Arial lead to earth voltage

[kthomas7]

If I measure with a multi meter from earth to my aerial lead I get a voltage varies between 10v and 80v depandant on how mant TV’s are switched on. All TV\\\'s are connected to the same aerial distribution. But TV’s are on a different ring main, I can even measure a voltage to the gas pipes!!! Any idea what the problem is? somthing seems wrong.

 

[Big_Spark]

Nothing is wrong, this is perfectly normal as there is always some leakage to true Earth from TV tuners. nature of the beast.

Further, you will be able to measure a voltage from the main aerial lead to true earth if you check..that is why voltage is called Potential difference..it is the potential charge difference between two electrically conductive points, material etc.

Nothing to worry about at all.

 

[kthomas7]

Thanks for your comment. But it is enough to give a small electric shock. How do large multi TV setups get over this like TV shops. Where they could get some very high voltages. This voltage is also evident on Audio interconnecting leads on the surround sound amp!

It seams if I can get a electric shock (how I discovered it) I should be worried?

 

[GaryB]

As Big_Spark says, this is normal. It happens on any double insulated appliance and is caused by very slight leakage through the capacitors that are present to ensure EMC (Electromagnetic Compatibility) compliance. As you've noticed, the current is tiny and is only usually measurable with a high impedance meter. It's an induced voltage - basically leakage, and is a tiny current and not dangerous.

I work for a TV/VCR/Camcorder manufacturer and we get regular calls about it.

Take a look at:
http://www.unitechelectronics.com/sparks.htm

 

[superstardeejay]

In large installations (pubs, hotels, prisons) where many TVs and other double insulated appliances are connected with conductive parts all commoned together through a distribution system, that distribution system normally has the screen wire earthed to mains earth, (usually as a design of the distribution amplifier) so the system as a whole cannot suffer from the effects of cumulative leakage currents reaching dangerous or annoying levels.

Other systems use galvanically isolated aerial outlets which means that the aerial connections into the wall are isolated by a small inbuilt transformer that cannot convey anything but the high frequency TV signal to the TV or anything back into the system.

Individual appliances are not allowed to leak a dangerous current to any exposed conductive part although many modern things allow enough voltage to appear on the aerial or other metal parts such that you get a shock if you touch it. It also happens with DVD players, Playstations, Mobile Phone Chargers, Laptop Adapters etc.

Some people are much more sensitive to this current than others and if it is enough of a concern to you, then introducing an earth to one point in the system will cure it. That earth may be something as simple as an AV feed from your TV to a class-1 audio amplifier or surround sound system which will drag the stray leakage down.

You have the RFI filtering and EMC regulations mandated by europe to thank for your little shocks.

 

[ericmark]

As already said this is common and the current is under the value that can be dangerous but there are some measures that can be taken to stop it.

The main problem is Sky who use a DC signal from their box to a digi-eye which allows one to change channel remotely from other rooms. Using any de-coupler either capacitive or inductive will stop this system from working.

Many wall plates do have de-coupling capacitors and these are really necessary in the first device that brings the signal in from outside. They stop static build up from atmospheric disturbances (Thunder Storms) from giving you a shock and this can build up to a lethal charge so for first termination (Likely a pre-amp) then some de-coupling is required.

After the first it can cause problems and I have removed all my band pass filters so the digi-eyes will work.

Today there is a whole host of filters available and it is common to combine Radio both VHF and digital plus standard TV and Sky TV together into one cable from aerials and LNB (Sky Dish) then separate again as they come out of the wall. These band pass filters do also stop and static build up being transferred.

From what you say I would assume in your case there is no problem. One can earth cables after the de-coupling capacitor but never before the de-coupling capacitor as if you earth an aerial it increases the chance of a lighting strike. In theory aerials should never be mounted on sharp edges of a building like the gable end or on conducting surfaces like a soot lined chimney. But you only have to look up to see most people don't worry about it.

As a radio amateur we tend to stick up more metal than most people so we are a little more aware of the problems. But there is only one method to protect totally from the damage caused by a lighting near miss or strike and that's insurance.

 

[Alarm]

As a radio amateur we tend to stick up more metal than most people so we are a little more aware of the problems. But there is only one method to protect totally from the damage caused by a lighting near miss or strike and that's insurance.

Rubber wellies, a kite tied to next doors antenna and insurance

 

[ericmark]

As a radio amateur we tend to stick up more metal than most people so we are a little more aware of the problems. But there is only one method to protect totally from the damage caused by a lighting near miss or strike and that's insurance.

Rubber wellies, a kite tied to next doors antenna and insurance

Be very wary of rubber wellies! Many have graphite in the mix to stop the problem of a belt from static so they are not as good of an insulator as one may expect.