 289
Germany
I discovered in the last time, that there's 90VAC on the screens of my hifi cables, where my PC, TV, VCR, Hifi Appliance are connected to. you can feel it well when holding a Chinch plug and touching the metal backside of my satellite receiver [Linked Image]

I measured a current of 9 mA to Ground. Is this normal???

the problem is, none of all the said Appliances is grounded, except the PC. but i guess there is no connection from the Jack Plug receptacle on the Soundcard to the case ground.


Just a few miutes ago, i discovered something very intreresting. My laptop always made a slight special disturbance noise when connected to a TV or Loudspeakers.

I have just running the Laptop in my parents livingroom for watching DVD, and guess what. I can hear the Noise of the Laptop when I turn up my Amplifier.

So the noise must travel from the 1st floor to the roof via the satellite cable, and from the Sat dish to my room at the 2nd floor, into the Receiver, via the Scart cable to the TV, from the TV via the Chinch cable to the AMP. Hell.

connecting the coax cable shield at the Receiver to the Wall outlet's ground does only eliminate the noise about a third.


Question: Would i solve the Problem by grounding the satellite dish's support, as it is not?
Do i need to hook it up the Lightning Protector at the roof?
there's the Support for our Overhead line 5 meters next to the dish. it's connected to the lightning protector outside.
Can i also connect the two supports inside of the roof? Or is this illegal?


[This message has been edited by :andy: (edited 11-23-2003).]


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#139526 - 11/23/03 07:38 PM Re: Grounding shields of Hifi/Video cables  
djk  Offline
Member
Joined: Dec 2002
Posts: 1,237
Ireland
No, I definitely don't think that's normal. Are you by any chance touching the cable feeding the satellite dish ? The LNB in the satellite reciever sometimes is controlled and/or powered through the co-ax cable by the sat. box. The voltages are applied to switch from vertical to horizontal polarization when you change channels.

If you are getting a "buzz" from the cable it could be from this.

However, there should not be any stray AC voltages on your hifi cables this would completely mess up the sound output on the speakers and 90V AC sounds rather serious and would actually potentially damage sensitive equipment. I most definitley would not like that connected to the sound card of my laptop!!

Perhaps you have a faulty satellite reciever? It could be a ground fault to the case from the satellite co-ax ?

I would suggest that you unplug all of your equipment and check each peice individually with a voltage meter to see where this 90V AC is comming from.

Also please note that 90V AC is not considered safe to touch so do not use your fingers!

-------

I don't quite understand what you mean when you mention a lightening conductor and overhead lines.

However, it sounds like you are talking about something to do with the overhead lines that are carrying power into your home.

DO NOT under any circumstances connect anything to any part of these.

It is EXTREMELY illegal and you risk not only prosecution but also death and could put yourself and others at risk.

In all parts of Europe it is completely illegal to connect anything to any part of the public electricity network and only the power company is allowed to work with these cables / poles / supports etc etc.

Power on overhead lines is potentially leathal. In your home you are protected by all sorts of devices that minimise your risk of shock / death
Typically:
(Powerline 230V)----(80A service fuse)--(meter)---(main fuse/switch)---(RCD 30mA)----(16A MCB/Fuse)---(socket outlet) (you)

You are generally not allowed to touch anything before the meter (or main fuse / switch (in Ireland))
-------------
Also:
Satellite dishes and antennas are generally not grounded btw. If you ground the dish you will only cause signal problems. On communications gear lightening protection, where used, normally consists of a high lightening conductor located near the dish / antenna which would attract any strikes before they hit the antenna/dish (just like on a church spire)

Lightening arrestors can be used on the cable from the dish to prevent any build up of static or currents being induced by near-by lightening strikes. However, they're quite unusual in domestic installations over here.

You need to check your system for the source of AC. It sounds faulty to me!



[This message has been edited by djk (edited 11-23-2003).]


