Why do I have power/current through my aerial cable?

[scoobasteve]

Hi,

I'm a bit confused, have just been tidying up some aerial cabling whilst doing some work in my house and whilst connecting it up with an electric test screwdriver I noticed it suddenly lighting up!

I appear to have a current in the cable - the cable is a link which goes from where my aerial feed comes in house up to a bedroom where it plugs into a freeview box.

Is it normal for there to be power present at the freeview box aerial IN point or do I have a faulty box?

Steve

 

[ColJack]

don't know about freeview, but SKY has a voltage output from one of the aerial out's to power the "magic eye" remote relay in another room..

does your aerial have a splitter on it somewhere? that may add power to it..

 

[ericmark]

Aerial boosters and Digi eyes both need power and some boxes do have a function where power is sent up the aerial feed wire. This is no more than 12vdc.

Also any electrical storms or high wind can generate quite high voltages on the aerial enough to light a neon screwdriver in fact a near miss can blow up nearly every electrical item in the house. But in UK this is very rare.

Also the switch mode power supplies used with TV's and other RF boxes can also generate a small voltage enough at very low current to light a neon. With a 20,000 ohms to volt AVO you may measure 50v at times but very little current so does no harm.

Depending on what type of screwdriver you were using it could be any of the three. Do not earth as to earth will increase the likely hood of a lighting strike. Aerials should not be mounted on sharp corners like gable ends or on conductors like soot lined chimneys yet it seems that is where most are mounted in UK. But as radio hams we stick a bit more metal in the air than used with TV aerials so we have to be more careful.

Some TV plates include de-coupling capacitors to stop static but they also stop wanted DC for mast head amps so one has to be selective as to where fitted.

 

[davelx]

Those neon test screwdrivers are complete rubbish and if they develop a fault can be lethal. Also they are not reliable enough to be used to test for dead on a mains circuit - bin it!

They will light up simply from capacitive coupling between a mains cable and the aerial cable, or due to the filter capacitors in the equipment.

 

[ericmark]

Those neon test screwdrivers are complete rubbish and if they develop a fault can be lethal. Also they are not reliable enough to be used to test for dead on a mains circuit - bin it!

They will light up simply from capacitive coupling between a mains cable and the aerial cable, or due to the filter capacitors in the equipment.

My Martindale tester is little different to a test screwdriver and it is accepted as a valid method to test for dead when used with a proving unit.

Although neon screwdrivers which rely on ones body acting as an aerial can't really test for dead in same was as Martindale testers they are very good secondary safety device able to detect while working on items when power has returned.

We all know we should have detailed plans and should work out where the supply comes from but mistakes do happen and I love using a neon screwdriver as my terminal driver as a secondary test.

Here is a good example as aerials should not have voltage high enough to light any neon screwdriver. The post does not say if it was a neon screwdriver or one with a 12/24V bulb as used for testing on a car.

So the voltage if shown with a neon is likely to be static and the neon would have likely allowed it to slowly leak away and possibly prevented the user from getting a nasty belt. With that in mind the neon screwdriver may have saved their live and you say don't use them!

If it was a low voltage unit then likely the supply to digi eye or the like.

So to warn not to rely on a neon screwdriver is a valid comment. But to say "Bin it" is rather dangerous as it could well save their live.

Very early neon screwdrivers 1950's did not have a limiting resistor and were dangerous but all latter one do have limiting resistor and even if neon goes short circuit will not cause danger. You will always find one in my tool bag together with clamp on and Martindale tester they all have their place. I use a inverter and 12V alarm battery as proving unit.

 

[davelx]

ericmark - what has your Martindale tester got to do with anything I posted??? Can it be used to undo screws? No? Well it's not an screwdriver then, is it???

The OP said he used an ELECTRIC TEST SCREWDRIVER which is what I commented on. You've read plenty of posts about those on Electrics UK so you are perfectly well aware of their limitations.

 

[RF Lighting]

You can search all my tool bags and boxes. I don't own one, and would never want to. They are dangerous and misleading.

I was nearly killed as an apprentice because of a stupid neon screwdriver.

