by ericmark »
Wed Dec 16, 2009 1:41 pm
You must realise that Sparx and myself both have a hobby which means we take more note of lighting and EMC (Was called interference) then most. The regulations refer to where you have more than 25 electric storms per year and at the moment we don’t.
I went to a very interesting lecture and this basically split it into two.
1) prevention.
2) reduction of damage.
I would say in the normal home for latter we rely on insurance. For houses I know of three where lighting did damage and one of them had a long wire aerial running across his garden. The problem is 1000’s of amps need very thick and good conductors to lead them away safely and the cost of a lighting conductor on a normal house is not worth the money. Especially until people are given discount on their insurance for fitting them.
So back to 1) Prevention. Lighting like all electric takes the easy path. Which is normally the shortest to earth. Any metal sticking up in the air and grounded is likely to be targeted by lighting. But during a storm the air is moving and the ionised air is also moving so often although the metal starts the ionisation track the actual strike does not hit metal but something near by. This is why you should not shelter under a single tree. It can miss the tree and get you instead. As the metal becomes higher the chances of being hit increases.
For years we all earthed out aerial metalwork mainly so it would not electrocute you if touched it. As not only do storms produce lighting but generally a voltage differential in the air and we have all had (Static) shocks where this has charged up some metal work like a car. Steel radial tyres have reduced that. So as with so much in life we need to strike a balance. So we have a large resistor between aerial and earth to leak the voltage away so once the storm has passed it can be touched in safety and a spark gap in case it’s too much for resistor. You can see these in a master telephone socket.
TV is a problem. Many TV sockets have de-coupling capacitors, which in the main stop you getting a belt of aerial plug. But mast head amplifiers and LNB’s need DC supply so there are also versions without the capacitor or at least with some by-pass. I had band pass filters in my TV aerial which all had to be removed in order for the Sky Digi-eye to work.
Which brings us to EMC filters. Many are fitted from new by the manufactures. For example a good speaker network will have a band pass filter which directs the difference frequencies to bass, treble, and twitter speakers and in so doing also removes all or most of the interference (EMC) the speaker wires pick up. The power supply has to rectify and smooth the incoming AC to a DC and remove the ripples and really it does not matter too much what is on the incoming AC as the power supply will remove it.
Where the problem lies is the wires around your house act as a broadcast transmitting aerial and the speaker wires act as a receiving aerial. Twisting the cables and encasing in earthed braid will help. The same with the mains cables running the line with neutral (Which is not always the case with two way lighting) and Ali-tube cable will help.
But in the main if you buy good equipment in first place that’s all you need to do.
Fitting car radio’s was the first time I found this. I had earth the bonnet, put suppressers on ignition coil, and on alternator, and used a choke to supply radio but I could not get rid of engine noise. The old mechanic with me said maybe it’s the radio lets try fitting the one I have in my locker. It was perfect not a crackle. So as an experiment I removed all the suppressors I had tried and still perfect. So we had to tell customer the radio was rubbish. After that I have found again and again how cheap radios pick up interference and more expensive one’s filter it all out.
If you think of your house buying a Hi-Fi (I know old name) costing £2000 is cheaper than re-wiring the house with Ali-tube cable.
So although we may fit a filtered socket to stop spikes that is about the limit. Just buy a good audio system to start with.
Yes do obvious like don’t fit an aerial on a chimney which is stainless steel or soot lined and try to avoid gable end. But in real terms in this country all you need is insurance.