#139527 - 11/24/03 08:56 AM Re: Grounding shields of Hifi/Video cables  
Texas_Ranger  Offline
Member
Joined: Dec 2001
Posts: 2,392
Vienna, Austria
What he says is this: 5m from the satellite dish there's a _support_ for the OH power line, and the support is grounded to the lightning conductor. Now he asks whether it is advisable to ground the satellite dish support to the lightning conductor as well. Nothing to do with the OH wires themselves.
Anyway, 90V, 9mA is way too much for something like that. You could try to find the source by unplugging each part of the system until you've found the faulty part.


#139528 - 11/25/03 11:31 AM Re: Grounding shields of Hifi/Video cables  
:andy:  Offline
Member
Joined: Oct 2003
Posts: 289
Germany
thanks for clearing up, ranger. Isn't lightning conductor the right word? Of yourse i would never get in mind to work on my overhead lines [Linked Image].

I didnt know that sat dishes are usually not grounded. In a technical book i have here, it says the supports of antennas should be grounded.

i re-checked the whole stuff today, there's still the 90VAC but i made a mistake when checking the current. its not 9 mA but 1,7mA.
when i set my multimeter to mA AC, but have the positive wire still plugged to the Voltage measuring jack, it says 9mA.

I felt the current again when connecting the Audio cables from the PC. When the PC is connected, the voltage goes from 90 to 0.

can 1,7mA be normal?


i asked my boss today (master, technician) and he said the 9mA is too much, but 90VAC can be ok and come from Capacitors (of the power supplies?) in the hifi/tv appliances.

but even if everything is off i have 63V left. can't this be stray voltages of all the cables connected?

[This message has been edited by :andy: (edited 11-25-2003).]

[This message has been edited by :andy: (edited 11-25-2003).]


#139529 - 11/25/03 11:42 AM Re: Grounding shields of Hifi/Video cables  
SvenNYC  Offline
Member
Joined: Aug 2002
Posts: 1,691
New York City
Just a weird question...does the voltage reading in question go away when you reverse the AC plug in the socket?


#139530 - 11/25/03 12:04 PM Re: Grounding shields of Hifi/Video cables  
:andy:  Offline
Member
Joined: Oct 2003
Posts: 289
Germany
sven, no change with reversed plug of the schuko distributor.


#139531 - 11/25/03 12:43 PM Re: Grounding shields of Hifi/Video cables  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
It does sound as though it's leakage due to filter capacitors. Many of the modern units do have quite noticeable touch leakage current. I've often felt it while brushing against the rear panel of a device with a grounded plug in my hand. 90V is undoubtedly high, but if it's via a high impedance it will drop considerably when touched.

On the satellite power feed, I don't know about in Germany but the most common domestic installations (Sky Digi-Box set on the Astra 2 cluster at 29.2 deg. E) here do indeed feed power to the LNB along the satellite coax with different voltages for switching polarization. They also use a 22kHz signal to switch bands on the LNB, but that shouldn't cause any problems.

I always ground antenna masts and supports. It shouldn't affect the signal. If it does, then there is a problem elsewhere on the system.


#139532 - 11/25/03 02:19 PM Re: Grounding shields of Hifi/Video cables  
:andy:  Offline
Member
Joined: Oct 2003
Posts: 289
Germany
paul, we use the same system to switch polarisations by signal over the coax cable.


#139533 - 11/29/03 08:04 PM Re: Grounding shields of Hifi/Video cables  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,211
SI,New Zealand
Paul,
The polarisation voltages for an LNB are only in the area of 12-14V aren't they?.
With this size of voltage (90V), It makes me wonder if the SMPS in the satellite decoder isn't faulty?, no appliance should leak these sorts of voltages!.
Just with regard to stray voltages, I've lost count of the number of times that I've recieved a small zap, through either an ear or the side of my face when contacting a TV aerial with a masthead amp on it.
Some of the amps here use up to 22VAC and a shock like this can be quite disheartening, I once nearly fell off the roof of a 3 storey house, because of this. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin


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