 

[ericmark]

You can search all my tool bags and boxes. I don't own one, and would never want to. They are dangerous and misleading.

I was nearly killed as an apprentice because of a stupid neon screwdriver.

And I was saved. I was to move a socket. House empty so turned off main isolator. Then started on socket never bothered to test as whole house dead. But it was connected to next doors supply. Neon screwdriver lit so I was saved.

Yes I should have tested but I was only young then and I have always tried to used a neon screwdriver as terminal driver since then.

And I have had circuits made live while working on them. Big site and incorrect information. But mistakes do happen and I see no reason for not edging ones bets and using a neon screwdriver as a second string to bow to high light mistakes.

I would agree the standard method with martindale is to connect wandering lead to neutral or earth but aerial method still works the same as screwdriver.

Working on a IT system in wet conditions the neon screwdriver was the best tool invented to stop one getting shocks. Used it all the time working on Robins TBM in Hong Kong.

They have their place. I would still test for dead in normal method and I would not rely on one. But one would not normally expect mains power on an aerial socket and as a result would be unlikely to test for dead so this is a very good example for where using a neon screwdriver as ones terminal driver could save one from a nasty belt. And if caused by static will also help to leak it away safely.

But the poster has never returned to say if it was a low 12 volt supply or higher voltage so could have been type with bulb and wandering lead as used with vehicles. It said nothing about using a neon screwdriver.

 

[Grizzly]

It said nothing about using a neon screwdriver.

... whilst connecting it up with an electric test screwdriver I noticed it suddenly lighting up!

 

[ericmark]

It said nothing about using a neon screwdriver.

... whilst connecting it up with an electric test screwdriver I noticed it suddenly lighting up!

Showing a test screwdriver which is Not a neon test screwdriver. Of course "scoobasteve" may have been using a neon type. But since he has never come back it hardly matters. Except to show how the use of neon screwdrivers may help avoid a shock when used on items where one would not expect there to be any connection to mains power.

See here for similar post where shocks have been received from coax cables. Seems this may be more of a problem than I had considered!

 

[sparky09]

HI
I was working on a property today which i had rewired and was getting a very slight "tingle" of the co ax s ! i have thoroughly tested the house and all readings were very satisfactory but this slight voltage has extremely worried me ? admittidely i did use a neon and it lit up ( i now theyre not gospel) so i put my multimeter from the back of the aerial outlet to the earth and was getting 30 volts - any ideas Guys
Sparky

 

[ericmark]

As a radio amateur I have had a few times specially in high winds where there has been a static build up on aerials. I used a set of large value resistors to leak this away and would not touch the lead in high winds.

The TV can also produce a voltage at the aerial and I have measured 50 volts on an AVO Mk8 set on 250 volt range. So at 10,000 ohms per volt if I remember correctly that's a load of 2,500,000 ohms so current of 0.02 mA which is so small it is unlikely to do anyone any harm. When I measured on a lower range the volts were much reduced today with digital meters we have really no idea of the load they put on a circuit and they act more like a valve volt meter.

The voltage from TV is caused because of the capacitors and inductances in the power supply and there is no direct connection.

The same can happen if the cable runs parallel to a mains cable.

 

[Sam Gangee]

Is it normal for there to be power present at the freeview box aerial IN point or do I have a faulty box

Yes, see http://www.satcure.co.uk/tech/shock.htm

 

[sparky09]

HI
I was working on a property today which i had rewired and was getting a very slight "tingle" of the co ax s ! i have thoroughly tested the house and all readings were very satisfactory but this slight voltage has extremely worried me ? admittidely i did use a neon and it lit up ( i now theyre not gospel) so i put my multimeter from the back of the aerial outlet to the earth and was getting 30 volts - any ideas Guys
Sparky

 

[sparky09]

HI
I was working on a property today which i had rewired and was getting a very slight "tingle" of the co ax s ! i have thoroughly tested the house and all readings were very satisfactory but this slight voltage has extremely worried me ? admittidely i did use a neon and it lit up ( i now theyre not gospel) so i put my multimeter from the back of the aerial outlet to the earth and was getting 30 volts - any ideas Guys
Sparky