Who asked about lightening and disconnecting TV aerials?

[ARW]

Just I have in my hand a 1964 copy of Aerial Handbook.

And it has this to say

maybe we need Bill's opinion.

[Peter Able]

Dodgy things whether there's lightning - or even lightening - about, or not.

At that time, the chassis was often connected to one of the mains wires. Too bad if you found out that the set was wrongly wired to the mains when attempting to pull the Belling-Lee connector out of the back of the set.

Some distribution systems leaked, too. Then you'd be zapped at the instant that the Belling-Lee disconnected from the TV.

That's for the metal-shell Belling-Lees - and for the guy who didn't make sure that his only electrical contact was with the plug.

PA

[ARW]

I spelt aerial correctly:-)

[NY]

"All receiver designs are obliged to maintain a d.c. path between the aerial terminal and the chassis in order to drain off such a charge". I wonder how well that worked in practice, if you were unlucky enough for the aerial to be struck. I suppose it is there for lightning *prevention*, like the mis-named lightning conductor on a building which is there to minimise the chance of the metal roof etc being struck, but which probably can't handle the huge current if it *is* struck.

[Peter Able]

Red herring time! It worked - but it never was intended to deal with direct strikes. What is?

PA

[ARW]

More than 3 amps?

[tony sayer]

Yes there is IIRC something like 2 meg ohms that will have a path to drain off any static charges from an aerial installation but if its struck then thats is an insignificance on the scale of things.

Best bet if you know there is a storm about is to unplug the aerial lead and drop it on the floor away from the TV not perfect but lightning contains very large voltages and currents and can and does cause a lot of damage even nearby strikes let alone direct ones.

Fix up a lightning conductor system if you like, got one here under construction as its been for bloody years !...

Need some earth rods and a down conductor usually ally tape around 25 mm wide and 4 mill thick much cheaper than copper!..

The idea is to shunt all the mega volts and amps past what you want to be protected..

[Brian Gaff (Sofa]

Well, the problem always has been one of how close a lightning strike must be to make any difference. I remember some time ago back in the analogue days, there was an aerial which was effectively a stack of little dipoles with a large reflector behind them, was it called the colour king or something like that?

The anecdotal evidence suggested more TV tuners died using them than with traditional yagi or logs. I always presumed that this was maybe due to the dc circuit of a normal aerial was short circuit, but not of the dipole kind. Also anywhere fringe where aerial amps on the mast were used, particularly the early ally can Antiference ones, tended to get the amp trashed at the slightest hint of a storm. I blue up three myself attempting to get Hannington from Surrey.

So the answer is, if you want to, pull the aerial out but its not going to save any active devices like masthead amps oar distribution amps still in the system. There do seem to be some places where a lot of lightning damage occurs, some of the WElsh Valleys for example. Also a lot of cable boxes die where TV services have overhead lines when there are storms. Brian

[Brian Gaff (Sofa]

No all aerial sockets had insulation capacitors in them to stop getting a severe shock. This was due to use of autotransformers or droppers rather than total isolation by a proper transformer. I could not read the doc as it was a picture, but I do know that its not just TVs, FM/DAB tuners are also affected if its too close. Its just a risk. I have over 150 metres of short wave aerials here and only in very bad storms would I unplug them from the receiver. Modern front ends are more durable lets say than the early transistor or fet ones were, but not as hardy as valves of course!

Brian

[tony sayer]

In article <sk635g$7ps$ [e-mail], Brian Gaff (Sofa) [e-mail]> scribeth thus

Thats one reason why the Russians used them in their Aircraft radios, they had great immunity to the EMP caused by Nuclear bomb detonations!!!..

[Martin Brown]

I have known my TV aerial lead to have enough potential on it to light my neon screwdriver and just about feel it as a little nip touching it. It was the latter that had me reaching for the neon screwdriver.

I guess there is sufficient capacitance in the coax downlead to store enough energy to actually feel it.

There was a thunderstorm later on that day. Perhaps not the best time to have been tweaking an aerial but there you go...

I'd expect old valve based sets to be fairly tolerant of lightning. I am less confident about the outcome for modern solid state fet front ends. Generally I have found lightning protection wanting in all the situations where a building I was in took a direct hit. The chunky surge arresters saved themselves by allowing more delicate line drivers to be destroyed in the main computer - not the intention of having them at all :(

[Mark Carver]

I've lost three masthead amp transistors in my time due to lightning (BFR-90 from memory)

They may well have saved the telly itself from EMP damage

[Sysadmin]

There is in a cathode ray tube (~500pf)

[charles]

it ws indeeed the Colour King made - by Wolsey. It had 4 double dipoles. The Cour Prince had only 2. They were wideband aerials with a good F/B ratio.

[NY]

I had more than "a little nip" when I unplugged an aerial cable from a tuner that was connected to my (earthed) PC, while touching the PC case with my other hand. I eventually tracked it down to my TV which was on another leg of the aerial cable (loft-mounted amplifier and splitter - unearthed). There were a lot of culprits to check because my VCR was connected by phono cable to my hifi, and all these devices were unearthed (double insulated).

When I measured the voltage on the screen of TV's aerial socket wrt ground, it was about 130 V (!). There was evidently a large resistor, because when I added a "human-sized" resistor where my body would have been (*) if I'd had mains earth in one hand and aerial cable screen in the other, the voltage dropped to about 50 V - still enough to give a fairly strong jolt when you're not expecting it.

I added a wire from the earth pin of the amplifier to the screen of the aerial cable (just one earth point, to avoid hum loops) to make sure the TV's aerial/phono screen was grounded.

This was a modern TV - I bought it new in 2000 - so it wasn't one of those ancient TVs with a live chassis. I was very underwhelmed with the reply I got from Panasonic when I reported the problem: 130 V no-load voltage on a part of the TV that the customer might touch, reducing to 50 V with a human load, was not what I would regard as a safe design. I did check that the plug was correctly wired, and that live and neutral hadn't got swapped.

(*) The shock was strong enough that I didn't want to repeat it, even as part of the test!

[NY]

The only time I've suffered lightning damage was to the power supply for my PC. And that was due to a lightning strike about a mile away (judging by the

5-second delay between lightning flash and very loud bang). So it was via mains rather than through aerial, and was a strike on an EHT pylon, so it had come via several step-down transformers.

I happened to have a spare PSU, and I had the old one disconnected and removed, and the new one in place and connected, and the PC booted up again, all within the 10-minute gap between one reading and the next from my weather station which the PC was logging.

I doubt whether any form of surge protection will protect against a direct lightning strike (*), but it should remove the sort of spikes that a lightning strike on a distant power line can cause.

(*) eg to an overhead 240 V mains supply wire from street to house

[Harry Bloomfield, Esq.]

NY explained :

I'm not surprised you got no joy from Panasonic, you will find that almost all domestic TV's do have ac connection to the antenna socket, via a capacitor.

[Harry Bloomfield, Esq.]

NY explained :

There was a major strike 100 yards from here, which hit the church spire. It blew their conductor out, then tracked down the spire doing considerable damage to the plaster work inside.

Here the surge took out my sat systems front end, a modem, an electronic phone, answering machine, plus some other minor items. Locally, it damaged several phone lines too- BT spent a week up and down the street repairing the damage.

[Martin Brown]

Some TV's like Freesat ones provide phantom power for the LNB but I'd have expected something more like 12-24v no load maximum. I remember the days of live chassis mains TVs full of glowing valves and in the back of colour sets a hefty EHT rectifier that would double as an X-ray tube!

[charles]

I found that on mine some years ago. Using a decent multimeter I found about 30uA to ground.